Showing 1–50 of 301 results for author: Dai, P

Magnetic-field induced momentum-dependent symmetry breaking in CsV$_3$Sb$_5$ revealed by magneto-ARPES

Authors: Jianwei Huang, Zheng Ren, Hengxin Tan, Jounghoon Hyun, Yichen Zhang, Thomas Hulse, Zhaoyu Liu, Jonathan M. DeStefano, Yaofeng Xie, Ziqin Yue, Junichiro Kono, Pengcheng Dai, Yu He, Aki Pulkkinen, Ján Minár, Jiun-Haw Chu, Ziqiang Wang, Binghai Yan, Rafael M. Fernandes, Ming Yi

Abstract: In quantum materials with multiple degrees of freedom with similar energy scales, intertwined electronic orders with distinct broken symmetries often appear in a strongly coupled fashion. Recently, in a class of kagome superconductors represented by CsV$_3$Sb$_5$, experimental reports have suggested rotational symmetry breaking and time reversal symmetry breaking associated with a charge density w… ▽ More In quantum materials with multiple degrees of freedom with similar energy scales, intertwined electronic orders with distinct broken symmetries often appear in a strongly coupled fashion. Recently, in a class of kagome superconductors represented by CsV$_3$Sb$_5$, experimental reports have suggested rotational symmetry breaking and time reversal symmetry breaking associated with a charge density wave (CDW) order, revealing an exotic nature of this CDW order. Here, utilizing our recently developed capability of performing angle-resolved photoemission spectroscopy in a tunable magnetic field (magneto-ARPES), we reveal momentum-selective response of the electronic structure of CsV$_3$Sb$_5$ to an external magnetic field. While the response in the electronic structure is clearly compatible with piezomagnetism, strong orbital-selectivity is observed. Specifically, bands associated with the vanadium $d$-orbitals contributing to the Van Hove singularities (VHS) near the Brillouin zone (BZ) boundary exhibit selective spectral broadening that breaks C$_6$ rotational symmetry and is odd in magnetic field, disappearing above the CDW transition. Meanwhile, the antimony $p$-orbital dominated electron pocket at the BZ center becomes elongated under an applied field -- an effect that persists above the $T_{\rm CDW}$. Our observations delineate the origin of the time-reversal symmetry breaking associated with the vanadium VHS-bands at the onset of the CDW order, while the field-induced rotational symmetry breaking largely associated with the antimony $p$ orbitals reflects fluctuations beyond the CDW ordering temperature. Our magneto-ARPES work demonstrates a novel tuning knob for disentangling intertwined orders in the momentum space for quantum materials. △ Less

Submitted 12 December, 2025; originally announced December 2025.

Comments: 37 pages, 12 figures, accepted for publication in Nature Physics

Evidence for itinerant electron-local moment interaction in Li-doped $α$-MnTe

Authors: Tingjun Zhang, Steven J. Gomez Alvarado, Sijie Xu, Travis J. Williams, Xiaoping Wang, Junhong He, Matthew B. Stone, Colin Sarkis, Feng Ye, Zhaoyu Liu, Jinyulin Li, Aparna Jayakumar, Zehao Wang, Yaofeng Xie, Ching-Wu Chu, Liangzi Deng, Emilia Morosan, Pengcheng Dai

Abstract: We use inelastic neutron scattering to study the impact of Li doping on the semiconducting altermagnet $α$-MnTe. Introducing Li results in a spin reorientation from in-plane to out-of-plane and increases the density of itinerant carriers. While the spin waves in Li-doped $α$-MnTe remain largely Heisenberg-like, there is significant spin wave broadening across the entire Brillouin zone, signaling e… ▽ More We use inelastic neutron scattering to study the impact of Li doping on the semiconducting altermagnet $α$-MnTe. Introducing Li results in a spin reorientation from in-plane to out-of-plane and increases the density of itinerant carriers. While the spin waves in Li-doped $α$-MnTe remain largely Heisenberg-like, there is significant spin wave broadening across the entire Brillouin zone, signaling enhanced magnon damping induced by itinerant carriers. By extracting the local dynamic susceptibility and applying the total moment sum rule, we find that both undoped and Li-doped $α$-MnTe exhibit the full expected Mn$^{2+}$ local moment of $\approx5.9~μ_\mathrm{B}$ with $S=5/2$. These results demonstrate that Li-doped $α$-MnTe hosts robust local-moment magnetism whose interactions are mediated via Ruderman-Kittel-Kasuya-Yosida-type interactions, highlighting the importance of itinerant carriers in magneto-transport and spin dynamic properties of altermagnets. △ Less

Submitted 30 November, 2025; originally announced December 2025.

Comments: 7 pages, 4 figures

Bi-altermagnetism unveiled by sublattice-specific circular dichroism in resonant inelastic X-ray scattering

Authors: G. Channagowdra, A. Singh, H. Y. Huang, M. Furo, Bin Gao, Pengcheng Dai, C. T. Chen, J. Kunes, A. Fujimori, S-W. Cheong, A. Hariki, D. J. Huang

Abstract: An altermagnet is a recently identified class of magnets that exhibit a zero net magnetic moment but break symmetry under the combined operations of parity and time reversal. It typically consists of two magnetic sites of opposite spins related by rotation within the unit cell. Here, we use circular dichroism (CD) in resonant inelastic X-ray scattering (RIXS) to identify a new form of altermagneti… ▽ More An altermagnet is a recently identified class of magnets that exhibit a zero net magnetic moment but break symmetry under the combined operations of parity and time reversal. It typically consists of two magnetic sites of opposite spins related by rotation within the unit cell. Here, we use circular dichroism (CD) in resonant inelastic X-ray scattering (RIXS) to identify a new form of altermagnetism, namely bi-altermagnetism, in the correlated insulator Fe2Mo3O8, which comprises two altermagnetic sublattices: one with alternating quasi-octahedral Fe environments and the other with alternating tetrahedral Fe environments. We experimentally revealed the emergence of CD in an achiral, zero-magnetization system, thereby probing mirror-symmetry breaking associated with altermagnetic order. Notably, the CD appeared at sublattice-specific excitations of the octahedral and tetrahedral sites, indicating symmetry breaking in both altermagnetic sublattices. Calculations based on a model with the bi-altermagnetic order along the c axis successfully reproduce the observed CD. Our findings provide compelling evidence for bi-altermagnetism in Fe2Mo3O8, and showcase the use of RIXS-CD as a probe of magnetic sublattices in systems with zero net magnetization. △ Less

Submitted 30 November, 2025; originally announced December 2025.

Comments: 8 pages, 4 figures. Comments are welcome

Interacting spin and charge density waves in kagome metal FeGe

Authors: Mason L. Klemm, Tingjun Zhang, Barry L. Winn, Fankang Li, Feng Ye, Sijie Xu, Xiaokun Teng, Bin Gao, Ming Yi, Pengcheng Dai

Abstract: Unveiling the interplay between spin density wave (SDW) and charge density wave (CDW) orders in correlated electron materials is important to obtain a comprehensive understanding of their electronic, structural, and magnetic properties. Kagome lattice materials are interesting because their flat electronic bands, Dirac points, and van Hove singularities can enable a variety of exotic electronic an… ▽ More Unveiling the interplay between spin density wave (SDW) and charge density wave (CDW) orders in correlated electron materials is important to obtain a comprehensive understanding of their electronic, structural, and magnetic properties. Kagome lattice materials are interesting because their flat electronic bands, Dirac points, and van Hove singularities can enable a variety of exotic electronic and magnetic phenomena. The kagome metal FeGe, which exhibits a CDW order deep within an A-type antiferromagnetic (AFM) phase, was found to respond dramatically to post-growth annealing - with the ability to tune the CDW repeatedly from long-range order to no (or extremely weak) order. Additionally, neutron scattering studies suggest that incommensurate magnetic peaks that onsets at $T_{Canting}$ = $T_{SDW} \approx$ 60 K in the system arise from a SDW order instead of the AFM double cone structure. Here we use inelastic neutron scattering to show two distinct spin excitations exist below $T_{Canting}$ corresponding to two coexisting magnetic orders in the system in both sets of annealed samples with and without CDW. While CDW order or no order can dramatically affect the onset temperature of $T_{Canting}$ and elastic incommensurate magnetic scattering, its impact on low-energy spin fluctuations is more limited. In both samples, a pair of gapless incommensurate spin excitations arising from the SDW order wavevector coexist with gapped commensurate spin waves from the A-type AFM order across $T_{Canting}$. Low-energy spin excitations for both samples couple dynamically to the lattice through enhanced magnetic scattering intensity on cooling below $T_{CDW}$, regardless the status of the static long-range CDW order. The incommensurate SDW order in the long-range CDW ordered sample also induces a tiny in-plane lattice distortion of the kagome lattice that is absent in the no CDW ordered sample. △ Less

Submitted 1 October, 2025; originally announced October 2025.

Strain-tunable anomalous Hall effect in hexagonal MnTe

Authors: Zhaoyu Liu, Sijie Xu, Jonathan M. DeStefano, Elliott Rosenberg, Tingjun Zhang, Jinyulin Li, Matthew B. Stone, Feng Ye, Rong Cong, Siyu Pan, Ching-Wu Chu, Liangzi Deng, Emilia Morosan, Rafael M. Fernandes, Jiun-Haw Chu, Pengcheng Dai

Abstract: The ability to control and manipulate time-reversal ($T$) symmetry-breaking phases with near-zero net magnetization is a sought-after goal in spintronic devices. The recently discovered hexagonal altermagnet manganese telluride ($α$-MnTe) is a prime example. It has a compensated altermagnetic ground state where the magnetic moments are aligned in each layer and stacked antiparallel along the $c$ a… ▽ More The ability to control and manipulate time-reversal ($T$) symmetry-breaking phases with near-zero net magnetization is a sought-after goal in spintronic devices. The recently discovered hexagonal altermagnet manganese telluride ($α$-MnTe) is a prime example. It has a compensated altermagnetic ground state where the magnetic moments are aligned in each layer and stacked antiparallel along the $c$ axis, yet it exhibits a spontaneous anomalous Hall effect (AHE) that breaks the $T$-symmetry with a vanishingly small $c$-axis ferromagnetic (FM) moment. However, the presence of three 120$^\circ$ separated in-plane magnetic domains presents a challenge in understanding the origin of the AHE and the effective control of the altermagnetic state. Here we use neutron scattering to show that a compressive uniaxial strain along the next-nearest-neighbor Mn-Mn bond direction detwins $α$-MnTe into a single in-plane magnetic domain, aligning the in-plane moments along the same axis. Furthermore, we find that uniaxial strain (-0.2% to 0.1%) significantly sharpens the magnetic hysteresis loop and switches the sign of the AHE near room temperature. Remarkably, this is achieved without altering the altermagnetic phase-transition temperature or substantially changing the small $c$-axis FM moment. Combined with our phenomenological model, we argue that these effects result from the modification of the electronic Berry curvature by a combination of both spin-orbit coupling and strain. Our work not only unambiguously establishes the relationship between the in-plane moment direction and the AHE in $α$-MnTe but also paves the way for future applications in highly scalable, strain-tunable magnetic sensors and spintronic devices. △ Less

Submitted 15 October, 2025; v1 submitted 23 September, 2025; originally announced September 2025.

Comments: 21 pages, 13 figures, theoretical model added

Signatures of three-state Potts nematicity in spin excitations of the van der Waals antiferromagnet FePSe$_3$

Authors: Weiliang Yao, Viviane Peçanha Antonio, Devashibhai Adroja, S. J. Gomez Alvarado, Bin Gao, Sijie Xu, Ruixian Liu, Xingye Lu, Pengcheng Dai

Abstract: In two-dimensional (2D) nearly square-lattice quantum materials, electron correlations can induce an electronic nematic phase with twofold rotational ($C_2$) symmetry that profoundly impacts their properties. For 2D materials with threefold rotational ($C_3$) symmetry, such as the honeycomb lattice, a vestigial three-state Potts nematic order has been observed in the van der Waals antiferromagnet… ▽ More In two-dimensional (2D) nearly square-lattice quantum materials, electron correlations can induce an electronic nematic phase with twofold rotational ($C_2$) symmetry that profoundly impacts their properties. For 2D materials with threefold rotational ($C_3$) symmetry, such as the honeycomb lattice, a vestigial three-state Potts nematic order has been observed in the van der Waals antiferromagnet (AFM) FePSe$_3$ via optical and thermodynamic methods under uniaxial strain. Here, we use neutron scattering to study the magnetic order and spin excitations of FePSe$_3$ under uniaxial strain. In the AFM ordered state, we find that $\sim$0.6% tensile strain significantly suppresses one zigzag domain and promotes the other two, lowering the AFM order and spin waves to $C_2$ symmetry. The broken $C_3$ symmetry in spin excitations persists slightly above $T_{\rm{N}}\approx 108.6$ K, where the zigzag AFM order is absent. Our results thus provide direct evidence of magnetoelastic coupling and suggest that the three-state Potts nematicity in paramagnetic spin excitations arises from the vestigial order associated with the low-temperature zigzag AFM order. △ Less

Submitted 2 September, 2025; originally announced September 2025.

Thermoelectric evidence of the electronic structure changes from the charge-density-wave transition in FeGe

Authors: Kaila Jenkins, Yuan Zhu, Dechen Zhang, Guoxin Zheng, Kuan-Wen Chen, Aaron Chan, Sijie Xu, Mason L. Klemm, Bin Gao, Ming Yi, Pengcheng Dai, Lu Li

Abstract: Kagome metals provide a material platform for probing new correlated quantum phenomena due to the naturally incorporated linear dispersions, flat bands, and Van Hove singularities in their electronic structures. Among these quantum phenomena is the charge density wave (CDW), or the distortion of the lattice structure due to the motion of correlated electrons through the material. CDWs lower the en… ▽ More Kagome metals provide a material platform for probing new correlated quantum phenomena due to the naturally incorporated linear dispersions, flat bands, and Van Hove singularities in their electronic structures. Among these quantum phenomena is the charge density wave (CDW), or the distortion of the lattice structure due to the motion of correlated electrons through the material. CDWs lower the energy of the compound, creating an energy gap that facilitates behaviors akin to superconductivity, nonlinear transport, or other quantum correlated phenomena. The kagome metal FeGe has been shown to host a CDW transition at approximately 100 K, and its occurrence is strongly influenced by the sample annealing conditions. However, a notable gap in the literature is the lack of clear thermoelectric transport evidence for electronic structure changes associated with this CDW transition. Here we present evidence of electron behavior modification due to annealing disorder via thermoelectric measurements on FeGe crystals presenting a CDW transition and those without a CDW. The observed Nernst effect and Seebeck effect under sufficient annealing demonstrate modified electrical transport properties resulting from induced disorder, including a change in carrier sign and an enhancement of the Nernst effect due to the CDW. Our results provide evidence of multiple phase transitions, which confirms the influence of CDW on the thermal properties of FeGe and demonstrates the suppression of CDW with sufficient disordering. △ Less

Submitted 26 August, 2025; originally announced August 2025.

Comments: 4 figures

Dichotomy of flat bands in the van der Waals ferromagnet Fe$_5$GeTe$_2$

Authors: Han Wu, Jianwei Huang, Chaowei Hu, Lei Chen, Yiqing Hao, Yue Shi, Paul Malinowski, Yucheng Guo, Bo Gyu Jang, Jian-Xin Zhu, Andrew F. May, Siqi Wang, Xiang Chen, Yaofeng Xie, Bin Gao, Yichen Zhang, Ziqin Yue, Zheng Ren, Makoto Hashimoto, Donghui Lu, Alexei Fedorov, Sung-Kwan Mo, Junichiro Kono, Yu He, Robert J. Birgeneau , et al. (6 additional authors not shown)

Abstract: Quantum materials with bands of narrow bandwidth near the Fermi level represent a promising platform for exploring a diverse range of fascinating physical phenomena, as the high density of states within the small energy window often enables the emergence of many-body physics. On one hand, flat bands can arise from strong Coulomb interactions that localize atomic orbitals. On the other hand, quantu… ▽ More Quantum materials with bands of narrow bandwidth near the Fermi level represent a promising platform for exploring a diverse range of fascinating physical phenomena, as the high density of states within the small energy window often enables the emergence of many-body physics. On one hand, flat bands can arise from strong Coulomb interactions that localize atomic orbitals. On the other hand, quantum destructive interference can quench the electronic kinetic energy. Although both have a narrow bandwidth, the two types of flat bands should exhibit very distinct spectral properties arising from their distinctive origins. So far, the two types of flat bands have only been realized in very different material settings and chemical environments, preventing a direct comparison. Here, we report the observation of the two types of flat bands within the same material system--an above-room-temperature van der Waals ferromagnet, Fe$_{5-x}$GeTe$_2$, distinguishable by a switchable iron site order. The contrasting nature of the flat bands is also identified by the remarkably distinctive temperature-evolution of the spectral features, indicating that one arises from electron correlations in the Fe(1) site-disordered phase, while the other geometrical frustration in the Fe(1) site-ordered phase. Our results therefore provide a direct juxtaposition of the distinct formation mechanism of flat bands in quantum materials, and an avenue for understanding the distinctive roles flat bands play in the presence of magnetism, topology, and lattice geometrical frustration, utilizing sublattice ordering as a key control parameter. △ Less

Submitted 6 August, 2025; v1 submitted 4 August, 2025; originally announced August 2025.

Comments: The manuscript was submitted on June 12 2024

Magnetism of kagome metals $\left(\text{Fe}_{1-x} \text{Co}_{x}\right) \text{Sn}$ studied by $μ$SR

Authors: Yipeng Cai, Sungwon Yoon, Qi Sheng, Guoqiang Zhao, Eric Francis Seewald, Sanat Ghosh, Julian Ingham, Abhay Narayan Pasupathy, Raquel Queiroz, Hechang Lei, Yaofeng Xie, Pengcheng Dai, Takashi Ito, Ruyi Ke, Robert J. Cava, Sudarshan Sharma, Mathew Pula, Graeme M. Luke, Kenji M. Kojima, Yasutomo J. Uemura

Abstract: We study the magnetic properties of the metallic kagome system $\left(\mathrm{Fe}_{1-x} \mathrm{Co}_{x}\right) \mathrm{Sn}$ by a combination of Muon Spin Relaxation ($μ\mathrm{SR}$), magnetic susceptibility and Scanning Tunneling Microscopy (STM) measurements, in single crystal specimens with Co concentrations $\mathrm{x}=0,0.11,0.8$. In the undoped antiferromagnetic compound FeSn, we find possibl… ▽ More We study the magnetic properties of the metallic kagome system $\left(\mathrm{Fe}_{1-x} \mathrm{Co}_{x}\right) \mathrm{Sn}$ by a combination of Muon Spin Relaxation ($μ\mathrm{SR}$), magnetic susceptibility and Scanning Tunneling Microscopy (STM) measurements, in single crystal specimens with Co concentrations $\mathrm{x}=0,0.11,0.8$. In the undoped antiferromagnetic compound FeSn, we find possible signatures for a previously unidentified phase that sets in at $T^*\sim 50$ K, well beneath the Neel temperature $T_N \sim 376$ K, as indicated by a peak in the relaxation rate $1/T_1$ observed in zero field (ZF) and longitudinal field (LF) $μ\mathrm{SR}$ measurements, with a corresponding anomaly in the ac and dc-susceptibility, and an increase in the static width $1/T_2$ in ZF measurements. No signatures of spatial symmetry breaking are found in STM down to $7$ K. In $\mathrm{Fe}_{0.2} \mathrm{Co}_{0.8} \mathrm{Sn}$, we find canonical spin glass behavior with freezing temperature $T_{g} \sim 3.5 \mathrm{~K}$; the ZF and LF time spectra exhibit results similar to those observed in dilute alloy spin glasses CuMn and AuFe, with a critical behavior of $1 / T_{1}$ at $T_{g}$ and $1 / \mathrm{T}_{1}\rightarrow 0$ as $T \rightarrow 0$. The absence of spin dynamics at low temperatures makes a clear contrast to the spin dynamics observed by $μ\mathrm{SR}$ in many geometrically frustrated spin systems on insulating kagome, pyrochlore, and triangular lattices. The spin glass behavior of CoSn doped with dilute Fe moments is shown to originate primarily from the randomness of doped Fe moments rather than due to geometrical frustration of the underlying lattice. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Journal ref: Phys. Rev. B 111, 214412 (2025)

Linear scaling relation between two-dimensional massless Dirac fermion Fermi velocity and Fe-As bond length in iron arsenide superconductor systems

Authors: Chengpu Lv, Jianzhou Zhao, Yueshan Xu, Yu Song, Chenglin Zhang, Mykhaylo Ozerov, Pengcheng Dai, Nan-Lin Wang, Zhi-Guo Chen

Abstract: Two-dimensional (2D) massless Dirac fermions (MDF), which represent a type of quasi-particles with linear energy-momentum dispersions only in 2D momentum space, provide a fertile ground for realizing novel quantum phenomena. However, 2D MDF were seldom observed in the superconducting bulk states of 3D materials. Furthermore, as a cornerstone for accurately tuning the quantum phenomena based on 2D… ▽ More Two-dimensional (2D) massless Dirac fermions (MDF), which represent a type of quasi-particles with linear energy-momentum dispersions only in 2D momentum space, provide a fertile ground for realizing novel quantum phenomena. However, 2D MDF were seldom observed in the superconducting bulk states of 3D materials. Furthermore, as a cornerstone for accurately tuning the quantum phenomena based on 2D MDF, a quantitative relationship between 2D MDF and a structural parameter has rarely been revealed so far. Here, we report magneto-infrared spectroscopy studies of the iron-arsenide-superconductor systems NaFeAs and $A\mathrm{Fe_2As_2} (A = \mathrm{Ca, Ba})$ at temperature $T \sim 4.2 $ K and at magnetic fields ($B$) up to 17.5 T. Our results demonstrate the existence of 2D MDF in the superconducting bulk state of NaFeAs. Moreover, the 2D-MDF Fermi velocities in NaFeAs and $A\mathrm{Fe_2As_2} (A = \mathrm{Ca, Ba})$, which are extracted from the slopes of the linear $\sqrt{B}$ dependences of the Landau-level transition energies, scale linearly with the Fe-As bond lengths. The linear scaling between the 2D-MDF Fermi velocities and the Fe-As bond lengths is supported by (i) the linear relationship between the square root of the effective mass of the $d_{xy}$ electrons and the Fe-As bond length and (ii) the linear dependence of the square root of the calculated tight-binding hopping energy on the Fe-As bond length. Our results open up new avenues for exploring and tuning novel quantum phenomena based on 2D MDF in the superconducting bulk states of 3D materials. △ Less

Submitted 21 May, 2025; originally announced May 2025.

Comments: Accepted in Physical Review B (Letter)

Anomalous Electrical Transport in the Kagome Magnet YbFe$_6$Ge$_6$

Authors: Weiliang Yao, Supeng Liu, Hodaka Kikuchi, Hajime Ishikawa, Øystein S. Fjellvåg, David W. Tam, Feng Ye, Douglas L. Abernathy, George D. A. Wood, Devashibhai Adroja, Chun-Ming Wu, Chien-Lung Huang, Bin Gao, Yaofeng Xie, Yuxiang Gao, Karthik Rao, Emilia Morosan, Koichi Kindo, Takatsugu Masuda, Kenichiro Hashimoto, Takasada Shibauchi, Pengcheng Dai

Abstract: Two-dimensional (2D) kagome metals offer a unique platform for exploring electron correlation phenomena derived from quantum many-body effects. Here, we report a combined study of electrical magnetotransport and neutron scattering on YbFe$_6$Ge$_6$, where the Fe moments in the 2D kagome layers exhibit an $A$-type collinear antiferromagnetic order below $T_{\rm{N}} \approx 500$ K. Interactions betw… ▽ More Two-dimensional (2D) kagome metals offer a unique platform for exploring electron correlation phenomena derived from quantum many-body effects. Here, we report a combined study of electrical magnetotransport and neutron scattering on YbFe$_6$Ge$_6$, where the Fe moments in the 2D kagome layers exhibit an $A$-type collinear antiferromagnetic order below $T_{\rm{N}} \approx 500$ K. Interactions between the Fe ions in the layers and the localized Yb magnetic ions in between reorient the $c$-axis aligned Fe moments to the kagome plane below $T_{\rm{SR}} \approx 63$ K. Our magnetotransport measurements show an intriguing anomalous Hall effect (AHE) that emerges in the spin-reorientated collinear state, accompanied by the closing of the spin anisotropy gap as revealed from inelastic neutron scattering. The gapless spin excitations and the Yb-Fe interaction are able to support a dynamic scalar spin chirality, which explains the observed AHE. Therefore, our study demonstrates spin fluctuations may provide an additional scattering channel for the conduction electrons and give rise to AHE even in a collinear antiferromagnet. △ Less

Submitted 16 April, 2025; originally announced April 2025.

Comments: 7 pages, 5 figures, to be published at PRL

Correlation between Complex Spin Textures and the Magnetocaloric and Hall Effects in Eu(Ga$_{1-x}$Al$_x$)$_4$ ($x$ = 0.9, 1)

Authors: Kelly J. Neubauer, Kevin Allen, Jaime M. Moya, Mason L. Klemm, Feng Ye, Zachary Morgan, Lisa DeBeer-Schmitt, Wei Tian, Emilia Morosan, Pengcheng Dai

Abstract: Determining the electronic phase diagram of a quantum material as a function of temperature (T) and applied magnetic field (H) forms the basis for understanding the microscopic origin of transport properties, such as the anomalous Hall effect (AHE) and topological Hall effect (THE). For many magnetic quantum materials, including EuAl$_4$, a THE arises from a topologically protected magnetic skyrmi… ▽ More Determining the electronic phase diagram of a quantum material as a function of temperature (T) and applied magnetic field (H) forms the basis for understanding the microscopic origin of transport properties, such as the anomalous Hall effect (AHE) and topological Hall effect (THE). For many magnetic quantum materials, including EuAl$_4$, a THE arises from a topologically protected magnetic skyrmion lattice with a non-zero scalar spin chirality. We identified a square skyrmion lattice (sSkL) peak in Eu(Ga$_{1-x}$Al$_x$)$_4$ ($x$ = 0.9) identical to the peak previously observed in EuAl$_4$ by performing neutron scattering measurements throughout the phase diagram. Comparing these neutron results with transport measurements, we found that in both compounds the maximal THE does not correspond to the sSkL area. Instead of the maximal THE, the maximal magnetocaloric effect (MCE) boundaries better identify the sSkL lattice phase observed by neutron scattering measurements. The maximal THE therefore arises from interactions of itinerant electrons with frustrated spin fluctuations in a topologically trivial magnetic state. △ Less

Submitted 9 January, 2025; originally announced January 2025.

Absence of Acoustic Phonon Anomaly in a Kagome Metal with Short-ranged Structural Modulation

Authors: Weiliang Yao, Supeng Liu, Zifan Xu, Daisuke Ishikawa, Zehao Wang, Bin Gao, Sijie Xu, Feng Ye, Kenichiro Hashimoto, Takasada Shibauchi, Alfred Q. R. Baron, Pengcheng Dai

Abstract: Kagome lattice $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) superconductors without magnetism from vanadium $d$-electrons are intriguing because they have a novel charge density wave (CDW) order around 90 K and display superconductivity at $\sim$3 K that competes with the CDW order. Recently, CsCr$_3$Sb$_5$, isostructural to $A$V$_3$Sb$_5$, was found to have concurrent structural and magnetic phase transi… ▽ More Kagome lattice $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) superconductors without magnetism from vanadium $d$-electrons are intriguing because they have a novel charge density wave (CDW) order around 90 K and display superconductivity at $\sim$3 K that competes with the CDW order. Recently, CsCr$_3$Sb$_5$, isostructural to $A$V$_3$Sb$_5$, was found to have concurrent structural and magnetic phase transition at $T^{\ast}\approx$ 55 K that can be suppressed by pressure to induce superconductivity [Liu \textit{et al.}, \href{https://doi.org/10.1038/s41586-024-07761-x}{Nature \textbf{632}, 1032 (2024)}]. Here, we use elastic and inelastic X-ray scattering to study the microscopic origin of the structural transition in CsCr$_3$Sb$_5$. Although our elastic measurements confirm the 4$\times$1$\times$1 superlattice order below $T^{\ast}$, its underlying correlation is rather short-ranged. Moreover, our inelastic measurements at the superlattice wavevectors around (3, 0, 0) find no evidence of a significant acoustic phonon anomaly below $T^{\ast}$, similar to the case of $A$V$_3$Sb$_5$. The absence of acoustic phonon anomaly indicates a weak electron-phonon coupling in CsCr$_3$Sb$_5$, suggesting that the structural transition is likely associated with an unconventional CDW order. △ Less

Submitted 21 October, 2024; originally announced October 2024.

Comments: Six pages, four figures

Vacancy-induced suppression of CDW order and its impact on magnetic order in kagome antiferromagnet FeGe

Authors: Mason L. Klemm, Saif Siddique, Yuan-Chun Chang, Sijie Xu, Yaofeng Xie, Tanner Legvold, Mehrdad T. Kiani, Feng Ye, Huibo Cao, Yiqing Hao, Wei Tian, Hubertus Luetkens, Masaaki Matsuda, Douglas Natelson, Zurab Guguchia, Chien-Lung Huang, Ming Yi, Judy J. Cha, Pengcheng Dai

Abstract: Two-dimensional (2D) kagome lattice metals are interesting because they display flat electronic bands, Dirac points, Van Hove singularities, and can have interplay between charge density wave (CDW), magnetic order, and superconductivity. In kagome lattice antiferromagnet FeGe, a short-range CDW order was found deep within an antiferromagnetically ordered state, interacting with the magnetic order.… ▽ More Two-dimensional (2D) kagome lattice metals are interesting because they display flat electronic bands, Dirac points, Van Hove singularities, and can have interplay between charge density wave (CDW), magnetic order, and superconductivity. In kagome lattice antiferromagnet FeGe, a short-range CDW order was found deep within an antiferromagnetically ordered state, interacting with the magnetic order. Surprisingly, post-growth annealing of FeGe at 560$^{\circ}$C can suppress the CDW order while annealing at 320$^{\circ}$C induces a long-range CDW order, with the ability to cycle between the states repeatedly by annealing. Here we perform transport, neutron scattering, scanning transmission electron microscopy (STEM), and muon spin rotation ($μ$SR) experiments to unveil the microscopic mechanism of the annealing process and its impact on magneto-transport, CDW, and magnetic properties of FeGe. We find that 560$^{\circ}$C annealing creates germanium vacancies uniformly distributed throughout the FeGe kagome lattice, which prevent the formation of Ge-Ge dimers necessary for the CDW order. Upon annealing at 320$^{\circ}$C, the system segregates into stoichiometric FeGe regions with long-range CDW order and regions with stacking faults that act as nucleation sites for the CDW. The presence or absence of CDW order greatly affects the anomalous Hall effect, incommensurate magnetic order, and spin-lattice coupling in FeGe, thus placing FeGe as the only known kagome lattice material with a tunable CDW and magnetic order, potentially useful for sensing and information transmission. △ Less

Submitted 17 October, 2024; originally announced October 2024.

Persistent flat band splitting and strong selective band renormalization in a kagome magnet thin film

Authors: Zheng Ren, Jianwei Huang, Hengxin Tan, Ananya Biswas, Aki Pulkkinen, Yichen Zhang, Yaofeng Xie, Ziqin Yue, Lei Chen, Fang Xie, Kevin Allen, Han Wu, Qirui Ren, Anil Rajapitamahuni, Asish Kundu, Elio Vescovo, Junichiro Kono, Emilia Morosan, Pengcheng Dai, Jian-Xin Zhu, Qimiao Si, Ján Minár, Binghai Yan, Ming Yi

Abstract: Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X=Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While pa… ▽ More Magnetic kagome materials provide a fascinating playground for exploring the interplay of magnetism, correlation and topology. Many magnetic kagome systems have been reported including the binary FemXn (X=Sn, Ge; m:n = 3:1, 3:2, 1:1) family and the rare earth RMn6Sn6 (R = rare earth) family, where their kagome flat bands are calculated to be near the Fermi level in the paramagnetic phase. While partially filling a kagome flat band is predicted to give rise to a Stoner-type ferromagnetism, experimental visualization of the magnetic splitting across the ordering temperature has not been reported for any of these systems due to the high ordering temperatures, hence leaving the nature of magnetism in kagome magnets an open question. Here, we probe the electronic structure with angle-resolved photoemission spectroscopy in a kagome magnet thin film FeSn synthesized using molecular beam epitaxy. We identify the exchange-split kagome flat bands, whose splitting persists above the magnetic ordering temperature, indicative of a local moment picture. Such local moments in the presence of the topological flat band are consistent with the compact molecular orbitals predicted in theory. We further observe a large spin-orbital selective band renormalization in the Fe d_xy+d_(x^2-y^2 ) spin majority channel reminiscent of the orbital selective correlation effects in the iron-based superconductors. Our discovery of the coexistence of local moments with topological flat bands in a kagome system echoes similar findings in magic-angle twisted bilayer graphene, and provides a basis for theoretical effort towards modeling correlation effects in magnetic flat band systems. △ Less

Submitted 8 October, 2024; originally announced October 2024.

Journal ref: Nature Communications 15, 9376 (2024)

Spin Excitation Continuum in the Exactly Solvable Triangular-Lattice Spin Liquid CeMgAl11O19

Authors: Bin Gao, Tong Chen, Chunxiao Liu, Mason L. Klemm, Shu Zhang, Zhen Ma, Xianghan Xu, Choongjae Won, Gregory T. McCandless, Naoki Murai, Seiko Ohira-Kawamura, Stephen J. Moxim, Jason T. Ryan, Xiaozhou Huang, Xiaoping Wang, Julia Y. Chan, Sang-Wook Cheong, Oleg Tchernyshyov, Leon Balents, Pengcheng Dai

Abstract: In magnetically ordered insulators, elementary quasiparticles manifest as spin waves - collective motions of localized magnetic moments propagating through the lattice - observed via inelastic neutron scattering. In effective spin-1/2 systems where geometric frustrations suppress static magnetic order, spin excitation continua can emerge, either from degenerate classical spin ground states or from… ▽ More In magnetically ordered insulators, elementary quasiparticles manifest as spin waves - collective motions of localized magnetic moments propagating through the lattice - observed via inelastic neutron scattering. In effective spin-1/2 systems where geometric frustrations suppress static magnetic order, spin excitation continua can emerge, either from degenerate classical spin ground states or from entangled quantum spins characterized by emergent gauge fields and deconfined fractionalized excitations. Comparing the spin Hamiltonian with theoretical models can unveil the microscopic origins of these zero-field spin excitation continua. Here, we use neutron scattering to study spin excitations of the two-dimensional (2D) triangular-lattice effective spin-1/2 antiferromagnet CeMgAl11O19. Analyzing the spin waves in the field-polarized ferromagnetic state, we find that the spin Hamiltonian is close to an exactly solvable 2D triangular-lattice XXZ model, where degenerate 120$^\circ$ ordered ground states - umbrella states - develop in the zero temperature limit. We then find that the observed zero-field spin excitation continuum matches the calculated ensemble of spin waves from the umbrella state manifold, and thus conclude that CeMgAl11O19 is the first example of an exactly solvable spin liquid on a triangular lattice where the spin excitation continuum arises from the ground state degeneracy. △ Less

Submitted 28 August, 2024; originally announced August 2024.

Comments: 11 pages, 5 figures

Evidence chain for time-reversal symmetry-breaking kagome superconductivity

Authors: Hanbin Deng, Guowei Liu, Z. Guguchia, Tianyu Yang, Jinjin Liu, Zhiwei Wang, Yaofeng Xie, Sen Shao, Haiyang Ma, William Liège, Frédéric Bourdarot, Xiao-Yu Yan, Hailang Qin, C. Mielke III, R. Khasanov, H. Luetkens, Xianxin Wu, Guoqing Chang, Jianpeng Liu, Morten Holm Christensen, Andreas Kreisel, Brian Møller Andersen, Wen Huang, Yue Zhao, Philippe Bourges , et al. (3 additional authors not shown)

Abstract: Superconductivity and magnetism are antagonistic quantum matter, while their intertwining has long been considered in frustrated-lattice systems1-3. In this work, we utilize scanning tunneling microscopy and muon spin resonance to discover time-reversal symmetry-breaking superconductivity in kagome metal Cs(V,Ta)3Sb5, where the Cooper pairing exhibits magnetism and is modulated by it. In the magne… ▽ More Superconductivity and magnetism are antagonistic quantum matter, while their intertwining has long been considered in frustrated-lattice systems1-3. In this work, we utilize scanning tunneling microscopy and muon spin resonance to discover time-reversal symmetry-breaking superconductivity in kagome metal Cs(V,Ta)3Sb5, where the Cooper pairing exhibits magnetism and is modulated by it. In the magnetic channel, we observe spontaneous internal magnetism in a full-gap superconducting state. Under perturbations of inverse magnetic fields, we detect a time-reversal asymmetrical interference of Bogoliubov quasi-particles at a circular vector. At this vector, the pairing gap spontaneously modulates, which is distinct from pair density waves occurring at a point vector and consistent with the theoretical proposal of unusual interference effect under time-reversal symmetry-breaking. The correlation between internal magnetism, Bogoliubov quasi-particles, and pairing modulation provides a chain of experimental clues for time-reversal symmetry-breaking kagome superconductivity. △ Less

Submitted 5 August, 2024; originally announced August 2024.

Comments: To appear in Nature Materials (2024)

Journal ref: Nat. Mater. 23, 1639-1644 (2024)

Search for orbital magnetism in the kagome superconductor ${\rm CsV_3Sb_5}$ using neutron diffraction

Authors: William Liège, Yaofeng Xie, Dalila Bounoua, Yvan Sidis, Frédéric Bourdarot, Yongkai Li, Zhiwei Wang, Jia-Xin Yin, Pengcheng Dai, Philippe Bourges

Abstract: As many Kagome metals, the topological superconductor AV$_3$Sb$_5$ with (A = K,Rb,Cs) hosts a charge density wave . A related chiral flux phase that breaks the time-reversal symmetry has been further theoretically predicted in these materials. The flux phase is associated with loop currents that produce ordered orbital magnetic moments, which would occur at the momentum points, $\bf M$, characteri… ▽ More As many Kagome metals, the topological superconductor AV$_3$Sb$_5$ with (A = K,Rb,Cs) hosts a charge density wave . A related chiral flux phase that breaks the time-reversal symmetry has been further theoretically predicted in these materials. The flux phase is associated with loop currents that produce ordered orbital magnetic moments, which would occur at the momentum points, $\bf M$, characterizing the charge-density wave state. Polarized neutron-diffraction experiments have been performed on an assembly of single crystals of ${\rm CsV_3Sb_5}$ to search for such orbital magnetic moments. No evidence for the existence of a three-dimensionally ordered moment is found at any temperature at the first ${\bf M_1}$=(1/2,0,0) point in the Brillouin zone within an excellent experimental uncertainty, ${\it i.e.}$ ${\bf m}=0 \pm 0.01μ_B$ per vanadium atom. However, a hint to a magnetic orbital moment is found in the second Brillouin zone at {\bf M$_2$}=(1/2,1/2,0) at the detection limit of the experiment. Some loop currents patterns flowing ${\it only}$ on vanadium triangles are able to account for this finding suggesting an ordered orbital magnetic moment of, at most, $\sim 0.02 \pm 0.01μ_B$ per vanadium triangle. △ Less

Submitted 19 July, 2024; originally announced July 2024.

Comments: 11 figures

Journal ref: Phys. Rev. B 110, 195109 (2024)

Spin Excitations and Flat Electronic Bands in a Cr-based Kagome Superconductor

Authors: Zehao Wang, Yucheng Guo, Hsiao Yu Huang, Fang Xie, Yuefei Huang, Bin Gao, Ji Seop Oh, Han Wu, Jun Okamoto, Ganesha Channagowdra, Chien Te Chen, Feng Ye, Xingye Lu, Zhaoyu Liu, Zheng Ren, Yuan Fang, Yiming Wang, Ananya Biswas, Yichen Zhang, Ziqin Yue, Cheng Hu, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Makoto Hashimoto , et al. (11 additional authors not shown)

Abstract: In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FB… ▽ More In the quest for topology- and correlation-driven quantum states, kagome lattice materials have garnered significant interest for their band structures, featuring flat bands (FBs) from the quantum destructive interference of the electronic wavefunction. Tuning an FB to the chemical potential could induce electronic instabilities and emergent orders. Despite extensive studies, direct evidence of FBs tuned to the chemical potential and their role in emergent orders in bulk materials remains lacking. Using angle-resolved photoemission spectroscopy, resonant inelastic X-ray scattering, and density functional theory, we show that the low-energy structure of the Cr-based kagome metal superconductor {\Cr} is dominated by FBs at the Fermi level. We also observe low-energy magnetic excitations evolving across the low-temperature transition, largely consistent with the FB shift. Our results suggest that the low-temperature order contains a magnetic origin and that the kagome FBs may play a role in the emergence of this order. △ Less

Submitted 24 November, 2025; v1 submitted 7 June, 2024; originally announced June 2024.

Comments: Published in Nature Communications 16, 7573 (2025). 11 pages, 4 main figures, 13 supplementary figures

Journal ref: Nat Commun 16, 7573 (2025)

Correlations among physical properties of pervious concrete with different aggregate sizes and mix proportions

Authors: Qifeng Lyu, Pengfei Dai, Anguo Chen

Abstract: Permeable pavement material can benefit urban environment. Here in this work, different aggregate sizes and mix proportions were used to manufacture pervious pavement concrete and investigate correlations among its properties. The porosity, permeability, compressive strength, inner structure, thermal conductivity, and abrasion resistance of the specimens were obtained. Results showed lower aggrega… ▽ More Permeable pavement material can benefit urban environment. Here in this work, different aggregate sizes and mix proportions were used to manufacture pervious pavement concrete and investigate correlations among its properties. The porosity, permeability, compressive strength, inner structure, thermal conductivity, and abrasion resistance of the specimens were obtained. Results showed lower aggregate-to-cement ratios and higher water-to-cement ratios led to porosity reduction, which decreased the permeability coefficient but increased the compressive strength, thermal conductivity, and abrasion resistance of the pervious concrete. Compared to the mixes, the aggregate sizes affected the physical properties of pervious concrete less. However, the sizes of pores and cement in the pervious concrete were more affected by aggregate sizes than by mixes. Moreover, the porosity, permeability coefficient, and compressive strength of the pervious concrete can be correlated by the power law, whereas the correlation between the porosity and abrasion resistance index can be fitted by a linear law. △ Less

Submitted 2 June, 2024; originally announced June 2024.

Comments: This article is published on https://doi.org/10.1080/14680629.2024.2331193

Journal ref: Road Materials and Pavement Design, 2024

Anomalous properties of spark plasma sintered boron nitride solids

Authors: Abhijit Biswas, Peter Serles, Gustavo A. Alvarez, Jesse Schimpf, Michel Hache, Jonathan Kong, Pedro Guerra Demingos, Bo Yuan, Tymofii S. Pieshkov, Chenxi Li, Anand B. Puthirath, Bin Gao, Tia Gray, Xiang Zhang, Jishnu Murukeshan, Robert Vajtai, Pengcheng Dai, Chandra Veer Singh, Jane Howe, Yu Zou, Lane W. Martin, James Patrick Clancy, Zhiting Tian, Tobin Filleter, Pulickel M. Ajayan

Abstract: Hexagonal boron nitride (h-BN) is a brittle ceramic with a layered structure, however, recent experiments have suggested that inter-layer structural engineering could be key to new structural and functional properties. Here we report the scalable bulk synthesis of high-density crystalline h-BN solids, by using high-temperature spark plasma sintering (SPS) of h-BN powders, which show high values of… ▽ More Hexagonal boron nitride (h-BN) is a brittle ceramic with a layered structure, however, recent experiments have suggested that inter-layer structural engineering could be key to new structural and functional properties. Here we report the scalable bulk synthesis of high-density crystalline h-BN solids, by using high-temperature spark plasma sintering (SPS) of h-BN powders, which show high values of mechanical strength, ductility, dielectric constant, thermal conductivity, and exceptional neutron radiation shielding capability. Through exhaustive characterizations we reveal that SPS induces non-basal plane crystallinity, twisting of layers, and facilitates inter-grain fusion with a high degree of in-plane alignment across macroscale dimensions, resulting in near-theoretical density and improved properties. Our findings highlight the importance of material design, via new approaches such as layer twisting and interlayer interconnections, to create novel ceramics with properties that could go beyond their intrinsic limits. △ Less

Submitted 20 May, 2025; v1 submitted 9 May, 2024; originally announced May 2024.

Comments: Authors revised version, 46 pages, 4 Main figures, 16 SI figures

Journal ref: Materials Today 86, 42-51 (2025)

Spin-charge-lattice coupling across the charge density wave transition in a Kagome lattice antiferromagnet

Authors: Xiaokun Teng, David W. Tam, Lebing Chen, Hengxin Tan, Yaofeng Xie, Bin Gao, Garrett E. Granroth, Alexandre Ivanov, Philippe Bourges, Binghai Yan, Ming Yi, Pengcheng Dai

Abstract: Understanding spin and lattice excitations in a metallic magnetic ordered system form the basis to unveil the magnetic and lattice exchange couplings and their interactions with itinerant electrons. Kagome lattice antiferromagnet FeGe is interesting because it displays rare charge density wave (CDW) deep inside the antiferromagnetic ordered phase that interacts with the magnetic order. We use neut… ▽ More Understanding spin and lattice excitations in a metallic magnetic ordered system form the basis to unveil the magnetic and lattice exchange couplings and their interactions with itinerant electrons. Kagome lattice antiferromagnet FeGe is interesting because it displays rare charge density wave (CDW) deep inside the antiferromagnetic ordered phase that interacts with the magnetic order. We use neutron scattering to study the evolution of spin and lattice excitations across the CDW transition $T_{\rm CDW}$ in FeGe. While spin excitations below $\sim$100 meV can be well described by spin waves of a spin-1 Heisenberg Hamiltonian, spin excitations at higher energies are centered around the Brillouin zone boundary and extend up to $\sim180$ meV consistent with quasiparticle excitations across spin-polarized electron-hole Fermi surfaces. Furthermore, $c$-axis spin wave dispersion and Fe-Ge optical phonon modes show a clear hardening below $T_{\rm CDW}$ due to spin-charge-lattice coupling but with no evidence for a phonon Kohn anomaly. By comparing our experimental results with density functional theory calculations in absolute units, we conclude that FeGe is a Hund's metal in the intermediate correlated regime where magnetism has contributions from both itinerant and localized electrons arising from spin polarized electronic bands near the Fermi level. △ Less

Submitted 5 April, 2024; originally announced April 2024.

Journal ref: Phys. Rev. Lett. 133, 046502 (2024)

Emergent photons and fractionalized excitations in a quantum spin liquid

Authors: Bin Gao, Félix Desrochers, David W. Tam, Paul Steffens, Arno Hiess, Yixi Su, Sang-Wook Cheong, Yong Baek Kim, Pengcheng Dai

Abstract: A quantum spin liquid (QSL) arises from a highly entangled superposition of many degenerate classical ground states in a frustrated magnet, and is characterized by emergent gauge fields and deconfined fractionalized excitations (spinons). Because such a novel phase of matter is relevant to high-transition-temperature superconductivity and quantum computation, the microscopic understanding of QSL s… ▽ More A quantum spin liquid (QSL) arises from a highly entangled superposition of many degenerate classical ground states in a frustrated magnet, and is characterized by emergent gauge fields and deconfined fractionalized excitations (spinons). Because such a novel phase of matter is relevant to high-transition-temperature superconductivity and quantum computation, the microscopic understanding of QSL states is a long-sought goal in condensed matter physics. The 3D pyrochlore lattice of corner-sharing tetrahedra can host a QSL with U(1) gauge fields called quantum spin ice (QSI), which is a quantum (with effective $S=1/2$) analog of the classical (with large effective moment) spin ice. A key difference between QSI and classical spin ice is the predicted presence of the linearly dispersing collective excitations near zero energy, dubbed the "photons", arising from emergent quantum electrodynamics, in addition to the spinons at higher energies. Recently, 3D pyrochlore systems Ce2M2O7 (M = Sn, Zr, Hf) have been suggested as effective $S=1/2$ QSI candidates, but there has been no evidence of quasielastic magnetic scattering signals from photons, a key signature for a QSI. Here, we use polarized neutron scattering experiments on single crystals of Ce2Zr2O7 to conclusively demonstrate the presence of magnetic excitations near zero energy at 50 mK in addition to signatures of spinons at higher energies. By comparing the energy (E), wave vector (Q), and polarization dependence of the magnetic excitations with theoretical calculations, we conclude that Ce2Zr2O7 is the first example of a dipolar-octupolar $π$ flux QSI with dominant dipolar Ising interactions, therefore identifying a microscopic Hamiltonian responsible for a QSL. △ Less

Submitted 5 April, 2024; originally announced April 2024.

Tunability of charge density wave in a magnetic kagome metal

Authors: Ji Seop Oh, Ananya Biswas, Mason Klemm, Hengxin Tan, Makoto Hashimoto, Donghui Lu, Binghai Yan, Pengcheng Dai, Robert J. Birgeneau, Ming Yi

Abstract: The discovery of the charge density wave order (CDW) within a magnetically ordered phase in the kagome lattice FeGe has provided a promising platform to investigate intertwined degrees of freedom in kagome lattices. Recently, a method based on post-annealing has been suggested to manipulate the CDW order in kagome FeGe towards either long-range or suppressed orders. Here, we provide a comprehensiv… ▽ More The discovery of the charge density wave order (CDW) within a magnetically ordered phase in the kagome lattice FeGe has provided a promising platform to investigate intertwined degrees of freedom in kagome lattices. Recently, a method based on post-annealing has been suggested to manipulate the CDW order in kagome FeGe towards either long-range or suppressed orders. Here, we provide a comprehensive comparison of the experimentally measured electronic structures of FeGe crystals that have undergone different post-annealing procedures and demonstrate the remarkable effectiveness on tuning the CDW gap without strong perturbation on the underlying electronic structure. Moreover, we observe an additional low temperature transition that only appears in crystals with a long-range CDW order, which we associate with a lattice-spin coupled order. Our work indicates a likely strong sensitivity of the CDW order to disorder in FeGe, and provides evidence for strong coupling between the electronic, lattice, and spin degrees of freedom in this kagome magnet. △ Less

Submitted 2 April, 2024; originally announced April 2024.

Journal ref: Science Advances 11, eadt2195 (2025)

Charge Dynamics of an Unconventional Three-Dimensional Charge Density Wave in Kagome FeGe

Authors: Shaohui Yi, Zhiyu Liao, Qi Wang, Haiyang Ma, Jianpeng Liu, Xiaokun Teng, Bin Gao, Pengcheng Dai, Yaomin Dai, Jianzhou Zhao, Yanpeng Qi, Bing Xu, Xianggang Qiu

Abstract: We report on the charge dynamics of kagome FeGe, an antiferromagnet with a charge density wave (CDW) transition at $T_{\mathrm{CDW}} \simeq 105$ K, using polarized infrared spectroscopy and band structure calculations. We reveal pronounced optical anisotropy along the $a$- and $c$-axis, as well as an unusual response associated with three-dimensional CDW order. Above $T_{\mathrm{CDW}}$, there is a… ▽ More We report on the charge dynamics of kagome FeGe, an antiferromagnet with a charge density wave (CDW) transition at $T_{\mathrm{CDW}} \simeq 105$ K, using polarized infrared spectroscopy and band structure calculations. We reveal pronounced optical anisotropy along the $a$- and $c$-axis, as well as an unusual response associated with three-dimensional CDW order. Above $T_{\mathrm{CDW}}$, there is a notable transfer of spectral weight (SW) from high to low energies, promoted by the magnetic splitting-induced shift in bands. Across the CDW transition, we observe a sudden SW transfer from low to high energies over a broad range, along with the emergence of new excitations around 1200 cm$^{-1}$. These results contrast with observations from other kagome metals like CsV$_3$Sb$_5$, where the nesting of VHSs leads to a clear CDW gap feature. Instead, our findings can be accounted for by a $2\times2\times2$ CDW ground state driven by a first-order structural transition involving large partial Ge1-dimerization. Our study thus unveils a complex interplay among structure, magnetism, and charge order, offering valuable insights for a comprehensive understanding of CDW order in FeGe. △ Less

Submitted 28 February, 2025; v1 submitted 14 March, 2024; originally announced March 2024.

Comments: 8 pages, 3 figures

Journal ref: Phys. Rev. Lett. 134, 086902 (2025)

Conventional Superconductivity in the Doped Kagome Superconductor Cs(V0.86Ta0.14)3Sb5 from Vortex Lattice Studies

Authors: Yaofeng Xie, Nathan Chalus, Zhiwei Wang, Weiliang Yao, Jinjin Liu, Yugui Yao, Jonathan S. White, Lisa M. DeBeer-Schmitt, Jia-Xin Yin, Pengcheng Dai, Morten Ring Eskildsen

Abstract: A hallmark of unconventional superconductors is their complex electronic phase diagrams where "intertwined orders" of charge-spin-lattice degrees of freedom compete and coexist as in copper oxides and iron pnictides. While the electronic phase diagram of kagome lattice superconductor such as CsV3Sb5 also exhibits complex behavior involving coexisting and competing charge density wave order and sup… ▽ More A hallmark of unconventional superconductors is their complex electronic phase diagrams where "intertwined orders" of charge-spin-lattice degrees of freedom compete and coexist as in copper oxides and iron pnictides. While the electronic phase diagram of kagome lattice superconductor such as CsV3Sb5 also exhibits complex behavior involving coexisting and competing charge density wave order and superconductivity, much is unclear about the microscopic origin of superconductivity. Here, we study the vortex lattice (VL) in superconducting state of Cs(V0.86Ta0.14)3Sb5, where the Ta-doping suppresses charge order and enhances superconductivity. Using small-angle neutron scattering, a strictly bulk probe, we show that the VL exhibits a strikingly conventional behavior. This includes a triangular VL with a period consistent with 2e-pairing, a field dependent scattering intensity that follows a London model, and a temperature dependence consistent with a uniform superconducting gap expected for s-wave pairing. These results suggest that optimal bulk superconductivity in Cs(V1-xTax)3Sb5 arises from a conventional Bardeen-Cooper-Schrieffer electron-lattice coupling, different from spin fluctuation mediated unconventional copper and iron based superconductors. △ Less

Submitted 24 July, 2024; v1 submitted 9 March, 2024; originally announced March 2024.

Comments: Multiple revisions

Journal ref: Nature Communications 15, 6467 (2024)

Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe3

Authors: Lebing Chen, Xiaokun Teng, Ding Hu, Feng Ye, Garrett E. Granroth, Ming Yi, Jae-Ho Chung, Robert J. Birgeneau, Pengcheng Dai

Abstract: We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe$_3$. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe$_3$ is first-order in nature. In addition, our INS measurements reveal that the sp… ▽ More We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe$_3$. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe$_3$ is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming across $T_N$, we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations above $T_N$. These results suggest that the first-order AF phase transition in FePSe$_3$ may arise from the competition between $C_3$ symmetric AF and $C_1$ symmetric FM spin fluctuations around $T_N$, in place of a conventional second-order AF phase transition. △ Less

Submitted 26 February, 2024; originally announced February 2024.

In-plane anisotropic magnetoresistance in detwinned $BaFe_{2-x}Ni_{x}As_{2}$ ($x$ = 0, 0.6)

Authors: Kelly J. Neubauer, Mason L. Klemm, Shirin Mozaffari, Lin Jiao, Alexei E. Koshelev, Alexander Yaresko, Ming Yi, Luis Balicas, Pengcheng Dai

Abstract: Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including $BaFe_2As_2$, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by… ▽ More Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including $BaFe_2As_2$, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic (AF) ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned $BaFe_2As_2$ and compare the results to the non-magnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, $BaFe_2As_2$ exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in $BaFe_2As_2$ that is not present in the Ni-doped material. △ Less

Submitted 1 February, 2024; originally announced February 2024.

Nematic quantum disordered state in FeSe

Authors: Ruixian Liu, Matthew B. Stone, Shang Gao, Mitsutaka Nakamura, Kazuya Kamazawa, Aleksandra Krajewska, Helen C. Walker, Peng Cheng, Rong Yu, Qimiao Si, Pengcheng Dai, Xingye Lu

Abstract: The unusual quantum-disordered magnetic ground state intertwined with superconductivity and electronic nematicity in FeSe has been a research focus in iron-based superconductors. However, the intrinsic spin excitations across the entire Brillouin zone in detwinned FeSe, which forms the basis for a microscopic understanding of the magnetic state and superconductivity, remain to be determined. Here,… ▽ More The unusual quantum-disordered magnetic ground state intertwined with superconductivity and electronic nematicity in FeSe has been a research focus in iron-based superconductors. However, the intrinsic spin excitations across the entire Brillouin zone in detwinned FeSe, which forms the basis for a microscopic understanding of the magnetic state and superconductivity, remain to be determined. Here, we use inelastic neutron scattering to map out the spin excitations of FeSe dewtinned with a uniaxial-strain device. We find that the stripe spin excitations (Q=(1, 0)/(0, 1)) exhibit the $C_2$ symmetry up to $E\approx120$ meV, while the N{é}el spin excitations (Q=(1, 1)) retain their $C_4$ symmetry in the nematic state. The temperature dependence of the difference in the spin excitations at Q=(1, 0) and (0, 1) for temperatures above the structural phase transition unambiguously shows the establishment of the nematic quantum disordered state. The similarity of the Néel excitations in FeSe and NaFeAs suggests that the Néel excitations are driven by the enhanced electron correlations in the $3d_{xy}$ orbital. By determining the key features of the stripe excitations and fitting their dispersions using a Heisenberg Hamiltonian with biquadratic interaction ($J_1$-$K$-$J_2$), we establish a spin-interaction phase diagram and conclude that FeSe is close to a crossover region between the antiferroquadrupolar, Néel, and stripe ordering regimes. The results provide an experimental basis for establishing a microscopic theoretical model to describe the origin and intertwining of the emergent orders in iron-based superconductors. △ Less

Submitted 1 April, 2025; v1 submitted 10 January, 2024; originally announced January 2024.

Comments: 9 pages, 5 figures

Multi-condensate lengths with degenerate excitation gaps in BaNi$_2$As$_2$ revealed by muon spin relaxation study

Authors: Kaiwen Chen, Zihao Zhu, Yaofeng Xie, Adrian D. Hillier, James S. Lord, Pengcheng Dai, Lei Shu

Abstract: The recently discovered (Ba,Sr)Ni$_2$As$_2$ family provides an ideal platform for investigating the interaction between electronic nematicity and superconductivity. Here we report the muon spin relaxation ($μ$SR) measurements on BaNi$_2$As$_2$. Transverse-field $μ$SR experiments indicate that the temperature dependence of superfluid density is best fitted with a single-band $s$-wave model. On the… ▽ More The recently discovered (Ba,Sr)Ni$_2$As$_2$ family provides an ideal platform for investigating the interaction between electronic nematicity and superconductivity. Here we report the muon spin relaxation ($μ$SR) measurements on BaNi$_2$As$_2$. Transverse-field $μ$SR experiments indicate that the temperature dependence of superfluid density is best fitted with a single-band $s$-wave model. On the other hand, the magnetic penetration depth $λ$ shows magnetic field dependence, which contradicts with the single-band fully-gapped scenario. Zero-field $μ$SR experiments indicate the absence of spontaneous magnetic field in the superconducting state, showing the preservation of time-reversal symmetry in the superconducting state. Our $μ$SR experiments suggest that BaNi$_2$As$_2$ is a fully-gapped multiband superconductor. The superconducting gap amplitudes of each band are nearly the same while different bands exhibit different coherence lengths. The present work helps to elucidate the controversial superconducting property of this parent compound, paving the way for further research on doping the system with Sr to enhance superconductivity. △ Less

Submitted 9 January, 2024; originally announced January 2024.

Comments: Accepted by Phys. Rev. B

Two-Step Electronic Response to Magnetic Ordering in a van der Waals Ferromagnet

Authors: Han Wu, Jian-Xin Zhu, Lebing Chen, Matthew W Butcher, Ziqin Yue, Dongsheng Yuan, Yu He, Ji Seop Oh, Jianwei Huang, Shan Wu, Cheng Gong, Yucheng Guo, Sung-Kwan Mo, Jonathan D. Denlinger, Donghui Lu, Makoto Hashimoto, Matthew B. Stone, Alexander I. Kolesnikov, Songxue Chi, Junichiro Kono, Andriy H. Nevidomskyy, Robert J. Birgeneau, Pengcheng Dai, Ming Yi

Abstract: The two-dimensional (2D) material Cr$_2$Ge$_2$Te$_6$ is a member of the class of insulating van der Waals magnets. Here, using high resolution angle-resolved photoemission spectroscopy in a detailed temperature dependence study, we identify a clear response of the electronic structure to a dimensional crossover in the form of two distinct temperature scales marking onsets of modifications in the e… ▽ More The two-dimensional (2D) material Cr$_2$Ge$_2$Te$_6$ is a member of the class of insulating van der Waals magnets. Here, using high resolution angle-resolved photoemission spectroscopy in a detailed temperature dependence study, we identify a clear response of the electronic structure to a dimensional crossover in the form of two distinct temperature scales marking onsets of modifications in the electronic structure. Specifically, we observe Te $p$-orbital-dominated bands to undergo changes at the Curie transition temperature T$_C$ while the Cr $d$-orbital-dominated bands begin evolving at a higher temperature scale. Combined with neutron scattering, density functional theory calculations, and Monte Carlo simulations, we find that the electronic system can be consistently understood to respond sequentially to the distinct temperatures at which in-plane and out-of-plane spin correlations exceed a characteristic length scale. Our findings reveal the sensitivity of the orbital-selective electronic structure for probing the dynamical evolution of local moment correlations in vdW insulating magnets. △ Less

Submitted 20 December, 2023; v1 submitted 18 December, 2023; originally announced December 2023.

Comments: PRB, in press

Journal ref: Physical Review B 109, 045416 (2024)

Nematic superconductivity from selective orbital pairing in Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals

Authors: Mason Klemm, Shirin Mozaffari, Rui Zhang, Brian W. Casas, Alexei E. Koshelev, Ming Yi, Luis Balicas, Pengcheng Dai

Abstract: We use transport measurements to determine the in-plane anisotropy of the upper critical field Hc2 in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence (x) of the upper critical field anisotropy for fields along the inter-planar (c-axis) and in-plane field directions was found to increas… ▽ More We use transport measurements to determine the in-plane anisotropy of the upper critical field Hc2 in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence (x) of the upper critical field anisotropy for fields along the inter-planar (c-axis) and in-plane field directions was found to increase in the overdoped regime. For underdoped samples, which exhibit a spin nematic phase below the tetragonal to orthorhombic structural transition temperature Ts , we find that Hc2 along the a-axis is considerably lower than that along the b-axis. The upper critical field anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with inelastic neutron scattering studies of spin excitations, and angle-resolved photoemission spectroscopy, we conclude that superconductivity in under-doped iron pnictides is orbital selective - with a dominant contribution from electrons with the dyz orbital character and being intimately associated with spin excitations. △ Less

Submitted 13 November, 2023; originally announced November 2023.

Non-Fermi liquid behavior in a correlated flatband pyrochlore lattice

Authors: Jianwei Huang, Lei Chen, Yuefei Huang, Chandan Setty, Bin Gao, Yue Shi, Zhaoyu Liu, Yichen Zhang, Turgut Yilmaz, Elio Vescovo, Makoto Hashimoto, Donghui Lu, Boris I. Yakobson, Pengcheng Dai, Jiun-Haw Chu, Qimiao Si, Ming Yi

Abstract: Electronic correlation effects are manifested in quantum materials when either the onsite Coulomb repulsion is large or the electron kinetic energy is small. The former is the dominant effect in the cuprate superconductors or heavy fermion systems while the latter in twisted bilayer graphene or geometrically frustrated metals. However, the simultaneous cooperation of both effects in the same quant… ▽ More Electronic correlation effects are manifested in quantum materials when either the onsite Coulomb repulsion is large or the electron kinetic energy is small. The former is the dominant effect in the cuprate superconductors or heavy fermion systems while the latter in twisted bilayer graphene or geometrically frustrated metals. However, the simultaneous cooperation of both effects in the same quantum material--the design principle to produce a correlated topological flat bands pinned at the Fermi level--remains rare. Here, using angle-resolved photoemission spectroscopy, we report the observation of a flat band at the Fermi level in a 3$d$ pyrochlore metal CuV$_2$S$_4$. From a combination of first-principles calculations and slave-spin calculations, we understand the origin of this band to be a destructive quantum-interference effect associated with the V pyrochlore sublattice and further renormalization to the Fermi level by electron interactions in the partially filled V $t_{2g}$ orbitals. As a result, we find transport behavior that indicates a deviation from Fermi-liquid behavior as well as a large Sommerfeld coefficient. Our work demonstrates the pathway into correlated topology by constructing and pinning correlated flat bands near the Fermi level out of a pure $d$-electron system by the combined cooperation of local Coulomb interactions and geometric frustration in a pyrochlore lattice system. △ Less

Submitted 2 November, 2023; originally announced November 2023.

Comments: 23 pages, 4 figures, to appear in Nature Physics

Journal ref: Nature Physics 20, 603-609 (2024)

Absence of $E_{2g}$ nematic instability and dominant $A_{1g}$ response in the kagome metal CsV$_3$Sb$_5$

Authors: Zhaoyu Liu, Yue Shi, Qianni Jiang, Elliott W. Rosenberg, Jonathan M. DeStefano, Jinjin Liu, Chaowei Hu, Yuzhou Zhao, Zhiwei Wang, Yugui Yao, David Graf, Pengcheng Dai, Jihui Yang, Xiaodong Xu, Jiun-Haw Chu

Abstract: Ever since the discovery of the charge density wave (CDW) transition in the kagome metal CsV$_3$Sb$_5$, the nature of its symmetry breaking is under intense debate. While evidence suggests that the rotational symmetry is already broken at the CDW transition temperature ($T_{\rm CDW}$), an additional electronic nematic instability well below $T_{\rm CDW}$ has been reported based on the diverging el… ▽ More Ever since the discovery of the charge density wave (CDW) transition in the kagome metal CsV$_3$Sb$_5$, the nature of its symmetry breaking is under intense debate. While evidence suggests that the rotational symmetry is already broken at the CDW transition temperature ($T_{\rm CDW}$), an additional electronic nematic instability well below $T_{\rm CDW}$ has been reported based on the diverging elastoresistivity coefficient in the anisotropic channel ($m_{E_{2g}}$). Verifying the existence of a nematic transition below $T_{\rm CDW}$ is not only critical for establishing the correct description of the CDW order parameter, but also important for understanding low-temperature superconductivity. Here, we report elastoresistivity measurements of CsV$_3$Sb$_5$ using three different techniques probing both isotropic and anisotropic symmetry channels. Contrary to previous reports, we find the anisotropic elastoresistivity coefficient $m_{E_{2g}}$ is temperature-independent, except for a step jump at $T_{\rm CDW}$. The absence of nematic fluctuations is further substantiated by measurements of the elastocaloric effect, which show no enhancement associated with nematic susceptibility. On the other hand, the symmetric elastoresistivity coefficient $m_{A_{1g}}$ increases below $T_{\rm CDW}$, reaching a peak value of 90 at $T^* = 20$ K. Our results strongly indicate that the phase transition at $T^*$ is not nematic in nature and the previously reported diverging elastoresistivity is due to the contamination from the $A_{1g}$ channel. △ Less

Submitted 1 July, 2024; v1 submitted 25 September, 2023; originally announced September 2023.

Comments: 11 pages, 5 figures, to appear in Physical Review X

Journal ref: Phys. Rev. X 14, 031015 (2024)

Symmetry breaking and ascending in the magnetic kagome metal FeGe

Authors: Shangfei Wu, Mason Klemm, Jay Shah, Ethan T. Ritz, Chunruo Duan, Xiaokun Teng, Bin Gao, Feng Ye, Masaaki Matsuda, Fankang Li, Xianghan Xu, Ming Yi, Turan Birol, Pengcheng Dai, Girsh Blumberg

Abstract: Spontaneous symmetry breaking-the phenomenon where an infinitesimal perturbation can cause the system to break the underlying symmetry-is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the tempera… ▽ More Spontaneous symmetry breaking-the phenomenon where an infinitesimal perturbation can cause the system to break the underlying symmetry-is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. In the paramagnetic state at 460K, we confirm that the crystal structure is indeed hexagonal kagome lattice. On cooling to TN, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10^(-4) and the associated splitting of the double degenerate phonon mode of the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice shows a small negative thermal expansion, and the crystal structure becomes more symmetric gradually upon further cooling. Increasing the crystalline symmetry upon cooling is unusual, it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter, hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system and sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant. △ Less

Submitted 8 March, 2024; v1 submitted 25 September, 2023; originally announced September 2023.

Comments: 20 pages with 10 figures, replaced with journal version

Journal ref: Phys. Rev. X 14, 011043 (2024)

Disorder-induced excitation continuum in a spin-1/2 cobaltate on a triangular lattice

Authors: Bin Gao, Tong Chen, Chien-Lung Huang, Yiming Qiu, Guangyong Xu, Jesse Liebman, Lebing Chen, Matthew B. Stone, Erxi Feng, Huibo Cao, Xiaoping Wang, Xianghan Xu, Sang-Wook Cheong, Stephen M. Winter, Pengcheng Dai

Abstract: A spin-1/2 triangular-lattice antiferromagnet is a prototypical frustrated quantum magnet, which exhibits remarkable quantum many-body effects that arise from the synergy between geometric spin frustration and quantum fluctuations. It can host quantum frustrated magnetic topological phenomena like quantum spin liquid (QSL) states, highlighted by the presence of fractionalized quasiparticles within… ▽ More A spin-1/2 triangular-lattice antiferromagnet is a prototypical frustrated quantum magnet, which exhibits remarkable quantum many-body effects that arise from the synergy between geometric spin frustration and quantum fluctuations. It can host quantum frustrated magnetic topological phenomena like quantum spin liquid (QSL) states, highlighted by the presence of fractionalized quasiparticles within a continuum of magnetic excitations. In this work, we use neutron scattering to study CoZnMo$_3$O$_8$, which has a triangular lattice of Jeff = 1/2 Co2+ ions with octahedral coordination. We found a wave-vector-dependent excitation continuum at low energy that disappears with increasing temperature. Although these excitations are reminiscent of a spin excitation continuum in a QSL state, their presence in CoZnMo$_3$O$_8$ originates from magnetic intersite disorder-induced dynamic spin states with peculiar excitations. Our results, therefore, give direct experimental evidence for the presence of a disorder-induced spin excitation continuum. △ Less

Submitted 17 August, 2023; originally announced August 2023.

Journal ref: Phys. Rev. B 108, 024431 2023

Competing itinerant and local spin interactions in kagome metal FeGe

Authors: Lebing Chen, Xiaokun Teng, Hengxin Tan, Barry L. Winn, Garrett E. Granorth, Feng Ye, D. H. Yu, R. A. Mole, Bin Gao, Binghai Yan, Ming Yi, Pengcheng Dai

Abstract: Two-dimensional kagome metals consisting of corner-sharing triangles offer a unique platform for studying strong electron correlations and band topology due to its geometrically frustrated lattice structure. The similar energy scales between spin, lattice, and electronic degrees of freedom in these systems give rise to competing quantum phases such as charge density wave (CDW), magnetic order, and… ▽ More Two-dimensional kagome metals consisting of corner-sharing triangles offer a unique platform for studying strong electron correlations and band topology due to its geometrically frustrated lattice structure. The similar energy scales between spin, lattice, and electronic degrees of freedom in these systems give rise to competing quantum phases such as charge density wave (CDW), magnetic order, and superconductivity. For example, kagome metal FeGe first exhibits A-type collinear antiferromagnetic (AFM) order at T_N ~ 400 K, then establishes a CDW phase coupled with AFM ordered moment below T_CDW ~ 100 K, and finally forms a $c$-axis double cone AFM structure around T_Canting ~ 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure at temperatures well above T_Canting and T_CDW that merge into gapped commensurate spin waves from the A-type AFM order. While commensurate spin waves follow the Bose population factor and can be well described by a local moment Heisenberg Hamiltonian, the incommensurate spin excitations first appear below T_N where AFM order is commensurate, start to deviate from the Bose population factor around T_CDW, and peaks at T_Canting, consistent with a critical scattering of a second order magnetic phase transition, as a function of decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order. The temperature dependence of the incommensurate spin excitations suggests a coupling between spin density wave and CDW order, likely due to flat electronic bands near the Fermi level around T_N and associated electron correlation effects. △ Less

Submitted 9 August, 2023; originally announced August 2023.

Comments: Submitted for publication in April 2023

Journal ref: Nat Commun 15, 1918 (2024)

Spectroscopic evidence for topological band structure in FeTe$_{0.55}$Se$_{0.45}$

Authors: Y. -F. Li, S. -D. Chen, M. Garcia-Diez, M. I. Iraola, H. Pfau, Y. -L. Zhu, Z. -Q. Mao, T. Chen, M. Yi, P. -C. Dai, J. A. Sobota, M. Hashimoto, M. G. Vergniory, D. -H. Lu, Z. -X. Shen

Abstract: FeTe$_{0.55}$Se$_{0.45}$(FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial thanks to the band inversion between the $d_{xz}$ and $p_z$ bands along $Γ$-$Z$. However, there remain debates in both the authenticity… ▽ More FeTe$_{0.55}$Se$_{0.45}$(FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial thanks to the band inversion between the $d_{xz}$ and $p_z$ bands along $Γ$-$Z$. However, there remain debates in both the authenticity of the Dirac surface states (DSS) and the experimental deviations of band structure from the theoretical band inversion picture. Here we resolve these debates through a comprehensive ARPES investigation. We first observe a persistent DSS independent of $k_z$. Then, by comparing FTS with FeSe which has no band inversion along $Γ$-$Z$, we identify the spectral weight fingerprint of both the presence of the $p_z$ band and the inversion between the $d_{xz}$ and $p_z$ bands. Furthermore, we propose a reconciling band structure under the framework of a tight-binding model preserving crystal symmetry. Our results highlight the significant influence of correlation on modifying the band structure and make a strong case for the existence of topological band structure in this unconventional superconductor. △ Less

Submitted 22 August, 2023; v1 submitted 7 July, 2023; originally announced July 2023.

Reversible Non-Volatile Electronic Switching in a Near Room Temperature van der Waals Ferromagnet

Authors: Han Wu, Lei Chen, Paul Malinowski, Jianwei Huang, Qinwen Deng, Kirsty Scott, Bo Gyu Jang, Jacob P. C. Ruff, Yu He, Xiang Chen, Chaowei Hu, Ziqin Yue, Ji Seop Oh, Xiaokun Teng, Yucheng Guo, Mason Klemm, Chuqiao Shi, Yue Shi, Chandan Setty, Tyler Werner, Makoto Hashimoto, Donghui Lu, T. Yilmaz, Elio Vescovo, Sung-Kwan Mo , et al. (15 additional authors not shown)

Abstract: The ability to reversibly toggle between two distinct states in a non-volatile method is important for information storage applications. Such devices have been realized for phase-change materials, which utilizes local heating methods to toggle between a crystalline and an amorphous state with distinct electrical properties. To expand such kind of switching between two topologically distinct phases… ▽ More The ability to reversibly toggle between two distinct states in a non-volatile method is important for information storage applications. Such devices have been realized for phase-change materials, which utilizes local heating methods to toggle between a crystalline and an amorphous state with distinct electrical properties. To expand such kind of switching between two topologically distinct phases requires non-volatile switching between two crystalline phases with distinct symmetries. Here we report the observation of reversible and non-volatile switching between two stable and closely-related crystal structures with remarkably distinct electronic structures in the near room temperature van der Waals ferromagnet Fe$_{5-δ}$GeTe$_2$. From a combination of characterization techniques we show that the switching is enabled by the ordering and disordering of an Fe site vacancy that results in distinct crystalline symmetries of the two phases that can be controlled by a thermal annealing and quenching method. Furthermore, from symmetry analysis as well as first principle calculations, we provide understanding of the key distinction in the observed electronic structures of the two phases: topological nodal lines compatible with the preserved global inversion symmetry in the site-disordered phase, and flat bands resulting from quantum destructive interference on a bipartite crystaline lattice formed by the presence of the site order as well as the lifting of the topological degeneracy due to the broken inversion symmetry in the site-ordered phase. Our work not only reveals a rich variety of quantum phases emergent in the metallic van der Waals ferromagnets due to the presence of site ordering, but also demonstrates the potential of these highly tunable two-dimensional magnets for memory and spintronics applications. △ Less

Submitted 6 July, 2023; originally announced July 2023.

Journal ref: Nat Commun 15, 2739 (2024)

Spectral Evidence for Local-Moment Ferromagnetism in van der Waals Metals Fe$_3$GaTe$_2$ and Fe$_3$GeTe$_2$

Authors: Han Wu, Chaowei Hu, Yaofeng Xie, Bo Gyu Jang, Jianwei Huang, Yucheng Guo, Shan Wu, Cheng Hu, Ziqin Yue, Yue Shi, Zheng Ren, T. Yilmaz, Elio Vescovo, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Alexei Fedorov, Jonathan Denlinger, Christoph Klewe, Padraic Shafer, Donghui Lu, Makoto Hashimoto, Junichiro Kono, Robert J. Birgeneau, Xiaodong Xu , et al. (4 additional authors not shown)

Abstract: Magnetism in two-dimensional (2D) materials has attracted considerable attention recently for both fundamental understanding of magnetism and their tunability towards device applications. The isostructural Fe$_3$GeTe$_2$ and Fe$_3$GaTe$_2$ are two members of the Fe-based van der Waals (vdW) ferromagnet family, but exhibit very different Curie temperatures (T$_C$) of 210 K and 360 K, respectively.… ▽ More Magnetism in two-dimensional (2D) materials has attracted considerable attention recently for both fundamental understanding of magnetism and their tunability towards device applications. The isostructural Fe$_3$GeTe$_2$ and Fe$_3$GaTe$_2$ are two members of the Fe-based van der Waals (vdW) ferromagnet family, but exhibit very different Curie temperatures (T$_C$) of 210 K and 360 K, respectively. Here, by using angle-resolved photoemission spectroscopy and density functional theory, we systematically compare the electronic structures of the two compounds. Qualitative similarities in the Fermi surface can be found between the two compounds, with expanded hole pockets in Fe$_3$GaTe$_2$ suggesting additional hole carriers compared to Fe$_3$GeTe$_2$. Interestingly, we observe no band shift in Fe$_3$GaTe$_2$ across its T$_C$ of 360 K, compared to a small shift in Fe$_3$GeTe$_2$ across its T$_C$ of 210 K. The weak temperature-dependent evolution strongly deviates from the expectations of an itinerant Stoner mechanism. Our results suggest that itinerant electrons have minimal contributions to the enhancement of T$_C$ in Fe$_3$GaTe$_2$ compared to Fe$_3$GeTe$_2$, and that the nature of ferromagnetism in these Fe-based vdW ferromagnets must be understood with considerations of the electron correlations. △ Less

Submitted 2 December, 2023; v1 submitted 1 July, 2023; originally announced July 2023.

Journal ref: Physical Review B 109, 104410 (2024)

Uniaxial-Strain Tuning of the Intertwined Orders in BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$

Authors: Zinan Zhao, Ding Hu, Xue Fu, Kaijuan Zhou, Yanhong Gu, Guotai Tan, Xingye Lu, Pengcheng Dai

Abstract: An experimental determination of electronic phase diagrams of high-transition temperature (high-$T_c$) superconductors forms the basis for a microscopic understanding of unconventional superconductivity. For most high-$T_c$ superconductors, the electronic phase diagrams are established through partial chemical substitution, which also induces lattice disorder. Here we show that symmetry-specific u… ▽ More An experimental determination of electronic phase diagrams of high-transition temperature (high-$T_c$) superconductors forms the basis for a microscopic understanding of unconventional superconductivity. For most high-$T_c$ superconductors, the electronic phase diagrams are established through partial chemical substitution, which also induces lattice disorder. Here we show that symmetry-specific uniaxial strain can be used to study electronic phases in iron-based superconductors, composed of two-dimensional nearly square iron lattice planed separated by other elements. By applying tunable uniaxial strain along different high symmetry directions and carrying out transport measurements, we establish strain-tuning dependent electronic nematicity, antiferromagnetic (AF) order, and superconductivity of BaFe$_2$(As$_{1-x}$P$_{x}$)$_2$ superconductor. We find that uniaxial strain along the nearest Fe-Fe direction can dramatically tune the AF order and superconductivity, producing an electronic phase diagram clearly different from the chemical substitution-induced one. Our results thus establish strain tuning as a way to study the intertwined orders in correlated electron materials without using chemical substitution. △ Less

Submitted 7 May, 2023; originally announced May 2023.

Observation of Flat Bands and Dirac Cones in a Pyrochlore Lattice Superconductor

Authors: Jianwei Huang, Chandan Setty, Liangzi Deng, Jing-Yang You, Hongxiong Liu, Sen Shao, Ji Seop Oh, Yucheng Guo, Yichen Zhang, Ziqin Yue, Jia-Xin Yin, Makoto Hashimoto, Donghui Lu, Sergey Gorovikov, Pengcheng Dai, Jonathan D. Denlinger, M. Zahid Hasan, Yuan-Ping Feng, Robert J. Birgeneau, Youguo Shi, Ching-Wu Chu, Guoqing Chang, Qimiao Si, Ming Yi

Abstract: Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases… ▽ More Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional kagome lattice, including magnetic order, time-reversal symmetry breaking charge order, nematicity, and superconductivity. However, the interlayer coupling of the kagome layers disrupts the destructive interference needed to completely quench the kinetic energy. Here we demonstrate that an interwoven kagome network-a pyrochlore lattice-can host a three dimensional (3D) localization of electron wavefunctions. Meanwhile, the nonsymmorphic symmetry of the pyrochlore lattice guarantees all band crossings at the Brillouin zone X point to be 3D gapless Dirac points, which was predicted theoretically but never yet observed experimentally. Through a combination of angle-resolved photoemission spectroscopy, fundamental lattice model and density functional theory calculations, we investigate the novel electronic structure of a Laves phase superconductor with a pyrochlore sublattice, CeRu$_2$. We observe flat bands originating from both the Ce 4$f$ orbitals as well as from the 3D destructive interference of the Ru 4$d$ orbitals. We further observe the nonsymmorphic symmetry-protected 3D gapless Dirac cones at the X point. Our work establishes the pyrochlore structure as a promising lattice platform to realize and tune novel emergent phases intertwining topology and many-body interactions. △ Less

Submitted 14 December, 2023; v1 submitted 18 April, 2023; originally announced April 2023.

Comments: 27 pages, 4 figures

Journal ref: npj Quantum Materials 9, 71 (2024)

Phase Stability of Hexagonal/cubic Boron Nitride Nanocomposites

Authors: Abhijit Biswas, Rui Xu, Joyce Christiansen-Salameh, Eugene Jeong, Gustavo A. Alvarez, Chenxi Li, Anand B. Puthirath, Bin Gao, Arushi Garg, Tia Gray, Harikishan Kannan, Xiang Zhang, Jacob Elkins, Tymofii S. Pieshkov, Robert Vajtai, A. Glen Birdwell, Mahesh R. Neupane, Bradford B. Pate, Tony Ivanov, Elias J. Garratt, Pengcheng Dai, Hanyu Zhu, Zhiting Tian, Pulickel M. Ajayan

Abstract: Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show… ▽ More Boron nitride (BN) is an exceptional material and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the co-existence of two phases can lead to strong non-linear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN grain sizes governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorphs based nanocomposites with desirable properties for optoelectronics and thermal energy management applications. △ Less

Submitted 17 April, 2023; originally announced April 2023.

Comments: 29 pages, 5 figures

Journal ref: Nano Lett. 2023, 23, 15, 6927

Diffusive Excitonic Bands from Frustrated Triangular Sublattice in a Singlet-Ground-State System

Authors: Bin Gao, Tong Chen, Xiao-Chuan Wu, Michael Flynn, Chunruo Duan, Lebing Chen, Chien-Lung Huang, Jesse Liebman, Shuyi Li, Feng Ye, Matthew B. Stone, Andrey Podlesnyak, Douglas L. Abernathy, Devashibhai T. Adroja, Manh Duc Le, Qingzhen Huang, Andriy H. Nevidomskyy, Emilia Morosan, Leon Balents, Pengcheng Dai

Abstract: Magnetic order in most materials occurs when magnetic ions with finite moments in a crystalline lattice arrange in a particular pattern below the ordering temperature determined by exchange interactions between the ions. However, when the crystal electric field (CEF) effect results in a spin-singlet ground state on individual magnetic sites, the collective ground state of the system can either rem… ▽ More Magnetic order in most materials occurs when magnetic ions with finite moments in a crystalline lattice arrange in a particular pattern below the ordering temperature determined by exchange interactions between the ions. However, when the crystal electric field (CEF) effect results in a spin-singlet ground state on individual magnetic sites, the collective ground state of the system can either remain non-magnetic, or more intriguingly, the exchange interactions between neighboring ions, provided they are sufficiently strong, can admix the excited CEF levels, resulting in a magnetically ordered ground state. The collective magnetic excitations in such a state are so-called spin excitons that describe the CEF transitions propagating through the lattice. In most cases, spin excitons originating from CEF levels of a localized single ion are dispersion-less in momentum (reciprocal) space and well-defined in both the magnetically ordered and paramagnetic states. Here we use thermodynamic and neutron scattering experiments to study stoichiometric Ni2Mo3O8 without site disorder, where Ni2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has long-range magnetic order. Furthermore, CEF spin excitons from the triangular-lattice arrangement of tetrahedral sites form, in both the antiferromagnetic and paramagnetic states, a dispersive diffusive pattern around the Brillouin zone boundary in reciprocal space. The present work thus demonstrates that spin excitons in an ideal triangular lattice magnet can have dispersive excitations, irrespective of the existence of static magnetic order, and this phenomenon is most likely due to spin entanglement and geometric frustrations. △ Less

Submitted 17 March, 2023; originally announced March 2023.

Imaging real-space flat band localization in kagome magnet FeSn

Authors: Daniel Multer, Jia-Xin Yin, Md. Shafayat Hossain, Xian Yang, Brian C Sales, Hu Miao, William R Meier, Yu-Xiao Jiang, Yaofeng Xie, Pengcheng Dai, Jianpeng Liu, Hanbin Deng, Hechang Lei, Biao Lian, M. Zahid Hasan

Abstract: Kagome lattices host flat bands due to their frustrated lattice geometry, which leads to destructive quantum interference of electron wave functions. Here, we report imaging of the kagome flat band localization in real-space using scanning tunneling microscopy. We identify both the Fe3Sn kagome lattice layer and the Sn2 honeycomb layer with atomic resolution in kagome antiferromagnet FeSn. On the… ▽ More Kagome lattices host flat bands due to their frustrated lattice geometry, which leads to destructive quantum interference of electron wave functions. Here, we report imaging of the kagome flat band localization in real-space using scanning tunneling microscopy. We identify both the Fe3Sn kagome lattice layer and the Sn2 honeycomb layer with atomic resolution in kagome antiferromagnet FeSn. On the Fe3Sn lattice, at the flat band energy determined by the angle resolved photoemission spectroscopy, tunneling spectroscopy detects an unusual state localized uniquely at the Fe kagome lattice network. We further show that the vectorial in-plane magnetic field manipulates the spatial anisotropy of the localization state within each kagome unit cell. Our results are consistent with the real-space flat band localization in the magnetic kagome lattice. We further discuss the magnetic tuning of flat band localization under the spin-orbit coupled magnetic kagome lattice model. △ Less

Submitted 24 December, 2022; originally announced December 2022.

Comments: To appear in Communications Materials

Evidence for gapless quantum spin liquid in a honeycomb lattice

Authors: Chengpeng Tu, Dongzhe Dai, Xu Zhang, Chengcheng Zhao, Xiaobo Jin, Bin Gao, Tong Chen, Pengcheng Dai, Shiyan Li

Abstract: One main theme in current condensed matter physics is the search of quantum spin liquid (QSL), an exotic magnetic state with strongly-fluctuating and highly-entangled spins down to zero temperature without static order. However, there is no consensus on the existence of a QSL ground state in any real material so far. The disorders and competing exchange interactions may prevent the formation of an… ▽ More One main theme in current condensed matter physics is the search of quantum spin liquid (QSL), an exotic magnetic state with strongly-fluctuating and highly-entangled spins down to zero temperature without static order. However, there is no consensus on the existence of a QSL ground state in any real material so far. The disorders and competing exchange interactions may prevent the formation of an ideal QSL state on frustrated spin lattices. Here we report systematic heat transport measurements on a honeycomb-lattice compound BaCo2(AsO4)2, which manifests magnetic order in zero field. In a narrow field range after the magnetic order is nearly suppressed by an in-plane field, in both perpendicular and parallel to the zigzag direction, a finite residual linear term of thermal conductivity is clearly observed, which is attributed to the mobile fractionalized spinon excitations. This provides smoking-gun evidence for a gapless QSL state in BaCo2(AsO4)2. We discuss the underlying physics to form this exotic gapless QSL state in Co2+ honeycomb lattice. △ Less

Submitted 9 January, 2023; v1 submitted 14 December, 2022; originally announced December 2022.

Spin structure and dynamics of the topological semimetal Co$_{3}$Sn$_{2-x}$In$_{x}$S$_{2}$

Authors: Kelly J. Neubauer, Feng Ye, Yue Shi, Paul Malinowski, Bin Gao, Keith M. Taddei, Philippe Bourges, Alexandre Ivanov, Jiun-Haw Chu, Pengcheng Dai

Abstract: The anomalous Hall effect (AHE), typically observed in ferromagnetic (FM) metals with broken time-reversal symmetry, depends on electronic and magnetic properties. In Co$_{3}$Sn$_{2-x}$In$_{x}$S$_{2}$, a giant AHE has been attributed to Berry curvature associated with the FM Weyl semimetal phase, yet recent studies report complicated magnetism. We use neutron scattering to determine the spin dynam… ▽ More The anomalous Hall effect (AHE), typically observed in ferromagnetic (FM) metals with broken time-reversal symmetry, depends on electronic and magnetic properties. In Co$_{3}$Sn$_{2-x}$In$_{x}$S$_{2}$, a giant AHE has been attributed to Berry curvature associated with the FM Weyl semimetal phase, yet recent studies report complicated magnetism. We use neutron scattering to determine the spin dynamics and structures as a function of $x$ and provide a microscopic understanding of the AHE and magnetism interplay. Spin gap and stiffness indicate a contribution from Weyl fermions consistent with the AHE. The magnetic structure evolves from $c$-axis ferromagnetism at $x$ = 0 to a canted antiferromagnetic (AFM) structure with reduced $c$-axis moment and in-plane AFM order at $x$ = 0.12 and further reduced $c$-axis FM moment at $x$ = 0.3. Since noncollinear spins can induce non-zero Berry curvature in real space acting as a fictitious magnetic field, our results revealed another AHE contribution, establishing the impact of magnetism on transport. △ Less

Submitted 15 November, 2022; originally announced November 2022.

Intertwined magnetism and charge density wave order in kagome FeGe

Authors: Xiaokun Teng, Ji Seop Oh, Hengxin Tan, Lebing Chen, Jianwei Huang, Bin Gao, Jia-Xin Yin, Jiun-Haw Chu, Makoto Hashimoto, Donghui Lu, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Garrett E. Granroth, Binghai Yan, Robert J. Birgeneau, Pengcheng Dai, Ming Yi

Abstract: Electron correlations often lead to emergent orders in quantum materials. Kagome lattice materials are emerging as an exciting platform for realizing quantum topology in the presence of electron correlations. This proposal stems from the key signatures of electronic structures associated with its lattice geometry: flat band induced by destructive interference of the electronic wavefunctions, topol… ▽ More Electron correlations often lead to emergent orders in quantum materials. Kagome lattice materials are emerging as an exciting platform for realizing quantum topology in the presence of electron correlations. This proposal stems from the key signatures of electronic structures associated with its lattice geometry: flat band induced by destructive interference of the electronic wavefunctions, topological Dirac crossing, and a pair of van Hove singularities (vHSs). A plethora of correlated electronic phases have been discovered amongst kagome lattice materials, including magnetism, charge density wave (CDW), nematicity, and superconductivity. These materials can be largely organized into two types: those that host magnetism and those that host CDW order. Recently, a CDW order has been discovered in the magnetic kagome FeGe, providing a new platform for understanding the interplay between CDW and magnetism. Here, utilizing angle-resolved photoemission spectroscopy, we observe all three types of electronic signatures of the kagome lattice: flat bands, Dirac crossings, and vHSs. From both the observation of a temperature-dependent shift of the vHSs towards the Fermi level as well as guidance via first-principle calculations, we identify the presence of the vHSs near the Fermi level (EF) to be driven by the development of underlying magnetic exchange splitting. Furthermore, we show spectral evidence for the CDW order as gaps that open on the near-EF vHS bands, as well as evidence of electron-phonon coupling from a kink on the vHS band together with phonon hardening observed by inelastic neutron scattering. Our observation points to the magnetic interaction-driven band modification resulting in the formation of the CDW order, indicating an intertwined connection between the emergent magnetism and vHS charge order in this moderately-correlated kagome metal. △ Less

Submitted 12 October, 2022; originally announced October 2022.

Comments: submitted on April 22, 2022

Magnetic field effects in an octupolar quantum spin liquid candidate

Authors: Bin Gao, Tong Chen, Han Yan, Chunruo Duan, Chien-Lung Huang, Xu Ping Yao, Feng Ye, Christian Balz, J. Ross Stewart, Kenji Nakajima, Seiko Ohira-Kawamura, Guangyong Xu, Xianghan Xu, Sang-Wook Cheong, Emilia Morosan, Andriy H. Nevidomskyy, Gang Chen, Pengcheng Dai

Abstract: Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising… ▽ More Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to the external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here we use neutron scattering to show that the application of a large external magnetic field to Ce2Zr2O7, an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wavevectors in Ce2Sn2O7 pyrochlore, and its absence in Ce2Zr2O7. Therefore, our results identify Ce2Zr2O7 as a strong candidate for an octupolar U (1) QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets. △ Less

Submitted 10 September, 2022; originally announced September 2022.

Journal ref: Physical Review B 2022

Anisotropic magnon damping by zero-temperature quantum fluctuations in ferromagnetic CrGeTe$_3$

Authors: Lebing Chen, Chengjie Mao, Jae-Ho Chung, Matthew B. Stone, Alexander I. Kolesnikov, Xiaoping Wang, Naoki Murai, Bin Gao, Olivier Delaire, Pengcheng Dai

Abstract: Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at t… ▽ More Spin and lattice are two fundamental degrees of freedom in a solid, and their fluctuations about the equilibrium values in a magnetic ordered crystalline lattice form quasiparticles termed magnons (spin waves) and phonons (lattice waves), respectively. In most materials with strong spin-lattice coupling (SLC), the interaction of spin and lattice induces energy gaps in the spin wave dispersion at the nominal intersections of magnon and phonon modes. Here we use neutron scattering to show that in the two-dimensional (2D) van der Waals honeycomb lattice ferromagnetic CrGeTe3, spin waves propagating within the 2D plane exhibit an anomalous dispersion, damping, and break-down of quasiparticle conservation, while magnons along the c axis behave as expected for a local moment ferromagnet. These results indicate the presence of dynamical SLC arising from the zero-temperature quantum fluctuations in CrGeTe3, suggesting that the observed in-plane spin waves are mixed spin and lattice quasiparticles fundamentally different from pure magnons and phonons. △ Less

Submitted 23 June, 2022; originally announced June 2022.

Comments: 12 pages, 4 figures; Nature Communication in press