Showing 1–50 of 132 results for author: Chi, S

Uniaxial strain tuning of polar lattice vibrations in KTaO$_3$ and SrTiO$_3$

Authors: I. Khayr, N. Somun, S. Hameed, Z. Van Fossan, X. He, R. Spieker, S. Chi, E. Clements, D. M. Pajerowski, M. Minola, B. Keimer, T. Birol, D. Pelc, M. Greven

Abstract: The interplay of electronic and structural degrees of freedom is a prominent feature of many quantum materials and of particular interest in systems with strong ferroelectric fluctuations, such as SrTiO$_3$ (STO) and KTaO$_3$ (KTO). Both materials are close to a ferroelectric transition, but despite six decades of extensive research, pivotal questions regarding the nature of this transition and of… ▽ More The interplay of electronic and structural degrees of freedom is a prominent feature of many quantum materials and of particular interest in systems with strong ferroelectric fluctuations, such as SrTiO$_3$ (STO) and KTaO$_3$ (KTO). Both materials are close to a ferroelectric transition, but despite six decades of extensive research, pivotal questions regarding the nature of this transition and of the associated fluctuations remain debated. Here we combine inelastic neutron scattering, Raman spectroscopy, and ab initio calculations to study the evolution of soft polar phonons across the strain-induced ferroelectric transition in STO and KTO. We find that the modes remain underdamped and at nonzero energy, consistent with a first-order quantum phase transition. We also reveal a strong violation of the well-known Lyddane-Sachs-Teller relation between the phonon energies and static dielectric permittivities in insulating KTO and STO, which is not captured by ab initio calculations and points to the presence of slow mesoscale fluctuations induced by long-range interactions. In metallic STO, we uncover a first-order transition at a remarkably low critical stress, in qualitative agreement with recent theoretical predictions. The present work resolves several long-standing questions pertaining to the model systems STO and KTO and is relevant to numerous other materials with soft polar phonons. △ Less

Submitted 13 November, 2025; originally announced November 2025.

Comments: 36 pages, supplementary information included

Magnetic dynamics in NiTiO3 honeycomb antiferromagnet using neutron scattering

Authors: Srimal Rathnayaka, Luke Daemen, Tao Hong, Songxue Chi, Stuart Calder, John A. Schneeloch, Yongqiang Cheng, Bing Li, Despina Louca

Abstract: The ilmenite NiTiO3 consists of a buckled honeycomb lattice, with the Ni spins aligned ferromagnetically in-plane and antiferromagnetically out-of-plane. Using neutron spectroscopy, the magnetic structure and the dynamics were investigated as a function of temperature. Dispersive acoustic bands and nearly dispersionless optical bands at ~3.7 meV are described by a highly anisotropy Heisenberg mode… ▽ More The ilmenite NiTiO3 consists of a buckled honeycomb lattice, with the Ni spins aligned ferromagnetically in-plane and antiferromagnetically out-of-plane. Using neutron spectroscopy, the magnetic structure and the dynamics were investigated as a function of temperature. Dispersive acoustic bands and nearly dispersionless optical bands at ~3.7 meV are described by a highly anisotropy Heisenberg model with stronger antiferromagnetic (AFM) out-of-plane, weaker ferromagnetic (FM) in-plane interactions and an anisotropy gap of 0.95 meV. The order parameter yields a critical exponent between the Heisenberg and two-dimensional Ising models, consistent with highly anisotropic Heisenberg systems. The frustration parameter ~ 2 supports a weakly frustrated system. △ Less

Submitted 30 August, 2025; originally announced September 2025.

Comments: 12 pages, 10 figures

Quasiparticle Interference in LiFeAs: Signature of Inelastic Tunneling through Spin Fluctuations

Authors: Shun Chi, Carolina A. Marques, Walter N. Hardy, Ruixing Liang, Pinder Dosanjh, Doug A. Bonn, Sarah A. Burke, Peter Wahl

Abstract: Quasi-particle interference (QPI) is a powerful tool to characterize the symmetry of the superconducting order parameter in unconventional superconductors, by mapping the spatial dependence of elastic tunneling of electrons between the tip of a scanning tunneling microscope and a sample. Here, we consider the influence of inelastic tunneling on quasi-particle interference, exemplarily for the iron… ▽ More Quasi-particle interference (QPI) is a powerful tool to characterize the symmetry of the superconducting order parameter in unconventional superconductors, by mapping the spatial dependence of elastic tunneling of electrons between the tip of a scanning tunneling microscope and a sample. Here, we consider the influence of inelastic tunneling on quasi-particle interference, exemplarily for the iron-based superconductor LiFeAs. We clearly observe replica features in both experimental QPI maps and the dispersion extracted from QPI, which from comparison with theoretical model calculations can be attributed to inelastic tunneling. Analysis of the QPI dispersion shows that the inelastic mode that gives rise to these replica features exhibits a resonance between 8 and 10 meV. Comparison of the energy scale of the resonance energy estimated from QPI with inelastic neutron scattering indicates that the replica features arise from interaction with spin fluctuations. △ Less

Submitted 15 August, 2025; originally announced August 2025.

Comments: 18 pages and 6 figures including supplementary

Nature of the ferromagnet-paramagnet transition in Y$_{1-x}$Ca$_{x}$TiO$_{3}$

Authors: S. Hameed, I. Khayr, J. Joe, G. Q. Zhao, Y. Cai, K. M. Kojima, S. Chi, T. J. Williams, M. Matsuda, Y. J. Uemura, M. Greven

Abstract: Neutron scattering, magnetometry, and muon spin rotation ($μ$SR) measurements were performed to investigate the magnetic order and spin dynamics across the ferromagnet-to-paramagnet transition in the hole-doped Mott insulator Y$_{1-x}$Ca$_x$TiO$_3$. We find that the transition proceeds through a volume-wise phase separation into ferromagnetic and paramagnetic regions. Spin fluctuations with a char… ▽ More Neutron scattering, magnetometry, and muon spin rotation ($μ$SR) measurements were performed to investigate the magnetic order and spin dynamics across the ferromagnet-to-paramagnet transition in the hole-doped Mott insulator Y$_{1-x}$Ca$_x$TiO$_3$. We find that the transition proceeds through a volume-wise phase separation into ferromagnetic and paramagnetic regions. Spin fluctuations with a characteristic timescale of $\sim$ 0.1 $μ$s, as detected via $μ$SR, are observed to appear at Ca concentrations $x \geq 0.10$. The magnetic phase separation, accompanied by a modest dynamic response, represents a novel behavior in Mott systems near the loss of magnetic order. It is linked to a previously observed insulator-metal transition and the associated electronic phase separation into hole-poor Mott insulating and hole-rich metallic phases for $0 < x < 0.50$. In particular, the $x$-dependence of the paramagnetic volume fraction strongly correlates with that of the volume fraction of the hole-rich metallic phase. The spin-wave spectra reveal a doping-induced crossover from isotropic to two-dimensional anisotropic exchange interactions, reflecting substantial changes in the orbital state with increasing Ca content. △ Less

Submitted 17 February, 2025; originally announced February 2025.

Comments: 6 figures; supplement included

Pervasive symmetry-lowering nanoscale structural fluctuations in the cuprate La$_{2-x}$Sr$_{x}$CuO$_{4}$

Authors: R. J. Spieker, M. Spaić, I. Khayr, X. He, D. Zhai, Z. W. Anderson, N. Bielinski, F. Ye, Y. Liu, S. Chi, S. Sarker, M. J. Krogstad, R. Osborn, D. Pelc, M. Greven

Abstract: The cuprate superconductors are among the most widely studied quantum materials, yet there remain fundamental open questions regarding their electronic properties and the role of the structural degrees of freedom. Recent neutron and x-ray scattering measurements uncovered exponential scaling with temperature of the strength of orthorhombic fluctuations in the tetragonal phase of… ▽ More The cuprate superconductors are among the most widely studied quantum materials, yet there remain fundamental open questions regarding their electronic properties and the role of the structural degrees of freedom. Recent neutron and x-ray scattering measurements uncovered exponential scaling with temperature of the strength of orthorhombic fluctuations in the tetragonal phase of $La_{2-x}Sr_xCuO_4$ and $Tl_2Ba_2CuO_{6+y}$, unusual behavior that closely resembles prior results for the emergence of superconducting fluctuations, and that points to a common origin rooted in inherent correlated structural inhomogeneity. Here we extend the measurements of $La_{2-x}Sr_xCuO_4$ to higher temperatures in the parent compound (x=0) and to optimal doping (x=0.155), and we furthermore investigate the effects of in-situ in-plane uniaxial stress. Our neutron scattering result for undoped $La_2CuO_4$ complement prior x-ray data and reveal that the structural fluctuations persist to the maximum experimental temperature of nearly 1000K, i.e., to a significant fraction of the melting point. At this temperature, the spatial correlation length extracted from the momentum-space data is still about three lattice constants. The neutron scattering experiment enables quasistatic discrimination and reveals that the response is increasingly dynamic at higher temperatures. We also find that uniaxial stress up to 500 MPa along the tetragonal [110] direction, which corresponds to a strain of about 0.2%, does not significantly alter this robust behavior. Overall, these results support the notion that subtle, underlying inhomogeneity underpins the cuprate phase diagram. Finally, we uncover (for x=0.2) low-energy structural fluctuations at a nominally forbidden reflection. While the origin of these fluctuations is not clear, they might be related to the presence of extended defects such as dislocations or stacking faults. △ Less

Submitted 5 February, 2025; originally announced February 2025.

Comments: 20 pages, 6 figures

Measuring pair correlations in Bose and Fermi gases via atom-resolved microscopy

Authors: Ruixiao Yao, Sungjae Chi, Mingxuan Wang, Richard J. Fletcher, Martin Zwierlein

Abstract: We demonstrate atom-resolved detection of itinerant bosonic $^{23}$Na and fermionic $^6$Li quantum gases, enabling the direct in situ measurement of interparticle correlations. In contrast to prior work on lattice-trapped gases, here we realize microscopy of quantum gases in the continuum. We reveal Bose-Einstein condensation with single-atom resolution, measure the enhancement of two-particle… ▽ More We demonstrate atom-resolved detection of itinerant bosonic $^{23}$Na and fermionic $^6$Li quantum gases, enabling the direct in situ measurement of interparticle correlations. In contrast to prior work on lattice-trapped gases, here we realize microscopy of quantum gases in the continuum. We reveal Bose-Einstein condensation with single-atom resolution, measure the enhancement of two-particle $g^{(2)}$ correlations of thermal bosons, and observe the suppression of $g^{(2)}$ for fermions; the Fermi or exchange hole. For strongly interacting Fermi gases confined to two dimensions, we directly observe non-local fermion pairs in the BEC-BCS crossover. We obtain the pairing gap, the pair size, and the short-range contact directly from the pair correlations. In situ thermometry is enabled via the fluctuation-dissipation theorem. Our technique opens the door to the atom-resolved study of strongly correlated quantum gases of bosons, fermions, and their mixtures. △ Less

Submitted 9 April, 2025; v1 submitted 13 November, 2024; originally announced November 2024.

Comments: 9 pages, 7 figures

Journal ref: Phys. Rev. Lett. 134, 183402 (2025)

A topological Hund nodal line antiferromagnet

Authors: Xian P. Yang, Yueh-Ting Yao, Pengyu Zheng, Shuyue Guan, Huibin Zhou, Tyler A. Cochran, Che-Min Lin, Jia-Xin Yin, Xiaoting Zhou, Zi-Jia Cheng, Zhaohu Li, Tong Shi, Md Shafayat Hossain, Shengwei Chi, Ilya Belopolski, Yu-Xiao Jiang, Maksim Litskevich, Gang Xu, Zhaoming Tian, Arun Bansil, Zhiping Yin, Shuang Jia, Tay-Rong Chang, M. Zahid Hasan

Abstract: The interplay of topology, magnetism, and correlations gives rise to intriguing phases of matter. In this study, through state-of-the-art angle-resolved photoemission spectroscopy, density functional theory and dynamical mean-field theory calculations, we visualize a fourfold degenerate Dirac nodal line at the boundary of the bulk Brillouin zone in the antiferromagnet YMn2Ge2. We further demonstra… ▽ More The interplay of topology, magnetism, and correlations gives rise to intriguing phases of matter. In this study, through state-of-the-art angle-resolved photoemission spectroscopy, density functional theory and dynamical mean-field theory calculations, we visualize a fourfold degenerate Dirac nodal line at the boundary of the bulk Brillouin zone in the antiferromagnet YMn2Ge2. We further demonstrate that this gapless, antiferromagnetic Dirac nodal line is enforced by the combination of magnetism, space-time inversion symmetry and nonsymmorphic lattice symmetry. The corresponding drumhead surface states traverse the whole surface Brillouin zone. YMn2Ge2 thus serves as a platform to exhibit the interplay of multiple degenerate nodal physics and antiferromagnetism. Interestingly, the magnetic nodal line displays a d-orbital dependent renormalization along its trajectory in momentum space, thereby manifesting Hund coupling. Our findings offer insights into the effect of electronic correlations on magnetic Dirac nodal lines, leading to an antiferromagnetic Hund nodal line. △ Less

Submitted 15 August, 2024; originally announced August 2024.

Journal ref: Nature Communications volume 15, Article number: 7052 (2024)

First Order Preemptive Ising-nematic Transition in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$

Authors: N. Giles-Donovan, Y. Chen, H. Fukui, T. Manjo, D. Ishikawa, A. Q. R. Baron, S. Chi, H. Zhong, S. Cao, Y. Tang, Y. Wang, X. Lu, Y. Song, R. J. Birgeneau

Abstract: Employing inelastic X-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a second-order magnetic transition at $T_{\textrm{N}}$ $\approx$ 34.6 K. The temperature difference between these two pha… ▽ More Employing inelastic X-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a second-order magnetic transition at $T_{\textrm{N}}$ $\approx$ 34.6 K. The temperature difference between these two phase transitions is $\sim$ 1 K. The observed phenomenon can be attributed to a distinctive first-order preemptive Ising-nematic transition, a characteristic unique to a quasi-two-dimensional scenario marked by strong out-of-plane spatial anisotropy due to weak coupling. Our studies establish K$_{5}$Fe$_{4}$Ag$_{6}$Te$_{10}$ as the first material in the family of iron pnictides and chalcogenides that possesses a nematic tricritical point preceding the magnetic one upon decreasing nematic coupling. △ Less

Submitted 19 July, 2024; originally announced July 2024.

Comments: 7 pages, 5 figures

Magic angle of Sr$_2$RuO$_4$: Optimizing correlation-driven superconductivity

Authors: Jonas B. Profe, Luke C. Rhodes, Matteo Dürrnagel, Rebecca Bisset, Carolina A. Marques, Shun Chi, Tilman Schwemmer, Ronny Thomale, Dante M. Kennes, Chris Hooley, Peter Wahl

Abstract: Understanding of unconventional superconductivity is crucial for engineering materials with specific order parameters or elevated superconducting transition temperatures. However, for many materials, the pairing mechanism and symmetry of the order parameter remain unclear: reliable and efficient methods of predicting the order parameter and its response to tuning parameters are lacking. Here, we i… ▽ More Understanding of unconventional superconductivity is crucial for engineering materials with specific order parameters or elevated superconducting transition temperatures. However, for many materials, the pairing mechanism and symmetry of the order parameter remain unclear: reliable and efficient methods of predicting the order parameter and its response to tuning parameters are lacking. Here, we investigate the response of superconductivity in Sr$_2$RuO$_4$ to structural distortions via the random phase approximation (RPA) and functional renormalization group (FRG), starting from realistic models of the electronic structure. Our results suggest that RPA misses the interplay of competing fluctuation channels. FRG reproduces key experimental findings. We predict a magic octahedral rotation angle, maximizing the superconducting $T_c$ and a dominant $d_{x^2-y^2}$ pairing symmetry. To enable experimental verification, we provide calculations of the phase-referenced Bogoliubov Quasiparticle Interference imaging. Our work demonstrates a designer approach to tuning unconventional superconductivity with relevance and applicability for a wide range of quantum materials. △ Less

Submitted 23 October, 2024; v1 submitted 23 May, 2024; originally announced May 2024.

Comments: 16 pages, 7 figures in main text, published version

Journal ref: Physical Review Materials 6, 043057 (2024)

Giant Anomalous Hall and Nernst Effects in a Heavy Fermion Ferromagnet

Authors: Longfei Li, Shuyue Guan, Shengwei Chi, Jiawei Li, Xinxuan Lin, Gang Xu, Shuang Jia

Abstract: The anomalous Hall and Nernst effects describe the voltage drop perpendicular to an applied current and temperature gradient due to the magnetization of a magnetic material. These effects can be utilized to measure the Berry curvature at the Fermi energy, and have potential applications in future electronic devices and thermoelectric energy conversion. In this paper, we report giant anomalous Hall… ▽ More The anomalous Hall and Nernst effects describe the voltage drop perpendicular to an applied current and temperature gradient due to the magnetization of a magnetic material. These effects can be utilized to measure the Berry curvature at the Fermi energy, and have potential applications in future electronic devices and thermoelectric energy conversion. In this paper, we report giant anomalous Hall conductivity and anomalous Nernst coefficient, as high as about 1000 $Ω^{-1}$ cm$^{-1}$ and 10 $μ$V K$^{-1}$, respectively, in a heavy fermion ferromagnet, CeCrGe$_3$. This compound uniquely manifests strong hybridization between the 4$f$ and conduction electrons, leading to a Kondo lattice state in the presence of ferromagnetic order. Unlike conventional topological semimetals in which the electron correlation is weak, CeCrGe$_3$ manifests a strong Berry curvature field of the heavy fermion with an extremely low Fermi energy. Our findings pave the way for exploring correlation-driven topological responses in a ferromagnetic Kondo lattice environment. △ Less

Submitted 31 January, 2024; originally announced January 2024.

Comments: 22 pages, 5 figures

Incipient nematicity from electron flat bands in a kagome metal

Authors: Nathan Drucker, Thanh Nguyen, Manasi Mandal, Phum Siriviboon, Yujie Quan, Artittaya Boonkird, Ryotaro Okabe, Fankang Li, Kaleb Buragge, Fumiaki Funuma, Masaaki Matsuda, Douglas Abernathy, Travis Williams, Songxue Chi, Feng Ye, Christie Nelson, Bolin Liao, Pavel Volkov, Mingda Li

Abstract: Engineering new quantum phases requires fine tuning of the electronic, orbital, spin, and lattice degrees of freedom. To this end, the kagome lattice with flat bands has garnered great attention by hosting various topological and correlated phases, when the flat band is at the Fermi level. Here we discover unconventional nematiciy in kagome metal CoSn, where flat bands are fully occupied below the… ▽ More Engineering new quantum phases requires fine tuning of the electronic, orbital, spin, and lattice degrees of freedom. To this end, the kagome lattice with flat bands has garnered great attention by hosting various topological and correlated phases, when the flat band is at the Fermi level. Here we discover unconventional nematiciy in kagome metal CoSn, where flat bands are fully occupied below the Fermi level. Thermodynamic, dilatometry, resonant X-ray scattering, inelastic neutron scattering, Larmor diffraction, and thermoelectric measurements consistently hint at rotational symmetry-breaking and nematic order that is pronounced only near T=225 K. These observations, principally the nematic's finite temperature stability -- incipience -- can be explained by a phenomenological model which reveals that thermally excited flat bands promote symmetry breaking at a characteristic temperature. Our work shows that thermal fluctuations, which are typically detrimental for correlated electron phases, can induce new ordered states of matter, avoiding the requirements for fine tuning of electronic bands. △ Less

Submitted 30 January, 2024; originally announced January 2024.

Comments: 37 pages, 5 main figures, 14 supplementary figures

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)

Electronic and topological characters of the ideal magnetic topological materials EuAuX with X = P, As, Sb, and Bi

Authors: Shengwei Chi, Gang Xu

Abstract: Ideal magnetic topological materials have great significance in both fundamental physics and technical applications, due to their abundant exotic quantum properties and facilitation of control. Using first-principles calculations, we find several ideal magnetic topological materials in EuTX (T = Cu, Ag, and Au; X= P, As, Sb, and Bi) family. Particularly, EuAuP is the ferromagnetic Weyl semimetal,… ▽ More Ideal magnetic topological materials have great significance in both fundamental physics and technical applications, due to their abundant exotic quantum properties and facilitation of control. Using first-principles calculations, we find several ideal magnetic topological materials in EuTX (T = Cu, Ag, and Au; X= P, As, Sb, and Bi) family. Particularly, EuAuP is the ferromagnetic Weyl semimetal, and EuAuX (X=As, Sb, and Bi) in their ground state with in-plane moments are the antiferromagnetic semimetals hosting the topological gap near the Fermi level. By tuning the magnetic moments to z-axis, EuAuX (X=As, Sb, and Bi) could further evolve into triple degenerate nodal points (TDNPs) semimetal states. The main characteristics of antiferromagnetic TDNP semimetal, including the Fermi arcs, and tangent Fermi surfaces with opposite spin winding numbers, are also studied. Our work provides a promising platform to modulate the magnetism, topological electronic structures and emergent quantum states. △ Less

Submitted 4 December, 2023; originally announced December 2023.

Quantum to classical crossover in generalized spin systems -- the temperature-dependent spin dynamics of FeI$_2$

Authors: D. Dahlbom, D. Brooks, M. S. Wilson, S. Chi, A. I. Kolesnikov, M. B. Stone, H. Cao, Y. -W. Li, K. Barros, M. Mourigal, C. D. Batista, X. Bai

Abstract: Simulating quantum spin systems at finite temperatures is an open challenge in many-body physics. This work studies the temperature-dependent spin dynamics of a pivotal compound, FeI$_2$, to determine if universal quantum effects can be accounted for by a phenomenological renormalization of the dynamical spin structure factor $S(\mathbf{q}, ω)$ measured by inelastic neutron scattering. Renormaliza… ▽ More Simulating quantum spin systems at finite temperatures is an open challenge in many-body physics. This work studies the temperature-dependent spin dynamics of a pivotal compound, FeI$_2$, to determine if universal quantum effects can be accounted for by a phenomenological renormalization of the dynamical spin structure factor $S(\mathbf{q}, ω)$ measured by inelastic neutron scattering. Renormalization schemes based on the quantum-to-classical correspondence principle are commonly applied at low temperatures to the harmonic oscillators describing normal modes. However, it is not clear how to extend this renormalization to arbitrarily high temperatures. Here we introduce a temperature-dependent normalization of the classical moments, whose magnitude is determined by imposing the quantum sum rule, i.e. $\int dωd\mathbf{q} S(\mathbf{q}, ω) = N_S S (S+1)$ for $N_S$ dipolar magnetic moments. We show that this simple renormalization scheme significantly improves the agreement between the calculated and measured $S(\mathbf{q}, ω)$ for FeI$_{2}$ at all temperatures. Due to the coupled dynamics of dipolar and quadrupolar moments in that material, this renormalization procedure is extended to classical theories based on SU(3) coherent states, and by extension, to any SU(N) coherent state representation of local multipolar moments. △ Less

Submitted 30 October, 2023; originally announced October 2023.

Comments: Associated source code for reproducing calculations available at: https://github.com/SunnySuite/SunnyContributed

Anomalous Hall effect in the antiferromagnetic Weyl semimetal SmAlSi

Authors: Yuxiang Gao, Shiming Lei, Eleanor M. Clements, Yichen Zhang, Xue-Jian Gao, Songxue Chi, Kam Tuen Law, Ming Yi, Jeffrey W. Lynn, Emilia Morosan

Abstract: The intrinsic anomalous Hall effect (AHE) has been reported in numerous ferromagnetic (FM) Weyl semimetals. However, AHE in the antiferromagnetic (AFM) or paramagnetic (PM) state of Weyl semimetals has been rarely observed experimentally, and only in centrosymmetric materials. Different mechanisms have been proposed to establish the connection between the AHE and the type of magnetic order. In thi… ▽ More The intrinsic anomalous Hall effect (AHE) has been reported in numerous ferromagnetic (FM) Weyl semimetals. However, AHE in the antiferromagnetic (AFM) or paramagnetic (PM) state of Weyl semimetals has been rarely observed experimentally, and only in centrosymmetric materials. Different mechanisms have been proposed to establish the connection between the AHE and the type of magnetic order. In this paper, we report AHE in both the AFM and PM states of non-centrosymmetric compound SmAlSi. To account for the AHE in non-centrosymmetric Weyl semimetals without FM, we introduce a new mechanism based on magnetic field-induced Weyl nodes evolution. Angle-dependent quantum oscillations in SmAlSi provide evidence for the Weyl points and large AHE in both the PM and the AFM states. The proposed mechanism qualitatively explains the temperature dependence of the anomalous Hall conductivity (AHC), which displays unconventional power law behavior of the AHC in both AFM and PM states of SmAlSi. △ Less

Submitted 24 August, 2024; v1 submitted 13 October, 2023; originally announced October 2023.

Journal ref: Phys. Rev. Materials 9, L061201 (2025)

Helical magnetic state in the vicinity of the pressure-induced superconducting phase in MnP

Authors: S. E. Dissanayake, M. Matsuda, K. Yoshimi, S. Kasamatsu, F. Ye, S. Chi, W. Steinhardt, G. Fabbris, S. Haravifard, J. -G. Cheng, J. -Q. Yan, J. Gouchi, Y. Uwatoko

Abstract: MnP is a metal that shows successive magnetic transitions from paramagnetic to ferromagnetic and helical magnetic phases at ambient pressure with decreasing temperature. With applied pressure, the magnetic transition temperatures decrease and superconductivity appears around 8 GPa where the magnetic order is fully suppressed and the quantum critical behavior is observed. These results suggest that… ▽ More MnP is a metal that shows successive magnetic transitions from paramagnetic to ferromagnetic and helical magnetic phases at ambient pressure with decreasing temperature. With applied pressure, the magnetic transition temperatures decrease and superconductivity appears around 8 GPa where the magnetic order is fully suppressed and the quantum critical behavior is observed. These results suggest that MnP is an unconventional superconductor in which magnetic fluctuations may be relevant to the superconducting pairing mechanism. In order to elucidate the magnetic ground state adjacent to the superconducting phase first discovered in Mn-based materials, high-pressure neutron diffraction measurements have been performed in hydrostatic pressure up to 7.5 GPa. The helical magnetic structure with the propagation vector along the $b$ axis, reported previously at 3.8 GPa, was found to be robust up to 7.5 GPa. First principles and classical Monte Carlo calculations have also been performed to understand how the pressure-driven magnetic phase transitions are coupled with change of the exchange interactions. The calculations, which qualitatively reproduce the magnetic structures as a function of pressure, suggest that the exchange interactions change drastically with applied pressure and the further-neighbor interactions become more influential at high pressures. Combining the experimental and theoretical results, we describe the detail of exchange interactions in the vicinity of the superconducting phase which is critical to understand the pairing mechanism of the unconventional superconductivity in MnP. △ Less

Submitted 25 August, 2023; originally announced August 2023.

Comments: 15 pages, 10 figures

Journal ref: Physical Review Research 5, 043026 (2023)

Static and dynamical properties of the spin-5/2 nearly ideal triangular lattice antiferromagnet Ba3MnSb2O9

Authors: Mingfang Shu, Weicen Dong, Jinlong Jiao, Jiangtao Wu, Gaoting lin, Tao Hong, Huibo Cao, Masaaki Matsuda, Wei Tian, Songxue Chi, Georg Ehlers, Zhongwen Ouyang, Hongwei Chen, Youming Zou, Zhe Qu, Qing Huang, Haidong Zhou, Yoshitomo Kamiya, Jie Ma

Abstract: We study the ground state and spin excitations in Ba3MnSb2O9, an easy-plane S = 5/2 triangular lattice antiferromagnet. By combining single-crystal neutron scattering, electric spin resonance (ESR), and spin wave calculations, we determine the frustrated quasi-two-dimensional spin Hamiltonian parameters describing the material. While the material has a slight monoclinic structural distortion, whic… ▽ More We study the ground state and spin excitations in Ba3MnSb2O9, an easy-plane S = 5/2 triangular lattice antiferromagnet. By combining single-crystal neutron scattering, electric spin resonance (ESR), and spin wave calculations, we determine the frustrated quasi-two-dimensional spin Hamiltonian parameters describing the material. While the material has a slight monoclinic structural distortion, which could allow for isosceles-triangular exchanges and biaxial anisotropy by symmetry, we observe no deviation from the behavior expected for spin waves in the in-plane 120o state. Even the easy-plane anisotropy is so small that it can only be detected by ESR in our study. In conjunction with the quasi-two-dimensionality, our study establishes that Ba3MnSb2O9 is a nearly ideal triangular lattice antiferromagnet with the quasi-classical spin S = 5/2, which suggests that it has the potential for an experimental study of Z- or Z2-vortex excitations. △ Less

Submitted 7 September, 2023; v1 submitted 9 June, 2023; originally announced June 2023.

Effect of random antiferromagnetic exchange on the spin waves in a three-dimensional Heisenberg ferromagnet

Authors: S. Hameed, Z. Wang, D. M. Gautreau, J. Joe, K. P. Olson, S. Chi, P. M. Gehring, T. Hong, D. M. Pajerowski, T. J. Williams, Z. Xu, M. Matsuda, T. Birol, R. M. Fernandes, M. Greven

Abstract: Neutron scattering is used to study spin waves in the three-dimensional Heisenberg ferromagnet YTiO$_3$, with spin-spin exchange disorder introduced $via$ La-substitution at the Y site. No significant changes are observed in the spin-wave dispersion up to a La concentration of 20%. However, a strong broadening of the spectrum is found, indicative of shortened spin-wave lifetimes. Density-functiona… ▽ More Neutron scattering is used to study spin waves in the three-dimensional Heisenberg ferromagnet YTiO$_3$, with spin-spin exchange disorder introduced $via$ La-substitution at the Y site. No significant changes are observed in the spin-wave dispersion up to a La concentration of 20%. However, a strong broadening of the spectrum is found, indicative of shortened spin-wave lifetimes. Density-functional theory calculations predict minimal changes in exchange constants as a result of average structural changes due to La substitution, in agreement with the data. The absence of significant changes in the spin-wave dispersion, the considerable lifetime effect, and the reduced ordered magnetic moment previously observed in the La-substituted system are qualitatively captured by an isotropic, nearest-neighbor, three-dimensional Heisenberg ferromagnet model with random antiferromagnetic exchange. We therefore establish Y$_{1-x}$La$_x$TiO$_3$ as a model system to study the effect of antiferromagnetic spin-exchange disorder in a three-dimensional Heisenberg ferromagnet. △ Less

Submitted 7 May, 2023; originally announced May 2023.

Comments: 6 pages and 5 figures in main text. Supplementary material included

Journal ref: Phys. Rev. B 108, 134406 (2023)

Magnetism and Fermiology of Kagome Magnet YMn6Sn4Ge2

Authors: Hari Bhandari, Rebecca L. Dally, Peter E. Siegfried, Resham B. Regmi, Kirrily C. Rule, Songxue Chi, Jeffery W. Lynn, I. I. Mazin, Nirmal J. Ghimire

Abstract: Kagome lattice magnets are an interesting class of materials as they can host topological properties in their magnetic and electronic structures. YMn6Sn6 is one such compound in which a series of competing magnetic phases is stabilized by an applied magnetic field, and both an enigmatic topological Hall effect and a Dirac crossing close to the Fermi energy have been realized. This material also sh… ▽ More Kagome lattice magnets are an interesting class of materials as they can host topological properties in their magnetic and electronic structures. YMn6Sn6 is one such compound in which a series of competing magnetic phases is stabilized by an applied magnetic field, and both an enigmatic topological Hall effect and a Dirac crossing close to the Fermi energy have been realized. This material also shows a magnetization-induced Lifshitz transition and evidence of a unique charge spin coupling in one of the magnetic phases, namely the fan-like phase. Tuning the magnetism, and thus the interplay with the electronic states, opens new avenues for precise control of these novel properties. Here, we demonstrate the extreme sensitivity of the magnetic phases in YMn6Sn4Ge2 through the investigation of structural, magnetic, and transport properties. The high sensitivity to small doping provides great potential for engineering the magnetic phases and associated electronic properties in this family of rare-earth kagome magnets. △ Less

Submitted 22 April, 2023; originally announced April 2023.

Journal ref: npj Quantum Materials 9, 6 (2024)

Observation of chiral edge transport in a rapidly-rotating quantum gas

Authors: Ruixiao Yao, Sungjae Chi, Biswaroop Mukherjee, Airlia Shaffer, Martin Zwierlein, Richard J. Fletcher

Abstract: The frictionless, directional propagation of particles at the boundary of topological materials is one of the most striking phenomena in transport. These chiral edge modes lie at the heart of the integer and fractional quantum Hall effects, and their extraordinary robustness against noise and disorder reflects the quantization of Hall conductivity in these systems. Despite their central importance… ▽ More The frictionless, directional propagation of particles at the boundary of topological materials is one of the most striking phenomena in transport. These chiral edge modes lie at the heart of the integer and fractional quantum Hall effects, and their extraordinary robustness against noise and disorder reflects the quantization of Hall conductivity in these systems. Despite their central importance, controllable injection of edge modes, and direct imaging of their propagation, structure, and dynamics, is challenging. Here, we demonstrate the distillation of individual chiral edge states in a rapidly-rotating bosonic superfluid confined by an optical boundary. Tuning the wall sharpness, we reveal the smooth crossover between soft wall behaviour in which the propagation speed is proportional to wall steepness, and the hard wall regime exhibiting chiral free particles. From the skipping motion of atoms along the boundary, we spectroscopically infer the energy gap between the ground and first excited edge bands, and reveal its evolution from the bulk Landau level splitting for a soft boundary, to the hard wall limit. △ Less

Submitted 1 May, 2024; v1 submitted 20 April, 2023; originally announced April 2023.

Comments: 9 pages, 5+2 figures, v3 added a new figure

Journal ref: Nature Physics 20, 1726-1731 (2024)

Probing site-resolved correlations in a spin system of ultracold molecules

Authors: Lysander Christakis, Jason S. Rosenberg, Ravin Raj, Sungjae Chi, Alan Morningstar, David A. Huse, Zoe Z. Yan, Waseem S. Bakr

Abstract: Synthetic quantum systems with interacting constituents play an important role in quantum information processing and in elucidating fundamental phenomena in many-body physics. Following impressive advances in cooling and trapping techniques, ensembles of ultracold polar molecules have emerged as a promising synthetic system that combines several advantageous properties. These include a large set o… ▽ More Synthetic quantum systems with interacting constituents play an important role in quantum information processing and in elucidating fundamental phenomena in many-body physics. Following impressive advances in cooling and trapping techniques, ensembles of ultracold polar molecules have emerged as a promising synthetic system that combines several advantageous properties. These include a large set of internal states for encoding quantum information, long nuclear and rotational coherence times and long-range, anisotropic interactions. The latter are expected to allow the exploration of intriguing phases of correlated quantum matter, such as topological superfluids, quantum spin liquids, fractional Chern insulators and quantum magnets. Probing correlations in these phases is crucial to understand their microscopic properties, necessitating the development of new experimental techniques. Here we use quantum gas microscopy to measure the site-resolved dynamics of quantum correlations in a gas of polar molecules in a two-dimensional optical lattice. Using two rotational states of the molecules, we realize a spin-1/2 system where the particles are coupled via dipolar interactions, producing a quantum spin-exchange model. Starting with the synthetic spin system prepared far from equilibrium, we study the evolution of correlations during the thermalization process for both spatially isotropic and anisotropic interactions. Furthermore, we study the correlation dynamics in a spin-anisotropic Heisenberg model engineered from the native spin-exchange model using Floquet techniques. These experiments push the frontier of probing and controlling interacting systems of ultracold molecules, with prospects for exploring new regimes of quantum matter and characterizing entangled states useful for quantum computation and metrology. △ Less

Submitted 19 July, 2022; originally announced July 2022.

Journal ref: Nature 614, 64 (2023)

Gapless spin-excitations in the superconducting state of a quasi-one-dimensional spin-triplet superconductor

Authors: Keith M. Taddei, Bing-Hua Lei, Michael A. Susner, Hui-Fei Zhai, Thomas J. Bullard, Liurukara D. Sanjeewa, Qiang Zheng, Athena S. Sefat, Songxue Chi, Clarina dela Cruz, David J. Singh, Bing Lv

Abstract: Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first principles calculations which suggest that quasi-one-dimensional K$_2$Cr$_3$As$_3$ may be such a superconductor. Using inelastic neutron scattering we probe the dynam… ▽ More Majorana zero modes form as intrinsic defects in an odd-orbital one-dimensional superconductor thus motivating the search for such materials in the pursuit of Majorana physics. Here, we present combined experimental results and first principles calculations which suggest that quasi-one-dimensional K$_2$Cr$_3$As$_3$ may be such a superconductor. Using inelastic neutron scattering we probe the dynamic spin-susceptibilities of K$_2$Cr$_3$As$_3$ and K$_2$Mo$_3$As$_3$ and show the presence of antiferromagnetic spin-fluctuations in both compounds. Below the superconducting transition, these fluctuations gap in K$_2$Mo$_3$As$_3$ but not in K$_2$Cr$_3$As$_3$. Using first principles calculations, we show that these fluctuations likely arise from nesting on one dimensional features of the Fermi surface. Considering these results we propose that while K$_2$Mo$_3$As$_3$ is a conventional superconductor, K$_2$Cr$_3$As$_3$ is likely a spin-triplet, and consequently, topological superconductor. △ Less

Submitted 23 June, 2022; originally announced June 2022.

Comments: 8 pages, 4 figures

Strongly-overdoped La$_{2-x}$Sr$_x$CuO$_4$: Evidence for Josephson-coupled grains of strongly-correlated superconductor

Authors: Yangmu Li, A. Sapkota, P. M. Lozano, Zengyi Du, Hui Li, Zebin Wu, Asish K. Kundu, R. J. Koch, Lijun Wu, B. L. Winn, Songxue Chi, M. Matsuda, M. Frontzek, E. S. Bozin, Yimei Zhu, I. Bozovic, Abhay N. Pasupathy, Ilya K. Drozdov, Kazuhiro Fujita, G. D. Gu, Igor Zaliznyak, Qiang Li, J. M. Tranquada

Abstract: The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with… ▽ More The interpretation of how superconductivity disappears in cuprates at large hole doping has been controversial. To address this issue, we present an experimental study of single-crystal and thin film samples of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) with $x\ge0.25$. In particular, measurements of bulk susceptibility on LSCO crystals with $x=0.25$ indicate an onset of diamagnetism at $T_{c1}=38.5$ K, with a sharp transition to a phase with full bulk shielding at $T_{c2}=18$ K, independent of field direction. Strikingly, the in-plane resistivity only goes to zero at $T_{c2}$. Inelastic neutron scattering on $x=0.25$ crystals confirms the presence of low-energy incommensurate magnetic excitations with reduced strength compared to lower doping levels. The ratio of the spin gap to $T_{c2}$ is anomalously large. Our results are consistent with a theoretical prediction for strongly overdoped cuprates by Spivak, Oreto, and Kivelson, in which superconductivity initially develops within disconnected self-organized grains characterized by a reduced hole concentration, with bulk superconductivity occurring only after superconductivity is induced by proximity effect in the surrounding medium of higher hole concentration. Beyond the superconducting-to-metal transition, local differential conductance measurements on an LSCO thin film suggest that regions with pairing correlations survive, but are too dilute to support superconducting order. Future experiments will be needed to test the degree to which these results apply to overdoped cuprates in general. △ Less

Submitted 21 December, 2022; v1 submitted 3 May, 2022; originally announced May 2022.

Comments: 21 pages, 15 figures; added more sample characterizations, results for another sample, more discussion; accepted version

Journal ref: Phys. Rev. B 106, 224515 (2022)

A two-dimensional programmable tweezer array of fermions

Authors: Zoe. Z. Yan, Benjamin M. Spar, Max L. Prichard, Sungjae Chi, Hao-Tian Wei, Eduardo Ibarra-García-Padilla, Kaden R. A. Hazzard, Waseem S. Bakr

Abstract: We prepare high-filling two-component arrays of up to fifty fermionic atoms in optical tweezers, with the atoms in the ground motional state of each tweezer. Using a stroboscopic technique, we configure the arrays in various two-dimensional geometries with negligible Floquet heating. Full spin- and density-resolved readout of individual sites allows us to post-select near-zero entropy initial stat… ▽ More We prepare high-filling two-component arrays of up to fifty fermionic atoms in optical tweezers, with the atoms in the ground motional state of each tweezer. Using a stroboscopic technique, we configure the arrays in various two-dimensional geometries with negligible Floquet heating. Full spin- and density-resolved readout of individual sites allows us to post-select near-zero entropy initial states for fermionic quantum simulation. We prepare a correlated state in a two-by-two tunnel-coupled Hubbard plaquette, demonstrating all the building blocks for realizing a programmable fermionic quantum simulator. △ Less

Submitted 28 March, 2022; originally announced March 2022.

Journal ref: Phys. Rev. Lett. 129, 123201 (2022)

Antiferromagnetic fluctuations and orbital-selective Mott transition in the van der Waals ferromagnet Fe3-xGeTe2

Authors: Xiaojian Bai, Frank Lechermann, Yaohua Liu, Yongqiang Cheng, Alexander I. Kolesnikov, Feng Ye, Travis J. Williams, Songxue Chi, Tao Hong, Garrett E. Granroth, Andrew F. May, Stuart Calder

Abstract: Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. Here, we demonstrate that antiferromagnetic spin fluct… ▽ More Fe3-xGeTe2 is a layered magnetic van der Waals material of interest for both fundamental and applied research. Despite the observation of intriguing physical properties, open questions exist even on the basic features related to magnetism: is it a simple ferromagnet or are there antiferromagnetic regimes and are the moments local or itinerant. Here, we demonstrate that antiferromagnetic spin fluctuations coexist with the ferromagnetism through comprehensive elastic and inelastic neutron scattering and thermodynamic measurements. Our realistic dynamical mean-field theory calculations reveal that the competing magnetic fluctuations are driven by an orbital selective Mott transition, where only the plane-perpendicular a1g orbital of the Fe(3d) manifold remains itinerant. Our results highlight the multi-orbital character in Fe3-xGeTe2 that supports a rare coexistence of local and itinerant physics within this material. △ Less

Submitted 11 November, 2022; v1 submitted 28 January, 2022; originally announced January 2022.

Comments: 16 pages, 3 main figures

Journal ref: Phys. Rev. B 106, L180409 (2022)

Field-Induced Lifshitz Transition in the Magnetic Weyl Semimetal Candidate PrAlSi

Authors: Lei Wu, Shengwei Chi, Huakun Zuo, Gang Xu, Lingxiao Zhao, Yongkang Luo, Zengwei Zhu

Abstract: Lifshitz transition (LT) refers to an abrupt change in the electronic structure and Fermi surface, and is associated to a variety of emergent quantum phenomena. Amongst the LTs observed in known materials, the field-induced LT has been rare and its origin remains elusive. To understand the origin of field-induced LT, it is important to extend the material basis beyond the usual setting of heavy fe… ▽ More Lifshitz transition (LT) refers to an abrupt change in the electronic structure and Fermi surface, and is associated to a variety of emergent quantum phenomena. Amongst the LTs observed in known materials, the field-induced LT has been rare and its origin remains elusive. To understand the origin of field-induced LT, it is important to extend the material basis beyond the usual setting of heavy fermion metals. Here, we report on a field-induced LT in PrAlSi, a magnetic Weyl semimetal candidate with localized 4$f$ electrons, through a study of magnetotransport up to 55 T. The quantum oscillation analysis reveals that across a threshold field $B^*\approx$14.5 T the oscillation frequency ($F_1$ = 43 T) is replaced by two new frequencies ($F_2$ = 62 T and $F_3$ = 103 T). Strikingly, the LT occurs well below quantum limit, with obvious temperature-dependent oscillation frequency and field-dependent cyclotron mass. Our work not only enriches the rare examples of field-induced LTs, but also paves the way for further investigation on the interplay among topology, magnetism and electronic correlation. △ Less

Submitted 19 January, 2023; v1 submitted 13 January, 2022; originally announced January 2022.

Comments: 8 pages, 5 figures, Supplemental Material included

Journal ref: npj Quantum Materials 8, 4 (2023)

Multiple lattice instabilities and complex ground state in Cs$_2$AgBiBr$_6$

Authors: Xing He, Matthew Krogstad, Mayanak K Gupta, Tyson Lanigan-Atkins, Chengjie Mao, Feng Ye, Yaohua Liu, Tao Hong, Songxue Chi, Haotong Wei, Jinsong Huang, Stephan Rosenkranz, Raymond Osborn, Olivier Delaire

Abstract: Metal halides perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with Cs$_2$AgBiBr$_6$ one of the main contenders among lead-free systems. Cs$_2$AgBiBr$_6$ crystallizes in a nominally double-perovskite structure, but exhibits a soft lattice with large atomic fluctuations characteristic of MHPs. While crucial to understand electron-phonon and phonon-phonon coup… ▽ More Metal halides perovskites (MHPs) are attracting considerable interest for optoelectronic applications, with Cs$_2$AgBiBr$_6$ one of the main contenders among lead-free systems. Cs$_2$AgBiBr$_6$ crystallizes in a nominally double-perovskite structure, but exhibits a soft lattice with large atomic fluctuations characteristic of MHPs. While crucial to understand electron-phonon and phonon-phonon couplings, the spatio-temporal correlations of these fluctuations remain largely unknown. Here, we reveal these correlations using comprehensive neutron and x-ray scattering measurements on Cs$_2$AgBiBr$_6$ single-crystals, complemented with first-principles simulations augmented with machine-learned neural-network potentials. We report the discovery of an unexpected complex modulated ground state structure containing several hundred atoms, arising from a soft-phonon instability of the low-temperature tetragonal phase. Further, our experiments and simulations both reveal extensive correlated 2D fluctuations of Br octahedra at finite temperature, arising from soft anharmonic optic phonons, reflecting very shallow potential wells. These results provide new insights into the atomic structure and fluctuations in MHPs, critical to understand and control their thermal and optoelectronic properties. △ Less

Submitted 10 June, 2023; v1 submitted 9 December, 2021; originally announced December 2021.

Comments: 12 pages, 4 figures

A Fermi-Hubbard Optical Tweezer Array

Authors: Benjamin M. Spar, Elmer Guardado-Sanchez, Sungjae Chi, Zoe Z. Yan, Waseem S. Bakr

Abstract: We use lithium-6 atoms in an optical tweezer array to realize an eight-site Fermi-Hubbard chain near half filling. We achieve single site detection by combining the tweezer array with a quantum gas microscope. By reducing disorder in the energy offsets to less than the tunneling energy, we observe Mott insulators with strong antiferromagnetic correlations. The measured spin correlations allow us t… ▽ More We use lithium-6 atoms in an optical tweezer array to realize an eight-site Fermi-Hubbard chain near half filling. We achieve single site detection by combining the tweezer array with a quantum gas microscope. By reducing disorder in the energy offsets to less than the tunneling energy, we observe Mott insulators with strong antiferromagnetic correlations. The measured spin correlations allow us to put an upper bound on the entropy of 0.26(4)$k_\mathrm{B}$ per atom, comparable to the lowest entropies achieved with optical lattices. Additionally, we establish the flexibility of the tweezer platform by initializing atoms on one tweezer and observing tunneling dynamics across the array for different 1D geometries. △ Less

Submitted 28 October, 2021; originally announced October 2021.

Comments: 10 pages, 7 figures

Journal ref: Phys. Rev. Lett. 128, 223202 (2022)

Incommensurate magnetic orders and topological Hall effect in the square-net centrosymmetric EuGa$_2$Al$_2$ system

Authors: Jaime M. Moya, Shiming Lei, Eleanor M. Clements, Caitlin S. Kengle, Stella Sun, Kevin Allen, Qizhi Li, Y. Y. Peng, Ali A. Husain, Matteo Mitrano, Matthew J. Krogstad, Raymond Osborn, Anand B. Puthirath, Songxue Chi, L. Debeer-Schmitt, J. Gaudet, P. Abbamonte, Jeffrey W. Lynn, E. Morosan

Abstract: Neutron diffraction on the centrosymmetric square-net magnet EuGa$_2$Al$_2$ reveals multiple incommensurate magnetic states (AFM1,2,3) in zero field. In applied field, a new magnetic phase (A) is identified from magnetization and transport measurements, bounded by two of the $μ_0H$~=~0 incommensurate magnetic phases (AFM1,helical and AFM3, cycloidal) with different moment orientations. Moreover, m… ▽ More Neutron diffraction on the centrosymmetric square-net magnet EuGa$_2$Al$_2$ reveals multiple incommensurate magnetic states (AFM1,2,3) in zero field. In applied field, a new magnetic phase (A) is identified from magnetization and transport measurements, bounded by two of the $μ_0H$~=~0 incommensurate magnetic phases (AFM1,helical and AFM3, cycloidal) with different moment orientations. Moreover, magneto-transport measurements indicate the presence of a topological Hall effect, with maximum values centered in the A phase. Together, these results render EuGa$_2$Al$_2$ a material with non-coplanar or topological spin texture in applied field. X-ray diffraction reveals an out-of-plane (OOP) charge density wave (CDW) below $T_{CDW} \sim$ 50 K while the magnetic propagation vector lies in plane below $T_N$ = 19.5 K. Together these data point to a new route to realizing in-plane non-collinear spin textures through an OOP CDW. In turn, these non-collinear spin textures may be unstable against the formation of topological spin textures in an applied field. △ Less

Submitted 22 September, 2022; v1 submitted 22 October, 2021; originally announced October 2021.

Journal ref: Phys. Rev. Materials 6, 074201 (2022)

A magnetic excitation linking quasi-1D Chevrel-type selenide and arsenide superconductors

Authors: Logan M. Whitt, Tyra C. Douglas, Songxue Chi, Keith M. Taddei, Jared M. Allred

Abstract: The quasi-one-dimensional Chevrel phases, A$_2$Mo$_6$Se$_6$ (A = Tl, In, K, Rb, Cs), are of interest due to their atypical electronic properties. The Tl and In analogues undergo a superconducting transition whereas the alkali metal analogues show charge gapping of another, not well understood type. We report the results of inelastic neutron scattering on polycrystalline In$_2$Mo$_6$Se$_6$ (… ▽ More The quasi-one-dimensional Chevrel phases, A$_2$Mo$_6$Se$_6$ (A = Tl, In, K, Rb, Cs), are of interest due to their atypical electronic properties. The Tl and In analogues undergo a superconducting transition whereas the alkali metal analogues show charge gapping of another, not well understood type. We report the results of inelastic neutron scattering on polycrystalline In$_2$Mo$_6$Se$_6$ ($T_c=2.85\,$K) and Rb$_2$Mo$_6$Se$_6$ (non-superconducting) samples, which reveal a column of intensity with linear dispersion from [0 0 1/2] to [0 0 1] in both compounds. The observed temperature and |$Q$| independence together suggest the presence of unconventional carriers with a spin contribution to the excitation. This is contrary to the prevailing model for these materials, which is that they are non-magnetic. The excitation has similar dispersion and $S(Q,E,T)$ behavior as one observed in the structurally related superconducting compounds A$_2$Cr$_3$As$_3$ and A$_2$Mo$_3$As$_3$ (A = K, Rb, Cs), which has been interpreted as magnetic in origin and related to Fermi surface nesting. The connection is unexpected because the calculated Fermi surface of the arsenides differs substantially from the A$_2$Mo$_6$Se$_6$ compounds, and many consider them distinct classes of materials. The new observation suggests a hidden link in the physics between both classes of superconductors, perhaps originating from their quasi-low-dimensional character. △ Less

Submitted 27 June, 2024; v1 submitted 19 October, 2021; originally announced October 2021.

Comments: Replaced with accepted article: 12 pages, 3 figures. Appendix: 4 pages, 1 figure

Journal ref: Phys. Rev. Materials 6, 124804, 2022

The Three-Dimensional Electronic Structure of LiFeAs: Strong-coupling Superconductivity and Topology in the Iron Pnictides

Authors: Ryan P. Day, MengXing Na, Manuel Zingl, Berend Zwartsenberg, Matteo Michiardi, Giorgio Levy, Michael Schneider, Doug Wong, Pinder Dosanjh, Tor M. Pedersen, Sergey Gorovikov, Shun Chi, Ruixing Liang, Walter N. Hardy, Douglas A. Bonn, Sergey Zhdanovich, Ilya S. Elfimov, Andrea Damascelli

Abstract: Amongst the iron-based superconductors, LiFeAs is unrivalled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here, we explore the challenge of interpret… ▽ More Amongst the iron-based superconductors, LiFeAs is unrivalled in the simplicity of its crystal structure and phase diagram. However, our understanding of this canonical compound suffers from conflict between mutually incompatible descriptions of the material's electronic structure, as derived from contradictory interpretations of the photoemission record. Here, we explore the challenge of interpretation in such experiments. By combining comprehensive photon energy- and polarization- dependent angle-resolved photoemission spectroscopy (ARPES) measurements with numerical simulations, we establish the providence of several contradictions in the present understanding of this and related materials. We identify a confluence of surface-related issues which have precluded unambiguous identification of both the number and dimensionality of the Fermi surface sheets. Ultimately, we arrive at a scenario which supports indications of topologically non-trivial states, while also being incompatible with superconductivity as a spin-fluctuation driven Fermi surface instability. △ Less

Submitted 27 September, 2021; originally announced September 2021.

Comments: 8 pages, 4 figures

Report number: Phys. Rev. B 105, 155142

Magnetic order and its interplay with structure phase transition in van der Waals ferromagnet VI$_3$

Authors: Yiqing Hao, Yiqing Gu, Yimeng Gu, Erxi Feng, Huibo Cao, Songxue Chi, Hua Wu, Jun Zhao

Abstract: Van der Waals magnet VI$_3$ demonstrates intriguing magnetic properties that render it great for use in various applications. However, its microscopic magnetic structure has not been determined yet. Here, we report neutron diffraction and susceptibility measurements in VI$_3$ that revealed a ferromagnetic order with the moment direction tilted from the $c$-axis by ~36° at 4 K. A spin reorientation… ▽ More Van der Waals magnet VI$_3$ demonstrates intriguing magnetic properties that render it great for use in various applications. However, its microscopic magnetic structure has not been determined yet. Here, we report neutron diffraction and susceptibility measurements in VI$_3$ that revealed a ferromagnetic order with the moment direction tilted from the $c$-axis by ~36° at 4 K. A spin reorientation accompanied by a structure distortion within the honeycomb plane is observed at a temperature of ~27 K, before the magnetic order completely disappears at $T_C$ = 50 K. The refined magnetic moment of ~1.3 $μ_B$ at 4 K is considerably lower than the fully ordered spin moment of 2 $μ_B$/ V$^{3+}$, suggesting the presence of a considerable orbital moment antiparallel to the spin moment and strong spin-orbit coupling in VI$_3$. This results in strong magnetoelastic interactions that make the magnetic properties of VI$_3$ easily tunable via strain and pressure. △ Less

Submitted 13 July, 2021; originally announced July 2021.

Comments: 8 pages, 4 figures

Journal ref: Chin. Phys. Lett. 38, 096101 (2021)

Growth and characterization of large (Y,La)TiO$_3$ and (Y,Ca)TiO$_3$ single crystals

Authors: S. Hameed, J. Joe, L. R. Thoutam, J. Garcia-Barriocanal, B. Yu, G. Yu, S. Chi, T. Hong, T. J. Williams, J. W. Freeland, P. M. Gehring, Z. Xu, M. Matsuda, B. Jalan, M. Greven

Abstract: The Mott-insulating rare-earth titanates (RTiO$_3$, R being a rare-earth ion) are an important class of materials that encompasses interesting spin-orbital phases as well as ferromagnet-antiferromagnet and insulator-metal transitions. The growth of these materials has been plagued by difficulties related to overoxidation, which arises from a strong tendency of Ti$^{3+}$ to oxidize to Ti$^{4+}$. We… ▽ More The Mott-insulating rare-earth titanates (RTiO$_3$, R being a rare-earth ion) are an important class of materials that encompasses interesting spin-orbital phases as well as ferromagnet-antiferromagnet and insulator-metal transitions. The growth of these materials has been plagued by difficulties related to overoxidation, which arises from a strong tendency of Ti$^{3+}$ to oxidize to Ti$^{4+}$. We describe our efforts to grow sizable single crystals of YTiO$_3$ and its La-substituted and Ca-doped variants with the optical travelling-solvent floating-zone technique. We present sample characterization $via$ chemical composition analysis, magnetometry, charge transport, neutron scattering, x-ray absorption spectroscopy and x-ray magnetic circular dichroism to understand macroscopic physical property variations associated with overoxidation. Furthermore, we demonstrate a good signal-to-noise ratio in inelastic magnetic neutron scattering measurements of spin-wave excitations. A superconducting impurity phase, found to appear in Ca-doped samples at high doping levels, is identified as TiO. △ Less

Submitted 18 June, 2021; originally announced June 2021.

Journal ref: Phys. Rev. Materials 5, 125003 (2021)

Fluctuation-driven, topology-stabilized order in a correlated nodal semimetal

Authors: Nathan C. Drucker, Thanh Nguyen, Fei Han, Xi Luo, Nina Andrejevic, Ziming Zhu, Grigory Bednik, Quynh T. Nguyen, Zhantao Chen, Linh K. Nguyen, Travis J. Williams, Matthew B. Stone, Alexander I. Kolesnikov, Songxue Chi, Jaime Fernandez-Baca, Tom Hogan, Ahmet Alatas, Alexander A. Puretzky, David B. Geohegan, Shengxi Huang, Yue Yu, Mingda Li

Abstract: The interplay between strong electron correlation and band topology is at the forefront of condensed matter research. As a direct consequence of correlation, magnetism enriches topological phases and also has promising functional applications. However, the influence of topology on magnetism remains unclear, and the main research effort has been limited to ground state magnetic orders. Here we repo… ▽ More The interplay between strong electron correlation and band topology is at the forefront of condensed matter research. As a direct consequence of correlation, magnetism enriches topological phases and also has promising functional applications. However, the influence of topology on magnetism remains unclear, and the main research effort has been limited to ground state magnetic orders. Here we report a novel order above the magnetic transition temperature in magnetic Weyl semimetal (WSM) CeAlGe. Such order shows a number of anomalies in electrical and thermal transport, and neutron scattering measurements. We attribute this order to the coupling of Weyl fermions and magnetic fluctuations originating from a three-dimensional Seiberg-Witten monopole, which qualitatively agrees well with the observations. Our work reveals a prominent role topology may play in tailoring electron correlation beyond ground state ordering, and offers a new avenue to investigate emergent electronic properties in magnetic topological materials. △ Less

Submitted 19 July, 2023; v1 submitted 15 March, 2021; originally announced March 2021.

Comments: Errors found in theoretical section of the paper

Unconventional short-range structural fluctuations in cuprate high-$T_c$ superconductors

Authors: D. Pelc, R. J. Spieker, Z. W. Anderson, M. J. Krogstad, N. Biniskos, N. G. Bielinski, B. Yu, T. Sasagawa, L. Chauviere, P. Dosanjh, R. Liang, D. A. Bonn, A. Damascelli, S. Chi, Y. Liu, R. Osborn, M. Greven

Abstract: The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic pr… ▽ More The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic properties. Here we use neutron and X-ray diffuse scattering to uncover universal structural fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ and Tl$_2$Ba$_2$CuO$_{6+δ}$, two cuprate superconductors with distinct point disorder effects and optimal superconducting transition temperatures. The fluctuations are present in wide doping and temperature ranges, including compositions that maintain high average structural symmetry, and they exhibit unusual, yet simple scaling behavior. The scaling regime is robust and universal, similar to the well-known critical fluctuations close to second-order phase transitions, but with a distinctly different physical origin. We relate this behavior to pre-transitional phenomena in a broad class of systems with structural and magnetic transitions, and propose an explanation based on rare structural fluctuations caused by intrinsic nanoscale inhomogeneity. We also uncover parallels with superconducting fluctuations, which indicates that the underlying inhomogeneity plays an important role in cuprate physics. △ Less

Submitted 5 December, 2022; v1 submitted 9 March, 2021; originally announced March 2021.

Comments: 24 pages, 4 figures

Journal ref: Sci. Rep. 12, 20483 (2022)

Canted antiferromagnetic order and spin dynamics in the honeycomb-lattice Tb2Ir3Ga9

Authors: Feng Ye, Zachary Morgan, Wei Tian, Songxue Chi, Xiaoping Wang, Michael E. Manley, David Parker, Mojammel A. Khan, J. F. Mitchell, Randy Fishman

Abstract: Single crystal neutron diffraction, inelastic neutron scattering, bulk magnetization measurements, and first-principles calculations are used to investigate the magnetic properties of the honeycomb lattice $\rm Tb_2Ir_3Ga_9$. While the $R\ln2$ magnetic contribution to the low-temperature entropy indicates a $\rm J_{eff}=1/2$ moment for the lowest-energy crystal-field doublet, the Tb$^{3+}$ ions fo… ▽ More Single crystal neutron diffraction, inelastic neutron scattering, bulk magnetization measurements, and first-principles calculations are used to investigate the magnetic properties of the honeycomb lattice $\rm Tb_2Ir_3Ga_9$. While the $R\ln2$ magnetic contribution to the low-temperature entropy indicates a $\rm J_{eff}=1/2$ moment for the lowest-energy crystal-field doublet, the Tb$^{3+}$ ions form a canted antiferromagnetic structure below 12.5 K. Due to the Dzyalloshinskii-Moriya interactions, the Tb moments in the $ab$ plane are slightly canted towards $b$ by $6^\circ$ with a canted moment of 1.22 $μ_{\rm B} $ per formula unit. A minimal $xxz$ spin Hamiltonian is used to simultaneously fit the spin-wave frequencies along the high symmetry directions and the field dependence of the magnetization along the three crystallographic axes. Long-range magnetic interactions for both in-plane and out-of-plane couplings up to the second nearest neighbors are needed to account for the observed static and dynamic properties. The $z$ component of the exchange interactions between Tb moments are larger than the $x$ and $y$ components. This compound also exhibits bond-dependent exchange with negligible nearest exchange coupling between moments parallel and perpendicular to the 4$f$ orbitals. Despite the $J_{\rm eff}=1/2$ moments, the spin Hamiltonian is denominated by a large in-plane anisotropy $K_z \sim -1$ meV. DFT calculations confirm the antiferromagnetic ground state and the substantial inter-plane coupling at larger Tb-Tb distances. △ Less

Submitted 4 March, 2021; originally announced March 2021.

Comments: 6 figures

Journal ref: Phys. Rev. B 103, 184413 (2021)

Parallel Spin Stripes and Their Coexistance with Superconducting Ground States at Optimal and High Doping in La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$

Authors: Qianli Ma, Kirrily C. Rule, Zachary W. Cronkwright, Mirela Dragomir, Gabrielle Mitchell, Evan M. Smith, Songxue Chi, Alexander I. Kolesnikov, Matthew B. Stone, Bruce D. Gaulin

Abstract: Quasi-two dimensional quantum magnetism is clearly highly correlated with superconducting ground states in cuprate-based High T$_c$ superconductivity. Three dimensional, commensurate long range magnetic order in La$_2$CuO$_4$ quickly evolves to quasi-two dimensional, incommensurate correlations on doping with mobile holes, and superconducting ground states follow for x as small as 0.05 in the La… ▽ More Quasi-two dimensional quantum magnetism is clearly highly correlated with superconducting ground states in cuprate-based High T$_c$ superconductivity. Three dimensional, commensurate long range magnetic order in La$_2$CuO$_4$ quickly evolves to quasi-two dimensional, incommensurate correlations on doping with mobile holes, and superconducting ground states follow for x as small as 0.05 in the La$_{2-x}$Sr$_x$/Ba$_x$CuO$_4$ family of superconductors. It has long been known that the onset of superconducting ground states in these systems is coincident with a remarkable rotation of the incommensurate spin order from "diagonal stripes" below x = 0.05, to "parallel stripes" above. However, little is known about the spin correlations at optimal and high doping levels, where the dome of superconductivity draws to an end. Here we present new elastic and inelastic neutron scattering measurements on single crystals of La$_{1.6-x}$Nd$_{0.4}$Sr$_x$CuO$_4$ with x = 0.125, 0.19, 0.24 and 0.26, and show that two dimensional, quasi-static, parallel spin stripes are observed to onset at temperatures such that the parallel spin stripe phase envelopes all superconducting ground states in this system. Parallel spin stripes stretch across 0.05 < < 0.26, with rapidly decreasing moment size and onset temperatures for x > 0.125. We also show that the low energy, parallel spin stripe fluctuations for optimally doped x = 0.19 display dynamic spectral weight which grows with decreasing temperature and saturates below its superconducting T$_c$. The elastic order parameter for x = 0.19 also shows plateau behavior coincident with the onset of superconductivity. This set of observations assert the foundational role played by two dimensional parallel spin stripe order and fluctuations in High T$_c$ cuprate superconductivity. △ Less

Submitted 9 September, 2020; originally announced September 2020.

Journal ref: Phys. Rev. Research 3, 023151 (2021)

Competition of three-dimensional magnetic phases in Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$: A structural perspective

Authors: Songxue Chi, Feng Ye, Gang Cao, Huibo Cao, Jaime A. Fernandez-Baca

Abstract: The crystalline and magnetic structures of Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ (x=0.02, 0.05, 0.08 and 0.12) have been studied using neutron and X-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru(Fe)O$_6$ octahedron leads to its reduced distortion. The $Pbca$ space group is maintained in all the Fe-dopings, so is the octahedral flattening. Wa… ▽ More The crystalline and magnetic structures of Ca$_2$Ru$_{1-x}$Fe$_x$O$_4$ (x=0.02, 0.05, 0.08 and 0.12) have been studied using neutron and X-ray diffraction. The Fe-doping reduces the Ru-O bond length in both apical and planar directions. The smaller Ru(Fe)O$_6$ octahedron leads to its reduced distortion. The $Pbca$ space group is maintained in all the Fe-dopings, so is the octahedral flattening. Warming has a similar effect on the lattice to that of the Fe-doping in releasing the distorted octahedra but precipitates an abrupt octahedral elongation near the N$\acute{e}$el temperature. Two competing antiferromagnetic orders, $A$- and $B$-centered phases have been observed. The Fe-doping-relaxed crystal structure prefers the latter to the former. As the doping increases, the $B$-centered phase continuously grows at the cost of the $A$-centered one and eventually replaces it at x=0.12. The absence of the two-dimensional antiferromagnetic critical fluctuations above the magnetic transition temperature and the three-dimensional magnetic correlation below the transition, together with the anomalous lattice response, point to an important role of orbital degree of freedom in driving the magnetic phase competition. △ Less

Submitted 30 July, 2020; originally announced July 2020.

Journal ref: Physical Review B 102, 014452 (2020)

In-plane magnetic field induced double fan spin structure with $\textit{c}$-axis component in metallic kagome antiferromagnet YMn$_{6}$Sn$_{6}$

Authors: Kelly J. Neubauer, Chunruo Duan, Feng Ye, Rui Zhang, Songxue Chi, Qi Wang, Kathryn Krycka, Hechang Lei, Pengcheng Dai

Abstract: The geometrical frustration nature of the kagome lattice makes it a great host to flat electronic band, non-trivial topological properties, and novel magnetisms. Metallic kagome antiferromagnet YMn$_{6}$Sn$_{6}$ exhibits the topological Hall effect (THE) when an in-plane magnetic field is applied. THE is typically associated with the nanometer-sized non-coplanar spin structure of skyrmions in non-… ▽ More The geometrical frustration nature of the kagome lattice makes it a great host to flat electronic band, non-trivial topological properties, and novel magnetisms. Metallic kagome antiferromagnet YMn$_{6}$Sn$_{6}$ exhibits the topological Hall effect (THE) when an in-plane magnetic field is applied. THE is typically associated with the nanometer-sized non-coplanar spin structure of skyrmions in non-centrosymmetric magnets with large Dzyaloshinskii-Moriya interaction. Here we use single crystal neutron diffraction to determine the field/temperature dependence of the magnetic structure in YMn$_{6}$Sn$_{6}$. We find that the observed THE cannot arise from a magnetic skyrmion lattice, but instead from an in-plane field-induced double fan spin structure with $\textit{c}$-axis components (DFC). Our work provides the experimental basis from which a microscopic theory can be established to understand the observed THE. △ Less

Submitted 23 July, 2020; originally announced July 2020.

Journal ref: Phys. Rev. B 103, 014416 (2021)

Anisotropic effect of a magnetic field on the neutron spin resonance in FeSe

Authors: Tong Chen, Youzhe Chen, David W. Tam, Bin Gao, Yiming Qiu, Astrid Schneidewind, Igor Radelytskyi, Karel Prokes, Songxue Chi, Masaaki Matsuda, Collin Broholm, Pengcheng Dai

Abstract: We use inelastic neutron scattering to study the effect of a magnetic field on the neutron spin resonance (Er = 3.6 meV) of superconducting FeSe (Tc = 9 K). While a field aligned along the in-plane direction broadens and suppresses the resonance, a c-axis aligned field does so much more efficiently, consistent with the anisotropic field-induced suppression of the superfluid density from the heat c… ▽ More We use inelastic neutron scattering to study the effect of a magnetic field on the neutron spin resonance (Er = 3.6 meV) of superconducting FeSe (Tc = 9 K). While a field aligned along the in-plane direction broadens and suppresses the resonance, a c-axis aligned field does so much more efficiently, consistent with the anisotropic field-induced suppression of the superfluid density from the heat capacity measurements. These results suggest that the resonance in FeSe is associated with the superconducting electrons arising from orbital selective quasi-particle excitations between the hole and electron Fermi surfaces. △ Less

Submitted 25 March, 2020; originally announced March 2020.

Comments: To be published as a Rapid Communication in Physical Review B

Noncollinear magnetic structure and magnetoelectric coupling in buckled honeycomb Co$_4$Nb$_2$O$_9$:A single crystal neutron diffraction study

Authors: Lei Ding, Minseong Lee, Tao Hong, Zhiling Dun, Ryan Sinclair, Songxue Chi, Harish K. Agrawal, Eun Sang Choi, Bryan C. Chakoumakos, Haidong Zhou, Huibo Cao

Abstract: Through analysis of single crystal neutron diffraction data, we present the magnetic structures of magnetoelectric Co4Nb2O9 under various magnetic fields. In zero-field, neutron diffraction experiments below TN=27 K reveal that the Co2+ moments order primarily along the a* direction without any spin canting along the c axis, manifested by the magnetic symmetry C2/c'. The moments of nearest neighbo… ▽ More Through analysis of single crystal neutron diffraction data, we present the magnetic structures of magnetoelectric Co4Nb2O9 under various magnetic fields. In zero-field, neutron diffraction experiments below TN=27 K reveal that the Co2+ moments order primarily along the a* direction without any spin canting along the c axis, manifested by the magnetic symmetry C2/c'. The moments of nearest neighbor Co atoms order ferromagnetically with a small cant away from the next nearest neighbor Co moments along the c axis. In the applied magnetic field H//a, three magnetic domains were aligned with their major magnetic moments perpendicular to the magnetic field with no indication of magnetic phase transitions. The influences of magnetic fields on the magnetic structures associated with the observed magnetoelectric coupling are discussed. △ Less

Submitted 24 February, 2020; originally announced February 2020.

Flat band magnetism and helical magnetic order in Ni-doped SrCo$_2$As$_2$

Authors: Yu Li, Zhonghao Liu, Zhuang Xu, Yu Song, Yaobo Huang, Dawei Shen, Ni Ma, Ang Li, Songxue Chi, Matthias Frontzek, Huibo Cao, Qingzhen Huang, Weiyi Wang, Yaofeng Xie, Yan Rong, David P. Young, J. F. DiTusa, Pengcheng Dai

Abstract: A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic order with magnetic moments ferromagnetically (FM) aligned in the $ab$ plane and a helimagnetic wavevector of $q=(0,0,0.56)$ for $x$ = 0.1. The c… ▽ More A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic order with magnetic moments ferromagnetically (FM) aligned in the $ab$ plane and a helimagnetic wavevector of $q=(0,0,0.56)$ for $x$ = 0.1. The combination of neutron diffraction and angle-resolved photoemission spectroscopy (ARPES) measurements show that the tuning of a flat band with $d_{x^2-y^2}$ orbital character drives the helimagnetism and indicates the possibility of a quantum order-by-disorder mechanism. △ Less

Submitted 20 September, 2019; v1 submitted 22 July, 2019; originally announced July 2019.

Comments: 9 pages, 12 figures, Supplementary Material available upon request, accepted by Phys. Rev. B

Journal ref: Phys. Rev. B 100, 094446 (2019)

Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal

Authors: Thanh Nguyen, Fei Han, Nina Andrejevic, Ricardo Pablo-Pedro, Anuj Apte, Yoichiro Tsurimaki, Zhiwei Ding, Kunyan Zhang, Ahmet Alatas, Ercan E. Alp, Songxue Chi, Jaime Fernandez-Baca, Masaaki Matsuda, David Alan Tennant, Yang Zhao, Zhijun Xu, Jeffrey W. Lynn, Shengxi Huang, Mingda Li

Abstract: The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. How… ▽ More The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wavevector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide (TaP). Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials. △ Less

Submitted 15 May, 2020; v1 submitted 2 June, 2019; originally announced June 2019.

Comments: 30 pages, 4 main figures, 11 supplementary figures and 1 theoretical derivation. Feedback most welcome

Journal ref: Phys. Rev. Lett. 124, 236401 (2020)

Quantized Thermoelectric Hall Effect Induces Giant Power Factor in a Topological Semimetal

Authors: Fei Han, Nina Andrejevic, Thanh Nguyen, Vladyslav Kozii, Quynh Nguyen, Tom Hogan, Zhiwei Ding, Ricardo Pablo-Pedro, Shreya Parjan, Brian Skinner, Ahmet Alatas, Ercan Alp, Songxue Chi, Jaime Fernandez-Baca, Shengxi Huang, Liang Fu, Mingda Li

Abstract: Thermoelectrics are promising by directly generating electricity from waste heat. However, (sub-)room-temperature thermoelectrics have been a long-standing challenge due to vanishing electronic entropy at low temperatures. Topological materials offer a new avenue for energy harvesting applications. Recent theories predicted that topological semimetals at the quantum limit can lead to a large, non-… ▽ More Thermoelectrics are promising by directly generating electricity from waste heat. However, (sub-)room-temperature thermoelectrics have been a long-standing challenge due to vanishing electronic entropy at low temperatures. Topological materials offer a new avenue for energy harvesting applications. Recent theories predicted that topological semimetals at the quantum limit can lead to a large, non-saturating thermopower and a quantized thermoelectric Hall conductivity approaching a universal value. Here, we experimentally demonstrate the non-saturating thermopower and quantized thermoelectric Hall effect in the topological Weyl semimetal (WSM) tantalum phosphide (TaP). An ultrahigh longitudinal thermopower Sxx= 1.1x10^3 muV/K and giant power factor ~525 muW/cm/K^2 are observed at ~40K, which is largely attributed to the quantized thermoelectric Hall effect. Our work highlights the unique quantized thermoelectric Hall effect realized in a WSM toward low-temperature energy harvesting applications. △ Less

Submitted 27 October, 2020; v1 submitted 5 April, 2019; originally announced April 2019.

Comments: 54 pages total, 5 main figures + 22 supplementary figures. To appear in Nature Communications (2020)

Journal ref: Nat Commun 11, 6167 (2020)

Anomalous Magnetic Behavior in Ba2CoO4 with Isolated CoO4 Tetrahedra

Authors: Qiang Zhang, Guixin Cao, Feng Ye, Huibo Cao, Masaaki Matsuda, D. A. Tennant, Songxue Chi, S. E. Nagler, W. A. Shelton, Rongying Jin, E. W. Plummer, Jiandi Zhang

Abstract: The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) Ba2CoO4. In spite of isolated CoO4 tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in… ▽ More The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) Ba2CoO4. In spite of isolated CoO4 tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to TN = 26 K. More unexpectedly, spin dynamics displays quasi-2D spin wave dispersion with an unusually large spin gap, and 1D magnetoelastic coupling. Our results indicate that Ba2CoO4 is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and novel spin states in oxides. △ Less

Submitted 6 March, 2019; originally announced March 2019.

Comments: to be published in Physical Review B

ω/T scaling and magnetic quantum criticality in BaFe2(As0.7P0.3)2

Authors: Ding Hu, Haoyu Hu, Wenliang Zhang, Yuan Wei, Shiliang Li, Yanhong Gu, Xiaoyan Ma, Douglas L. Abernathy, Songxue Chi, Travis J. Williams, Yu Li, Qimiao Si, Pengcheng Dai

Abstract: We used transport and inelastic neutron scattering to study the optimally phosphorus-doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ superconductor ($T_c = 30$ K). In the normal state, we find that the previously reported linear temperature dependence of the resistivity below room temperature extends to $\sim$ 500 K. Our analysis of the temperature and energy ($E=\hbarω$) dependence of spin dynamical susc… ▽ More We used transport and inelastic neutron scattering to study the optimally phosphorus-doped BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$ superconductor ($T_c = 30$ K). In the normal state, we find that the previously reported linear temperature dependence of the resistivity below room temperature extends to $\sim$ 500 K. Our analysis of the temperature and energy ($E=\hbarω$) dependence of spin dynamical susceptibility at the antiferromagnetic (AF) ordering wave vector $χ^{\prime\prime}({\bf Q}_{\rm AF},ω)$ reveal an $ω/ T$ scaling within $1.1<E/k_BT<110$. These results suggest that the linear temperature dependence of the resistivity is due to the presence of a magnetic quantum critical point in the cleanest iron pnictides near optimal superconductivity. Moreover, the results reconcile the strange-metal temperature dependences with the weakly first-order nature of the quantum transition out of the AF and nematic orders. △ Less

Submitted 31 December, 2018; originally announced December 2018.

Comments: 4 figures

Spin-Liquid-Like State in Pure and Mn-Doped TbInO3 with Nearly Triangular Lattice

Authors: M. G. Kim, B. Winn, S. Chi, A. T. Savici, J. A. Rodriguez-Rivera, W. C. Chen, X. Xu, Y. Li, J. W. Kim, S. -W. Cheong, V. Kiryukhin

Abstract: Inelastic neutron scattering studies in single crystals of TbInO3 and TbIn0.95Mn0.05O3 with nearly-triangular antiferromagnetic lattice are reported. At low energies, a broad and apparently gapless continuum of magnetic excitations, located at the triangular lattice (TL) Brillouin zone boundary, is observed. The data are well described by the uncorrelated nearest-neighbor valence bonds model. At h… ▽ More Inelastic neutron scattering studies in single crystals of TbInO3 and TbIn0.95Mn0.05O3 with nearly-triangular antiferromagnetic lattice are reported. At low energies, a broad and apparently gapless continuum of magnetic excitations, located at the triangular lattice (TL) Brillouin zone boundary, is observed. The data are well described by the uncorrelated nearest-neighbor valence bonds model. At higher energies, a broad excitation branch dispersing from the TL zone boundary is observed. No signs of static magnetic order are found down to the temperatures two orders of magnitude smaller than the effective interaction energy. The fluctuating magnetic moment exceeds two thirds of the Tb3+ free-ion value and is confined to the TL plane. These observations are consistent with a TL-based spin liquid state in TbInO3. △ Less

Submitted 28 June, 2019; v1 submitted 3 December, 2018; originally announced December 2018.

Journal ref: Phys. Rev. B 100, 024405 (2019)

Lattice distortion in the spin-orbital entangled state in RVO3 perovskites

Authors: J. -Q. Yan, W. Tian, H. B. Cao, S. Chi, F. Ye, A. Llobet, Q. Chen, J. Ma, Y. Ren, J. -G. Cheng, J. -S. Zhou, M. A. McGuire, R. J. McQueeney

Abstract: We report a thorough study of Y$_{0.7}$La$_{0.3}$VO$_3$ single crystals by measuring magnetic properties, specific heat, thermal conductivity, x-ray and neutron diffraction with the motivation of revealing the lattice response to the spin-orbital entanglement in \textit{R}VO$_3$. Upon cooling from room temperature, the orbitally disordered paramagnetic state changes around T*$\sim$220\,K to spin-o… ▽ More We report a thorough study of Y$_{0.7}$La$_{0.3}$VO$_3$ single crystals by measuring magnetic properties, specific heat, thermal conductivity, x-ray and neutron diffraction with the motivation of revealing the lattice response to the spin-orbital entanglement in \textit{R}VO$_3$. Upon cooling from room temperature, the orbitally disordered paramagnetic state changes around T*$\sim$220\,K to spin-orbital entangled state which is then followed by a transition at T$_N$=116\,K to C-type orbital ordered (OO) and G-type antiferromagnetic ordered (AF) ground state. In the temperature interval T$_N<T<T^*$, the VO$_{6/2}$ octahedra have two comparable in-plane V-O bonds which are longer than the out-of-plane V-O1 bond. This local structural distortion supports the spin-orbital entanglement of partially filled and degenerate yz/zx orbitals. However, this distortion is incompatible with the steric octahedral site distortion intrinsic to orthorhombic perovskites. Their competition induces a second order transition from the spin-orbital entangled state to C-OO/G-AF ground state where the long range OO suppresses the spin-orbital entanglement. Our analysis suggests that the spin-orbital entangled state and G-OO are comparable in energy and compete with each other. Rare earth site disorder favors the spin-orbital entanglement rather than a cooperative Jahn-Teller distortion. The results also indicate for LaVO$_3$ a C-OO/G-AF state in T$_t$\,$\leq$\,T\,$\leq$T$_N$ and an orbital flipping transition at T$_t$. △ Less

Submitted 10 September, 2018; originally announced September 2018.

Comments: 9 pages, 9 figures

Journal ref: Phys. Rev. B 100, 184423 (2019)

Neutron spin resonance as a probe of Fermi surface nesting and superconducting gap symmetry in Ba$_{0.67}$K$_{0.33}$(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$

Authors: Rui Zhang, Weiyi Wang, Thomas Maier, Meng Wang, Matthew B. Stone, Songxue Chi, Barry Winn, Pengcheng Dai

Abstract: We use inelastic neutron scattering to study energy and wave vector dependence of the superconductivity-induced resonance in hole-doped Ba$_{0.67}$K$_{0.33}$(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0,0.08$ with $T_c\approx 37, 28$ K, respectively). In previous work on electron-doped Ba(Fe$_{0.963}$Ni$_{0.037}$)$_2$As$_2$ ($T_N=26$ K and $T_c=17$ K), the resonance is found to peak sharply at the anti… ▽ More We use inelastic neutron scattering to study energy and wave vector dependence of the superconductivity-induced resonance in hole-doped Ba$_{0.67}$K$_{0.33}$(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ ($x=0,0.08$ with $T_c\approx 37, 28$ K, respectively). In previous work on electron-doped Ba(Fe$_{0.963}$Ni$_{0.037}$)$_2$As$_2$ ($T_N=26$ K and $T_c=17$ K), the resonance is found to peak sharply at the antiferromagnetic (AF) ordering wave vector ${\bf Q}_{\rm AF}$ along the longitudinal direction, but disperses upwards away from ${\bf Q}_{\rm AF}$ along the transverse direction. For hole doped $x=0, 0.08$ without AF order, we find that the resonance displays ring-like upward dispersion away from ${\bf Q}_{\rm AF}$ along both the longitudinal and transverse directions. By comparing these results with calculations using the random phase approximation, we conclude that the dispersive resonance is a direct signature of isotropic superconducting gaps arising from nested hole-electron Fermi surfaces. △ Less

Submitted 18 August, 2018; originally announced August 2018.

Comments: 5 pages, 4 figures

Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe$_{1-x}$Ni$_x$As

Authors: Weiyi Wang, Yu Song, Chongde Cao, Kuo-Feng Tseng, Thomas Keller, Yu Li, L. W. Harriger, Wei Tian, Songxue Chi, Rong Yu, Andriy H. Nevidomskyy, Pengcheng Dai

Abstract: Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe$_{1-x}$Ni$_x$As, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for t… ▽ More Understanding the interplay between nematicity, magnetism and superconductivity is pivotal for elucidating the physics of iron-based superconductors. Here we use neutron scattering to probe magnetic and nematic orders throughout the phase diagram of NaFe$_{1-x}$Ni$_x$As, finding that while both static antiferromagnetic and nematic orders compete with superconductivity, the onset temperatures for these two orders remain well-separated approaching the putative quantum critical points. We uncover local orthorhombic distortions that persist well above the tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ in underdoped samples and extend well into the overdoped regime that exhibits neither magnetic nor structural phase transitions. These unexpected local orthorhombic distortions display Curie-Weiss temperature dependence and become suppressed below the superconducting transition temperature $T_{\rm c}$, suggesting they result from a large nematic susceptibility near optimal superconductivity. Our results account for observations of rotational symmetry-breaking above $T_{\rm s}$, and attest to the presence of significant nematic fluctuations near optimal superconductivity. △ Less

Submitted 13 July, 2018; originally announced July 2018.

Comments: Supplementary Information available upon request

Journal ref: Nature Communications 9, 3128 (2018)