Showing 1–50 of 165 results for author: Sun, F

Electronic Structure of Multilayer Graphene with Arbitrary Stackings

Authors: Fred Sun, Jia-An Yan

Abstract: Stacking geometry in multilayer graphene (MLG) provides an interesting degree of freedom to engineer its electronic structure near the Fermi level, wherein the linear bands in single layer graphene could retain or evolve into parabolic or flat bands. Using a tight-binding model, we carried out a detailed analytical analysis of the electronic band structures for arbitrarily stacked MLGs. We show th… ▽ More Stacking geometry in multilayer graphene (MLG) provides an interesting degree of freedom to engineer its electronic structure near the Fermi level, wherein the linear bands in single layer graphene could retain or evolve into parabolic or flat bands. Using a tight-binding model, we carried out a detailed analytical analysis of the electronic band structures for arbitrarily stacked MLGs. We show that their low energy band dispersions near the Fermi level may be deduced from its substacks in isolation. The analytical solutions of the momenta with zero eigenvalue for an AA stacking allows us to generalize the results of the zero energy momenta for arbitrarily stacked MLGs. Moreover, we find that an interplay of parallel and rhombohedral stackings allows for flat band engineering and enhancement in arbitrarily stacked MLGs. The existence of flat bands in MLGs might offer another interesting platform for exploring the superconductivity in graphene systems beyond the twisted bilayer graphene. △ Less

Submitted 13 February, 2026; originally announced February 2026.

Comments: 13 pages, 7 figures

Resolving Quantum Criticality in the Honeycomb Hubbard Model

Authors: Fo-Hong Wang, Fanjie Sun, Chenghao He, Xiao Yan Xu

Abstract: Quantum phase transitions driven by electronic correlations are central to understanding the physics of graphene and related two-dimensional materials. A paradigmatic example is the semimetal-to-Mott-insulator transition on the honeycomb lattice, governed by the Gross-Neveu-Heisenberg universality class, yet consensus on its critical exponents has remained elusive for over a decade due to severe f… ▽ More Quantum phase transitions driven by electronic correlations are central to understanding the physics of graphene and related two-dimensional materials. A paradigmatic example is the semimetal-to-Mott-insulator transition on the honeycomb lattice, governed by the Gross-Neveu-Heisenberg universality class, yet consensus on its critical exponents has remained elusive for over a decade due to severe finite-size effects and the absence of rigorous conformal bootstrap benchmarks. Here we try to resolve this long-standing controversy by performing projector determinant quantum Monte Carlo simulations on lattices of unprecedented size, reaching 10,368 sites. By developing a novel projected submatrix update algorithm, we achieve a significant algorithmic speedup that enables us to access the thermodynamic limit with high precision. We observe that the fermion anomalous dimension and the correlation length exponent converge rapidly, while the boson anomalous dimension exhibits a systematic size dependence that we resolve via linear extrapolation. To validate our analysis, we perform parallel large-scale simulations of the spinless $t$-$V$ model on the honeycomb lattice, which belongs to the Gross-Neveu-Ising class. Our results for the $t$-$V$ model show agreement with conformal bootstrap predictions, thereby corroborating the robustness of our methodology. Our work provides state-of-the-art critical exponents for the honeycomb Hubbard model and establishes a systematic finite-size scaling workflow applicable to a broad class of strongly correlated quantum systems, paving the way for resolving other challenging fermionic quantum critical phenomena. △ Less

Submitted 9 February, 2026; v1 submitted 3 February, 2026; originally announced February 2026.

Comments: 12+20 pages, 6+16 figures

Frustrated Magnetism in FeGe$_3$O$_4$ with a Chiral Trillium Network

Authors: Matt Boswell, Mingyu Xu, Haozhe Wang, Mouyang Cheng, N. Li, X. F. Sun, Haidong Zhou, Huibo Cao, Mingda Li, Weiwei Xie

Abstract: The discovery of new magnetic ground states in geometrically frustrated lattices remains a central challenge in materials science. Here, we report the synthesis, structural characterization, and frustrated magnetic properties of FeGe$_3$O$_4$, a newly identified compound that crystallizes in the noncentrosymmetric cubic space group $P2_13$. In this structure, Fe atoms form an intricate double-tril… ▽ More The discovery of new magnetic ground states in geometrically frustrated lattices remains a central challenge in materials science. Here, we report the synthesis, structural characterization, and frustrated magnetic properties of FeGe$_3$O$_4$, a newly identified compound that crystallizes in the noncentrosymmetric cubic space group $P2_13$. In this structure, Fe atoms form an intricate double-trillium lattice with nearest-neighbor Fe--Fe distances of $\sim$4.2~Å, while Ge$^{2+}$ ions mediate magnetic interactions through Fe-Ge-Fe pathways. Field-dependent magnetization at 2~K shows a pronounced nonlinearity, reaching a maximum moment of 2.55(3)~$μ_\mathrm{B}$/Fe$^{2+}$ at 70~kOe without evidence of saturation. Magnetic susceptibility, heat capacity, and neutron scattering collectively reveal the onset of short-range magnetic interactions near 5~K, with no long-range ordering detected down to 0.06~K. Specific heat measurements demonstrate strong frustration: only $\sim$34\% of the expected magnetic entropy is recovered at 2.4~K. Taken together, these results establish FeGe$_3$O$_4$ as a rare example of a geometrically frustrated trillium-lattice magnet, offering a promising platform for exploring exotic quantum magnetic phenomena. △ Less

Submitted 13 January, 2026; originally announced January 2026.

Comments: 26 pages, 4+6 figures

Dominant Excitonic Superconductivity in a Three-component Hubbard Chain

Authors: Sheng Chen, Qiao Yang, Wéi Wú, Fadi Sun

Abstract: Understanding superconductivity emerging from repulsive fermions remains a major challenge in condensed matter physics. In this paper, we investigate the pairing tendencies in a one-dimensional, three component repulsive Hubbard model, using the density matrix renormalization group method. At half-filling, the system exhibits density wave ground state due to strong Hubbard repulsions. Upon doping,… ▽ More Understanding superconductivity emerging from repulsive fermions remains a major challenge in condensed matter physics. In this paper, we investigate the pairing tendencies in a one-dimensional, three component repulsive Hubbard model, using the density matrix renormalization group method. At half-filling, the system exhibits density wave ground state due to strong Hubbard repulsions. Upon doping, we find that Cooper pairs can emerge, whose fluctuations predominate the long-range physics in the system across a wide parameter range. The effective attractions between Cooper pairs are mediated by the particle-hole fluctuations in the third non-pairing component, resembling an excitonic mechanism of superconductivity. The coexistence of multiple density waves and superconductivity at different fermion fillings is explored. We also present an analytical study of the pairing mechanism in both weak and strong coupling limits. Our results provide a new perspective for understanding and exploring unconventional superconductivities in strongly correlated fermionic systems. △ Less

Submitted 9 December, 2025; originally announced December 2025.

Nonperturbative quantum field theory for pseudo-Goldstone modes, slow-Goldstone modes, and their quantum chaos

Authors: Fadi Sun, Jinwu Ye

Abstract: In this work, we develop a novel form of non-perturbative theory to identify a light pseudo-Goldstone mode with a small mass, as well as a new type of Goldstone mode with a tiny slope (termed the slow-Goldstone mode), which may not be obtained via traditional perturbative methods. We demonstrate our formalism in the context of superfluids formed by Rashba spin-orbit coupled spinor bosons in a squa… ▽ More In this work, we develop a novel form of non-perturbative theory to identify a light pseudo-Goldstone mode with a small mass, as well as a new type of Goldstone mode with a tiny slope (termed the slow-Goldstone mode), which may not be obtained via traditional perturbative methods. We demonstrate our formalism in the context of superfluids formed by Rashba spin-orbit coupled spinor bosons in a square lattice weakly interacting with a spin-anisotropic interaction. The experimental detections of these two modes, especially their roles leading to the quantum information scramblings at a finite temperature are discussed. The slow-Goldstone mode is compared with the slow light and the soft mode in the Sachdev-Ye-Kitaev models. This non-perturbative formalism can be widely applied to study other emergent particles in various quantum matter. △ Less

Submitted 20 August, 2025; originally announced August 2025.

Comments: 16 pages, 5 figures, 4 appendices

Journal ref: Physical Review B 112, 085127 (2025)

Observe novel tricritical phenomena in self-organized Fermi gas induced by higher order Fermi surface nesting

Authors: Yilun Xu, Feng-Xiao Sun, Qiongyi He

Abstract: Cold atom systems in optical lattices have long been recognized as an ideal platform for bridging condense matter physics and quantum optics. Here, we investigate the 1D fermionic superradiance in an optical lattice, and observe novel tricritical phenomena and multistability in finite-temperature cases. As a starting point, which can be analytically calculated, we compare the 1D and 2D Fermi gases… ▽ More Cold atom systems in optical lattices have long been recognized as an ideal platform for bridging condense matter physics and quantum optics. Here, we investigate the 1D fermionic superradiance in an optical lattice, and observe novel tricritical phenomena and multistability in finite-temperature cases. As a starting point, which can be analytically calculated, we compare the 1D and 2D Fermi gases in zero-temperature limit. It turns out that the tricritical point originates from the higher-order Fermi surface nesting (FSN), and the infrared divergence in 1D systems is absent in 2D cases. When extending to finite-temperature cases, our numerical results reveal that both quantum- and classical-type trcritical phenomena can be observed simultaneously. Moreover, there exists an optimal temperature for observing superradiance. This work provides a new approach to understanding the relation between quantum and classical phase transitions. △ Less

Submitted 12 January, 2026; v1 submitted 7 August, 2025; originally announced August 2025.

Journal ref: Phys. Rev. A 113, 023717 (2026)

Crystal structures and electronic and magnetic properties of Janus bilayer Cl3Cr2I3

Authors: Suqi Liu, Feng Sun, Aijun Hong

Abstract: Two-dimensional (2D) magnetic material CrI3 has aroused extensive attention, because it could provide a new platform for investigating the relations between crystal structures and electronic and magnetic properties. Here, we study crystal structures and electronic and magnetic properties of three configurations (Cl-Cl I-I and Cl-I) of Janus bilayer Cl3Cr2I3 with two stacking orders (AB and AA1=3)… ▽ More Two-dimensional (2D) magnetic material CrI3 has aroused extensive attention, because it could provide a new platform for investigating the relations between crystal structures and electronic and magnetic properties. Here, we study crystal structures and electronic and magnetic properties of three configurations (Cl-Cl I-I and Cl-I) of Janus bilayer Cl3Cr2I3 with two stacking orders (AB and AA1=3) by using the first principles approach incorporating the spin-orbit coupling (SOC) effect and the dipole correction. It is found that the spin polarization and the SOC effect can expand the lattice constant and the interlayer distance (ID) of the three configurations. Especially, the ID of the I-I configuration is 1 Å larger than that of the Cl-Cl configuration. The total energy calculation results show that the atomic configuration, the SOC effect and the stacking order play an important role in determining the magnetic ground states of the Janus layers. Our results indicate the atomic configurations of the Janus bilayers are not conducive to the increase of critical temperature. Interestingly, the Cl-I configuration has a vertical dipole moment, and its AFM state has large spin splitting. It is revealed that the non-periodic structure, the symmetry breaking of the average potential and the weak interlayer interaction lead to the vertical dipole moment and the abnormal AFM state that is not the most stable state. △ Less

Submitted 19 June, 2025; originally announced June 2025.

Novel bipartite entanglement in the quantum dimer magnet Yb$_2$Be$_2$SiO$_7$

Authors: A. Brassington, Q. Ma, G. Duan, S. Calder, A. I. Kolesnikov, K. M. Taddei, G. Sala, E. S. Choi, H. Wang, W. Xie, B. A. Frandsen, N. Li, X. F. Sun, C. Liu, R. Yu, H. D. Zhou, A. A. Aczel

Abstract: The quantum dimer magnet, with antiferromagnetic intradimer and interdimer Heisenberg exchange between spin-1/2 moments, is known to host an up/down - down/up singlet ground state when the intradimer exchange is dominant. Rare-earth-based quantum dimer systems with strong spin-orbit coupling offer the opportunity for tuning their magnetic properties by using magnetic anisotropy as a control knob.… ▽ More The quantum dimer magnet, with antiferromagnetic intradimer and interdimer Heisenberg exchange between spin-1/2 moments, is known to host an up/down - down/up singlet ground state when the intradimer exchange is dominant. Rare-earth-based quantum dimer systems with strong spin-orbit coupling offer the opportunity for tuning their magnetic properties by using magnetic anisotropy as a control knob. Here, we present bulk characterization and neutron scattering measurements of the quantum dimer magnet Yb$_2$Be$_2$SiO$_7$. We find that the Yb$^{3+}$ ions can be described by an effective spin-1/2 model at low temperatures and the system does not show signs of magnetic order down to 50 mK. The magnetization, heat capacity, and neutron spectroscopy data can be well-described by an isolated dimer model with highly anisotropic exchange that stabilizes a singlet ground state with a wavefunction up/up - down/down or up/up + down/down. Our results show that strong spin-orbit coupling can induce novel entangled states of matter in quantum dimer magnets. △ Less

Submitted 8 September, 2025; v1 submitted 1 May, 2025; originally announced May 2025.

Comments: 11 pages, 4 figures, and SM

Reentrant phase transition in quasiperiodic photonic waveguides

Authors: Yang Chen, Ze-Zheng Li, Hua-Yu Bai, Shuai-Peng Guo, Tian-Yang Zhang, Xu-Lin Zhang, Qi-Dai Chen, Guang-Can Guo, Fang-Wen Sun, Zhen-Nan Tian, Ming Gong, Xi-Feng Ren, Hong-Bo Sun

Abstract: Anderson transition in quasiperiodic potentials and the associated mobility edges have been a central focus in quantum simulation across multidisciplinary physical platforms. While these transitions have been experimentally observed in ultracold atoms, acoustic systems, optical waveguides, and superconducting junctions, their interplay between quasiperiodic potential and long-range hopping remains… ▽ More Anderson transition in quasiperiodic potentials and the associated mobility edges have been a central focus in quantum simulation across multidisciplinary physical platforms. While these transitions have been experimentally observed in ultracold atoms, acoustic systems, optical waveguides, and superconducting junctions, their interplay between quasiperiodic potential and long-range hopping remains unexplored experimentally. In this work, we report the observation of localization-delocalization transition induced by the hopping between the next-nearest neighboring sites using quasiperiodic photonic waveguides. Our findings demonstrate that increasing the next-nearest hopping strength induces a reentrant phase transition, where the system transitions from an initially extended phase into a localized phase before eventually returning to an extended phase. This remarkable interplay between hopping and quasiperiodic potential in the lattice models provides crucial insights into the mechanism of Anderson transition. Furthermore, our numerical simulation reveals that this phase transition exhibits a critical exponent of $ν\simeq 1/3$, which is experimentally observable for system sizes $L\sim10^3$ - $10^4$. These results establish a framework for direct observation of the Anderson transition and precise determination of its critical exponents, which can significantly advance our understanding of localization physics in quasiperiodic systems. △ Less

Submitted 16 April, 2025; originally announced April 2025.

Comments: 16 pages, 5 figures

Siamese Foundation Models for Crystal Structure Prediction

Authors: Liming Wu, Wenbing Huang, Rui Jiao, Jianxing Huang, Liwei Liu, Yipeng Zhou, Hao Sun, Yang Liu, Fuchun Sun, Yuxiang Ren, Jirong Wen

Abstract: Predicting crystal structures from chemical compositions is a fundamental challenge in materials discovery, complicated by complex 3D geometries that distinguish it from fields like protein folding. Here, we present Diffusion-based Crystal Omni (DAO), a pretrain-finetune framework for crystal structure prediction integrating two Siamese foundation models: a structure generator and an energy predic… ▽ More Predicting crystal structures from chemical compositions is a fundamental challenge in materials discovery, complicated by complex 3D geometries that distinguish it from fields like protein folding. Here, we present Diffusion-based Crystal Omni (DAO), a pretrain-finetune framework for crystal structure prediction integrating two Siamese foundation models: a structure generator and an energy predictor. The generator is pretrained via a two-stage pipeline on a vast dataset of stable and unstable structures, leveraging the predictor to relax unstable configurations and guide the generative sampling. Across two well-known benchmarks, pretraining significantly enhances performance across multiple backbone architectures. Ablation studies confirm that the synergy between the generator and predictor mutually benefits both components. We further validate DAO on three real-world superconductors ($\text{Cr}_6\text{Os}_2$, $\text{Zr}_{16}\text{Rh}_8\text{O}_4$, and $\text{Zr}_{16}\text{Pd}_8\text{O}_4$) typically inaccessible to conventional computation. For $\text{Cr}_6\text{Os}_2$, DAO achieves a 100\% match rate with experimental references and an atomic-position error of 0.0012 under 20-shot generation, performing over 2000$\times$ faster per iteration than DFT-based structure predictors. These compelling results collectively highlight the potential of our approach for advancing materials science research. △ Less

Submitted 13 April, 2026; v1 submitted 13 March, 2025; originally announced March 2025.

Single Atom Catalysts with Halogen Ligands: Elevating the HER Performance of Pd-anchored MoS2 monolayer

Authors: Feng Sun, Xuqiang Zhang, Jiangtao Chen, Yun Zhao, Yan Li

Abstract: Single-atom catalysts (SACs) have attracted ever-growing interest due to their high atom-utilization efficiency and potential for cost-effective of hydrogen production. However, enhancing the hydrogen evolution reaction (HER) performance remains a key challenge in developing SACs for HER technology. Herein, we employed first-principles calculations in conjunction with the climbing-image nudged ela… ▽ More Single-atom catalysts (SACs) have attracted ever-growing interest due to their high atom-utilization efficiency and potential for cost-effective of hydrogen production. However, enhancing the hydrogen evolution reaction (HER) performance remains a key challenge in developing SACs for HER technology. Herein, we employed first-principles calculations in conjunction with the climbing-image nudged elastic band (CI-NEB) method to explore the effect of surface ligands (F, Cl, Br, I) on the HER performance and mechanism of single-atom (Pd or Cu)-anchored MoS2 monolayer. The results indicate that the relative Gibbs free energy for the adsorbed hydrogen atom in the I-Pd@MoS2 system is an exceptionally low value of -0.13 eV, which is not only comparable to that of Pt-based catalysts but also significantly more favorable than the calculated 0.84 eV for Pd@MoS2. However, the introduction of ligands to Cu@MoS2 deteriorates HER performance due to strong coupling between the absorbed H and ligands. It reveals that the ligand I restructures the local chemical microenvironment surrounding the SAC Pd, leading to impurity bands near the Fermi level that couple favorably with the s states of H atoms, yielding numerous highly active sites to enhance catalytic performance. Furthermore, the CI-NEB method elucidates that the enhanced HER mechanism for the I-Pd@MoS2 catalyst should belong to the coexistence of the Volmer-Tafel and Volmer-Heyrovsky reactions. This investigation provides a valuable framework for the experimental design and development of innovative single-atom catalysts. △ Less

Submitted 8 March, 2025; originally announced March 2025.

Possible quantum spin liquid state of CeTa$_7$O$_{19}$

Authors: N. Li, A. Rutherford, Y. Y. Wang, H. Liang, Y. Zhou, Y. Sun, D. D. Wu, P. F. Chen, Q. J. Li, H. Wang, W. Xie, E. S. Choi, S. Z. Zhang, M. Lee, H. D. Zhou, X. F. Sun

Abstract: CeTa$_7$O$_{19}$ is a recently found two-dimensional triangular lattice antiferromagnet without showing magnetic order. We grew high-quality CeTa$_7$O$_{19}$ single crystals and studied the low-temperature magnetic susceptibility, specific heat and thermal conductivity. The dc magnetic susceptibility and magnetization reveal its nature of effective spin-1/2, easy axis anisotropy, and antiferromagn… ▽ More CeTa$_7$O$_{19}$ is a recently found two-dimensional triangular lattice antiferromagnet without showing magnetic order. We grew high-quality CeTa$_7$O$_{19}$ single crystals and studied the low-temperature magnetic susceptibility, specific heat and thermal conductivity. The dc magnetic susceptibility and magnetization reveal its nature of effective spin-1/2, easy axis anisotropy, and antiferromagnetic spin coupling. The ultralow-temperature ac susceptibility and specific heat data indicate the absence of any phase transition down to 20 mK. The ultralow-temperature thermal conductivity ($κ$) at zero magnetic field exhibits a non-zero residual term $κ_0/T =$ 0.0056 W/K$^2$m. Although the magnetic field dependence of $κ$ is rather weak, the 14 T thermal conductivity shows an essential zero residual term. All these results point to a possible ground state of quantum spin liquid. △ Less

Submitted 5 March, 2025; v1 submitted 2 March, 2025; originally announced March 2025.

Comments: 8 pages, 6 figures, accepted for publication in Phys. Rev. B

Reversibly Strain Engineering and Electric-Field Control of Crystal Symmetry in Multiferroic Oxides

Authors: Fei Sun, Chao Chen, Deyang Chen, Minghui Qin, Xubing Lu, Xingsen Gao, Christopher T Nelson, Jun-Ming Liu

Abstract: Multiferroic oxides, such as BiFeO3, have garnered significant attention due to their coupled ferroelectric, magnetic, and elastic properties, offering exciting opportunities for multifunctional device applications. Controlling phase transitions in these materials is critical for tuning their physical properties and achieving desired functionalities. While numerous studies have focused on ferroele… ▽ More Multiferroic oxides, such as BiFeO3, have garnered significant attention due to their coupled ferroelectric, magnetic, and elastic properties, offering exciting opportunities for multifunctional device applications. Controlling phase transitions in these materials is critical for tuning their physical properties and achieving desired functionalities. While numerous studies have focused on ferroelectric-ferroelectric transitions at rhombohedral-tetragonal morphotropic phase boundaries, far less attention has been given to the ferroelectric-antiferroelectric phase boundaries. Such systems hold promise for discovering novel physical phenomena, such as reversible phase transitions, enhanced piezoelectricity, and magnetoelectric coupling. In this work, we report a reversible antiferroelectric-to-ferroelectric phase transition in La doped BiFeO3 thin films. By modulating the residual strain via film thickness, an antiferroelectric orthorhombic phase is stabilized within a ferroelectric rhombohedral phase matrix. Under an external electric field, the phase transitions reversibly between these two states. This discovery not only enriches the understanding of orthorhombic-rhombohedral morphotropic phase boundaries but also provides a potential pathway for developing magnetoelectric devices with enhanced functionality. △ Less

Submitted 18 February, 2025; originally announced February 2025.

Thermodynamics and heat transport of quantum spin liquid candidates NaYbS$_2$ and NaYbSe$_2$

Authors: N. Li, M. T. Xie, Q. Huang, Z. W. Zhuo, Z. Zhang, E. S. Choi, Y. Y. Wang, H. Liang, Y. Sun, D. D. Wu, Q. J. Li, H. D. Zhou, G. Chen, X. Zhao, Q. M. Zhang, X. F. Sun

Abstract: We study the ultralow-temperature thermodynamics and thermal conductivity ($κ$) of the single-crystal rare-earth chalcogenides NaYbS$_2$ and NaYbSe$_2$, which have an ideal triangular lattice of the Yb$^{3+}$ ions and have been proposed to be quantum spin liquid candidates. The magnetic specific heat divided by temperature $C_{\rm{mag}}/T$ is nearly constant at $T <$ 200 mK, which is indeed the in… ▽ More We study the ultralow-temperature thermodynamics and thermal conductivity ($κ$) of the single-crystal rare-earth chalcogenides NaYbS$_2$ and NaYbSe$_2$, which have an ideal triangular lattice of the Yb$^{3+}$ ions and have been proposed to be quantum spin liquid candidates. The magnetic specific heat divided by temperature $C_{\rm{mag}}/T$ is nearly constant at $T <$ 200 mK, which is indeed the indication of the gapless magnetic excitations with a constant density of states. However, we observe a vanishingly small residual term $κ_0/T$, which points to the absence of mobile fermionic excitations in these materials. Both the weak temperature dependence of $κ$ and the strong magnetic-field dependence of $κ$ suggest the significant scattering between the spinons and phonons, which actually supports the existence of gapless or tiny-gapped quantum spin liquid. Moreover, the $κ(B)/κ(0)$ isotherms show a series of field-induced magnetic transitions for $B \parallel a$, confirming the easy-plane anisotropy, which is consistent with the results of ac magnetic susceptibility. We expect our results to inspire further interests in the understanding of the spinon-phonon coupling in the spin liquid systems. △ Less

Submitted 15 December, 2024; originally announced December 2024.

Comments: 10 pages, 10 figures

Report number: Phys. Rev. B 110, 224414 (2024)

Creation of independently controllable and long lifetime polar skyrmion textures in ferroelectric-metallic heterostructures

Authors: Fei Sun, Jianhua Ren, Hongfang Li, Yiwei Wu, Jianwei Liang, Hui Yang, Yi Zhang, Jianyi Liu, Linjie Liu, Mengjun Wu, Xiaoyue Zhang, Wenpeng Zhu, Weijin Chen, Yue Zheng

Abstract: Topological textures like vortices, labyrinths and skyrmions formed in ferroic materials have attracted extensive interests during the past decade for their fundamental physics, intriguing topology, and technological prospects. So far, polar skyrmions remain scarce in ferroelectrics as they require a delicate balance between various dipolar interactions. Here, we report that PbTiO3 thin films in a… ▽ More Topological textures like vortices, labyrinths and skyrmions formed in ferroic materials have attracted extensive interests during the past decade for their fundamental physics, intriguing topology, and technological prospects. So far, polar skyrmions remain scarce in ferroelectrics as they require a delicate balance between various dipolar interactions. Here, we report that PbTiO3 thin films in a metallic contact undergo a topological phase transition and stabilize a broad family of skyrmion-like textures (e.g., skyrmion bubbles, multiple π-twist target skyrmions, and skyrmion bags) with independent controllability, analogous to those reported in magnetic systems. Weakly-interacted skyrmion arrays with a density over 300 Gb/inch2 are successfully written, erased and read-out by local electrical and mechanical stimuli of a scanning probe. Interestingly, in contrast to the relatively short lifetime <20 hours of the skyrmion bubbles, the multiple π-twist target skyrmions and skyrmion bags show topology-enhanced stability with lifetime over two weeks. Experimental and theoretical analysis implies the heterostructures carry electric Dzyaloshinskii-Moriya interaction mediated by oxygen octahedral tiltings. Our results demonstrate ferroelectric-metallic heterostructures as fertile playground for topological states and emergent phenomena. △ Less

Submitted 23 September, 2024; originally announced September 2024.

Scalable Reshaping of Diamond Particles via Programmable Nanosculpting

Authors: Tongtong Zhang, Fuqiang Sun, Yaorong Wang, Yingchi Li, Jing Wang, Zhongqiang Wang, Kwai Hei Li, Ye Zhu, Qi Wang, Lei Shao, Ngai Wong, Dangyuan Lei, Yuan Lin, Zhiqin Chu

Abstract: Diamond particles have many interesting properties and possible applications. However, producing diamond particles with well-defined shapes at scale is challenging because diamonds are chemically inert and extremely hard. Here, we show air oxidation, a routine method for purifying diamonds, can be used to precisely shape diamond particles at scale. By exploiting the distinct reactivities of differ… ▽ More Diamond particles have many interesting properties and possible applications. However, producing diamond particles with well-defined shapes at scale is challenging because diamonds are chemically inert and extremely hard. Here, we show air oxidation, a routine method for purifying diamonds, can be used to precisely shape diamond particles at scale. By exploiting the distinct reactivities of different crystal facets and defects inside the diamond, layer-by-layer outward-to-inward and inward-to-outward oxidation produced diverse diamond shapes including sphere, twisted surface, pyramidal islands, inverted pyramids, nano-flowers, and hollow polygons. The nanosculpted diamonds had more and finer features that enabled them to outperform the original raw diamonds in various applications. Using experimental observations and Monte Carlo simulations, we built a shape library that guides the design and fabrication of diamond particles with well-defined shapes and functional value. Our study presents a simple, economical and scalable way to produce shape-customized diamonds for various photonics, catalysis, quantum and information technology applications. △ Less

Submitted 14 September, 2024; originally announced September 2024.

Two-dimensional superconductivity in a thick exfoliated kagome film

Authors: Fei Sun, Andrea Capa Salinas, Stephen D. Wilson, Haijing Zhang

Abstract: We report the observation of two-dimensional superconductivity (2D SC) in exfoliated kagome metal CsV$_3$Sb$_5$ with a thickness far thicker than the atomic limit. By examining the critical current and upper critical magnetic fields ($H_{c2}$) of 40-60 nm thick films in the superconducting state, we identify a pronounced Berezinskii-Kosterlitz-Thouless (BKT) transition behavior, i.e. a drastic dec… ▽ More We report the observation of two-dimensional superconductivity (2D SC) in exfoliated kagome metal CsV$_3$Sb$_5$ with a thickness far thicker than the atomic limit. By examining the critical current and upper critical magnetic fields ($H_{c2}$) of 40-60 nm thick films in the superconducting state, we identify a pronounced Berezinskii-Kosterlitz-Thouless (BKT) transition behavior, i.e. a drastic decrease of the superfluid stiffness near the transition, and a cusp-like feature of the angular dependent $H_{c2}$, both of which serve as direct evidence of 2D SC. In addition, an exceeding of the Pauli paramagnetic limit of the in-plane $H_{c2}$ is consistent with the 2D SC nature. The observed 2D SC occurs in thick films with the highest superconducting transition temperature $T_c$ and the lowest charge density wave transition temperature $T_{\rm {CDW}}$, which suggests that the charge density wave states are interrelated with the superconducting states. Our findings impose constraints in understanding the enhancement of SC in kagome superconductors, and illuminate pathways for achieving novel 2D superconducting states in more stable and much thicker systems. △ Less

Submitted 15 August, 2024; originally announced August 2024.

Comments: 6 pages and 4 figures for the main text; 10 pages and 9 figures for the Supplementary Materials

Ising-type quantum spin liquid state in PrMgAl$_{11}$O$_{19}$

Authors: N. Li, A. Rutherford, Y. Y. Wang, H. Liang, Q. J. Li, Z. J. Zhang, H. Wang, W. Xie, H. D. Zhou, X. F. Sun

Abstract: We have grown single crystals of PrMgAl$_{11}$O$_{19}$, an ideal triangular-lattice antiferromagnet, and performed magnetic susceptibility, specific heat and thermal conductivity measurements at low temperatures. The main results are as follows: (i) The temperature-dependent susceptibility shows a negligible in-plane response and the isothermal magnetization curves confirm the easy axis along the… ▽ More We have grown single crystals of PrMgAl$_{11}$O$_{19}$, an ideal triangular-lattice antiferromagnet, and performed magnetic susceptibility, specific heat and thermal conductivity measurements at low temperatures. The main results are as follows: (i) The temperature-dependent susceptibility shows a negligible in-plane response and the isothermal magnetization curves confirm the easy axis along the $c$ axis. (ii) The specific heat measurements reveal the absence of long-range magnetic order down to 60 mK, and the power-law temperature dependence indicates the existence of the gapless magnetic excitations in system. (iii) The ultralow-temperature thermal conductivity exhibits negligibly small residual term ($κ_0/T$) and strong spin-phonon scattering effect, suggesting that the spin excitations are also involved. Our results further demonstrate that PrMgAl$_{11}$O$_{19}$ is a rare quantum spin liquid candidate with Ising-like anisotropy. △ Less

Submitted 22 November, 2024; v1 submitted 15 July, 2024; originally announced July 2024.

Comments: 8 pages, 5 figures

Journal ref: Phys. Rev. B 110, 134401 (2024)

Impurity-level induced broadband photoelectric response in wide-band semiconductor SrSnO3

Authors: Yuyang Zhang, Lisheng Wang, Weijie Wu, Zhaoyang Wang, Fei Sun, He Jiang, Bangmin Zhang, Yue Zheng

Abstract: Broadband spectrum detectors exhibit great promise in fields such as multispectral imaging and optical communications. Despite significant progress, challenges like materials instability, complex manufacturing process and high costs still hinder further application. Here we present a method that achieves broadband spectral detect by impurity-level in SrSnO3. We report over 200 mA/W photo-responsiv… ▽ More Broadband spectrum detectors exhibit great promise in fields such as multispectral imaging and optical communications. Despite significant progress, challenges like materials instability, complex manufacturing process and high costs still hinder further application. Here we present a method that achieves broadband spectral detect by impurity-level in SrSnO3. We report over 200 mA/W photo-responsivity at 275 nm (ultraviolet C solar-bind) and 367 nm (ultraviolet A) and ~ 1 mA/W photo-responsivity at 532 nm and 700 nm (visible) with a voltage bias of 5V. Further transport and photoluminescence results indicate that the broadband response comes from the impurity levels and mutual interactions. Additionally, the photodetector demonstrates excellent robustness and stability under repeated tests and prolonged exposure in air. These findings show the potential of SSO photodetectors and propose a method to achieve broadband spectrum detection, creating new possibility for the development of single-phase, low-cost, simple structure and high-efficiency photodetectors. △ Less

Submitted 22 May, 2024; originally announced May 2024.

Comments: 5 Figures

Spinons in a new Shastry-Sutherland lattice magnet Pr$_2$Ga$_2$BeO$_7$

Authors: N. Li, A. Brassington, M. F. Shu, Y. Y. Wang, H. Liang, Q. J. Li, X. Zhao, P. J. Baker, H. Kikuchi, T. Masuda, G. Duan, C. Liu, H. Wang, W. Xie, R. Zhong, J. Ma, R. Yu, H. D. Zhou, X. F. Sun

Abstract: Identifying the elusive spinon excitations in quantum spin liquid (QSL) materials is what scientists have long sought for. Recently, thermal conductivity ($κ$) has emerged to be a decisive probe because the fermionic nature of spinons leads to a characteristic nonzero linear $κ_0/T$ term while approaching zero Kelvin. So far, only a few systems have been reported to exhibit such term. Here, we rep… ▽ More Identifying the elusive spinon excitations in quantum spin liquid (QSL) materials is what scientists have long sought for. Recently, thermal conductivity ($κ$) has emerged to be a decisive probe because the fermionic nature of spinons leads to a characteristic nonzero linear $κ_0/T$ term while approaching zero Kelvin. So far, only a few systems have been reported to exhibit such term. Here, we report a $κ_0/T \approx$ 0.01 WK$^{-2}$m$^{-1}$, the largest $κ_0/T$ value ever observed in magnetic oxide QSL candidates, in a new quantum magnet Pr$_2$Ga$_2$BeO$_7$ with a Shastry-Sutherland lattice (SSL). Its QSL nature is further supported by the power-law temperature dependence of the specific heat, a plateau of muon spin relaxation rate, and gapless inelastic neutron spectra. Our theoretical analysis reveals that the introduction of XY spin anisotropy is the key for Pr$_2$Ga$_2$BeO$_7$ to be the first QSL realized on the SSL, after more than four decades of extensive studies on this celebrated magnetically frustrated lattice. △ Less

Submitted 22 May, 2024; originally announced May 2024.

Comments: 20 pages, 6 figures, with Supplementary Information

Oxygen vacancies modulated VO2 for neurons and Spiking Neural Network construction

Authors: Liang Li, Ting Zhou, Tong Liu, Zhiwei Liu, Yaping Li, Shuo Wu, Shanguang Zhao, Jinglin Zhu, Meiling Liu, Zhihan Lin, Bowen Sun, Jianjun Li, Fangwen Sun, Chongwen Zou

Abstract: Artificial neuronal devices are the basic building blocks for neuromorphic computing systems, which have been motivated by realistic brain emulation. Aiming for these applications, various device concepts have been proposed to mimic the neuronal dynamics and functions. While till now, the artificial neuron devices with high efficiency, high stability and low power consumption are still far from pr… ▽ More Artificial neuronal devices are the basic building blocks for neuromorphic computing systems, which have been motivated by realistic brain emulation. Aiming for these applications, various device concepts have been proposed to mimic the neuronal dynamics and functions. While till now, the artificial neuron devices with high efficiency, high stability and low power consumption are still far from practical application. Due to the special insulator-metal phase transition, Vanadium Dioxide (VO2) has been considered as an idea candidate for neuronal device fabrication. However, its intrinsic insulating state requires the VO2 neuronal device to be driven under large bias voltage, resulting in high power consumption and low frequency. Thus in the current study, we have addressed this challenge by preparing oxygen vacancies modulated VO2 film(VO2-x) and fabricating the VO2-x neuronal devices for Spiking Neural Networks (SNNs) construction. Results indicate the neuron devices can be operated under lower voltage with improved processing speed. The proposed VO2-x based back-propagation SNNs (BP-SNNs) system, trained with the MNIST dataset, demonstrates excellent accuracy in image recognition. Our study not only demonstrates the VO2-x based neurons and SNN system for practical application, but also offers an effective way to optimize the future neuromorphic computing systems by defect engineering strategy. △ Less

Submitted 16 April, 2024; originally announced May 2024.

Comments: 18 pages,4 figures

Boosting Determinant Quantum Monte Carlo with Submatrix Updates: Unveiling the Phase Diagram of the 3D Hubbard Model

Authors: Fanjie Sun, Xiao Yan Xu

Abstract: Determinant Quantum Monte Carlo (DQMC) provides numerically exact solutions for strongly correlated fermionic systems but faces significant computational challenges with increasing system size. While submatrix updates were originally developed for Hirsch-Fye QMC with onsite interactions at finite temperatures [Phys. Rev. B 80, 195111 (2009)], their comprehensive application in DQMC has remained un… ▽ More Determinant Quantum Monte Carlo (DQMC) provides numerically exact solutions for strongly correlated fermionic systems but faces significant computational challenges with increasing system size. While submatrix updates were originally developed for Hirsch-Fye QMC with onsite interactions at finite temperatures [Phys. Rev. B 80, 195111 (2009)], their comprehensive application in DQMC has remained unexplored despite noted algorithmic similarities. We present the first comprehensive application of submatrix updates in DQMC, significantly extending beyond the original scope by enabling simulations with extended interactions and at zero temperature. Building upon conventional fast updates and delay updates, our generalized implementation achieves an order-of-magnitude improvement in computational efficiency, enabling simulations of the half-filled Hubbard model on lattices up to 8,000 sites - a scale previously challenging with standard DQMC implementations. This enhanced computational capability allows us to accurately determine the finite-temperature phase diagram of the 3D Hubbard model at half-filling. Our findings not only shed light on the phase transitions within these complex systems but also pave the way for more effective simulations of strongly correlated electrons, potentially guiding experimental efforts in cold atom simulations of the 3D Hubbard model. △ Less

Submitted 18 February, 2025; v1 submitted 15 April, 2024; originally announced April 2024.

Comments: 19 pages, 15 figures, 3 tables

Journal ref: SciPost Phys. 18, 055 (2025)

Re-understanding of the deformation potential constant in the single crystal silicon

Authors: Feng Sun, Aijun Hong

Abstract: The mobility formula based on deformation potential (DP) theory is of great importance in semiconductor physics. However, the related calculations for the DP constant are controversial. It is necessary to redo in-depth and comprehensive research on the mobility of single crystal silicon and the related parameters such as the effective mass and the DP constant. In this work the conductivity effecti… ▽ More The mobility formula based on deformation potential (DP) theory is of great importance in semiconductor physics. However, the related calculations for the DP constant are controversial. It is necessary to redo in-depth and comprehensive research on the mobility of single crystal silicon and the related parameters such as the effective mass and the DP constant. In this work the conductivity effective mass is redefined and a method based on the first principles is presented to evaluate the correction of the DP constant. It is found that the effective mass is closer to experimental data and the correction of the DP is a negligible value of about 0.3 eV. Using these parameters, we obtain the mobilities of the single crystal silicon in reasonable agreement with the experimental values. Our method can be effectively applied to the prediction for the mobility in bulk materials. △ Less

Submitted 29 January, 2024; v1 submitted 11 December, 2023; originally announced December 2023.

The Lorenz ratio as a guide to scattering contributions to Planckian transport

Authors: F. Sun, S. Mishra, U. Stockert, R. Daou, N. Kikugawa, R. S. Perry, E. Hassinger, S. A. Hartnoll, A. P. Mackenzie, V. Sunko

Abstract: In many physical situations in which many-body assemblies exist at temperature $T$, a characteristic quantum-mechanical time scale of approximately $\hbar/k_{B}T$ can be identified in both theory and experiment, leading to speculation that it may be the shortest meaningful time in such circumstances. When this behaviour is investigated by probing the scattering rate of strongly interacting electro… ▽ More In many physical situations in which many-body assemblies exist at temperature $T$, a characteristic quantum-mechanical time scale of approximately $\hbar/k_{B}T$ can be identified in both theory and experiment, leading to speculation that it may be the shortest meaningful time in such circumstances. When this behaviour is investigated by probing the scattering rate of strongly interacting electrons in metals, it is clear that in some cases only electron-electron scattering can be its cause, while in others it arises from high-temperature scattering of electrons from quantised lattice vibrations, i.e. phonons. In metallic oxides, which are among the most studied materials, analysis of electrical transport does not satisfactorily identify the relevant scattering mechanism at 'high' temperatures near room temperature. We employ a contactless optical method to measure thermal diffusivity in two Ru-based layered perovskites, Sr$_3$Ru$_2$O$_7$ and Sr$_2$RuO$_4$, and use the measurements to extract the dimensionless Lorenz ratio. By comparing our results to the literature data on both conventional and unconventional metals we show how the analysis of high-temperature thermal transport can both give important insight into dominant scattering mechanisms, and be offered as a stringent test of theories attempting to explain anomalous scattering. △ Less

Submitted 18 October, 2023; originally announced October 2023.

Comments: 27 pages, 8 figures

Delay Update in determinant quantum Monte Carlo

Authors: Fanjie Sun, Xiao Yan Xu

Abstract: Determinant quantum Monte Carlo (DQMC) is a widely used unbiased numerical method for simulating strongly correlated electron systems. However, the update process in DQMC is often a bottleneck for its efficiency. To address this issue, we propose a generalized delay update scheme that can handle both onsite and extended interactions. Our delay update scheme can be implemented in both zero-temperat… ▽ More Determinant quantum Monte Carlo (DQMC) is a widely used unbiased numerical method for simulating strongly correlated electron systems. However, the update process in DQMC is often a bottleneck for its efficiency. To address this issue, we propose a generalized delay update scheme that can handle both onsite and extended interactions. Our delay update scheme can be implemented in both zero-temperature and finite-temperature versions of DQMC. We apply the delay update scheme to various strongly correlated electron models and evaluate its efficiency under different conditions. Our results demonstrate that the proposed delay update scheme significantly improves the efficiency of DQMC simulations, enable it to simulate larger system size. △ Less

Submitted 5 June, 2025; v1 submitted 23 August, 2023; originally announced August 2023.

Comments: 12 pages, 7 figures

Journal ref: Phys. Rev. B 109, 235140 (2024)

Manipulation of Weyl Points in Reciprocal and Nonreciprocal Mechanical Lattices

Authors: Mingsheng Tian, Ivan Velkovsky, Tao Chen, Fengxiao Sun, Qiongyi He, Bryce Gadway

Abstract: We introduce feedback-measurement technologies to achieve flexible control of Weyl points and conduct the first experimental demonstration of Weyl type I-II transition in mechanical systems. We demonstrate that non-Hermiticity can expand the Fermi arc surface states from connecting Weyl points to Weyl rings, and lead to a localization transition of edge states influenced by the interplay between b… ▽ More We introduce feedback-measurement technologies to achieve flexible control of Weyl points and conduct the first experimental demonstration of Weyl type I-II transition in mechanical systems. We demonstrate that non-Hermiticity can expand the Fermi arc surface states from connecting Weyl points to Weyl rings, and lead to a localization transition of edge states influenced by the interplay between band topology and the non-Hermitian skin effect. Our findings offer valuable insights into the design and manipulation of Weyl points in mechanical systems, providing a promising avenue for manipulating topological modes in non-Hermitian systems. △ Less

Submitted 21 March, 2024; v1 submitted 15 August, 2023; originally announced August 2023.

Journal ref: Phys.Rev.Lett.132,126602 (2024)

Anisotropic in-plane heat transport of Kitaev magnet Na$_2$Co$_2$TeO$_6$

Authors: Shuangkui Guang, Na Li, Qing Huang, Ke Xia, Yiyan Wang, Hui Liang, Yan Sun, Qiuju Li, Xia Zhao, Rui Leonard Luo, Gang Chen, Haidong Zhou, Xuefeng Sun

Abstract: We report a study on low-temperature heat transport of Kitaev magnet Na$_2$Co$_2$TeO$_6$, with the heat current and magnetic fields along the honeycomb spin layer (the $ab$ plane). The zero-field thermal conductivity of $κ^a_{xx}$ and $κ^{a*}_{xx}$ display similar temperature dependence and small difference in their magnitudes; whereas, their magnetic field (parallel to the heat current) dependenc… ▽ More We report a study on low-temperature heat transport of Kitaev magnet Na$_2$Co$_2$TeO$_6$, with the heat current and magnetic fields along the honeycomb spin layer (the $ab$ plane). The zero-field thermal conductivity of $κ^a_{xx}$ and $κ^{a*}_{xx}$ display similar temperature dependence and small difference in their magnitudes; whereas, their magnetic field (parallel to the heat current) dependence are quite different and are related to the field-induced magnetic transitions. The $κ^a_{xx}(B)$ data for $B \parallel a$ at very low temperatures have an anomaly at 10.25--10.5 T, which reveals an unexplored magnetic transition. The planar thermal Hall conductivity $κ^a_{xy}$ and $κ^{a*}_{xy}$ show very weak signals at low fields and rather large values with sign change at high fields. This may point to a possible magnetic structure transition or the change of the magnon band topology that induces a radical change of magnon Berry curvature distribution before entering the spin polarized state. These results put clear constraints on the high-field phase and the theoretical models for Na$_2$Co$_2$TeO$_6$. △ Less

Submitted 12 July, 2023; originally announced July 2023.

Comments: 7 pages, 4 figures

Journal ref: Phys. Rev. B 109, 024419 (2024)

Bose-Einstein condensations of magnons in quantum magnets with spin-orbit coupling in a Zeeman field

Authors: Fadi Sun, Jinwu Ye

Abstract: We study the response of a quantum magnet with spin-orbit coupling (SOC) to a Zeeman field by constructing effective actions and performing Renormalization Group (RG) analysis. There are several novel classes of quantum phase transitions at a low $ h_{c1} $ and an upper critical field $ h_{c2} $ driven by magnon condensations at commensurate (C-) or in-commensurate (IC-) momenta $ 0 < k_0 < π$. Th… ▽ More We study the response of a quantum magnet with spin-orbit coupling (SOC) to a Zeeman field by constructing effective actions and performing Renormalization Group (RG) analysis. There are several novel classes of quantum phase transitions at a low $ h_{c1} $ and an upper critical field $ h_{c2} $ driven by magnon condensations at commensurate (C-) or in-commensurate (IC-) momenta $ 0 < k_0 < π$. The intermediate IC- Skyrmion crystal (IC-SkX) phase is controlled by a line of fixed points in the RG flows labeled by $ k_0 $. We derive the relations between the quantum spin and the order parameters of the effective actions which determine the spin-orbital structures of the IC-SkX phase. We also analyze the operator contents near $ h_{c1} $ and $ h_{c2} $ which determine the exotic excitation spectra inside the IC-SkX. The intrinsic differences between the magnon condensations at the C- and IC- momenta are explored. The two critical fields $ h_{c1} < h_{c2} $ and the intermediate IC-SkX phase could be a generic feature to any quantum magnets with SOC in a Zeeman field. Experimental implications to some materials or cold atom systems with SOC in a Zeeman field are presented. △ Less

Submitted 25 April, 2023; originally announced April 2023.

Comments: 12 PRB pages, 8 .eps figures, accepted for publication in Phys. Rev. B. arXiv admin note: text overlap with arXiv:2011.11287

Journal ref: Phys. Rev. B 107, 184425, Published 12 May 2023

Tutorial: Nonlinear magnonics

Authors: Shasha Zheng, Zhenyu Wang, Yipu Wang, Fengxiao Sun, Qiongyi He, Peng Yan, H. Y. Yuan

Abstract: Nonlinear magnonics studies the nonlinear interaction between magnons and other physical platforms (phonon, photon, qubit, spin texture) to generate novel magnon states for information processing. In this tutorial, we first introduce the nonlinear interactions of magnons in pure magnetic systems and hybrid magnon-phonon and magnon-photon systems. Then we show how these nonlinear interactions can g… ▽ More Nonlinear magnonics studies the nonlinear interaction between magnons and other physical platforms (phonon, photon, qubit, spin texture) to generate novel magnon states for information processing. In this tutorial, we first introduce the nonlinear interactions of magnons in pure magnetic systems and hybrid magnon-phonon and magnon-photon systems. Then we show how these nonlinear interactions can generate exotic magnonic phenomena. In the classical regime, we will cover the parametric excitation of magnons, bistability and multistability, and the magnonic frequency comb. In the quantum regime, we will discuss the single magnon state, Schrödinger cat state and the entanglement and quantum steering among magnons, photons and phonons. The applications of the hybrid magnonics systems in quantum transducer and sensing will also be presented. Finally, we outlook the future development direction of nonlinear magnonics. △ Less

Submitted 28 March, 2023; originally announced March 2023.

Comments: 50 pages, 26 figures

Journal ref: J. Appl. Phys. 134, 151101 (2023)

Topological phase transitions generated by the order from quantum disorder

Authors: Fadi Sun, Jinwu Ye

Abstract: The order from quantum disorder (OFQD) phenomenon was first discovered in quantum spin systems in geometric frustrated lattice. Similar phenomenon was also discovered in interacting bosonic systems or quantum spin systems with spin-orbit coupling in a bipartite lattice. Here we show that the OFQD also leads to a topological phase transition. We demonstrate this new connection in the experimentally… ▽ More The order from quantum disorder (OFQD) phenomenon was first discovered in quantum spin systems in geometric frustrated lattice. Similar phenomenon was also discovered in interacting bosonic systems or quantum spin systems with spin-orbit coupling in a bipartite lattice. Here we show that the OFQD also leads to a topological phase transition. We demonstrate this new connection in the experimentally realized weakly interacting Quantum Anomalous Hall system of spinor bosons in an optical lattice. There are two classes of topological phenomena: the first class is a perturbative one smoothly connected to the non-interacting limit. The second one is a non-perturbative one which has no analog in the non-interacting limit. Their experimental detections are also discussed. △ Less

Submitted 13 March, 2023; originally announced March 2023.

Comments: 6 pages main text + 5 pages SM

Topological phase transitions, invariants and enriched bulk-edge correspondences in fermionic gapless systems with extended Fermi surface

Authors: Fadi Sun, Jinwu Ye

Abstract: Topological phases and topological phase transitions (TPT) are among the most fantastic phenomena in Nature. Here we show that injecting a current may lead to new topological phases, especially new gapless topological metallic phases with extended Fermi surfaces (FSs) through novel class of TPTs in the bulk or the boundary. Specifically, we study the quantum anomalous Hall (QAH) system in a square… ▽ More Topological phases and topological phase transitions (TPT) are among the most fantastic phenomena in Nature. Here we show that injecting a current may lead to new topological phases, especially new gapless topological metallic phases with extended Fermi surfaces (FSs) through novel class of TPTs in the bulk or the boundary. Specifically, we study the quantum anomalous Hall (QAH) system in a square lattice under various forms of injecting currents. In addition to the previously known Chern insulator (which will be called even Chern insulator here), band insulator and band metal (BM), we find three new topological phases we name as: the gapped odd Chern insulator (Odd CI), the gapless odd Chern metal (Odd CM) and even Chern metal (Even CM). The Chern number may not be effective anymore in characterizing the topological gapless phases with extended FS. It is the Hall conductance which acts as the new topological invariant in such gapless systems. Its jump is a universal integer or non-integer across the even CM/BM or odd CM/BM TPT respectively where there is also a corresponding TPT in the Longitudinal (L-)edge modes. The Odd/even CM to BM transition is a novel class of TPT without any non-analyticity in the ground state energy density. This presents the first example of a TPT which is not a quantum phase transition (QPT). The original bulk-edge correspondence is enriched into bulk/Longitudinal (L-)/Transverse (T-) edge correspondence. The L-edge reconstruction may happen earlier, later or at the same time as the bulk TPT respectively in the even CI/odd CI/odd CM sequence with the edge dynamic exponent $ z_L=3 $, in the even CI/even CM/odd CM sequence with $ z_L=2 $ or a direct even CI/odd CM with a flat edge. The disappearance of the T-edge always happen at the same time as the bulk TPT with a universal edge critical behaviour. △ Less

Submitted 11 March, 2023; originally announced March 2023.

A spatially resolved optical method to measure thermal diffusivity

Authors: F. Sun, S. Mishra, P. H. McGuinness, Z. H. Filipiak, I. Markovic, D. A. Sokolov, N. Kikugawa, J. W. Orenstein, S. A. Hartnoll, A. P. Mackenzie, V. Sunko

Abstract: We describe an optical method to directly measure position-dependent thermal diffusivity of reflective single crystal samples across a broad range of temperatures for condensed matter physics research. Two laser beams are used, one as a source to locally modulate the sample temperature, and the other as a probe of sample reflectivity, which is a function of the modulated temperature. Thermal diffu… ▽ More We describe an optical method to directly measure position-dependent thermal diffusivity of reflective single crystal samples across a broad range of temperatures for condensed matter physics research. Two laser beams are used, one as a source to locally modulate the sample temperature, and the other as a probe of sample reflectivity, which is a function of the modulated temperature. Thermal diffusivity is obtained from the phase delay between source and probe signals. We combine this technique with a microscope setup in an optical cryostat, in which the sample is placed on a 3-axis piezo-stage, allowing for spatially resolved measurements. Furthermore, we demonstrate experimentally and mathematically that isotropic in-plane diffusivity can be obtained when overlapping the two laser beams instead of separating them in the traditional way, which further enhances the spatial resolution to a micron scale, especially valuable when studying inhomogeneous or multidomain samples. We discuss in detail the experimental conditions under which this technique is valuable, and demonstrate its performance on two stoichiometric bilayer ruthenates: Sr3Ru2O7 and Ca3Ru2O7. The spatial resolution allowed us to study the diffusivity in single domains of the latter, and we uncovered a temperature-dependent in-plane diffusivity anisotropy. Finally, we used the enhanced spatial resolution enabled by overlapping the two beams to measure temperature-dependent diffusivity of Ti-doped Ca3Ru2O7, which exhibits a metal-insulator transition. We observed large variations of transition temperature over the same sample, originating from doping inhomogeneity, and pointing to the power of spatially resolved techniques in accessing inherent properties. △ Less

Submitted 3 March, 2023; originally announced March 2023.

Comments: 30 pages, 5 figures in the main text, and 5 figures in Appendix

Low-temperature specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ single crystals

Authors: H. L. Che, S. J. Li, J. C. Wu, N. Li, S. K. Guang, K. Xia, X. Y. Yue, Y. Y. Wang, X. Zhao, Q. J. Li, X. F. Sun

Abstract: We report a study on the specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ ($x =$ 0, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of Tb$^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state… ▽ More We report a study on the specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ ($x =$ 0, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of Tb$^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state doublet and the gap between the ground state and first excited level. These crystals has an extremely low phonon thermal conductivity in a broad temperature range that can be attributed to the scattering by the magnetic excitations, which are mainly associated with the CEF levels. There is strong magnetic field dependence of thermal conductivity, which is more likely related to the field-induced changes of phonon scattering by the CEF levels than magnetic transitions or spin excitations. For magnetic field along the [111] direction, there is large thermal Hall conductivity at low temperatures which displays a broad peak around 8 T. At high fields up to 14 T, the thermal Hall conductivity decreases to zero, which supports its origin from either the spinon transport or the phonon skew scattering by CEF levels. The thermal Hall effect is rather robust with Zr doping up to 0.2 but is strongly weakened in higher Zr-doped sample. △ Less

Submitted 21 February, 2023; originally announced February 2023.

Comments: 9 pages, 8 figures, accepted for publication in Phys. Rev. B

Tripling energy storage density through order-disorder transition induced polar nanoregions in PbZrO3 thin films by ion implantation

Authors: Yongjian Luo, Changan Wang, Chao Chen, Yuan Gao, Fei Sun, Caiwen Li, Xiaozhe Yin, Chunlai Luo, Ulrich Kentsch, Xiangbin Cai, Mei Bai, Zhen Fan, Minghui Qin, Min Zeng, Jiyan Dai, Guofu Zhou, Xubing Lu, Xiaojie Lou, Shengqiang Zhou, Xingsen Gao, Deyang Chen, Jun-Ming Liu

Abstract: Dielectric capacitors are widely used in pulsed power electronic devices due to their ultrahigh power densities and extremely fast charge/discharge speed. To achieve enhanced energy storage density, both maximum polarization (Pmax) and breakdown strength (Eb) need to be improved simultaneously. However, these two key parameters are inversely correlated. In this study, order-disorder transition ind… ▽ More Dielectric capacitors are widely used in pulsed power electronic devices due to their ultrahigh power densities and extremely fast charge/discharge speed. To achieve enhanced energy storage density, both maximum polarization (Pmax) and breakdown strength (Eb) need to be improved simultaneously. However, these two key parameters are inversely correlated. In this study, order-disorder transition induced polar nanoregions (PNRs) have been achieved in PbZrO3 thin films by making use of the low-energy ion implantation, enabling us overcome the trade-off between high polarizability and breakdown strength, which leads to the tripling of the energy storage density from 20.5 J/cm3 to 62.3 J/cm3 as well as the great enhancement of breakdown strength. This approach could be extended to other dielectric oxides to improve the energy storage performance, providing a new pathway for tailoring the oxide functionalities. △ Less

Submitted 28 November, 2022; originally announced November 2022.

Journal ref: Applied Physics Reviews 2023

Thermal Transport of Fractionalized Antiferromagnetic and Field Induced States in the Kitaev Material Na$_2$Co$_2$TeO$_6$

Authors: S. K. Guang, N. Li, R. L. Luo, Q. Huang, Y. Y. Wang, X. Y. Yue, K. Xia, Q. J. Li, X. Zhao, G. Chen, H. D. Zhou, X. F. Sun

Abstract: We report an in-plane thermal transport study of the honeycomb Kitaev material Na$_2$Co$_2$TeO$_6$ at subKelvin temperatures. In zero field, the $κ(T)$ displays a rather weak $T$-dependence but has a non-zero residual term $κ_0/T$, indicating strong phonon scattering by magnetic excitation and the possibility of itinerant spinon-like excitations coexisting with an antiferromagnetic order below 27… ▽ More We report an in-plane thermal transport study of the honeycomb Kitaev material Na$_2$Co$_2$TeO$_6$ at subKelvin temperatures. In zero field, the $κ(T)$ displays a rather weak $T$-dependence but has a non-zero residual term $κ_0/T$, indicating strong phonon scattering by magnetic excitation and the possibility of itinerant spinon-like excitations coexisting with an antiferromagnetic order below 27 K. We propose the zero-field ground state is a novel fractionalized antiferromagnetic (AF*) state with both magnetic order and fractionalized excitations. With both the heat current and external field along the $a*$ (Co-Co bond) direction, the $κ_{a*}$ exhibits two sharp minima at 7.5 T and 10 T, and its value at 8.5 T is almost the same as the pure phononic transport for the high-field polarized state. This confirms the phase boundaries of the reported field-induced intermediate state and suggest its gapless continuum excitations possibly transport heat. No such intermediate phase was found in the $κ_a$ for the current and field along the $a$ (zigzag chain) direction. Finally, Na$_2$Co$_2$TeO$_6$ displays a strongly anisotropic magneto-thermal conductivity since the in-plane (out-of-plane) field strongly enhances (suppresses) the $κ_{a*}$ and $κ_a$. △ Less

Submitted 15 November, 2022; originally announced November 2022.

Comments: 7 pages, 3 figures

Journal ref: Phys. Rev. B 107, 184423 (2023)

Low-temperature transport properties of intermetallic compound HoAgGe with kagome spin ice state

Authors: N. Li, Q. Huang, X. Y. Yue, S. K. Guang, K. Xia, Y. Y. Wang, Q. J. Li, X. Zhao, H. D. Zhou, X. F. Sun

Abstract: We study the magnetic susceptibility, magnetization, resistivity and thermal conductivity of intermetallic HoAgGe single crystals at low temperatures and in magnetic fields along the $a$ and $c$ axis, while the electric and heat currents are along the $c$ axis. The magnetization curves show a series of metamagnetic transitions and small hysteresis at low field for $B \parallel a$, and a weak metam… ▽ More We study the magnetic susceptibility, magnetization, resistivity and thermal conductivity of intermetallic HoAgGe single crystals at low temperatures and in magnetic fields along the $a$ and $c$ axis, while the electric and heat currents are along the $c$ axis. The magnetization curves show a series of metamagnetic transitions and small hysteresis at low field for $B \parallel a$, and a weak metamagnetic transition for $B \parallel c$, respectively. Both the magnetic susceptibility and $ρ(T)$ curve show anomalies at the antiferromagnetic transition ($T\rm_N \sim$ 11.3 K) and spin reorientation transition ($\sim$ 7 K). In zero field and at very low temperatures, the electrons are found to be the main heat carriers. For $B \parallel a$, the $ρ(B)$ curves display large and positive transverse magnetoresistance (MR) with extraordinary field dependence between $B^2$ and $B$-linear, accompanied with anomalies at the metamagnetic transitions and low-field hysteresis; meanwhile, the $κ(B)$ mainly decrease with increasing field and display some anomalies at the metamagnetic transitions. For $B \parallel c$, there is weak and negative longitudinal MR while the $κ(B)$ show rather strong field dependence, indicating the role of phonon heat transport. △ Less

Submitted 25 July, 2022; originally announced July 2022.

Comments: 8 pages, 8 figures

Journal ref: Phys. Rev. B 106, 014416 (2022)

Emergent space-time meets emergent quantum phenomena: observing quantum phase transitions in a moving sample

Authors: Fad Sun, Jinwu Ye

Abstract: In material science, it was established that as the number of particles $ N $ in a material gets more and more, especially in the thermodynamic limit, various macroscopic quantum phenomena such as superconductivity, superfluidity, quantum magnetism, Fractional quantum Hall effects and various quantum or topological phase transitions (QPT) emerge in such non-relativistic quantum many-body systems.… ▽ More In material science, it was established that as the number of particles $ N $ in a material gets more and more, especially in the thermodynamic limit, various macroscopic quantum phenomena such as superconductivity, superfluidity, quantum magnetism, Fractional quantum Hall effects and various quantum or topological phase transitions (QPT) emerge in such non-relativistic quantum many-body systems. There is always a reservoir which exchanges energy and particles with the material. This is the essence of P. W. Anderson's great insight `` More is different ''. However, there is still a fundamental component missing in this general picture: How the `` More is different '' becomes different in a moving inertial frame or a moving sample? Here we address this outstanding problem.We demonstrate our claims by studying one of the simplest QPTs: Superfluid (SF)-Mott transitions of interacting bosons in a square lattice in a sample moving with a constant velocity $ \vec{v} $. We first elaborate the crucial difference between a moving sample and a moving inertial frame, stressing the crucial roles played by a reservoir in a grand canonical ensemble which is needed to study the SF-Mott transition in the first place. In this work, we mainly present the moving sample case and only discuss very briefly the moving inertial frame case. It is the moving which mixes the space and time. We also stress the important roles played by the underlying lattice. Doing various light or neutron scattering measurements in a moving sample may become an effective way not only measure various intrinsic properties of the materials, tune various quantum and topological phases through phase transitions, but also probe the new emergent space-time structure near any QPT. △ Less

Submitted 26 March, 2023; v1 submitted 21 July, 2022; originally announced July 2022.

Comments: 61 PRB pages, 17 figures. For a related work on the revival of the supersolid in Helium 4 and cold atom BEC in a driving superfluid, see arXiv:cond-mat/0512480

Nonreciprocal Amplification Transition in a Driven-Dissipative Quantum Network

Authors: Mingsheng Tian, Fengxiao Sun, Kaiye Shi, Haitan Xu, Qiongyi He, Wei Zhang

Abstract: We study the transport properties of a driven-dissipative quantum network, where multiple bosonic cavities such as photonic microcavities are coupled through a nonreciprocal bus with unidirectional transmission. For short-range coupling between the cavities, the occurrence of nonreciprocal amplification can be linked to a topological phase transition of the underlying dynamic Hamiltonian. However,… ▽ More We study the transport properties of a driven-dissipative quantum network, where multiple bosonic cavities such as photonic microcavities are coupled through a nonreciprocal bus with unidirectional transmission. For short-range coupling between the cavities, the occurrence of nonreciprocal amplification can be linked to a topological phase transition of the underlying dynamic Hamiltonian. However, for long-range coupling, we find that the nonreciprocal amplification transition deviates drastically from the topological phase transition. Nonetheless, we show that the nonreciprocal amplification transition can be connected to the emergence of zero-energy edge states of an auxiliary Hamiltonian with chiral symmetry even in the long-range coupling limit. We also investigate the stability, the crossover from short to long-range coupling, and the bandwidth of the nonreciprocal amplification. Our work has potential application in signal transmission and amplification, and also opens a window to non-Hermitian systems with long-range coupling and nontrivial boundary effects. △ Less

Submitted 14 July, 2022; originally announced July 2022.

Comments: 5 pages, 4 figures

Journal ref: Photonics Research 11, 852 (2023)

Lattice distortion inducing exciton splitting and coherent quantum beating in CsPbI3 perovskite quantum dots

Authors: Yaoyao Han, Wenfei Liang, Xuyang Lin, Yulu Li, Fengke Sun, Fan Zhang, Peter C. Sercel, Kaifeng Wu

Abstract: Anisotropic exchange-splitting in semiconductor quantum dots (QDs) results in bright-exciton fine-structure-splitting (FSS) important for quantum information processing. Direct measurement of FSS usually requires single/few QDs at liquid-helium temperatures, because of its sensitivity to QD size and shape, whereas measuring and controlling FSS at an ensemble-level seem to be impossible unless all… ▽ More Anisotropic exchange-splitting in semiconductor quantum dots (QDs) results in bright-exciton fine-structure-splitting (FSS) important for quantum information processing. Direct measurement of FSS usually requires single/few QDs at liquid-helium temperatures, because of its sensitivity to QD size and shape, whereas measuring and controlling FSS at an ensemble-level seem to be impossible unless all the dots are made to be nearly the same. Here we report strong bright-exciton FSS up to 1.6 meV in solution-processed CsPbI3 perovskite QDs, manifested as quantum beats in ensemble-level transient absorption at liquid-nitrogen to room temperatures. The splitting is robust to QD size and shape heterogeneity, and increases with decreasing temperature, pointing towards a mechanism associated with orthorhombic distortion of perovskite lattice. Effective-mass-approximation calculations reveal an intrinsic "fine-structure gap" that agrees well with the observed FSS. This gap stems from an avoided crossing of bright-excitons confined in orthorhombically-distorted QDs that are bounded by the pseudocubic {100} family of planes. △ Less

Submitted 27 June, 2022; originally announced June 2022.

Na$_5$Co$_{15.5}$Te$_6$O$_{36}$: an $S$ = 1/2 stacked Ising kagome antiferromagnet with a partially disordered ground state

Authors: Z. Y. Zhao, X. Y. Yue, J. Y. Li, N. Li, H. L. Che, X. F. Sun, Z. Z. He

Abstract: $S$ = 1/2 Ising kagome antiferromagnets (KAFs) are rare in reality. In this work, we report the magnetic ground state and field-induced transitions of an $S$ = 1/2 Ising antiferromagnet with a stacked kagome geometry, Na$_5$Co$_{15.5}$Te$_6$O$_{36}$. Upon cooling, magnetic susceptibility measurement reveals three successive magnetic transitions at $T_1$ = 45.2 K, $T_2$ = 35.4 K, and $T_3 \sim… ▽ More $S$ = 1/2 Ising kagome antiferromagnets (KAFs) are rare in reality. In this work, we report the magnetic ground state and field-induced transitions of an $S$ = 1/2 Ising antiferromagnet with a stacked kagome geometry, Na$_5$Co$_{15.5}$Te$_6$O$_{36}$. Upon cooling, magnetic susceptibility measurement reveals three successive magnetic transitions at $T_1$ = 45.2 K, $T_2$ = 35.4 K, and $T_3 \sim$ 20 K. When the magnetic field is applied along the Ising $c$ axis, a strikingly anomalous initial magnetization lying outside the hysteresis loop is observed below $T_3$. Through the detailed characterizations, the magnetic field $vs$ temperature phase diagram is established. A partially disordered antiferromagnetic state is proposed below $T_1$, which becomes frozen below $T_3$. Na$_5$Co$_{15.5}$Te$_6$O$_{36}$ is therefore a promising candidate for $S$ = 1/2 Ising KAFs, and this unique composite structure may shed light on the exploratory path for $S$ = 1/2 Ising KAFs. △ Less

Submitted 31 March, 2022; originally announced April 2022.

Comments: 9 pages, 9 pictures, to be published in PRB

Quantum Lifshitz transitions generated by order from quantum disorder in strongly correlated Rashba spin-orbital coupled systems

Authors: Fadi Sun, Jinwu Ye

Abstract: We study the system of strongly interacting spinor bosons in a square lattice subject to the isotropic Rashba SOC $ α=β$. It supports collinear spin-bond correlated magnetic Y-x phase, a gapped in-commensurate (IC-) co-planar IC-XY-y phase, a non-coplanar commensurate (C-) $ 3 \times 3 $ Skyrmion crystal phase (SkX). The state at the Abelian point $ α=β=π/2 $ is just an AFM state in a rotated basi… ▽ More We study the system of strongly interacting spinor bosons in a square lattice subject to the isotropic Rashba SOC $ α=β$. It supports collinear spin-bond correlated magnetic Y-x phase, a gapped in-commensurate (IC-) co-planar IC-XY-y phase, a non-coplanar commensurate (C-) $ 3 \times 3 $ Skyrmion crystal phase (SkX). The state at the Abelian point $ α=β=π/2 $ is just an AFM state in a rotated basis. Slightly away from the point, we identify a spurious $ U(1) $ symmetry, develop a novel and non-perturbative method to calculate not only the gap, but also the excitation spectrum due to the order from quantum disorder (OFQD) mechanism. We construct a symmetry based effective action to investigate the quantum Lifshitz transition from the Y-x state to the IC-XY-y state and establish the connection between the phenomenological parameters in the effective action and those evaluated by the microscopic non-perturbative OFQD analysis in the large $ S $ limits. Experimental implications on cold atoms and some 4d or 5d Kitaev materials are discussed. △ Less

Submitted 24 February, 2022; originally announced February 2022.

Comments: 5+2 pages. arXiv admin note: text overlap with arXiv:1603.00451

Journal ref: Phys. Rev. A ( Letter) 104, L031306 (2021)

Bone tumor suppression in rabbits by hyperthermia below the clinical safety limit using aligned magnetic bone cement

Authors: Xiang Yu, Shan Gao, Dian Wu, Zhengrui Li, Yan Mi, Tianyu Yang, Fan Sun, Lichen Wang, Ruoshui Liu, Shuli He, Qinggang Ge, Yang Lv, Andy, Xu, Hao Zeng

Abstract: Demonstrating highly efficient alternating current (AC) magnetic field heating of nanoparticles in physiological environments under clinically safe field parameters has remained a great challenge, hindering clinical applications of magnetic hyperthermia. In this work, we report exceptionally high loss power of magnetic bone cement under clinical safety limit of AC field parameters, incorporating D… ▽ More Demonstrating highly efficient alternating current (AC) magnetic field heating of nanoparticles in physiological environments under clinically safe field parameters has remained a great challenge, hindering clinical applications of magnetic hyperthermia. In this work, we report exceptionally high loss power of magnetic bone cement under clinical safety limit of AC field parameters, incorporating DC field-aligned soft magnetic Zn0.3Fe2.7O4 nanoparticles with low concentration. Under an AC field of 4 kA/m at 430 kHz, the aligned bone cement with 0.2 wt% nanoparticles achieved a temperature increase of 30 C in 180 s. This amounts to a specific loss power value of 327 W/gmetal and an intrinsic loss power of 47 nHm^2/kg, which is enhanced by 50-fold compared to randomly oriented samples. The high-performance magnetic bone cement allows for the demonstration of effective hyperthermia suppression of tumor growth in the bone marrow cavity of New Zealand White Rabbits subjecting to rapid cooling due to blood circulation, and significant enhancement of survival rate. △ Less

Submitted 6 February, 2022; originally announced February 2022.

Journal ref: Small 2022, 18, 2104626

Magnon-Polaron Driven Thermal Hall Effect in a Heisenberg-Kitaev Antiferromagnet

Authors: N. Li, R. R. Neumann, S. K. Guang, Q. Huang, J. Liu, K. Xia, X. Y. Yue, Y. Sun, Y. Y. Wang, Q. J. Li, Y. Jiang, J. Fang, Z. Jiang, X. Zhao, A. Mook, J. Henk, I. Mertig, H. D. Zhou, X. F. Sun

Abstract: The thermal Hall effect, defined as a heat current response transversal to an applied temperature gradient, is a central experimental probe of exotic electrically insulating phases of matter. A key question is how the interplay between magnetic and structural degrees of freedom gives rise to a nonzero thermal Hall conductivity (THC). Here, we present evidence for an intrinsic thermal Hall effect i… ▽ More The thermal Hall effect, defined as a heat current response transversal to an applied temperature gradient, is a central experimental probe of exotic electrically insulating phases of matter. A key question is how the interplay between magnetic and structural degrees of freedom gives rise to a nonzero thermal Hall conductivity (THC). Here, we present evidence for an intrinsic thermal Hall effect in the Heisenberg-Kitaev antiferromagnet and spin-liquid candidate Na$_2$Co$_2$TeO$_6$ brought about by the quantum-geometric Berry curvature of so-called magnon polarons, resulting from magnon-phonon hybridization. At low temperatures, our field- and temperature-dependent measurements show a negative THC for magnetic fields below 10 T and a sign change to positive THC above. Theoretically, the sign and the order of magnitude of the THC cannot be solely explained with magnetic excitations. We demonstrate that, by incorporating spin-lattice coupling into our theoretical calculations, the Berry curvature of magnon polarons counteracts the purely magnonic contribution, reverses the overall sign of the THC, and increases its magnitude, which significantly improves agreement with experimental data. Our work highlights the crucial role of spin-lattice coupling in the thermal Hall effect. △ Less

Submitted 27 February, 2023; v1 submitted 27 January, 2022; originally announced January 2022.

Comments: 19 pages, 4 figures

Remote generation of magnon Schrödinger cat state via magnon-photon entanglement

Authors: Feng-Xiao Sun, Sha-Sha Zheng, Yang Xiao, Qihuang Gong, Qiongyi He, Ke Xia

Abstract: Magnon cat state represents a macroscopic quantum superposition of collective magnetic excitations of large number spins that not only provides fundamental tests of macroscopic quantum effects but also finds applications in quantum metrology and quantum computation. In particular, remote generation and manipulation of Schrödinger cat states are particularly interesting for the development of long-… ▽ More Magnon cat state represents a macroscopic quantum superposition of collective magnetic excitations of large number spins that not only provides fundamental tests of macroscopic quantum effects but also finds applications in quantum metrology and quantum computation. In particular, remote generation and manipulation of Schrödinger cat states are particularly interesting for the development of long-distance and large-scale quantum information processing. Here, we propose an approach to remotely prepare magnon even/odd cat states by performing local non-Gaussian operations on the optical mode that is entangled with magnon mode through pulsed optomagnonic interaction. By evaluating key properties of the resulting cat states, we show that for experimentally feasible parameters they are generated with both high fidelity and nonclassicality, and with a size large enough to be useful for quantum technologies. Furthermore, the effects of experimental imperfections such as the error of projective measurements and dark count when performing single-photon operations have been discussed, where the lifetime of the created magnon cat states is expected to be $t\sim1\,μ$s. △ Less

Submitted 11 August, 2021; originally announced August 2021.

Comments: 14 pages, 10 figures

Journal ref: Phys. Rev. Lett. 127, 087203 (2021)

Survival of itinerant excitations and quantum spin state transitions in YbMgGaO$_4$ with chemical disorder

Authors: X. Rao, G. Hussain, Q. Huang, W. J. Chu, N. Li, X. Zhao, Z. Dun, E. S. Choi, T. Asaba, L. Chen, L. Li, X. Y. Yue, N. N. Wang, J. -G. Cheng, Y. H. Gao, Y. Shen, J. Zhao, G. Chen, H. D. Zhou, X. F. Sun

Abstract: A recent focus of quantum spin liquid (QSL) studies is how disorder/randomness in a QSL candidate affects its true magnetic ground state. The ultimate question is whether the QSL survives disorder or the disorder leads to a "spin-liquid-like" state, such as the proposed random-singlet (RS) state. Since disorder is a standard feature of most QSL candidates, this question represents a major challeng… ▽ More A recent focus of quantum spin liquid (QSL) studies is how disorder/randomness in a QSL candidate affects its true magnetic ground state. The ultimate question is whether the QSL survives disorder or the disorder leads to a "spin-liquid-like" state, such as the proposed random-singlet (RS) state. Since disorder is a standard feature of most QSL candidates, this question represents a major challenge for QSL candidates. YbMgGaO$_4$, a triangular lattice antiferromagnet with effective spin-1/2 Yb$^{3+}$ ions, is an ideal system to address this question, since it shows no long-range magnetic ordering with Mg/Ga site disorder. Despite the intensive study, it remains unresolved as to whether YbMgGaO$_4$ is a QSL or in the RS state. Here, through ultralow-temperature thermal conductivity and magnetic torque measurements, plus specific heat and DC magnetization data, we observed a residual $κ_0/T$ term and series of quantum spin state transitions in the zero temperature limit for YbMgGaO$_4$. These observations strongly suggest that a QSL state with itinerant excitations and quantum spin fluctuations survives disorder in YbMgGaO$_4$. △ Less

Submitted 4 July, 2021; originally announced July 2021.

Comments: 31 pages, 10 figures

Journal ref: Nat. Commun. 12, 4949 (2021)

Quantum spin state transitions in spin-1 equilateral triangular lattice antiferromagnet Na$_2$BaNi(PO$_4$)$_2$

Authors: N. Li, Q. Huang, A. Brassington, X. Y. Yue, W. J. Chu, S. K. Guang, X. H. Zhou, P. Gao, E. X. Feng, H. B. Cao, E. S. Choi, Y. Sun, Q. J. Li, X. Zhao, H. D. Zhou, X. F. Sun

Abstract: We have grown single crystals of Na$_2$BaNi(PO$_4$)$_2$, a new spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat and thermal conductivity measurements at ultralow temperatures. The main results are (i) at zero magnetic field, Na$_2$BaNi(PO$_4$)$_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the… ▽ More We have grown single crystals of Na$_2$BaNi(PO$_4$)$_2$, a new spin-1 equilateral triangular lattice antiferromagnet (ETLAF), and performed magnetic susceptibility, specific heat and thermal conductivity measurements at ultralow temperatures. The main results are (i) at zero magnetic field, Na$_2$BaNi(PO$_4$)$_2$ exhibits a magnetic ordering at 430 mK with a weak ferromagnetic moment along the $c$ axis. This suggests a canted 120$^\circ$ spin structure, which is in a plane including the crystallographic $c$ axis due to the existence of an easy-axis anisotropy and ferromagnetically stacked along the $c$ axis; (ii) with increasing field along the $c$ axis, a 1/3 magnetization plateau is observed which means the canted 120$^\circ$ spin structure is transformed to a up up down (UUD) spin structure. With even higher fields, the UUD phase further evolves to possible V and V' phases; (iii) with increasing field along the $a$ axis, the canted 120$^\circ$ spin structure is possibly transformed to a umbrella phase and a V phase. Therefore, Na$_2$BaNi(PO$_4$)$_2$ is a rare example of spin-1 ETLAF with single crystalline form to exhibit easy-axis spin anisotropy and series of quantum spin state transitions. △ Less

Submitted 24 August, 2021; v1 submitted 1 July, 2021; originally announced July 2021.

Comments: 9 pages, 10 figures, accepted for publication in Phys. Rev. B

Journal ref: Phys. Rev. B 104, 104403 (2021)

Femtosecond dynamics of a polariton bosonic cascade at room temperature

Authors: Fei Chen, Hang Zhou, Hui Li, Song Luo, Zheng Sun, Zhe Zhang, Fenghao Sun, Beier Zhou, Hongxing Dong, Huailiang Xu, Hongxing Xu, Alexey Kavokin, Zhanghai Chen, Jian Wu

Abstract: Whispering gallery modes in a microwire are characterized by a nearly equidistant energy spectrum. In the strong exciton-photon coupling regime, this system represents a bosonic cascade: a ladder of discrete energy levels that sustains stimulated transitions between neighboring steps. In this work, by using femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of polar… ▽ More Whispering gallery modes in a microwire are characterized by a nearly equidistant energy spectrum. In the strong exciton-photon coupling regime, this system represents a bosonic cascade: a ladder of discrete energy levels that sustains stimulated transitions between neighboring steps. In this work, by using femtosecond angle-resolved spectroscopic imaging technique, the ultrafast dynamics of polaritons in a bosonic cascade based on a one-dimensional ZnO whispering gallery microcavity is explicitly visualized. Clear ladder-form build-up process from higher to lower energy branches of the polariton condensates are observed, which are well reproduced by modeling using rate equations. Moreover, the polariton parametric scattering dynamics are distinguished on a timescale of hundreds of femtoseconds. Our understanding of the femtosecond condensation and scattering dynamics paves the way towards ultrafast coherent control of polaritons at room temperature, which will make it promising for high-speed all-optical integrated applications. △ Less

Submitted 18 May, 2021; originally announced May 2021.

Accommodation mechanisms in strain-transformable titanium alloys

Authors: Yolaine Danarda, Guilhem Martin, Lola Lilensten, Fan Sun, Anthony Seret, Régis Poulain, Srinivas Mantri, Raphaëlle Guillou, Dominique Thiaudière, Immanuel Freiherr von Thüngen, Daniel Galy, Michaël Piellard, Nathalie Bozzolo, Raj Banerjee, Frédéric Prima

Abstract: A new $β$-metastable Ti-alloy is designed with the aim to obtain a TWIP alloy but positioned at the limit between the TRIP/TWIP and the TWIP dominated regime. The designed alloy exhibits a large ductility combined with an elevated and stable work-hardening rate. Deformation occurring by formation and multiplication of {332}<113> twins is evidenced and followed by in-situ electron microscopy, and n… ▽ More A new $β$-metastable Ti-alloy is designed with the aim to obtain a TWIP alloy but positioned at the limit between the TRIP/TWIP and the TWIP dominated regime. The designed alloy exhibits a large ductility combined with an elevated and stable work-hardening rate. Deformation occurring by formation and multiplication of {332}<113> twins is evidenced and followed by in-situ electron microscopy, and no primary stress induced martensite is observed. Since microstructural investigations of the deformation mechanisms show a highly heterogeneous deformation, the reason of the large ductility is then investigated. The spatial strain distribution is characterized by micro-scale digital image correlation, and the regions highly deformed are found to stand at the crossover between twins, or at the intersection between deformation twins and grain boundaries. Detailed electron back-scattered imaging in such regions of interest finally allowed to evidence the formation of thin needles of stress induced martensite. The latter is thus interpreted as an accommodation mechanism, relaxing the local high strain fields, which ensures a large and stable plastic deformation of this newly designed Ti-alloy. △ Less

Submitted 12 May, 2021; v1 submitted 28 February, 2021; originally announced March 2021.

Covalent 2D Cr$_2$Te$_3$ ferromagnet

Authors: Mengying Bian, Aleksandr N. Kamenskii, Mengjiao Han, Wenjie Li, Sichen Wei, Xuezeng Tian, David B. Eason, Fan Sun, Keke He, Haolei Hui, Fei Yao, Renat Sabirianov, Jonathan P. Bird, Chunlei Yang, Jianwei Miao, Junhao Lin, Scott A. Crooker, Yanglong Hou, Hao Zeng

Abstract: To broaden the scope of van der Waals 2D magnets, we report the synthesis and magnetism of covalent 2D magnetic Cr$_2$Te$_3$ with a thickness down to one-unit-cell. The 2D Cr$_2$Te$_3$ crystals exhibit robust ferromagnetism with a Curie temperature of 180 K, a large perpendicular anisotropy of 7*105 J m-3, and a high coercivity of ~ 4.6 kG at 20 K. First-principles calculations further show a tran… ▽ More To broaden the scope of van der Waals 2D magnets, we report the synthesis and magnetism of covalent 2D magnetic Cr$_2$Te$_3$ with a thickness down to one-unit-cell. The 2D Cr$_2$Te$_3$ crystals exhibit robust ferromagnetism with a Curie temperature of 180 K, a large perpendicular anisotropy of 7*105 J m-3, and a high coercivity of ~ 4.6 kG at 20 K. First-principles calculations further show a transition from canted to collinear ferromagnetism, a transition from perpendicular to in-plane anisotropy, and emergent half-metallic behavior in atomically-thin Cr$_2$Te$_3$, suggesting its potential application for injecting carriers with high spin polarization into spintronic devices. △ Less

Submitted 27 February, 2021; originally announced March 2021.

Journal ref: MATER. RES. LETT.9 (2021), 205-212

Evolution of magnetic-field-induced ordering in the layered structure quantum Heisenberg triangular-lattice antiferromagnet Ba$_3$CoSb$_2$O$_9$

Authors: N. A. Fortune, Q. Huang, T. Hong, J. Ma, E. S. Choi, S. T. Hannahs, Z. Y. Zhao, X. F. Sun, Y. Takano, H. D. Zhou

Abstract: Quantum fluctuations in the effective spin one-half layered structure triangular-lattice quantum Heisenberg antiferromagnet Ba$_3$CoSb$_2$O$_9$ lift the classical degeneracy of the antiferromagnetic ground state in magnetic field, producing a series of novel spin structures for magnetic fields applied within the crystallographic ab plane. Theoretically unresolved, however, are the effects of inter… ▽ More Quantum fluctuations in the effective spin one-half layered structure triangular-lattice quantum Heisenberg antiferromagnet Ba$_3$CoSb$_2$O$_9$ lift the classical degeneracy of the antiferromagnetic ground state in magnetic field, producing a series of novel spin structures for magnetic fields applied within the crystallographic ab plane. Theoretically unresolved, however, are the effects of interlayer antferromagnetic coupling and transverse magnetic fields on the ground states of this system. To address these issues, we have used specific heat, neutron diffraction, thermal conductivity, and magnetic torque measurements to map out the phase diagram as a function of magnetic field intensity and orientation relative to the crystallographic ab plane. For H parallel to the ab plane, we have discovered an additional, previously unreported magnetic-field-induced phase transition at low temperature and an unexpected tetracritical point in the high field phase diagram, which - coupled with the apparent second-order nature of the phase transitions - eliminates several theoretically proposed spin structures for the high field phases. Our calorimetric measurements as a function of magnetic field orientation are in general agreement with theory for field-orientation angles close to plane parallel but diverge at angles near plane perpendicular; a predicted convergence of two phase boundaries at finite angle and a corresponding change in the order of the field induced phase transition is not observed experimentally. Our results emphasize the role of interlayer coupling in selecting and stabilizing field-induced phases, provide new guidance into the nature of the magnetic order in each phase, and reveal the need for new physics to account for the nature of magnetic ordering in this archetypal 2D spin one-half triangular lattice quantum Heisenberg antiferromagnet. △ Less

Submitted 7 May, 2021; v1 submitted 23 December, 2020; originally announced December 2020.

Comments: 10 pages, 11 figures

Journal ref: Phys. Rev. B 103, 184425 (2021)