Showing 1–50 of 179 results for author: Song, F

Elementary magnons and interacting multi-magnon quasiparticles in the effective spin-$\frac{1}{2}$ kagome-staircase magnet Co$_{3}$V$_{2}$O$_{8}$

Authors: Yuan Xiao, Jiajun Mo, Zhenmeng Jiang, Haoyang Leng, Otkur Omar, Yanjun Li, Fengyi Song, Jianjun Ying, Shang Gao

Abstract: The excitation spectrum of an anisotropic magnet provides a direct link between its microscopic Hamiltonian and interaction-driven quasiparticles. Here we use high-resolution time-domain terahertz spectroscopy to map the magnetic excitations of the three-dimensional kagome-staircase compound Co$_{3}$V$_{2}$O$_{8}$ as functions of temperature and magnetic field. At low energies, polarization-resolv… ▽ More The excitation spectrum of an anisotropic magnet provides a direct link between its microscopic Hamiltonian and interaction-driven quasiparticles. Here we use high-resolution time-domain terahertz spectroscopy to map the magnetic excitations of the three-dimensional kagome-staircase compound Co$_{3}$V$_{2}$O$_{8}$ as functions of temperature and magnetic field. At low energies, polarization-resolved spectra identify magnetic-dipole-active one-magnon modes and track their evolution across the ferromagnetic and spin-density-wave phases. Combining their field dependence with previously reported inelastic-neutron-scattering dispersions, we determine an effective spin-$\frac{1}{2}$ Hamiltonian with strongly anisotropic exchange that quantitatively reproduces the one-magnon spectrum. This model provides a noninteracting benchmark for the high-energy response, where we observe sharp branches with field slopes that are two to four times those of the one-magnon modes, together with anticrossings between branches of different magnon numbers. Their sharpness, polarization dependence, and departure from the calculated multi-magnon continua identify them as interacting multi-magnon quasiparticles that can be stabilized by strong exchange anisotropy. △ Less

Submitted 9 August, 2026; originally announced August 2026.

Gaplessness indicator by topologically trivial twisting operators

Authors: Yuan Yao, Linhao Li, Feng-Feng Song

Abstract: We propose several general necessary conditions for quantum many-body system in one dimension respecting U(1) symmetry to be gapped. We show that the ground-state expectation value of topologically trivial twisting operators must approach unity in the thermodynamic limit with a certain finite-size scaling. Equivalently, its violation can indicate gaplessness of U(1)-symmetric Hamiltonians. The top… ▽ More We propose several general necessary conditions for quantum many-body system in one dimension respecting U(1) symmetry to be gapped. We show that the ground-state expectation value of topologically trivial twisting operators must approach unity in the thermodynamic limit with a certain finite-size scaling. Equivalently, its violation can indicate gaplessness of U(1)-symmetric Hamiltonians. The topological triviality of such a twisting operator enables us to derive infinitely many other gaplessness indicators by static structure factor to any order in real experiments, which are impossible to obtain by earlier topologically nontrivial twisting operators. We also apply analytic and numerical calculations to test the efficiency and consistency of our results. △ Less

Submitted 23 July, 2026; originally announced July 2026.

Comments: 6 pages, 1 figure

Strain-Tuned Nodal Superconductivity in the Charge-Ordered Kagome Metal CsV$_3$Sb$_5$

Authors: Yusuke Takeuchi, Akito Kobayashi, Saki Uchida, Takumi Nagao, Seigo Ogawa, Rui Zhou, Shinji Kawasaki, Fei Song, Hao Ni, Yong Zhao, Guo-qing Zheng

Abstract: The nature of the superconducting pairing symmetry in the kagome metal CsV$_3$Sb$_5$ and its relationship with the charge density wave (CDW) order are central unresolved issues. Here, we investigate the evolution of superconductivity in CsV$_3$Sb$_5$ under in-situ uniaxial pressure using $^{121}$Sb nuclear quadrupole resonance (NQR). We find that tensile strain significantly enhances the supercond… ▽ More The nature of the superconducting pairing symmetry in the kagome metal CsV$_3$Sb$_5$ and its relationship with the charge density wave (CDW) order are central unresolved issues. Here, we investigate the evolution of superconductivity in CsV$_3$Sb$_5$ under in-situ uniaxial pressure using $^{121}$Sb nuclear quadrupole resonance (NQR). We find that tensile strain significantly enhances the superconducting transition temperature, $T_{\rm c}$, while the CDW remains unchanged, demonstrating that superconductivity can be tuned independently of the bulk charge order. At a tensile strain of $\varepsilon$ = +0.90%, the nuclear spin-lattice relaxation rate reveals a remarkable double transition: an upper transition at $T_{\rm c1}$ = 3.6 K to a nodal gap state, and a lower one at $T_{\rm c2}$ = 3.0 K characterized by a nodeless gap. These results evidence degenerate superconducting states with different gap symmetry in the kagome metal at ambient pressure which split under strain. Our work demonstrates a high tunability of superconductivity by uniaxial pressure. △ Less

Submitted 13 July, 2026; originally announced July 2026.

Comments: 8 pages, 8 figures, to appear in Phys. Rev. Lett

$5/9-$Magnetization Plateau and Spin Supersolidity in YCu$_3$(OD)$_{7-x}$Br$_{2+x}$ under Magnetic Fields up to 120~T

Authors: Hiroaki Hayashi, Han Li, Feng-Feng Song, Xu-Guang Zhou, Akira Matsuo, Taeyun Kim, Enze Lv, Yuto Ishii, Zhe Qu, Gang Su, Koichi Kindo, Kwang-Yong Choi, Wei Li, Yasuhiro H. Matsuda

Abstract: We performed high-precision magnetization measurements up to 120~T on three compositions of the newly discovered kagome antiferromagnet YCu$_3$(OD)$_{7-x}$Br$_{2+x}$ (YCOB), revealing a previously unobserved 5/9 fractional magnetization plateau. All YCOB samples with different Br$^-$ concentrations exhibit nearly identical magnetization curves below 60~T, whereas the 5/9 plateau appears at markedl… ▽ More We performed high-precision magnetization measurements up to 120~T on three compositions of the newly discovered kagome antiferromagnet YCu$_3$(OD)$_{7-x}$Br$_{2+x}$ (YCOB), revealing a previously unobserved 5/9 fractional magnetization plateau. All YCOB samples with different Br$^-$ concentrations exhibit nearly identical magnetization curves below 60~T, whereas the 5/9 plateau appears at markedly different fields in the ultrahigh-field regime. By modeling the experimental data using tensor-network calculations, we derive the effective spin Hamiltonians for the YCOB family with three spatially anisotropic Heisenberg couplings (the 3$J$-type model), which quantitatively reproduces the measured magnetization processes and captures the composition-dependent evolution of the 5/9 plateau. Furthermore, our theoretical analysis suggests the emergence of a spin supersolid phase in the field window between the 1/3 and 5/9 plateaus, which is sensitive to spin exchange parameters and accounts for the significant variation in the critical fields of the 5/9 plateau observed among different YCOB compositions. △ Less

Submitted 18 May, 2026; originally announced May 2026.

Comments: 9 pages, 7 figures

Higher odd-order nonlinear Hall effect in magnetic topological insulator Mn(Bi1-xSbx)2Te4

Authors: Xiubing Li, Zheng Dai, Shuai Zhang, Heng Zhang, Congcong Li, Boyuan Wei, Fengyi Guo, Chunfeng Li, Fucong Fei, Minhao Zhang, Xuefeng Wang, Huaiqiang Wang, Fengqi Song

Abstract: The nonlinear Hall effect is a new member of the Hall effect family, which attracts intense research interests, and it is closely related to the quantum geometry of quantum materials. The previous studies primarily concentrate on the second-order and third-order nonlinear Hall effect. However, the experimental study of higher-order nonlinear Hall effect is scarce at present. In this work, we repor… ▽ More The nonlinear Hall effect is a new member of the Hall effect family, which attracts intense research interests, and it is closely related to the quantum geometry of quantum materials. The previous studies primarily concentrate on the second-order and third-order nonlinear Hall effect. However, the experimental study of higher-order nonlinear Hall effect is scarce at present. In this work, we report the observations of the higher odd-order (third-, fifth-, seventh-order) nonlinear Hall effect in magnetic topological insulator Mn(Bi1-xSbx)2Te4 thin flakes. The higher odd-order nonlinear Hall voltage exhibits a twofold angular dependence and exists only below the Néel temperature. It reaches its maximum near the charge neutral point and decays exponentially as the order of the nonlinear Hall effect increases. Furthermore, such higher odd-order nonlinear Hall effect is observed in both odd- and even-layer samples with comparable magnitudes. Theoretical analysis indicates that the higher odd-order nonlinear Hall effect responses may arise from the Berry curvature multipoles. Our work paves the way for the study of the higher-order nonlinear transport phenomena. △ Less

Submitted 23 April, 2026; originally announced April 2026.

Room-temperature third-order nonlinear anomalous Hall effect in ferromagnetic metal Fe3GaTe2

Authors: Zheng Dai, Shuai Zhang, Jiajun Li, Xiubing Li, Congcong Li, Fengyi Guo, Heng Zhang, Ziqi Wang, Minhao Zhang, Xuefeng Wang, Huaiqiang Wang, Fengqi Song

Abstract: Berry curvature, as the imaginary component of quantum geometry, plays a crucial role in condensed matter physics. The spatial distribution of Berry curvature can be characterized by its dipole and multipole moments, which can induce the nonlinear anomalous Hall effect (NLAHE). To date, the NLAHE has been demonstrated in various materials, yet reports on room-temperature NLAHE are still limited. I… ▽ More Berry curvature, as the imaginary component of quantum geometry, plays a crucial role in condensed matter physics. The spatial distribution of Berry curvature can be characterized by its dipole and multipole moments, which can induce the nonlinear anomalous Hall effect (NLAHE). To date, the NLAHE has been demonstrated in various materials, yet reports on room-temperature NLAHE are still limited. In this work, we report the observation of the third-order NLAHE in ferromagnetic metal Fe3GaTe2. The third-order NLAHE shows hysteretic behavior with the variation of magnetic field, where the coercive field is the same as that of the anomalous Hall effect, and the third-order NLAHE remains observable up to the Curie temperature (~350 K). The scaling analysis suggests that the third-order NLAHE may be attributed to the Berry curvature quadrupole. Our work not only provides an approach to study magnetic materials through nonlinear electric transports, but also opens up possibilities for the future development of room-temperature third-order nonlinear electronic devices. △ Less

Submitted 23 April, 2026; originally announced April 2026.

A Sc2C2@C88 cluster based ultra-compact multi-level probabilistic bit for matrix multiplication

Authors: Haoran Qi, Guohao Xi, Yuan-Biao Zhou, Xinrong Liu, Yifu Mao, Jian Yang, Jun Chen, Kuojuei Hu, Weiwei Gao, Shuai Zhang, Xiaoqin Gao, Jianguo Wan, Da-Wei Zhou, Junhong An, Xuefeng Wang, De-Chuan Zhan, Minhao Zhang, Cong Wang, Wei ji, Yuan-Zhi Tan, Su-Yuan Xie, Fengqi Song

Abstract: Information units are progressively approaching the fundamental physical limits of the integration density, including in terms of extremely small sizes, multistates and probabilistic traversal. However, simultaneously encompassing all of these characteristics in a unit remains elusive. Here, via real-time in situ electrical monitoring, we clearly observed stochastic alterations of multiple conduct… ▽ More Information units are progressively approaching the fundamental physical limits of the integration density, including in terms of extremely small sizes, multistates and probabilistic traversal. However, simultaneously encompassing all of these characteristics in a unit remains elusive. Here, via real-time in situ electrical monitoring, we clearly observed stochastic alterations of multiple conductance states in Sc2C2@C88. The true random bit sequence generated exhibited an autocorrelation function whose confidence interval fell within \pm 0.02, demonstrating high-quality randomness. The alterations of multiple conductance states are controllable, that is, whose probability distributions could traverse from 0 to 1, enabling us to factorize 551 into its prime factors. Furthermore, we proposed a matrix-chain multiplication scheme and experimentally verified the multiplication of two 4 \times 4 state-transition matrices with a small maximum error < 0.05. Combined with theoretical calculations, the stochastic but controllable multistates are probably attributed to the rich energy landscape, which could be stepwise changed by the electric field. Our findings reveal extremely small multi-level probabilistic bit for matrix multiplication, which pave the way for ultracompact intelligent electronic devices. △ Less

Submitted 27 March, 2026; originally announced March 2026.

Boundary-sensitive non-Hermiticity of Floquet Hamiltonian: spectral transition and scale-free localization

Authors: Bo Li, He-Ran Wang, Fei Song

Abstract: We report a novel mechanism of boundary-sensitive PT symmetry breaking in one-dimensional Floquet systems. By designing a time-periodic driving protocol, we realize a Floquet Hamiltonian that is Hermitian under periodic boundary conditions yet acquires non-Hermitian boundary terms under open boundary conditions due to the non-commutativity of driving Hamiltonians. We establish that a PT symmetry b… ▽ More We report a novel mechanism of boundary-sensitive PT symmetry breaking in one-dimensional Floquet systems. By designing a time-periodic driving protocol, we realize a Floquet Hamiltonian that is Hermitian under periodic boundary conditions yet acquires non-Hermitian boundary terms under open boundary conditions due to the non-commutativity of driving Hamiltonians. We establish that a PT symmetry breaking transition occurs when the quasienergy bandwidth expands to cover the entire frequency Brillouin zone. This condition highlights a crucial difference from static non-Hermitian systems, where such transitions typically require band touching. Furthermore, we demonstrate that in the PT-broken phase, the eigenstates exhibit scale-free localization, a phenomenon arising from the specific system-size scaling of non-Hermitian terms. Finally, we provide a general framework for constructing multi-band models that exhibit this boundary-induced phase transition. △ Less

Submitted 23 March, 2026; originally announced March 2026.

Comments: 7 pages + 5 figures

Demonstration of robust chiral edge transport in Chern insulator MnBi2Te4 devices with engineered geometric defects

Authors: Pinyuan Wang, Jun Ge, Jiawei Luo, Xiaoqi Liu, Fucong Fei, Fengqi Song, Jian Wang

Abstract: Chiral edge states in Chern insulators are theoretically predicted to propagate unidirectionally along the sample boundary with inherent robustness against local perturbations, which manifests as the immunity to impurity-induced backscattering, a key factor for the development of robust, high-performance quantum devices. However, the direct experimental verification of the robustness of chiral edg… ▽ More Chiral edge states in Chern insulators are theoretically predicted to propagate unidirectionally along the sample boundary with inherent robustness against local perturbations, which manifests as the immunity to impurity-induced backscattering, a key factor for the development of robust, high-performance quantum devices. However, the direct experimental verification of the robustness of chiral edge states remains scarce. Here, we experimentally validate the robustness of the chiral edge states in MnBi2Te4 devices featuring engineered geometric defects introduced via atomic force microscope (AFM) nanomachining. Specifically, under a moderate perpendicular magnetic field, the MnBi2Te4 devices exhibit the Chern insulator state, characterized by a quantized Hall plateau and simultaneously vanishing longitudinal resistance. To verify the robustness of this topological state, we modify the device geometry by cutting a slit using AFM nanomachining that severs the original edge channel. Remarkably, the quantization behavior survives this drastic modification. The robust nature of the chiral edge transport is further confirmed by two-terminal, three-terminal and non-local measurements, fully demonstrating that the edge currents can bypass the artificial cut without dissipation. Our results unambiguously demonstrate the robustness of chiral edge states against geometric disruption and establish AFM nanomachining as a promising technique for topological quantum devices engineering. △ Less

Submitted 4 March, 2026; originally announced March 2026.

Ising Supercriticality and Universal Magnetocalorics in Spiral Antiferromagnet Nd$_3$BWO$_9$

Authors: Xinyang Liu, Enze Lv, Xueling Cui, Han Ge, Fangyuan Song, Zhaoming Tian, Gang Su, Kan Zhao, Junsen Xiang, Peijie Sun, Wei Li

Abstract: The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of a ferromagnet has long been a cornerstone for understanding universality of phase transitions and critical phenomena. Here we extend this analogy to a highly frustrated antiferromagnet, the spiral Ising compound Nd$_3$BWO$_9$ with kagome layers. In its phase diagr… ▽ More The celebrated analogy between the pressure-temperature phase diagram of a liquid-gas system and the field-temperature phase diagram of a ferromagnet has long been a cornerstone for understanding universality of phase transitions and critical phenomena. Here we extend this analogy to a highly frustrated antiferromagnet, the spiral Ising compound Nd$_3$BWO$_9$ with kagome layers. In its phase diagram, we identify a metamagnetic transition line with a critical endpoint (CEP) located at $μ_0H_{\mathrm{c}} \simeq 1.04$ T and $T_{\mathrm{c}} \simeq 0.3$ K. Above the CEP, an Ising supercritical regime emerges with crossover lines that follow a universal scaling law, as evidenced by the specific heat, magnetic susceptibility, and magnetocaloric measurements. Remarkably, we observe highly sensitive magnetic cooling near the emergent CEP, characterized by a divergent magnetic Grüneisen ratio $Γ_H \propto 1/t^{β+γ-1}$, with $β+ γ\simeq 1.563$ the sum of critical exponents of the 3D Ising universality class and $t \equiv (T-T_{\rm c})/T_{\rm c}$ the reduced temperature. Adiabatic demagnetization from 2 K and 4 T reaches a minimum temperature of 195 mK, via a self-cascading process that combines supercritical and topological cooling. Our findings open a new avenue for studying supercritical phenomena and magnetic refrigeration with the frustrated rare-earth compounds RE$_3$BWO$_9$ and, more broadly, in Ising-anisotropic antiferromagnets such as spin ices. △ Less

Submitted 17 June, 2026; v1 submitted 12 January, 2026; originally announced January 2026.

Comments: 5 pages, 4 figures

Stable boundary modes for fragile topology from spontaneous PT-symmetry breaking

Authors: Kang Yang, Fei Song, Piet W. Brouwer

Abstract: Two-dimensional topological insulators protected by nonlocal symmetries or with fragile topology usually do not admit robust in-gap edge modes due to the incompatibility between the symmetry and the boundary. Here, we show that in a parity-time (PT) symmetric system robust in-gap topological edge modes can be stably induced by non-Hermitian couplings that spontaneously break the PT symmetry of the… ▽ More Two-dimensional topological insulators protected by nonlocal symmetries or with fragile topology usually do not admit robust in-gap edge modes due to the incompatibility between the symmetry and the boundary. Here, we show that in a parity-time (PT) symmetric system robust in-gap topological edge modes can be stably induced by non-Hermitian couplings that spontaneously break the PT symmetry of the eigenstates. The topological edge modes traverse the imaginary spectral gap between a pair of fragile topological bands, which is opened by the presence of the non-Hermitian perturbation. We demonstrate that the net number of resulting in-gap modes is protected by an operator version of anomaly cancellation that extends beyond the Hermitian limit. The results imply that loss and gain can in principle drive fragile topological phenomena to stable topological phenomena. △ Less

Submitted 5 January, 2026; originally announced January 2026.

Comments: 5+6 pages

Observation of Magnetostatic Surface Spin Wave Solitons in Yttrium Iron Garnet Thin Film

Authors: Simin Pang, Zhengyi Li, Ziyu Wang, Yanpei Lv, Feilong Song, Peng Yan, Jun Zhang

Abstract: Magnetostatic surface spin wave (MSSW) solitons hold great promise for magnonic information processing, but their existence has long been debated. In this work, we resolve this issue by advanced time-resolved Brillouin light scattering (TR-BLS) spectroscopy. We observe long-period MSSW soliton trains in yttrium iron garnet (YIG) thin films by demonstrating their quasiparticle behavior, mode beatin… ▽ More Magnetostatic surface spin wave (MSSW) solitons hold great promise for magnonic information processing, but their existence has long been debated. In this work, we resolve this issue by advanced time-resolved Brillouin light scattering (TR-BLS) spectroscopy. We observe long-period MSSW soliton trains in yttrium iron garnet (YIG) thin films by demonstrating their quasiparticle behavior, mode beating, and periodic modulations. We reveal that the MSSW soliton originates from the dipole gap mechanism with a unique comb-like frequency spectrum caused by the four-magnon process. By varying the microwave power, we find significant changes in soliton periods linked to the spin wave dispersion renormalization. Additionally, we report exotic transverse solitons across the YIG thickness. These findings deepen the understanding of nonlinear physics, and pave the way for spin-wave-soliton-based information technologies. △ Less

Submitted 20 November, 2025; originally announced November 2025.

Comments: 11 pages, 9 figures

Exchange operation of Majorana zero modes in topological insulator-based Josephson trijunctions

Authors: Yunxiao Zhang, Zhaozheng Lyu, Xiang Wang, Yukun Shi, Duolin Wang, Xiaozhou Yang, Enna Zhuo, Bing Li, Yuyang Huang, Zenan Shi, Anqi Wang, Heng Zhang, Fucong Fei, Xiaohui Song, Peiling Li, Bingbing Tong, Ziwei Dou, Jie Shen, Guangtong Liu, Fanming Qu, Fengqi Song, Li Lu

Abstract: Majorana zero modes are anyons obeying non-Abelian exchange statistics distinct from fermions or bosons. While significant progresses have been achieved in the past two decades in searching for these exotic excitations in solid-state systems, their non-Abelian nature remains unverified, as definitive proof requires braiding operations. Here, we report preliminarily experimental advances in creatin… ▽ More Majorana zero modes are anyons obeying non-Abelian exchange statistics distinct from fermions or bosons. While significant progresses have been achieved in the past two decades in searching for these exotic excitations in solid-state systems, their non-Abelian nature remains unverified, as definitive proof requires braiding operations. Here, we report preliminarily experimental advances in creating, manipulating, and exchanging the presumed Majorana zero modes in an envelope-shaped Josephson device composed of multiple trijunctions on a topological insulator surface. We observed the signatures of in-gap states migration consistent with the expectations of the Fu-Kane model, supporting the realization of an exchange operation. This work would establish a critical pathway toward ultimately braiding Majorana zero modes in the Fu-Kane scheme of topological quantum computation. △ Less

Submitted 2 November, 2025; originally announced November 2025.

Comments: 4 pages, 4 figures

Field-free superconducting diode effect of NbSe2 induced by strain

Authors: Jiajun Li, Minhao Zou, Fengyi Guo, Dai Zheng, Yiying Zhang, Yu Du, Fuwei Zhou, Heng Zhang, Wuyi Qi, Tianqi Wang, YeFan Yu, Rui Wang, Fucong Fei, Hao Geng, Fengqi Song

Abstract: Superconducting diodes, similar to semiconductor diodes, possess unidirectional superconducting properties and are the fundamental units for constructing superconducting quantum computing, thus attracting widespread attention. At present, most of superconducting diodes require an external magnetic field or proximity effect to break time reversal symmetry (TRS). The cases of intrinsic superconducti… ▽ More Superconducting diodes, similar to semiconductor diodes, possess unidirectional superconducting properties and are the fundamental units for constructing superconducting quantum computing, thus attracting widespread attention. At present, most of superconducting diodes require an external magnetic field or proximity effect to break time reversal symmetry (TRS). The cases of intrinsic superconducting diode effect (SDE) under zero magnetic field are relatively scarce, and there are still some puzzles especially regarding the reasons for the TRS breaking. Here, we not only report field free SDE in NbSe2 induced by strain, but also large values of the difference of Ic+ and |Ic-| (ΔIc) of 286 μA and the superconducting diode efficiency (η) of 6.76 % are achieved. Interestingly, ΔIc varies with the magnetic field and exhibits two distinct evolutionary behaviors with B-odd or B-even symmetry in various devices. We attribute this to the selective activation of two independent, spatially-orthogonal mechanisms: a stress-induced real-space polarity and a field-induced reciprocal-space asymmetric energy bands. In general, we propose an extremely effectively method to produce field free SDE, even when the material itself does not possess field free SDE, and provide new perspectives to understand the SDE which build new avenues for superconducting quantum devices. △ Less

Submitted 28 September, 2025; originally announced September 2025.

Comments: 15 pages, 4 figures

Realization of large magnetocaloric effect in the Kagome antiferromagnet Gd3BWO9 for Sub-Kelvin cryogenic refrigeration

Authors: Fangyuan Song, Xinyang Liu, Chao Dong, Jin Zhou, Xinlong Shi, Yuyan Han, Langsheng Ling, Huifen Ren, Songliu Yuan, Shun Wang, Junsen Xiang, Peijie Sun, Zhaoming Tian

Abstract: Rare-earth (RE) based frustrated magnets have attracted great attention as excellent candidates for magnetic refrigeration at sub-Kelvin temperatures, while the experimental identification on systems exhibiting both large volumetric cooling capacity and reduced working temperatures far below 1 K remain to be a challenge. Here, through the ultra-low temperature magnetism and thermodynamic character… ▽ More Rare-earth (RE) based frustrated magnets have attracted great attention as excellent candidates for magnetic refrigeration at sub-Kelvin temperatures, while the experimental identification on systems exhibiting both large volumetric cooling capacity and reduced working temperatures far below 1 K remain to be a challenge. Here, through the ultra-low temperature magnetism and thermodynamic characterizations, we unveil the large magnetocaloric effect (MCE) realized at sub-Kelvin temperatures in the frustrated Kagome antiferromagnet Gd3BWO9 with TN~1.0 K. The isothermal magnetization curves indicate the existence of field (B) induced anisotropic magnetic phase diagrams, where four distinct magnetic phases for B // c-axis and five magnetic phases for B // ab-plane are identified at T< TN. The analysis of magnetic entropy S(B, T) data and direct adiabatic demagnetization tests reveal a remarkable cooling performance at sub-Kelvin temperatures featured by a large volumetric entropy density 502.2 mJ/K/cm3 and a low attainable minimal temperature Tmin~168 mK from the initial cooling condition of 2 K and 6 T, surpassing most of Gd-based refrigerants previously documented in temperature ranges of 0.25-4 K. The realized Tmin~168 mK far below TN ~ 1.0 K in Gd3BWO9 is related to the combined effects of magnetic frustration and criticality-enhanced MCE, which together leave a substantial magnetic entropy at reduced temperatures by enhancing spin fluctuations. △ Less

Submitted 14 September, 2025; originally announced September 2025.

Understanding the atomically precise evolution of the miscibility of newly prepared face-centered cubic W-Cu nanoalloys and its asymmetry

Authors: Yongxin Zhang, Weihan Zhang, Luneng Zhao, Zixiang Zhao, Siqi Lu, Yangrui Liu, Dongsheng Song, Changzheng Wei, Zhentao Pang, Yifeng Ren, Junfeng Gao, Weiwei Gao, Di Wu, Jijun Zhao, Kuo-Juei Hu, Wei Ji, Yu Deng, Binghui Ge, Fengqi Song

Abstract: According to classical Miedema theory, reducing crystals to the order of nanometer sizes might greatly modulate the mixing enthalpy of elements, thus enabling the invention of a lot of new bulk-immiscible alloys. Although numerous alloys with higher mixing enthalpies remain unexplored, this strategy is approaching its limit, as reflected by the critical diameter of recent alloys of 1.8 nm, which c… ▽ More According to classical Miedema theory, reducing crystals to the order of nanometer sizes might greatly modulate the mixing enthalpy of elements, thus enabling the invention of a lot of new bulk-immiscible alloys. Although numerous alloys with higher mixing enthalpies remain unexplored, this strategy is approaching its limit, as reflected by the critical diameter of recent alloys of 1.8 nm, which corresponds to ~150 atoms and hardly provides a crystalline order. Future development requires not only even smaller atomic-scale control but also a new surface energy-saving mechanism. Here, we report the formation of W-Cu nanoalloys with a very large miscibility gap in the bulk via the use of an atomically size-selected cluster beam source as an example. The face-centered cubic (FCC) structure was demonstrated through electron diffraction, which indicated a lattice constant of 3.88Å for W0.85Cu0.15 nanoalloys (~2280 atoms). In this comprehensive study that covers a large parameter space of W/Cu compositions and numbers of atoms, an asymmetric miscibility nanophase diagram in which W-rich compositions favor mixing and the critical size is approximately 6000 atoms, which far exceeds the approximately tens of atoms predicted via classical theory, was obtained for the first time. Density functional theory (DFT) calculations revealed a mutual strain-induced mechanism that simultaneously lowers the surface energies while reducing the size to the atomic scale. This approach paves the way for the development of new high-performance nonequilibrium phase alloys. △ Less

Submitted 28 August, 2025; originally announced August 2025.

Variational boundary based tensor network renormalization group

Authors: Feng-Feng Song, Naoki Kawashima

Abstract: We propose a real-space renormalization group algorithm for accurately coarse-graining two-dimensional tensor networks. The central innovation of our method lies in utilizing variational boundary tensors as a globally optimized environment for the entire system. Based on this optimized environment, we construct renormalization projectors that significantly enhance accuracy. By leveraging the canon… ▽ More We propose a real-space renormalization group algorithm for accurately coarse-graining two-dimensional tensor networks. The central innovation of our method lies in utilizing variational boundary tensors as a globally optimized environment for the entire system. Based on this optimized environment, we construct renormalization projectors that significantly enhance accuracy. By leveraging the canonical form of tensors, our algorithm maintains the same computational complexity as the original tensor renormalization group (TRG) method, yet achieves higher accuracy than existing approaches that do not incorporate entanglement filtering. Our work offers a practical pathway for extending TRG methods to higher dimensions while keeping computational costs manageable. △ Less

Submitted 14 August, 2025; originally announced August 2025.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 112, 235167 (2025)

Negative differential conductance in triangular molecular assemblies

Authors: Chao Li, Vladislav Pokorný, Prokop Hapala, Martin Žonda, Ping Zhou, Silvio Decurtins, Shi-Xia Liu, Fengqi Song, Rémy Pawlak, Ernst Meyer

Abstract: We report the creation and characterization of a molecular-scale negative differential conductance (NDC) device by assembling a triangular trimer of 4,5,9,10-tetrabromo-1,3,6,8-tetraazapyrene (TBTAP) molecules on a superconducting Pb(111) substrate. Using low-temperature scanning tunneling spectroscopy, we observe robust NDC behavior manifesting as a decrease in current with increasing voltage bet… ▽ More We report the creation and characterization of a molecular-scale negative differential conductance (NDC) device by assembling a triangular trimer of 4,5,9,10-tetrabromo-1,3,6,8-tetraazapyrene (TBTAP) molecules on a superconducting Pb(111) substrate. Using low-temperature scanning tunneling spectroscopy, we observe robust NDC behavior manifesting as a decrease in current with increasing voltage between 0.7-0.9 V arising from the interplay of Coulomb blockade and strong inter-molecular capacitive coupling within the molecular cluster. Gate-controlled charging and discharging processes are directly visualized via two-dimensional differential conductance mapping, which reveals the emergence of Coulomb rings and spatial regions of NDC. Theoretical modeling using a three-impurity Anderson model and master equation approach quantitatively reproduces the experimental observations and demonstrates that the NDC emerges purely from electron correlations, independent of the underlying superconductivity. By tuning the geometry to a hexamer structure, we further show that cluster topology provides versatile control over electronic properties at the molecular scale. These results establish a functional platform for implementing multifunctional molecular devices and highlight a strategy toward programmable and scalable nanoelectronics. △ Less

Submitted 7 August, 2025; originally announced August 2025.

Gap reopening as a possible signature of coupling between Majorana zero modes in Sn-(Bi,Sb)2(Te,S)3-based Josephson trijunctions

Authors: Duolin Wang, Xiang Zhang, Yunxiao Zhang, Heng Zhang, Fucong Fei, Xiang Wang, Bing Li, Xiaozhou Yang, Yukun Shi, Zhongmou Jia, Enna Zhuo, Yuyang Huang, Anqi Wang, Zenan Shi, Zhaozheng Lyu, Xiaohui Song, Peiling Li, Bingbing Tong, Ziwei Dou, Jie Shen, Guangtong Liu, Fanming Qu, Fengqi Song, Li Lu

Abstract: In the past two decades, enormous efforts have been made to search for possible platforms and schemes to implement topological quantum computation (TQC). In exploring the Fu-Kane scheme of TQC based on Josephson trijunctions constructed on topological insulators, the predicted Majorana phase diagram of an individual trijunction has already been verified experimentally. If Majorana zero modes indee… ▽ More In the past two decades, enormous efforts have been made to search for possible platforms and schemes to implement topological quantum computation (TQC). In exploring the Fu-Kane scheme of TQC based on Josephson trijunctions constructed on topological insulators, the predicted Majorana phase diagram of an individual trijunction has already been verified experimentally. If Majorana zero modes indeed exist in this kind of trijunction, coupling between them in multiple trijunction devices should be further expected. In this study, we fabricated Josephson devices containing two adjacent Josephson trijunctions on the surface of Sn-(Bi, Sb)2(Te, S)3 and observed a possible signature of the coupling effect manifesting as the reopening of a minigap in both trijunctions where a closure would otherwise be expected if the trijunctions existed individually. While alternative interpretations cannot be fully ruled out, our findings provide experimental support for the validity of the Fu-Kane theory and provide further motivation for advancing the TQC scheme proposed by Fu and Kane. △ Less

Submitted 18 June, 2026; v1 submitted 8 July, 2025; originally announced July 2025.

Comments: 4 figures

Journal ref: Physical Review B 113, 235424 (2026)

Superfluid dome in the spatially modulated two-dimensional XY model

Authors: Feng-Feng Song, Aditya Chugh, Hanggai Nuomin, Naoki Kawashima, Alexander Wietek

Abstract: In strongly correlated electron systems, superconductivity and charge density waves often coexist in close proximity, suggesting a deeper relationship between these competing phases. Recent research indicates that these orders can intertwine, with the superconducting order parameter coupling to modulations in the electronic density. To elucidate this interplay, we study a two-dimensional XY model… ▽ More In strongly correlated electron systems, superconductivity and charge density waves often coexist in close proximity, suggesting a deeper relationship between these competing phases. Recent research indicates that these orders can intertwine, with the superconducting order parameter coupling to modulations in the electronic density. To elucidate this interplay, we study a two-dimensional XY model with a periodic modulation of the coupling strength in one spatial direction. Using a combination of tensor network methods and Monte Carlo simulations, we reveal a non-monotonic, dome-like dependence of $T_c$ on the modulation wavelength, with the peak $T_c$ shifting to longer wavelengths as the modulation strength grows. The origin of this phenomenon is traced back to an effective pinning of vortices in the valleys of the modulation, confirmed by a comparison to modulated $q$-state clock models. These findings shed new light on the phase behavior of intertwined superconducting and charge-ordered states, offering a deeper understanding of their complex interactions. △ Less

Submitted 19 June, 2025; originally announced June 2025.

Comments: 7 pages, 5 figures + 14 pages, 9 figures

Journal ref: Phys. Rev. Lett. 135, 256001 (2025)

Synthesis of intrinsic magnetic topological insulator MnBi2nTe3n+1 family by chemical vapor transport method with feedback regulation

Authors: Heng Zhang, Yiying Zhang, Yong Zhang, Bo Chen, Jingwen Guo, Yu Du, Jiajun Li, Hangkai Xie, Zhixin Zhang, Fuwei Zhou, Tianqi Wang, Wuyi Qi, Xuefeng Wang, Fucong Fei, Fengqi Song

Abstract: MnBi2nTe3n+1 (MBT) is a representative family of intrinsic magnetic topological insulators, in which numerous exotic phenomena such as the quantum anomalous Hall effect are expected. The high-quality crystal growth and magnetism manipulation are the most essential processes. Here we develop a modified chemical vapor transport method using a feedback-regulated strategy, which provides the closed-lo… ▽ More MnBi2nTe3n+1 (MBT) is a representative family of intrinsic magnetic topological insulators, in which numerous exotic phenomena such as the quantum anomalous Hall effect are expected. The high-quality crystal growth and magnetism manipulation are the most essential processes. Here we develop a modified chemical vapor transport method using a feedback-regulated strategy, which provides the closed-loop control of growth temperature within +/- 0.1 degree Celsius. Single crystals of MnBi2Te4, MnBi4Te7, and MnBi6Te10 are obtained under different temperature intervals respectively, and show variable tunability on magnetism by finely tuning the growth temperatures. Specifically, the cold-end temperatures not only vary the strength of antiferromagnetic coupling in MnBi2Te4, but also induce magnetic ground state transitions from antiferromagnetism to ferromagnetism in MnBi4Te7 and MnBi6Te10. In MnBi2Te4 with optimized magnetism, quantized transport with Chern insulator state is also realized at the low field of 3.7 T. Our results provide a systematic picture for the crystal growth and the rich magnetic tunability of MBT family, providing richer platforms for the related researches combining magnetism and topological physics. △ Less

Submitted 19 May, 2025; v1 submitted 11 April, 2025; originally announced April 2025.

Journal ref: Adv. Mater. 2025, 2405686

Non-Bloch edge dynamics of non-Hermitian lattices

Authors: Wen-Tan Xue, Fei Song, Yu-Min Hu, Zhong Wang

Abstract: The non-Hermitian skin effect, i.e., the localization of nominally bulk modes, not only drastically reshapes the spectral properties of non-Hermitian systems, but also dramatically modifies the real-time dynamics therein. Here we investigate the time evolution of waves (or quantum-mechanical particles) initialized around the edge of non-Hermitian lattices. The non-Hermitian skin effect tends to lo… ▽ More The non-Hermitian skin effect, i.e., the localization of nominally bulk modes, not only drastically reshapes the spectral properties of non-Hermitian systems, but also dramatically modifies the real-time dynamics therein. Here we investigate the time evolution of waves (or quantum-mechanical particles) initialized around the edge of non-Hermitian lattices. The non-Hermitian skin effect tends to localize the wave to the edge, meaning that the real-time dynamics differs from the Bloch-theory picture. We focus on the long-time decay or growth rate of wave function, which is quantified by the Lyapunov exponents. These exponents can be obtained from the saddle points in the complex momentum space. We propose an efficient yet unambiguous criterion for identifying the dominant saddle point that determines the Lyapunov exponents. Our criterion can be precisely formulated in terms of a mathematical concept known as the Lefschetz thimble. Counterintuitively, the seemingly natural criterion based on the imaginary part of the energy fails. Our work provides a coherent theory for characterizing the real-time edge dynamics of non-Hermitian lattices. Our predictions are testable in various non-Hermitian physical platforms. △ Less

Submitted 17 March, 2025; originally announced March 2025.

Comments: 16 Pages, 10 figures

Discovery of unconventional charge-spin-intertwined density wave in magnetic kagome metal GdTi3Bi4

Authors: Xianghe Han, Hui Chen, Zhongyi Cao, Jingwen Guo, Fucong Fei, Hengxin Tan, Jianfeng Guo, Yanhao Shi, Runnong Zhou, Ruwen Wang, Zhen Zhao, Haitao Yang, Fengqi Song, Shiyu Zhu, Binghai Yan, Ziqiang Wang, Hong-Jun Gao

Abstract: The symmetry breaking and its interplay among spin, charge, and lattice degrees of freedom is crucial for understanding correlated quantum states such as charge density waves (CDWs) and unconventional superconductivity. Here, we report the discovery by low-temperature scanning tunneling microscopy/spectroscopy of unconventional charge-spin-intertwined density waves in magnetic kagome metal GdTi3Bi… ▽ More The symmetry breaking and its interplay among spin, charge, and lattice degrees of freedom is crucial for understanding correlated quantum states such as charge density waves (CDWs) and unconventional superconductivity. Here, we report the discovery by low-temperature scanning tunneling microscopy/spectroscopy of unconventional charge-spin-intertwined density waves in magnetic kagome metal GdTi3Bi4, which exhibits the one-third magnetization plateau. We reveal the emergence of 3Q CDWs incommensurate with the crystalline lattice in both periodicity and orientation, breaking all mirror and rotation symmetries. The CDW exhibits incommensurate-commensurate transitions in an applied magnetic field and transitions between 3Q and 1Q CDWs as a function of field and temperature, accompanied by changes in the spatial symmetries. Remarkably, the quantum and classic melting of the CDWs exhibits a phase structure which is consistent with the magnetization phase diagram of bulk GdTi3Bi4, providing strong evidence for the intertwined charge-spin density wave order. The origin of the charge-spin intertwinement is further evidenced by the observed hybridization between itinerant electrons and Gd local moments. Our findings uncover an unconventional form of charge-spin orders and offer new insights into a broad class of multi-components density wave formation in kagome and other correlated quantum materials. △ Less

Submitted 7 March, 2025; originally announced March 2025.

Phase coherence of charge-$6e$ superconductors via a frustrated Kagome XY antiferromagnet

Authors: Feng-Feng Song, Guang-Ming Zhang

Abstract: Recent experimental evidence for the charge-$6e$ condensed phase in kagome superconductors has generated significant interest. We investigate the unconventional superconductivity in the kagome superconductor $\mathrm{CsV_3Sb_5}$, focusing on the emergence of charge-$6e$ superconductivity (SC) at temperatures higher than the conventional charge-$2e$ SC state. By modeling the phase coherence of the… ▽ More Recent experimental evidence for the charge-$6e$ condensed phase in kagome superconductors has generated significant interest. We investigate the unconventional superconductivity in the kagome superconductor $\mathrm{CsV_3Sb_5}$, focusing on the emergence of charge-$6e$ superconductivity (SC) at temperatures higher than the conventional charge-$2e$ SC state. By modeling the phase coherence of the SC order parameter using a frustrated antiferromagnetic XY model on an emergent kagome lattice, we show that the condensation of fractional vortices with $1/3$ vorticity stabilizes phase coherence in $\exp(i3θ)$, giving rise to the charge-$6e$ SC state. Using a tensor network approach tailored for frustrated spin systems, we identify a Berezinskii-Kosterlitz-Thouless transition at $T_c/J \simeq 0.075$, where the unbinding of $1/3$ fractional vortex-antivortex pairs transforms the system from the charge-$6e$ SC phase to the normal phase. Below $T_c$, the $1/3$ fractional vortex correlations exhibit power-law decay, while the integer vortex correlations decay exponentially, reflecting the dominance of charge-$6e$ SC in the absence of charge-$2e$ SC. Our results provide a theoretical understanding of the charge-$6e$ SC in two-dimensional kagome superconductors, emphasizing the interplay between fractional vortices, frustration, and topology in stabilizing this exotic SC phase. △ Less

Submitted 24 February, 2025; originally announced February 2025.

Comments: 6 pages, 4 figures

Journal ref: Chin. Phys. Lett. 42, 037401 (2025)

Hartree-Fock approximation for bosons with symmetry-adapted variational wave functions

Authors: B. R. Que, J. M. Zhang, H. F. Song, Y. Liu

Abstract: The Hartree-Fock approximation for bosons employs variational wave functions that are a combination of permanents. These are bosonic counterpart of the fermionic Slater determinants, but with the significant distinction that the single-particle orbitals used to construct a permanent can be arbitrary and do not need to be orthogonal to each other. Typically, the variational wave function may break… ▽ More The Hartree-Fock approximation for bosons employs variational wave functions that are a combination of permanents. These are bosonic counterpart of the fermionic Slater determinants, but with the significant distinction that the single-particle orbitals used to construct a permanent can be arbitrary and do not need to be orthogonal to each other. Typically, the variational wave function may break the symmetry of the Hamiltonian, resulting in qualitative and quantitative errors in physical observables. A straightforward method to restore symmetry is projection after variation, where we project the variational wave function onto the desired symmetry sector. However, a more effective strategy is variation after projection, which involves first creating a symmetry-adapted variational wave function and then optimizing its parameters. We have devised a scheme to realize this strategy and have tested it on various models with symmetry groups ranging from $\mathbb{Z}_2$, $\text{C}_L$, to $\text{D}_L$. In all the models and symmetry sectors studied, the variational wave function accurately estimates not only the energy of the lowest eigenstate but also the single-particle correlation function, as it approximate the target eigenstate very well on the wave function level. We have applied this method to study few-body bound states, superfluid fraction, and Yrast lines of some Bose-Hubbard models. This approach should be valuable for studying few-body or mesoscopic bosonic systems. △ Less

Submitted 21 February, 2025; originally announced February 2025.

Comments: 22 pages, 12 figures

Journal ref: Physica A 664, 130449 (2025)

Large Anomalous Hall Effect in a Noncoplanar Magnetic Heterostructure

Authors: Anke Song, Jine Zhang, Yequan Chen, Zhizhong Zhang, Xinjuan Cheng, Ruijie Xu, Wenzhuo Zhuang, Wenxuan Sun, Yong Zhang, Xu Zhang, Zhongqiang Chen, Fengqi Song, Yue Zhang, Xuechao Zhai, Yongbing Xu, Weisheng Zhao, Rong Zhang, Xuefeng Wang

Abstract: The anomalous Hall effect (AHE) occurs in magnetic systems and also unexpectedly in non-magnetic materials adjacent to magnetic insulators via the heterointerface interactions. However, the AHE in heterostructures induced by magnetic proximity effect remains quite weak, restricting their practical device applications. Here, we report a large intrinsic AHE with a resistivity of 114 nΩ cm at 5 K in… ▽ More The anomalous Hall effect (AHE) occurs in magnetic systems and also unexpectedly in non-magnetic materials adjacent to magnetic insulators via the heterointerface interactions. However, the AHE in heterostructures induced by magnetic proximity effect remains quite weak, restricting their practical device applications. Here, we report a large intrinsic AHE with a resistivity of 114 nΩ cm at 5 K in noncoplanar magnetic heterostructures of Cr5Te6/Pt. This is the record-high AHE value among all the magnetic insulators/heavy metal heterostructures. A reversal of the AHE signal occurs due to the reconstruction of Berry curvature at the Fermi level, which is verified by the first-principles calculations. Topological spin textures at the interface are directly visualized via high-magnetic-field magnetic force microscopy, which accounts for the large AHE, as confirmed by the atomic simulations. These findings open a new avenue for exploring the large AHE in heterointerfaces and facilitate the potential applications in topological spintronic devices. △ Less

Submitted 17 January, 2025; v1 submitted 12 January, 2025; originally announced January 2025.

Comments: 23 pages, 15 figures

Journal ref: Adv. Funct. Mater. 35, 2422040 (2025)

Emergent Intermediate Phase in the $J_1$-$J_2$ XY model from Tensor Network Approaches

Authors: Feng-Feng Song, Hanggai Nuomin, Naoki Kawashima

Abstract: We investigate the finite-temperature phase diagram of the classical $J_1$-$J_2$ XY model on a square lattice using a tensor network approach designed for frustrated spin systems. This model, characterized by competing nearest-neighbor and next-to-nearest-neighbor interactions, exhibits a complex interplay between $U(1)$ and $Z_2$ symmetries. Our study reveals an emergent intermediate phase around… ▽ More We investigate the finite-temperature phase diagram of the classical $J_1$-$J_2$ XY model on a square lattice using a tensor network approach designed for frustrated spin systems. This model, characterized by competing nearest-neighbor and next-to-nearest-neighbor interactions, exhibits a complex interplay between $U(1)$ and $Z_2$ symmetries. Our study reveals an emergent intermediate phase around $J_2/J_1 \sim 0.505$, which is characterized by a $Z_2$ long-range stripe order without phase coherence in the XY spins. The intermediate phase features two well-separated phase transitions: a higher-temperature Ising transition and a lower-temperature Berezinskii-Kosterlitz-Thouless transition. The relative separation between these transitions is significantly larger than previously reported, enabling a clearer investigation of their distinct thermodynamic properties. For $0.5<J_2/J_1 < 0.501$, two transitions merge into a single first-order phase transition, a phenomenon that cannot be explained solely by mapping to the Ising-XY model. As $J_2/J_1 \to \infty$, the transition evolves continuously into the BKT universality class. These findings advance the understanding of the mechanisms driving phase transitions in frustrated spin systems and suggest potential experimental realizations in platforms such as ultracold atoms, Josephson junction arrays, and optical lattices. △ Less

Submitted 25 December, 2024; originally announced December 2024.

Comments: 12 pages, 9 figures

Journal ref: Phys. Rev. B 111, 104428 (2025)

Exciton Enhanced Giant Correlated Stoke AntiStokes Scattering of Multiorder Phonons in Semiconductor

Authors: Jia-Min Lai, Haonan Chang, Feilong Song, Xiaohong Xu, Ping-Heng Tan, Jun Zhang

Abstract: The correlated Stoke antiStokes (SaS) scattering plays a crucial role in quantum information processing, such as heralded light sources, Fock state dynamics, and write read protocol for quantum memory. However, several reported materials exhibit low degree of SaS correlation and require high-power pulse laser excitation, limiting further applications. Herein, we explore the giant correlated multio… ▽ More The correlated Stoke antiStokes (SaS) scattering plays a crucial role in quantum information processing, such as heralded light sources, Fock state dynamics, and write read protocol for quantum memory. However, several reported materials exhibit low degree of SaS correlation and require high-power pulse laser excitation, limiting further applications. Herein, we explore the giant correlated multiorder SaS scattering under low power continuous laser excitation through red-sideband resonance of exciton in semiconductor ZnTe nanobelts. At low temperatures, we observe an unexpectedly strong anti-Stokes signal for multiorder longitudinal optical phonons, with SaS correlations two or four orders of magnitude larger than reported results. Furthermore, we observed the mitigation of laser heating effect for longitudinal optical phonon in SaS scattering. This finding paves a new pathway to study multiorder quantum correlated photon pairs produced through exciton-resonant Raman scattering. △ Less

Submitted 13 November, 2024; originally announced November 2024.

Fragile non-Bloch spectrum and unconventional Green's function

Authors: Fei Song, Hong-Yi Wang, Zhong Wang

Abstract: In non-Hermitian systems, it is a counterintuitive feature of the non-Hermitian skin effect (NHSE) that the energy spectrum and eigenstates can be totally different under open or periodic boundary conditions, suggesting that non-Hermitian spectra can be extremely sensitive to non-local perturbations. Here, we show that a wide range of non-Hermitian models with NHSE can even be highly sensitive to… ▽ More In non-Hermitian systems, it is a counterintuitive feature of the non-Hermitian skin effect (NHSE) that the energy spectrum and eigenstates can be totally different under open or periodic boundary conditions, suggesting that non-Hermitian spectra can be extremely sensitive to non-local perturbations. Here, we show that a wide range of non-Hermitian models with NHSE can even be highly sensitive to local perturbation under open boundary conditions. The spectrum of these models is so fragile that it can be significantly modified by adding only exponentially small perturbations on boundaries. Intriguingly, we show that such fragile spectra are quantified by the Green's function exhibiting unconventional V-shape asymptotic behaviors. Accordingly, bi-directional exponential amplification can be observed. As an interesting consequence, we find a real-to-complex transition of the bulk spectrum induced by exponentially small boundary perturbations. Finally, we reveal a hierarchy of the asymptotic behaviors of non-Hermitian Green's functions, which restricts the frequency range for the presence of unconventional Green's functions. △ Less

Submitted 30 October, 2024; originally announced October 2024.

Comments: 7 pages, 3 figures. Supplemental Matriel will be added to the next version

Structure and magnetic properties of a family of two-leg spin ladder compounds Ba2RE2Ge4O13 (RE = Pr, Nd, and Gd-Ho) with strong rung interaction

Authors: Jin Zhou, Andi Liu, Fangyuan Song, Langsheng Ling, Jingxin Li, Wei Tong, Zhengcai Xia, Gaoshang Gong, Yongqiang Wang, Jinkui Zhao, Hanjie Guo, Zhaoming Tian

Abstract: Compared to the intensive investigation on the 3d transition-metal (TM)-based spin ladder compounds, less attention has been paid to the ones constructed by the rare-earth (RE) ions. Herein, we report a family of RE-based spin ladder compounds Ba2RE2Ge4O13 (RE = Pr, Nd, Gd-Ho) crystallized into the monoclinic structure with the space group C2/c. The RE ions are arranged on a two-leg spin ladder mo… ▽ More Compared to the intensive investigation on the 3d transition-metal (TM)-based spin ladder compounds, less attention has been paid to the ones constructed by the rare-earth (RE) ions. Herein, we report a family of RE-based spin ladder compounds Ba2RE2Ge4O13 (RE = Pr, Nd, Gd-Ho) crystallized into the monoclinic structure with the space group C2/c. The RE ions are arranged on a two-leg spin ladder motif along the b-axis, where the rung and leg exchange interactions are bridged via the RE-O-RE pathways and RE-O-Ge-O-RE routes, respectively. Moreover, the much shorter rung distance in the RE2O12 dimer units than the leg distance suggests Ba2RE2Ge4O13 to be a strong-rung spin ladder system. All the synthesized Ba2RE2Ge4O13 (RE = Pr, Nd, Gd-Ho) compounds exhibit the dominant antiferromagnetic (AFM) interactions and absence of magnetic order down to 1.8 K. Among the family members, Ba2Dy2Ge4O13 can be described by Jeff = 1/2 Kramers doublet states, the low temperature specific heat indicates the coexistence of spin dimerized state with broad maximum at ~ 2.4 K and long-range AFM order with TN = 0.81 K. This family of Ba2RE2Ge4O13 compounds thereby provides a rare platform to investigate the novel spin ladder physics constructed by 4f electrons. △ Less

Submitted 7 November, 2024; v1 submitted 15 September, 2024; originally announced September 2024.

Many-body Liouvillian dynamics with a non-Hermitian tensor-network kernel polynomial algorithm

Authors: Guangze Chen, Jose L. Lado, Fei Song

Abstract: Understanding the dynamics of open quantum many-body systems is a major problem in quantum matter. Specifically, efficiently solving the spectrum of the Liouvillian superoperator governing such dynamics remains a critical open challenge. Here, we put forward a method for solving the many-body Liouvillian spectrum and dynamics based on the non-Hermitian kernel polynomial method and tensor-network t… ▽ More Understanding the dynamics of open quantum many-body systems is a major problem in quantum matter. Specifically, efficiently solving the spectrum of the Liouvillian superoperator governing such dynamics remains a critical open challenge. Here, we put forward a method for solving the many-body Liouvillian spectrum and dynamics based on the non-Hermitian kernel polynomial method and tensor-network techniques. We demonstrate the faithfulness of our method by computing the dynamics of the dephasing quantum compass model with a gradient magnetic field and comparing it with exact results. In particular, we show that our method allows us to characterize the quantum Zeno crossover and the reduction of relaxation rate due to Stark localization in this model. We further demonstrate the ability of our method to go beyond exact results by exploring nearest-neighbor interaction effects on the Liouvillian dynamics, elucidating the interplay between Stark localization and many-body interactions. Our method provides an efficient solution to many-body Liouvillian spectrum and dynamics, establishing a methodology to explore large open quantum many-body systems. △ Less

Submitted 24 November, 2024; v1 submitted 8 July, 2024; originally announced July 2024.

Comments: 11 pages, 7 figures. Source codes are available at https://github.com/GUANGZECHEN/NHKPM.jl

Journal ref: Phys. Rev. Research 6, 043182 (2024)

Theory of charge-6e condensed phase in Kagome lattice superconductors

Authors: Tong-Yu Lin, Feng-Feng Song, Guang-Ming Zhang

Abstract: We develop a Ginzburg-Landau theory for commensurate pair density wave (PDW) states in a hexagonal lattice system, relevant to the kagome superconductors $\rm{AV_3Sb_5}$. Compared to previous theoretical frameworks, the commensurate wave vectors permit additional symmetric terms in the free energy, altering the system's ground state and its degeneracy. In particular, we analyze topological defects… ▽ More We develop a Ginzburg-Landau theory for commensurate pair density wave (PDW) states in a hexagonal lattice system, relevant to the kagome superconductors $\rm{AV_3Sb_5}$. Compared to previous theoretical frameworks, the commensurate wave vectors permit additional symmetric terms in the free energy, altering the system's ground state and its degeneracy. In particular, we analyze topological defects in the energetically favorable $ψ_{\text{kagome}}$ ground state and find that kinks on domain walls can carry $1/3$ topological charges. We further establish a correspondence between the SC fluctuations in these states and an effective $J_1-J_2$ frustrated XY model on the emergent kagome lattice. By employing a state-of-the-art numerical tensor network method, we rigorously solve this effective model at finite temperatures and confirm the existence of a vestigial phase characterized by $1/3$ vortex-antivortex pairs in low temperatures with the absence of phase coherence of Cooper pairs, which is dual to the charge-$6e$ condensed phase. Our theory provides a potential explanation for the vestigial charge-$6e$ magnetoresistance oscillations observed in recent experiments [J. Ge, et. al., Phys. Rev. X 14, 021025 (2024)]. △ Less

Submitted 25 November, 2024; v1 submitted 17 June, 2024; originally announced June 2024.

Comments: 16 pages, 11 figures; revised version and another author included

Journal ref: Physical Review B, 111, 054508 (2025)

Nonlinear Hall effect and scaling law in Sb-doped topological insulator MnBi4Te7

Authors: Shaoyu Wang, Xiubing Li, Heng Zhang, Bo Chen, Hangkai Xie, Congcong Li, Fucong Fei, Shuai Zhang, Fengqi Song

Abstract: Nonlinear Hall effect (NLHE), as a new member of Hall effect family, has been realized in many materials, attracting a great deal of attention. Here, we report the observation of NLHE in magnetic topological insulator Sb-doped MnBi4Te7 flakes. The NLHE generation efficiency can reach up to 0.06 V^-1, which is comparable to that observed in MnBi2Te4. Differently, the NLHE can survive up to 200 K, m… ▽ More Nonlinear Hall effect (NLHE), as a new member of Hall effect family, has been realized in many materials, attracting a great deal of attention. Here, we report the observation of NLHE in magnetic topological insulator Sb-doped MnBi4Te7 flakes. The NLHE generation efficiency can reach up to 0.06 V^-1, which is comparable to that observed in MnBi2Te4. Differently, the NLHE can survive up to 200 K, much larger than the magnetic transition temperature. We further study the scaling behavior of the NLHE with longitudinal conductivity. The linear relationship with opposite slope when temperature is below and above the magnetic transition temperature is uncovered. It reveals that the NLHE originates from skew scattering. Our work provides a platform to search NLHE with larger generation efficiency at higher temperatures. △ Less

Submitted 9 April, 2024; originally announced April 2024.

Journal ref: Appl. Phys. Lett. 124, 153102 (2024)

Even-Odd Layer-Dependent Exchange Bias Effect in MnBi2Te4 Chern Insulator Devices

Authors: Bo Chen, Xiaoda Liu, Yu-Hang Li, Han Tay, Takashi Taniguchi, Kenji Watanabe, Moses. H. W. Chan, Jiaqiang Yan, Fengqi Song, Ran Cheng, Cui-Zu Chang

Abstract: Magnetic topological materials with coexisting magnetism and non-trivial band structures exhibit many novel quantum phenomena, including the quantum anomalous Hall effect, the axion insulator state, and the Weyl semimetal phase. As a stoichiometric layered antiferromagnetic topological insulator, thin films of MnBi2Te4 show fascinating even-odd layer-dependent physics. In this work, we fabricate a… ▽ More Magnetic topological materials with coexisting magnetism and non-trivial band structures exhibit many novel quantum phenomena, including the quantum anomalous Hall effect, the axion insulator state, and the Weyl semimetal phase. As a stoichiometric layered antiferromagnetic topological insulator, thin films of MnBi2Te4 show fascinating even-odd layer-dependent physics. In this work, we fabricate a series of thin-flake MnBi2Te4 devices using stencil masks and observe the Chern insulator state at high magnetic fields and a square hysteresis loop near zero magnetic field in all these devices. Upon magnetic field training, a large exchange bias effect is observed in odd but not in even septuple layer (SL) devices. Our theoretical calculations interpret this even-odd layer-dependent exchange bias effect as a consequence of contrasting surface and bulk magnetic properties of MnBi2Te4 devices. Our findings reveal the microscopic magnetic configuration of MnBi2Te4 thin flakes and highlight the challenges in replicating the zero magnetic field quantum anomalous Hall effect in odd SL MnBi2Te4 devices. △ Less

Submitted 3 April, 2024; originally announced April 2024.

Comments: 23 pages, 4 figures, comments are very much welcome

Journal ref: Nano Lett. 24, 8320 (2024)

Electrically controlled nonvolatile switching of single-atom magnetism in a Dy@C84 single-molecule transistor

Authors: Feng Wang, Wangqiang Shen, Yuan Shui, Jun Chen, Huaiqiang Wang, Rui Wang, Yuyuan Qin, Xuefeng Wang, Jianguo Wan, Minhao Zhang, Xing Lu, Tao Yang, Fengqi Song

Abstract: Single-atom magnetism switching is a key technique towards the ultimate data storage density of computer hard disks and has been conceptually realized by leveraging the spin bistability of a magnetic atom under a scanning tunnelling microscope. However, it has rarely been applied to solid-state transistors, an advancement that would be highly desirable for enabling various applications. Here, we d… ▽ More Single-atom magnetism switching is a key technique towards the ultimate data storage density of computer hard disks and has been conceptually realized by leveraging the spin bistability of a magnetic atom under a scanning tunnelling microscope. However, it has rarely been applied to solid-state transistors, an advancement that would be highly desirable for enabling various applications. Here, we demonstrate realization of the electrically controlled Zeeman effect in Dy@C84 single-molecule transistors, thus revealing a transition in the magnetic moment from 3.8 μB to 5.1 μB for the ground-state GN at an electric field strength of 3-10 MV/cm. The consequent magnetoresistance significantly increases from 600% to 1100% at the resonant tunneling point. Density functional theory calculations further corroborate our realization of nonvolatile switching of single-atom magnetism, and the switching stability emanates from an energy barrier of 92 meV for atomic relaxation. These results highlight the potential of using endohedral metallofullerenes for high-temperature, high-stability, high-speed, and compact single-atom magnetic data storage. △ Less

Submitted 17 March, 2024; originally announced March 2024.

Comments: 26 pages, 4 figures

Journal ref: Nature Communications (2024)

Light-induced giant enhancement of nonreciprocal transport at KTaO3-based interfaces

Authors: Xu Zhang, Tongshuai Zhu, Shuai Zhang, Zhongqiang Chen, Anke Song, Chong Zhang, Rongzheng Gao, Wei Niu, Yequan Chen, Fucong Fei, Yilin Tai, Guoan Li, Binghui Ge, Wenkai Lou, Jie Shen, Haijun Zhang, Kai Chang, Fengqi Song, Rong Zhang, Xuefeng Wang

Abstract: Nonlinear transport is a unique functionality of noncentrosymmetric systems, which reflects profound physics, such as spin-orbit interaction, superconductivity and band geometry. However, it remains highly challenging to enhance the nonreciprocal transport for promising rectification devices. Here, we observe a light-induced giant enhancement of nonreciprocal transport at the superconducting and e… ▽ More Nonlinear transport is a unique functionality of noncentrosymmetric systems, which reflects profound physics, such as spin-orbit interaction, superconductivity and band geometry. However, it remains highly challenging to enhance the nonreciprocal transport for promising rectification devices. Here, we observe a light-induced giant enhancement of nonreciprocal transport at the superconducting and epitaxial CaZrO3/KTaO3 (111) interfaces. The nonreciprocal transport coefficient undergoes a giant increase with three orders of magnitude up to 105 A-1T-1. Furthermore, a strong Rashba spin-orbit coupling effective field of 14.7 T is achieved with abundant high-mobility photocarriers under ultraviolet illumination, which accounts for the giant enhancement of nonreciprocal transport coefficient. Our first-principles calculations further disclose the stronger Rashba spin-orbit coupling strength and the longer relaxation time in the photocarrier excitation process, bridging the light-property quantitative relationship. Our work provides an alternative pathway to boost nonreciprocal transport in noncentrosymmetric systems and facilitates the promising applications in opto-rectification devices and spin-orbitronic devices. △ Less

Submitted 7 March, 2024; originally announced March 2024.

Comments: 38 pages, 17 figures

Journal ref: Nature Communications (2024)

Phase Transition of single-layer vanadium diselenide on Au(111) with distinguished electronic structures

Authors: Jinbang Hu, Xiansi Wang, Chaoqin Huang, Fei Song, Justin W Wells

Abstract: Herein, we report the reversible structural transition of single-layer VSe2 grown on Au(111) through alternating thermal annealing and Se replenishment. Using scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we demonstrate the epitaxial growth of high-quality VSe2 on Au(111) with the octahedral (1T) structure and the Se-vacancy-induced transformation of VS… ▽ More Herein, we report the reversible structural transition of single-layer VSe2 grown on Au(111) through alternating thermal annealing and Se replenishment. Using scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), we demonstrate the epitaxial growth of high-quality VSe2 on Au(111) with the octahedral (1T) structure and the Se-vacancy-induced transformation of VSe2 from the metallic moiré (1T) phase to the semiconducting (2H) phase. With convincing agreement between the experimental results and DFT calculations, the nanostructure near the grain boundary in the defective intermediate phase is confirmed, as well as the reaction pathway with Se gradually depleting at elevated temperatures. Importantly, it is revealed that the density of the linear Se defects plays a crucial role in the formation of the 2H domain phase due to the increment of the in-plane lattice parameter after Se desorption and the better thermal stability of the 2H phase compared to the 1T phase. The proper control of the density of Se atoms in the topmost Se layer of VSe2 could feasibly manipulate the ratio between the 1T phase and the 2H phase in the steak-shaped domain, which is regarded as a good platform for 2D homojunctions in nanoelectronics. △ Less

Submitted 18 June, 2024; v1 submitted 30 January, 2024; originally announced January 2024.

Comments: 17 pages

Observation of a 1/3 Magnetisation Plateau Phase as Evidence for the Kitaev Interaction in a Honeycomb-Lattice Antiferromagnet

Authors: Yanyan Shangguan, Song Bao, Zhao-Yang Dong, Ning Xi, Yi-Peng Gao, Zhen Ma, Wei Wang, Zhongyuan Qi, Shuai Zhang, Zhentao Huang, Junbo Liao, Xiaoxue Zhao, Bo Zhang, Shufan Cheng, Hao Xu, Dehong Yu, Richard A. Mole, Naoki Murai, Seiko Ohira-Kawamura, Lunhua He, Jiazheng Hao, Qing-Bo Yan, Fengqi Song, Wei Li, Shun-Li Yu , et al. (2 additional authors not shown)

Abstract: Fractional magnetisation plateaus, in which the magnetisation is pinned at a fraction of its saturated value within a range of external magnetic field, are spectacular macroscopic manifestations of the collective quantum behaviours. One prominent example of the plateau phase is found in spin-1/2 triangular-lattice antiferromagnets featuring strong geometrical frustration, and is often interpreted… ▽ More Fractional magnetisation plateaus, in which the magnetisation is pinned at a fraction of its saturated value within a range of external magnetic field, are spectacular macroscopic manifestations of the collective quantum behaviours. One prominent example of the plateau phase is found in spin-1/2 triangular-lattice antiferromagnets featuring strong geometrical frustration, and is often interpreted as quantum-fluctuation-stabilised state in magnetic field via the "order-by-disorder" mechanism. Here, we observe an unprecedented 1/3 magnetisation plateau between 5.2 and 7.4 T at 2 K in a spin-1 antiferromagnet Na$_3$Ni$_2$BiO$_6$ with a honeycomb lattice, where conventionally no geometrical frustration is anticipated. By carrying out elastic neutron scattering measurements, we propose the spin structure of the plateau phase to be an unusual partial spin-flop ferrimagnetic order, transitioning from the zigzag antiferromagnetic order in zero field. Our theoretical calculations show that the plateau phase is stabilised by the bond-anisotropic Kitaev interaction. These results provide a new paradigm for the exploration of rich quantum phases in frustrated magnets and exotic Kitaev physics in high-spin systems. △ Less

Submitted 26 December, 2023; originally announced December 2023.

Comments: Submitted version, 10 pages, 5 figures. Final version has been published in Nature Physics

Journal ref: Nature Physics 19, 1883-1889 (2023)

Ba6RE2Ti4O17 (RE= Nd, Sm,Gd, Dy-Yb): A family of Rare-earth based layered triangular lattice magnets

Authors: Fangyuan Song, Andi Liu, Qiao Chen, Jin Zhou, Jingxin Li, Wei Tong, Shun Wang, Yanhong Wang, Hongcheng Lu, Songliu Yuan, Hanjie Guo, Zhaoming Tian

Abstract: Rare-earth-based triangular-lattice magnets provide the fertile ground to explore the exotic quantum magnetic state. Herein, we report a new family of RE-based triangular-lattice magnets Ba6RE2Ti4O17(RE= rare earth ions) crystallized into the hexagonal structure with space group of P63 mmc, where magnetic rare earth ions form an ideal triangular lattice within the ab-plane and stack in an AA -type… ▽ More Rare-earth-based triangular-lattice magnets provide the fertile ground to explore the exotic quantum magnetic state. Herein, we report a new family of RE-based triangular-lattice magnets Ba6RE2Ti4O17(RE= rare earth ions) crystallized into the hexagonal structure with space group of P63 mmc, where magnetic rare earth ions form an ideal triangular lattice within the ab-plane and stack in an AA -type fashion along the c-axis. The low-temperature magnetic susceptibility results reveal all the serial compounds have the dominant antiferromagnetic interactions and an absence of magnetic ordering down to 1.8 K. The magnetization and electron spin resonance results indicate distinct magnetic anisotropy for the compounds with different RE ions. Moreover, Ba6Nd2Ti4O17 single crystal is successfully grown and it exhibits strong Ising like anisotropy with magnetic easy-axis perpendicular to the triangle-lattice plane, being a candidate to explore quantum spin liquid state with dominant Ising-type interaction. △ Less

Submitted 8 March, 2024; v1 submitted 15 November, 2023; originally announced November 2023.

Comments: 20 pages, 8 figures

Magnetic-field tuned anisotropic quantum phase transition in the distorted kagome antiferromagnet Nd3BWO9

Authors: Fangyuan song, Han Ge, Andi Liu, Yuqi Qin, Yuyan Han, Langsheng Ling, Songliu Yuan, Zhongwen Ouyang, Jieming Sheng, Liusuo Wu, Zhaoming Tian

Abstract: Rare-earth (RE) kagome-lattice magnets offer an excellent platform to discover the novel magnetic phase as well as quantum phase transition tuned by non-thermal control parameters, while the experimental realizations remain largely unexplored. Here, we report the discovery of magnetic-field (B)-induced anisotropic quantum phase transition in a distorted kagome antiferromagnet Nd3BWO9 with TN~0.32… ▽ More Rare-earth (RE) kagome-lattice magnets offer an excellent platform to discover the novel magnetic phase as well as quantum phase transition tuned by non-thermal control parameters, while the experimental realizations remain largely unexplored. Here, we report the discovery of magnetic-field (B)-induced anisotropic quantum phase transition in a distorted kagome antiferromagnet Nd3BWO9 with TN~0.32 K. The isothermal magnetizations at 0.05 K exhibit the spin-flop like metamagnetic crossover behaviors with different fractional magnetization anomalies for B perpendicular (B // c-axis) and parallel (B // a*-axis) to the kagome plane, respectively. In combination with the thermodynamic measurements, the field-temperature (B-T) phase diagrams for both field directions are constructed and that reveal the existence of several field-induced magnetic states. Along the c-axis, a proximate quantum bicritical point is observed near the metamagnetic crossover, which separates the low-field antiferromagnetic (AFM) phase and the intermediate AFM phase. While, for B // a*, another intermediate magnetic phase (IAFM2) appears between the low-field AFM phase and intermediate AFM (IAFM1) phase, giving rise to a tetracritical point. These results support the anisotropic field-induced metamagnetic quantum criticalities in Nd3BWO9, making it as a rare kagome antiferromagnet to investigate the quantum multi-criticality driven by spin frustration. △ Less

Submitted 7 November, 2023; originally announced November 2023.

Comments: 22 pages, 7 figures

MSC Class: 81-05 ACM Class: E.1

Defect-induced helicity-dependent terahertz emission in Dirac semimetal PtTe2 thin films

Authors: Zhongqiang Chen, Hongsong Qiu, Xinjuan Cheng, Jizhe Cui, Zuanming Jin, Da Tian, Xu Zhang, Kankan Xu, Ruxin Liu, Wei Niu, Liqi Zhou, Tianyu Qiu, Yequan Chen, Caihong Zhang, Xiaoxiang Xi, Fengqi Song, Rong Yu, Xuechao Zhai, Biaobing Jin, Rong Zhang, Xuefeng Wang

Abstract: Nonlinear transport enabled by symmetry breaking in quantum materials has aroused considerable interest in condensed matter physics and interdisciplinary electronics. However, the nonlinear optical response in centrosymmetric Dirac semimetals via the defect engineering has remained highly challenging. Here, we observe the helicity-dependent terahertz (THz) emission in Dirac semimetal PtTe2 thin fi… ▽ More Nonlinear transport enabled by symmetry breaking in quantum materials has aroused considerable interest in condensed matter physics and interdisciplinary electronics. However, the nonlinear optical response in centrosymmetric Dirac semimetals via the defect engineering has remained highly challenging. Here, we observe the helicity-dependent terahertz (THz) emission in Dirac semimetal PtTe2 thin films via circular photogalvanic effect (CPGE) under normal incidence. This is activated by artificially controllable out-of-plane Te-vacancy defect gradient, which is unambiguously evidenced by the electron ptychography. The defect gradient lowers the symmetry, which not only induces the band spin splitting, but also generates the giant Berry curvature dipole (BCD) responsible for the CPGE. Such BCD-induced helicity-dependent THz emission can be manipulated by the Te-vacancy defect concentration. Furthermore, temperature evolution of the THz emission features the minimum of the THz amplitude due to the carrier compensation. Our work provides a universal strategy for symmetry breaking in centrosymmetric Dirac materials for efficient nonlinear transport and facilitates the promising device applications in integrated optoelectronics and spintronics. △ Less

Submitted 1 March, 2024; v1 submitted 15 October, 2023; originally announced October 2023.

Comments: 27 pages, 5 figures

Journal ref: Nature Communications (2024)

Unified tensor network theory for frustrated classical spin models in two dimensions

Authors: Feng-Feng Song, Tong-Yu Lin, Guang-Ming Zhang

Abstract: Frustration is a ubiquitous phenomenon in many-body physics that influences the nature of the system in a profound way with exotic emergent behavior. Despite its long research history, the analytical or numerical investigations on frustrated spin models remain a formidable challenge due to their extensive ground state degeneracy. In this work, we propose a unified tensor network theory to numerica… ▽ More Frustration is a ubiquitous phenomenon in many-body physics that influences the nature of the system in a profound way with exotic emergent behavior. Despite its long research history, the analytical or numerical investigations on frustrated spin models remain a formidable challenge due to their extensive ground state degeneracy. In this work, we propose a unified tensor network theory to numerically solve the frustrated classical spin models on various two-dimensional (2D) lattice geometry with high efficiency. We show that the appropriate encoding of emergent degrees of freedom in each local tensor is of crucial importance in the construction of the infinite tensor network representation of the partition function. The frustrations are thus relieved through the effective interactions between emergent local degrees of freedom. Then the partition function is written as a product of a one-dimensional (1D) transfer operator, whose eigen-equation can be solved by the standard algorithm of matrix product states rigorously, and various phase transitions can be accurately determined from the singularities of the entanglement entropy of the 1D quantum correspondence. We demonstrated the power of our unified theory by numerically solving 2D fully frustrated XY spin models on the kagome, square and triangular lattices, giving rise to a variety of thermal phase transitions from infinite-order Brezinskii-Kosterlitz-Thouless transitions, second-order transitions, to first-order phase transitions. Our approach holds the potential application to other types of frustrated classical systems like Heisenberg spin antiferromagnets. △ Less

Submitted 11 September, 2023; originally announced September 2023.

Comments: 20 pages, 19 figures

Journal ref: Physical Review B 108, 224404 (2023)

Magnetic kagome materials RETi3Bi4 family with weak interlayer interactions

Authors: Jingwen Guo, Liqin Zhou, Jianyang Ding, Gexing Qu, Zhengtai Liu, Yu Du, Heng Zhang, Jiajun Li, Yiying Zhang, Fuwei Zhou, Wuyi Qi, Fengyi Guo, Tianqi Wang, Fucong Fei, Yaobo Huang, Tian Qian, Dawei Shen, Hongming Weng, Fengqi Song

Abstract: Kagome materials have attracted a surge of research interest recently, especially for the ones combining with magnetism, and the ones with weak interlayer interactions which can fabricate thin devices. However, kagome materials combining both characters of magnetism and weak interlayer interactions are rare. Here we investigate a new family of titanium based kagome materials RETi3Bi4 (RE = Eu, Gd… ▽ More Kagome materials have attracted a surge of research interest recently, especially for the ones combining with magnetism, and the ones with weak interlayer interactions which can fabricate thin devices. However, kagome materials combining both characters of magnetism and weak interlayer interactions are rare. Here we investigate a new family of titanium based kagome materials RETi3Bi4 (RE = Eu, Gd and Sm). The flakes of nanometer thickness of RETi3Bi4 can be obtained by exfoliation due to the weak interlayer interactions. According to magnetic measurements, out-of-plane ferromagnetism, out-of-plane anti-ferromagnetism, and in-plane ferromagnetism are formed for RE = Eu, Gd, and Sm respectively. The magnetic orders are simple and the saturation magnetizations can be relatively large since the rare earth elements solely provide the magnetic moments. Further by angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations, the electronic structures of RETi3Bi4 are investigated. The ARPES results are consistent with the calculations, indicating the bands characteristic with kagome sublattice in RETi3Bi4. We expect these materials to be promising candidates for observation of the exotic magnetic topological phases and the related topological quantum transport studies. △ Less

Submitted 28 August, 2023; originally announced August 2023.

Journal ref: Science Bulletin 06.36.2024

Observation of giant nonreciprocal charge transport from quantum Hall states in a topological insulator

Authors: Chunfeng Li, Rui Wang, Shuai Zhang, Yuyuan Qin, Zhe Ying, Boyuan Wei, Zheng Dai, Fengyi Guo, Wei Chen, Rong Zhang, Baigeng Wang, Xuefeng Wang, Fengqi Song

Abstract: Symmetry breaking in quantum materials is of great importance and can lead to nonreciprocal charge transport. Topological insulators provide a unique platform to study nonreciprocal charge transport due to their surface states, especially quantum Hall states under external magnetic field. Here, we report the observation of nonreciprocal charge transport mediated by quantum Hall states in devices c… ▽ More Symmetry breaking in quantum materials is of great importance and can lead to nonreciprocal charge transport. Topological insulators provide a unique platform to study nonreciprocal charge transport due to their surface states, especially quantum Hall states under external magnetic field. Here, we report the observation of nonreciprocal charge transport mediated by quantum Hall states in devices composed of the intrinsic topological insulator Sn-Bi1.1Sb0.9Te2S, which is attributed to asymmetric scattering between quantum Hall states and Dirac surface states. A giant nonreciprocal coefficient of up to 2.26*10^5 A^-1 is found. Our work not only reveals the properties of nonreciprocal charge transport of quantum Hall states in topological insulators, but also paves the way for future electronic devices. △ Less

Submitted 17 March, 2024; v1 submitted 17 July, 2023; originally announced July 2023.

Journal ref: Nat Mater (2024)

Solvable BCS-Hubbard Liouvillians in arbitrary dimensions

Authors: Xu-Dong Dai, Fei Song, Zhong Wang

Abstract: We present the construction of a solvable Lindblad model in arbitrary dimensions, wherein the Liouvillian can be mapped to a BCS-Hubbard model featuring an imaginary interaction. The Hilbert space of the system can be divided into multiple sectors, each characterized by an onsite invariant configuration. The model exhibits bistable steady states in all spatial dimensions, which is guaranteed by th… ▽ More We present the construction of a solvable Lindblad model in arbitrary dimensions, wherein the Liouvillian can be mapped to a BCS-Hubbard model featuring an imaginary interaction. The Hilbert space of the system can be divided into multiple sectors, each characterized by an onsite invariant configuration. The model exhibits bistable steady states in all spatial dimensions, which is guaranteed by the fermion-number parity. Notably, the Liouvillian gap exhibits a Zeno transition, below which the Liouvillian gap is linear with respect to the dissipation. We also uncover a generic dimension-dependent gap behavior: In one dimension, the gap originates from multiple sectors with spectral crossing; in higher dimensions, a single sector determines the gap. △ Less

Submitted 14 September, 2023; v1 submitted 22 June, 2023; originally announced June 2023.

Comments: 7 pages, 2 figures

Journal ref: Phys. Rev. B 108, 115127 (2023)

Non-Bloch dynamics and topology in a classical non-equilibrium process

Authors: Bo Li, He-Ran Wang, Fei Song, Zhong Wang

Abstract: The non-Hermitian skin effect refers to the accumulation of eigenstates near the boundary in open boundary lattice models, which can be systematically characterized using the non-Bloch band theory. Here, we apply the non-Bloch band theory to investigate the stochastic reaction-diffusion process by mapping it to a non-Hermitian Kitaev chain. We exactly obtain the open boundary spectrum and the gene… ▽ More The non-Hermitian skin effect refers to the accumulation of eigenstates near the boundary in open boundary lattice models, which can be systematically characterized using the non-Bloch band theory. Here, we apply the non-Bloch band theory to investigate the stochastic reaction-diffusion process by mapping it to a non-Hermitian Kitaev chain. We exactly obtain the open boundary spectrum and the generalized Brillouin zone, and identify a robust zero mode arising from the non-Bloch topology. Notably, distinct from its Hermitian counterpart in the quantum context, the zero mode supports anomalous dynamical crossover in the Markov process. We quantitatively demonstrate the intriguing dynamical effects through the spectral decomposition of the Hamiltonian on the non-Bloch eigenstates, and confirm our findings by conducting stochastic simulations with high accuracy. Our study highlights the significant and general role of non-Bloch topology in non-equilibrium dynamics. △ Less

Submitted 9 September, 2024; v1 submitted 19 June, 2023; originally announced June 2023.

Comments: 6+13 pages, 3+2 figures

Journal ref: Phys. Rev. B 109, L201121 (2024)

Steady-state edge burst: From free-particle systems to interaction-induced phenomena

Authors: Yu-Min Hu, Wen-Tan Xue, Fei Song, Zhong Wang

Abstract: The interplay between the non-Hermitian skin effect and the imaginary gap of lossy lattices results in the edge burst, a boundary-induced dynamical phenomenon in which an exceptionally large portion of particle loss occurs at the edge. Here, we find that this intriguing non-Hermitian dynamical phenomenon can be exactly mapped into the steady-state density distribution of a corresponding open quant… ▽ More The interplay between the non-Hermitian skin effect and the imaginary gap of lossy lattices results in the edge burst, a boundary-induced dynamical phenomenon in which an exceptionally large portion of particle loss occurs at the edge. Here, we find that this intriguing non-Hermitian dynamical phenomenon can be exactly mapped into the steady-state density distribution of a corresponding open quantum system. Consequently, the bulk-edge scaling relation of loss probability in the edge burst maps to that of steady-state density. Furthermore, we introduce a many-body open-system model in which the two-body loss generates an interaction-induced non-Hermitian skin effect. Using the positive-$P$ method, we demonstrate the validity of the scaling relation for steady-state correlators. These results provide a unique perspective on the interaction-induced many-body non-Hermitian skin effect. Our predictions are testable in state-of-the-art experimental platforms. △ Less

Submitted 15 December, 2023; v1 submitted 14 June, 2023; originally announced June 2023.

Comments: 13 pages, 6 figures

Journal ref: Phys. Rev. B 108, 235422 (2023)

Ba9RE2(SiO4)6 (RE=Ho-Yb): A New Family of Rare-earth based Honeycomb Lattice Magnets

Authors: Andi Liu, Fangyuan Song, Zhaohu Li, Malik Ashtar, Yuqi Qin, Dingjun Liu, Zhengcai Xia, Jingxin Li, Zhitao Zhang, Wei Tong, Hanjie Guo, Zhaoming Tian

Abstract: Rare-earth (RE) based honeycomb-lattice materials with strong spin-orbit coupled Jeff=1/2 moments have attracted great interest as a platform to realize Kitaev quantum spin liquid (QSL) state. Herein, we report the discovery of a new family of RE based honeycomb-lattice magnets Ba9RE2(SiO4)6(RE=Ho-Yb), which crystallize into the rhombohedral structure with space group R-3. In these serial compound… ▽ More Rare-earth (RE) based honeycomb-lattice materials with strong spin-orbit coupled Jeff=1/2 moments have attracted great interest as a platform to realize Kitaev quantum spin liquid (QSL) state. Herein, we report the discovery of a new family of RE based honeycomb-lattice magnets Ba9RE2(SiO4)6(RE=Ho-Yb), which crystallize into the rhombohedral structure with space group R-3. In these serial compounds, magnetic RE3+ ions are arranged on a perfect honeycomb lattice within the ab-plane and stacked in the ABCABC-type fashion along the c-axis. All Ba9RE2(SiO4)6(RE=Ho-Yb) polycrystals exhibit the dominant antiferromagnetic interactions and absence of magnetic order down to 2 K. In combination with the magnetization and electron spin resonance (ESR) results, distinct anisotropic magnetic behaviors are proposed for compounds with different RE ions. Moreover, the synthesized Ba9Yb2Si6O24 single crystals show large magnetic frustration and no long-range magnetic ordering down to 0.15 K, being a possible QSL candidate state. These serial compounds are attractive for exploring the exotic magnetic phases of Kitaev materials with 4f electrons. △ Less

Submitted 20 May, 2023; originally announced May 2023.

Comments: 19 pages, 8 figures

Tensor network approach to the fully frustrated XY model on a kagome lattice with a fractional vortex-antivortex pairing transition

Authors: Feng-Feng Song, Guang-Ming Zhang

Abstract: We have developed a tensor network approach to the two-dimensional fully frustrated classical XY spin model on the kagome lattice, and clarified the nature of the possible phase transitions of various topological excitations.We find that the standard tensor network representation for the partition function does not work due to the strong frustrations in the low temperature limit. To avoid the dire… ▽ More We have developed a tensor network approach to the two-dimensional fully frustrated classical XY spin model on the kagome lattice, and clarified the nature of the possible phase transitions of various topological excitations.We find that the standard tensor network representation for the partition function does not work due to the strong frustrations in the low temperature limit. To avoid the direct truncation of the Boltzmann weight, based on the duality transformation, we introduce a new representation to build the tensor network with local tensors lying on the centers of the elementary triangles of the kagome lattice. Then the partition function is expressed as a product of one-dimensional transfer matrix operators, whose eigen-equation can be solved by the variational uniform matrix product state algorithm accurately. The singularity of the entanglement entropy for the one-dimensional quantum operator provides a stringent criterion for the possible phase transitions. Through a systematic numerical analysis of thermodynamic properties and correlation functions in the thermodynamic limit, we prove that the model exhibits a single Berezinskii-Kosterlitz-Thouless phase transition only, which is driven by the unbinding of $1/3$ fractional vortex-antivortex pairs determined at $T_{c}\simeq 0.075J_{1}$ accurately. The absence of long-range order of chirality or quasi-long range order of integer vortices has been verified in the whole finite temperature range. Thus the long-standing controversy about the phase transitions in this fully frustrated XY model on the kagome lattice is solved rigorously, which provides a plausible way to understand the charge-6e superconducting phase observed experimentally in the two-dimensional kagome superconductors. △ Less

Submitted 26 April, 2023; v1 submitted 24 April, 2023; originally announced April 2023.

Comments: 14 pages, 13 figures, submitted version for publication

Journal ref: Physical Review B, 108, 014424 (2023)

Observation of colossal topological Hall effect in noncoplanar ferromagnet Cr5Te6 thin films

Authors: Yequan Chen, Yingmei Zhu, Renju Lin, Wei Niu, Ruxin Liu, Wenzhuo Zhuang, Xu Zhang, Jinghua Liang, Wenxuan Sun, Zhongqiang Chen, Yongsheng Hu, Fengqi Song, Jian Zhou, Di Wu, Binghui Ge, Hongxin Yang, Rong Zhang, Xuefeng Wang

Abstract: The topological Hall effect (THE) is critical to the exploration of the spin chirality generated by the real-space Berry curvature, which has attracted worldwide attention for its prospective applications in spintronic devices. However, the prominent THE remains elusive at room temperature, which severely restricts the practical integration of chiral spin textures. Here, we show a colossal intrins… ▽ More The topological Hall effect (THE) is critical to the exploration of the spin chirality generated by the real-space Berry curvature, which has attracted worldwide attention for its prospective applications in spintronic devices. However, the prominent THE remains elusive at room temperature, which severely restricts the practical integration of chiral spin textures. Here, we show a colossal intrinsic THE in large-area ferromagnet Cr5Te6 thin films epitaxially grown by pulsed laser deposition. Such a THE can be maintained until 270 K, which is attributed to the field-stimulated noncoplanar spin textures induced by the interaction of the in-plane ferromagnet and antiferromagnet infrastructures. Our first-principles calculations further verify the considerable Dzyaloshinskii-Moriya interaction in Cr5Te6. This work not only paves the way for robust chiral spin textures near room temperature in large-area low-dimensional ferromagnetic films for practical applications, but also facilitates the development of high-density and dissipationless spintronic devices. △ Less

Submitted 23 April, 2023; originally announced April 2023.

Comments: 18 pages, 13 figures, 1 table

Journal ref: Advanced Functional Materials 33, 2302984 (2023)