Showing 1–50 of 221 results for author: Liu, P

Nonvolatile photoswitching of a Mott state via reversible stacking rearrangement

Authors: Junde Liu, Liwen Su, Pei Liu, Hui Liu, Mojun Pan, Yuchong Zhang, Famin Chen, Yueqian Chen, Zhaoyang Xie, Stefan Mathias, Tianping Ying, Lin Hu, Tian Qian, Xun Shi, Yugui Yao

Abstract: Nonvolatile control of the Mott transition is a central goal in correlated-electron physics, offering access to fascinating emergent states and great potential for technological applications. Compared to chemical or mechanical approaches, ultrafast optical excitation further promises a path to create and manipulate novel non-equilibrium phases with ultimate spatiotemporal precision. However, achie… ▽ More Nonvolatile control of the Mott transition is a central goal in correlated-electron physics, offering access to fascinating emergent states and great potential for technological applications. Compared to chemical or mechanical approaches, ultrafast optical excitation further promises a path to create and manipulate novel non-equilibrium phases with ultimate spatiotemporal precision. However, achieving a truly nonvolatile electronic phase transition in laser-excited Mott systems remains an elusive challenge. Here, we present a highly robust and reversible method for optical control of the Mott state in van der Waals systems. Specifically, using angle-resolved photoemission spectroscopy, we observe a nonvolatile Mott-to-metallic transition in the ultrafast laser-excited charge density wave (CDW) material 1T-TaSe2. Complementary theoretical calculations reveal that this transition originates from a rearrangement of the interlayer CDW stacking. This new stacking order, formed following the ultrafast quenching of the CDW, circumvents the need for large-scale atomic sliding. Intriguingly, it introduces a significant in-plane component to the electron hopping and effectively reduces the ratio of on-site Coulomb interaction to bandwidth, thereby suppressing the Mott state and stabilizing a metallic phase. Our results establish optical-control of interlayer stacking as a versatile strategy for inducing nonvolatile phase transitions, opening a new route to tailor correlated electronic phases and realize reconfigurable high-frequency devices. △ Less

Submitted 25 December, 2025; originally announced December 2025.

Bilayer B80 Structure: High Stability and Experimental Support for Existence

Authors: Yi-Sha Chen, Jing-Jing Guo, Peng-Bo Liu, Hui-Yan Zhao, Jing Wang, Ying Liu

Abstract: The recent experimental characterization of B80- via photoelectron spectroscopy stimulated renewed interest in exploring B80 clusters. Here, a D3h-symmetric B80 bilayer structure has been proposed using density functional theory calculations. Ab initio molecular dynamics simulations confirm that the bilayer structure maintain its structural integrity up to 1400 K, indicating superior thermodynamic… ▽ More The recent experimental characterization of B80- via photoelectron spectroscopy stimulated renewed interest in exploring B80 clusters. Here, a D3h-symmetric B80 bilayer structure has been proposed using density functional theory calculations. Ab initio molecular dynamics simulations confirm that the bilayer structure maintain its structural integrity up to 1400 K, indicating superior thermodynamic stability compared to previously known B80 configurations, including the B80 buckyball and volleyball-like structures. Vibrational frequency analysis confirms its kinetic stability. Electronic structure calculations reveals a HOMO-LUMO gap of 0.72 eV and pronounced aromaticity, further supported by a nucleus-independent chemical shift (NICS(0)) value of -44.3 ppm in the interlayer B-B bonds. The simulated photoelectron spectrum of the B80- bilayer reproduces key experimental features, with vertical detachment energies agreeing with experimental peaks within 0.04 eV. These findings support the potential existence of this bilayer configuration, and enrich the structural diversity of boron clusters, offering promising prospects for applications in nanoscale electronics. △ Less

Submitted 17 November, 2025; originally announced November 2025.

Non-altermagnetic spin texture in MnTe

Authors: Meng Zeng, Pengfei Liu, Ming-Yuan Zhu, Naifu Zheng, Xiang-Rui Liu, Yu-Peng Zhu, Tian-Hao Shao, Yu-Jie Hao, Xiao-Ming Ma, Gexing Qu, Rafał Kurleto, Dawid Wutke, Rong-Hao Luo, Yue Dai, Xiaoqian Zhang, Koji Miyamoto, Kenya Shimada, Taichi Okuda, Kiyohisa Tanaka, Yaobo Huang, Qihang Liu, Chang Liu

Abstract: Recently, altermagnets have emerged as promising candidates in spintronics, uniquely combining large spin-polarized electronic states with zero net magnetization. A prominent example is $α$-MnTe, whose altermagnetic spin splitting, i.e., the degeneracy lift in momentum space induced by collinear magnetic order, has been experimentally observed. However, the direct evidence of its $g$-wave spin pol… ▽ More Recently, altermagnets have emerged as promising candidates in spintronics, uniquely combining large spin-polarized electronic states with zero net magnetization. A prominent example is $α$-MnTe, whose altermagnetic spin splitting, i.e., the degeneracy lift in momentum space induced by collinear magnetic order, has been experimentally observed. However, the direct evidence of its $g$-wave spin polarization, the key property for altermagnetic spintronics, is thus far lacking. By combining high-resolution spin- and angle-resolved photoemission spectroscopy (SARPES) with first-principles calculations, we reveal a $k_z$-independent, Rashba-like spin texture in $α$-MnTe. Our results indicate that the observed spin polarization is primarily governed by spin-orbit coupling, whereas the magnetic order contributes to the splitting of energy bands but plays a much less dominant role in spin polarization due to the multi-domain nature. From this result, we further establish a way to prescreen altermagnet candidates that favor the formation of large antiferromagnetic domains based on symmetry analysis. Our work elucidates the interplay between magnetic order and spin-orbit coupling in governing spin polarization in altermagnet candidates, and thereby advances the materials design paradigm for spin-functional devices. △ Less

Submitted 4 November, 2025; originally announced November 2025.

Comments: 19 pages, 4 figures

Numerical Investigation of Single-Core to Split-Core Transitions in Nematic Liquid Crystals

Authors: Daniel Siebel-Cortopassi, Pei Liu

Abstract: We analyze single-core and split-core defect structures in nematic liquid crystals within the Landau-de Gennes framework by studying minimizers of the associated energy functional. A bifurcation occurs at a critical temperature threshold, below which both split-core and single-core configurations are solutions to the Euler-Lagrange equation, with the split-core defect possessing lower energy. Abov… ▽ More We analyze single-core and split-core defect structures in nematic liquid crystals within the Landau-de Gennes framework by studying minimizers of the associated energy functional. A bifurcation occurs at a critical temperature threshold, below which both split-core and single-core configurations are solutions to the Euler-Lagrange equation, with the split-core defect possessing lower energy. Above the threshold, the split-core configuration vanishes, leaving the single-core defect as the only stable solution. We analyze the dependence of such temperature threshold on the domain size and characterize the nature of the transition between the two defect types. We carry out a quantitative study of defect core sizes as functions of temperature and domain size for both single and split core defects. △ Less

Submitted 30 October, 2025; originally announced October 2025.

Entanglement Sum Rule from Higher-Form Symmetries

Authors: Pei-Yao Liu

Abstract: We prove an entanglement sum rule for $(d-1)$-dimensional quantum lattice models with finite abelian higher-form symmetries, obtained by minimally coupling a sector on $p$-simplices carrying a $p$-form $G$ symmetry to a sector on $(p+1)$-simplices carrying the dual $(d-p-2)$-form $\widehat G$ symmetry (with $\widehat G$ being the Pontryagin dual of $G$). The coupling is introduced by conjugation w… ▽ More We prove an entanglement sum rule for $(d-1)$-dimensional quantum lattice models with finite abelian higher-form symmetries, obtained by minimally coupling a sector on $p$-simplices carrying a $p$-form $G$ symmetry to a sector on $(p+1)$-simplices carrying the dual $(d-p-2)$-form $\widehat G$ symmetry (with $\widehat G$ being the Pontryagin dual of $G$). The coupling is introduced by conjugation with a symmetry-preserving operator $\mathcal{U}$ that dresses symmetry-invariant operators with appropriate Wilson operators. Our main result concerns symmetric eigenstates of the coupled model that arise by acting with $\mathcal{U}$ on direct-product symmetric eigenstates of the decoupled model: provided a topological criterion formulated via the Mayer--Vietoris sequence holds for the chosen bipartition, $\mathcal{U}$ factorizes across the cut when acting on the symmetric state, and the entanglement entropy equals the sum of the entropies of the two sectors. This framework explains and generalizes known examples in fermion-$\mathbb{Z}_2$ gauge theory, identifies when topology obstructs the sum rule, and provides a procedure to construct new examples by gauging higher-form symmetries. △ Less

Submitted 12 November, 2025; v1 submitted 20 October, 2025; originally announced October 2025.

Efficient small-cell sampling for machine-learning potentials of multi-principal element alloys

Authors: Yan Liu, Jiantao Wang, Hongkun Deng, Yan Sun, Xing-Qiu Chen, Peitao Liu

Abstract: Multi-principal element alloys (MPEAs) exhibit exceptional properties but face significant challenges in developing accurate machine-learning potentials (MLPs) due to their vast compositional and configurational complexity. Here, we introduce an efficient small-cell sampling (SCS) method, which allows for generating diverse and representative training datasets for MPEAs using only small-cell struc… ▽ More Multi-principal element alloys (MPEAs) exhibit exceptional properties but face significant challenges in developing accurate machine-learning potentials (MLPs) due to their vast compositional and configurational complexity. Here, we introduce an efficient small-cell sampling (SCS) method, which allows for generating diverse and representative training datasets for MPEAs using only small-cell structures with just one and two elements, thereby bypassing the computational overhead of iterative active learning cycles and large-cell density functional theory calculations. The efficacy of the method is carefully validated through principal component analysis, extrapolation grades evaluation, and root-mean-square errors and physical properties assessment on the TiZrHfCuNi system. Further demonstrations on TiZrVMo, CoCrFeMnNi, and AlTiZrNbHfTa systems accurately reproduce complex phenomena including phase transitions, chemical orderings, and thermodynamic properties. This work establishes an efficient one-shot protocol for constructing high-quality training datasets across multiple elements, laying a solid foundation for developing universal MLPs for MPEAs. △ Less

Submitted 18 October, 2025; originally announced October 2025.

Comments: 21 pages, 16 figures (including SM)

Hidden phonon-assisted charge density wave transition in BaFe2Al9 revealed by ultrafast optical spectroscopy

Authors: Lei Wang, Mingwei Ma, Jiangxu Li, Liucheng Chen, Bingru Lu, Xiang Li, Feng Jin, Elbert E. M. Chia, Jianlin Luo, Rongyan Chen, Peitao Liu, Fang Hong, Xinbo Wang

Abstract: The interplay between electronic and lattice degrees of freedom is fundamental to charge density wave (CDW) formation, yet the microscopic origin often remains elusive. Here, we investigate the transient optical response of the intermetallic compound BaFe2Al9 using polarization-resolved ultrafast optical spectroscopy. We identify a discontinuous sign reversal in the transient reflectivity at Tc ~… ▽ More The interplay between electronic and lattice degrees of freedom is fundamental to charge density wave (CDW) formation, yet the microscopic origin often remains elusive. Here, we investigate the transient optical response of the intermetallic compound BaFe2Al9 using polarization-resolved ultrafast optical spectroscopy. We identify a discontinuous sign reversal in the transient reflectivity at Tc ~ 110 K, providing unambiguous evidence for the first-order transition. The anisotropic quasiparticle relaxation establishes the three-dimensional nature of the ordered state. Below Tc, a single coherent 1.6 THz oscillation appears abruptly and remains confined to the CDW phase. This mode exhibits weak temperature dependence with negligible softening and is absent in Raman spectra. First-principles calculations imply that it is a precursor phonon at the CDW wave vector with strong electron-phonon coupling. Our results indicate that the CDW in BaFe2Al9 arises from intertwined electronic and lattice instabilities, assisted by a displacive mechanism mediated by a hidden strongly coupled phonon, distinct from conventional amplitude-mode softening scenarios. △ Less

Submitted 7 October, 2025; originally announced October 2025.

Comments: 7 pages, 4 figures

Twist dominates bending in the liquid crystal organization of bacteriophage DNA

Authors: Pei Liu, Tamara Christiani, Zhijie Wang, Fei Guo, Mariel Vazquez, M. Carme Calderer, Javier Arsuaga

Abstract: DNA frequently adopts liquid-crystalline conformations in both cells and viruses. The Oseen--Frank framework provides a powerful continuum description of these phases through three elastic moduli: splay ($K_1$), twist or cholesteric ($K_2$), and bending ($K_3$). While $K_1$ is typically assumed to dominate, the relative magnitude of $K_2$ and $K_3$ in confined DNA remains poorly understood. Here,… ▽ More DNA frequently adopts liquid-crystalline conformations in both cells and viruses. The Oseen--Frank framework provides a powerful continuum description of these phases through three elastic moduli: splay ($K_1$), twist or cholesteric ($K_2$), and bending ($K_3$). While $K_1$ is typically assumed to dominate, the relative magnitude of $K_2$ and $K_3$ in confined DNA remains poorly understood. Here, we combine cryo-electron microscopy, liquid-crystal modeling, and knot theory to quantify this relationship in bacteriophage P4, whose genome is partially organized in a spool-like liquid-crystalline phase. We first show experimentally that the ordered DNA occupies three concentric layers within the capsid. We then formulate an Oseen--Frank model for this geometry and use it, together with the measured layer radii, to estimate the elastic ratio $α= K_3/K_2$. We find $α\approx 0.0064$, indicating that twist elasticity overwhelmingly dominates bending. To validate this result, we perform Langevin dynamics simulations of DNA trajectories and classify the resulting knots. The predicted knot distribution agrees with experimental data from P4, demonstrating consistency between elasticity, topology, and observed genome organization. △ Less

Submitted 5 October, 2025; originally announced October 2025.

Knotted DNA Configurations in Bacteriophage Capsids: A Liquid Crystal Theory Approach

Authors: Pei Liu, Zhijie Wang, Tamara Christiani, Mariel Vazquez, M. Carme Calderer, Javier Arsuaga

Abstract: Bacteriophages, viruses that infect bacteria, store their micron long DNA inside an icosahedral capsid with a typical diameter of 40 nm to 100 nm. Consistent with experimental observations, such confinement conditions induce an arrangement of DNA that corresponds to a hexagonal chromonic liquid-crystalline phase, and increase the topological complexity of the genome in the form of knots. A mathema… ▽ More Bacteriophages, viruses that infect bacteria, store their micron long DNA inside an icosahedral capsid with a typical diameter of 40 nm to 100 nm. Consistent with experimental observations, such confinement conditions induce an arrangement of DNA that corresponds to a hexagonal chromonic liquid-crystalline phase, and increase the topological complexity of the genome in the form of knots. A mathematical model that implements a chromonic liquid-crystalline phase and that captures the changes in topology has been lacking. We adopt a mathematical model that represents the viral DNA as a pair of a vector field and a line. The vector field is a minimizer of the total Oseen-Frank energy for nematic liquid crystals under chromonic constraints, while the line is identified with the tangent to the field at selected locations, representing the central axis of the DNA molecule. The fact that the Oseen-Frank functional assigns infinite energy to topological defects (point defects in two dimensions and line defects in three dimensions) precludes the presence of singularities and, in particular, of knot structures. To address this issue, we begin with the optimal vector field and helical line, and propose a new algorithm to introduce knots through stochastic perturbations associated with splay and twist deformations, modeled by means of a Langevin system. We conclude by comparing knot distributions generated by the model and by interpreting them in the context of previously published experimental results. Altogether, this work relies on the synergy of modeling, analysis and computation in the study of viral DNA organization in capsids. △ Less

Submitted 12 September, 2025; originally announced September 2025.

Anomalous Magnetoresistance beyond the Jullière Model for Spin Selectivity in Chiral Molecules

Authors: Tian-Yi Zhang, Yue Mao, Peng-Yi Liu, Ai-Min Guo, Qing-Feng Sun

Abstract: The issue of anomalous high magnetoresistance, beyond the Jullière model, observed in nonmagnetic electrode-chiral molecular-ferromagnetic electrode devices has puzzled the community for a long time. Here, by considering the magnetic proximity effect which shifts the nonmagnetic-ferromagnetic interface toward chiral molecules, we show the anomalous high magnetoresistance beyond the spin polarizati… ▽ More The issue of anomalous high magnetoresistance, beyond the Jullière model, observed in nonmagnetic electrode-chiral molecular-ferromagnetic electrode devices has puzzled the community for a long time. Here, by considering the magnetic proximity effect which shifts the nonmagnetic-ferromagnetic interface toward chiral molecules, we show the anomalous high magnetoresistance beyond the spin polarization in ferromagnetic electrodes even in the very weak spin-orbit coupling. Our results are in excellent agreement with the experiments, demonstrating that the spin-orbit coupling plays a fundamental role in chiral-induced spin selectivity and the magnetic proximity effect can dramatically enhance the magnetoresistance. These results elucidate the interaction between chiral molecules and ferromagnetic electrodes and facilitate the design of chiral-based spintronic devices. △ Less

Submitted 2 December, 2025; v1 submitted 6 September, 2025; originally announced September 2025.

Comments: 6 pages, 4 figures

Journal ref: The Journal of Physical Chemistry Letters, 2025, 16, 12596-12602

Interplay of Altermagnetic Order and Wilson Mass in the Dirac Equation: Helical Edge States without Time-Reversal Symmetry

Authors: Yu-Hao Wan, Peng-Yi Liu, Qing-Feng Sun

Abstract: We investigate topological phases in three-dimensional topological insulator (3DTI) thin films interfaced with altermagnetic (AM) orders. Starting from a modified Dirac equation, we elucidate the interplay between the Wilson mass, arising from lattice regularization, and the altermagnetic mass, and show how this interplay fundamentally alters the band topology and boundary modes. In particular, we… ▽ More We investigate topological phases in three-dimensional topological insulator (3DTI) thin films interfaced with altermagnetic (AM) orders. Starting from a modified Dirac equation, we elucidate the interplay between the Wilson mass, arising from lattice regularization, and the altermagnetic mass, and show how this interplay fundamentally alters the band topology and boundary modes. In particular, we demonstrate that coupling a 3DTI thin film to AM order induces a topological phase transition: although the total Chern number remains zero across the transition, topological helical edge states emerge after the transition. These helical edge states arise from opposite Chern numbers at different high-symmetry points, and are distinct from both the chiral edge states of the quantum anomalous Hall phase and the helical edge states of the conventional quantum spin Hall states. The quantum transport simulations reveal robust, quantized nonlocal resistance plateaus associated with these helical edge states, which persist even under strong potential and magnetic disorder. Our results establish 3DTI/AM heterostructures as a feasible material platform for engineering and detecting helical topological edge transport without time-reversal symmetry, thus expanding the landscape of topological matter and providing new opportunities for quantum devices. △ Less

Submitted 4 September, 2025; originally announced September 2025.

Probing the Linewidth of the 12.4-keV Solid-State $^{45}$Sc Isomeric Resonance

Authors: Peifan Liu, Miriam Gerharz, Berit Marx-Glowna, Willi Hippler, Jan-Etienne Pudell, Alexey Zozulya, Brandon Stone, Deming Shu, Robert Loetzsch, Sakshath Sadashivaiah, Lars Bocklage, Christina Boemer, Shan Liu, Vitaly Kocharyan, Dietrich Krebs, Tianyun Long, Weilun Qin, Matthias Scholz, Kai Schlage, Ilya Sergeev, Hans-Christian Wille, Ulrike Boesenberg, Gianluca Aldo Geloni, Jörg Hallmann, Wonhyuk Jo , et al. (15 additional authors not shown)

Abstract: The $^{45}$Sc nuclear transition from the ground to the isomeric state at 12.389~keV, with a lifetime of 0.46~s, exhibits an extraordinarily narrow natural width of 1.4~feV and a quality factor $\simeq 10^{19}$ -- surpassing those of the most precise atomic clocks -- making $^{45}$Sc a compelling platform for advanced metrology and nuclear clocks. Here we investigate how closely the spectral width… ▽ More The $^{45}$Sc nuclear transition from the ground to the isomeric state at 12.389~keV, with a lifetime of 0.46~s, exhibits an extraordinarily narrow natural width of 1.4~feV and a quality factor $\simeq 10^{19}$ -- surpassing those of the most precise atomic clocks -- making $^{45}$Sc a compelling platform for advanced metrology and nuclear clocks. Here we investigate how closely the spectral width and quality factor of the solid-state $^{45}$Sc resonance can approach these natural limits. Using the European X-ray Free-Electron Laser, we confirm the isomer's lifetime via time-delayed incoherent $K_{α,β}$ fluorescence and observe previously unreported elastic fluorescence, yielding a partial internal conversion coefficient of 390(60). The absence of a clear nuclear forward scattering signal beyond a 2-ms delay implies environmental broadening of at least $500~Γ_{0}$ under experimental conditions, placing bounds on solid-state decoherence mechanisms. These findings set new experimental benchmarks for solid-state nuclear clock development. △ Less

Submitted 26 August, 2025; v1 submitted 24 August, 2025; originally announced August 2025.

Comments: 9 pages, 4 figures, 2 tables

Bending nanoribbon to induce large anisotropic magnetoconductance

Authors: Ponder Liu, Hao-Cheng Hung, You-Ting Huang, Jia-Cheng Li, Carmine Ortix, Ching-Hao Chang

Abstract: When a nanoribbon is bent under a homogeneous external magnetic field, the effective magnetic field inside becomes either homogeneous or inhomogeneous, depending on the direction of the field. This enables the selective creation of bulk, interface, and edge magnetic states in the bent structure, for a magnetic field with a strength. We establish theoretically that these tuneable states lead to a s… ▽ More When a nanoribbon is bent under a homogeneous external magnetic field, the effective magnetic field inside becomes either homogeneous or inhomogeneous, depending on the direction of the field. This enables the selective creation of bulk, interface, and edge magnetic states in the bent structure, for a magnetic field with a strength. We establish theoretically that these tuneable states lead to a strong geometry-induced anisotropic magnetoconductance (GAMC) in perpendicularly bent nanoribbon, which can reach up to 100\%. Moreover, the GAMC can be further enhanced to 200\%, 300\%, or even higher by either further bending or tuning the bending angle. The potential of this phenomenon for practical applications is demonstrated by its stable anisotropy, which remains consistent across a wide range of Fermi energies, can be observed even at weak magnetic fields and room temperature, and occurs in various systems such as two-dimensional electron gas (2DEG) and graphene. △ Less

Submitted 21 August, 2025; originally announced August 2025.

Atomistic mechanisms of phase transitions in all-temperature barocaloric material KPF$_6$

Authors: Jiantao Wang, Yi-Chi Zhang, Yan Liu, Hongkun Deng, Mingfeng Liu, Yan Sun, Bing Li, Xing-Qiu Chen, Peitao Liu

Abstract: Conventional barocaloric materials typically exhibit limited operating temperature ranges. In contrast, KPF$_6$ has recently been reported to achieve an exceptional all-temperature barocaloric effect (BCE) via pressure-driven phase transitions. Here, we elucidate the atomistic mechanisms underlying the phase transitions through first-principles calculations and machine-learning potential accelerat… ▽ More Conventional barocaloric materials typically exhibit limited operating temperature ranges. In contrast, KPF$_6$ has recently been reported to achieve an exceptional all-temperature barocaloric effect (BCE) via pressure-driven phase transitions. Here, we elucidate the atomistic mechanisms underlying the phase transitions through first-principles calculations and machine-learning potential accelerated molecular dynamics simulations. We identify four distinct phases: the room-temperature cubic (C) plastic crystal characterized by strong fluorine orientational disorder (FOD) and anharmonicity, the intermediate-temperature monoclinic (M-II) phase with decreasing FOD, the low-temperature monoclinic (M-I) phase with suppressed FOD, and the fully ordered rhombohedral (R) phase under pressure. Phonon calculations confirm the dynamic stability of the M-II, M-I, and R phases at 0 K, whereas the C phase requires thermal fluctuations for stabilization. Under pressure, all the C, M-II, and M-I phases transform to the R phase, which are driven by cooperative PF$_6$ octahedral rotations coupled with lattice modulations. These pressure-induced phase transitions result in persistent isothermal entropy changes across a wide temperature range, thereby explaining the experimentally observed all-temperature BCE in this material. Hybrid functional calculations reveal wide-bandgap insulating behavior across all phases. This work deciphers the interplay between FOD, anharmonicity, and phase transitions in KPF$_6$, providing important insights for the design of BCE materials with broad operational temperature spans. △ Less

Submitted 19 August, 2025; originally announced August 2025.

Comments: 16 pages, 15 figures, 4 tables (including Supplemental Material)

Journal ref: Journal of Materials Science & Technology 263 (2026) 211-219

High Entropy Engineering of Magnetic Kagome Lattice (Gd,Tb,Dy,Ho,Er)Mn6Sn6

Authors: Wenhao Liu, Nikhil Uday Dhale, Youzhe Chen, Pramanand Joshi, Zixin Zhai, Xiqu Wang, Ping Liu, Robert J. Birgeneau, Boris Maiorov, Christopher A. Mizzi, Bing Lv

Abstract: The magnetic kagome lattice compound RMn6Sn6 (R=rare earth) is an emerging platform to exploit the interplay between magnetism and topological electronic states where a variety of exciting findings such as flat bands, Dirac points as well as the dramatic dependence of magnetic order on the rare-earth element have been reported. High entropy through rare earth alloying, on the other hand, provides… ▽ More The magnetic kagome lattice compound RMn6Sn6 (R=rare earth) is an emerging platform to exploit the interplay between magnetism and topological electronic states where a variety of exciting findings such as flat bands, Dirac points as well as the dramatic dependence of magnetic order on the rare-earth element have been reported. High entropy through rare earth alloying, on the other hand, provides another knob to control over the physical properties in this system. Here, by the marriage of high entropy and the magnetic kagome lattice, we obtain (Gd,Tb,Dy,Ho,Er)Mn6Sn6 single crystals and systematically investigate their magnetic and transport properties. Different from the parent phases, the high entropy 166 material displays multiple novel magnetic transitions induced by temperature and external magnetic fields. Furthermore, linear magnetoresistance persisting up to 20 T has been revealed at 4 K. The intrinsic nontrivial band topology also survives in the high entropy form, as evidenced by the intrinsic anomalous Hall effect. Our results highlight high entropy as a powerful approach for tuning the interplay of charge, spin and lattice degree of freedom in magnetic topological materials. △ Less

Submitted 30 July, 2025; originally announced July 2025.

Diverse polymorphs and phase transitions in van der Waals In$_2$Se$_3$

Authors: Mingfeng Liu, Jiantao Wang, Peitao Liu, Qiang Wang, Zhibo Liu, Yan Sun, Xing-Qiu Chen

Abstract: Van der Waals In$_2$Se$_3$ has garnered significant attention due to its unique properties and wide applications associated with its rich polymorphs and polymorphic phase transitions. Despite extensive studies, the vast complex polymorphic phase space remains largely unexplored, and the underlying microscopic mechanism for their phase transformations remains elusive. Here, we develop a highly accu… ▽ More Van der Waals In$_2$Se$_3$ has garnered significant attention due to its unique properties and wide applications associated with its rich polymorphs and polymorphic phase transitions. Despite extensive studies, the vast complex polymorphic phase space remains largely unexplored, and the underlying microscopic mechanism for their phase transformations remains elusive. Here, we develop a highly accurate, efficient, and reliable machine-learning potential (MLP), which not only facilitates accurate exploration of the intricate potential energy surface (PES), but also enables us to conduct large-scale molecular dynamics (MD) simulations with first-principles accuracy. We identify the accurate structure of the $β''$ polymorph and uncover several previously unreported $β'$ polymorph variants exhibiting dynamic stability and competing energies, which are elucidated by characteristic flat imaginary phonon bands and the distinctive Mexican-hat-like PES in the $β$ polymorph. Through the MLP-accelerated MD simulations, we directly observe the polymorphic phase transformations among the $α$, $β$, $β'$, and $β''$ polymorphs under varying temperature and pressure conditions, and build for the first time an ab initio temperature-pressure phase diagram, showing good agreement with experiments. Furthermore, our MD simulations reveal a novel strain-induced reversible phase transition between the $β'$ and $β''$ polymorphs. This work not only unveils diverse polymorphs in van der Waals In$_2$Se$_3$, but also provides crucial atomic insights into their phase transitions, opening new avenues for the design of novel functional electronic devices. △ Less

Submitted 26 June, 2025; originally announced June 2025.

Comments: 32 pages, 20 figures (including Supplementary Information)

Construction of Kondo Chains by Engineering Porphyrin π-Radicals on Au(111)

Authors: Yan Zhao, Kaiyue Jiang, Peng-Yi Liu, Jie Li, Ruoning Li, Xin Li, Xinchen Fang, Anjing Zhao, Yutong Zhu, Hongxiang Xu, Ting Chen, Dong Wang, Xiaodong Zhuang, Shimin Hou, Kai Wu, Song Gao, Qing-Feng Sun, Yajie Zhang, Yongfeng Wang

Abstract: Quantum manipulation of molecular radical spins provides a crucial platform for exploring emergent phenomena in many-body systems. Here, we combine surface-confined synthesis with scanning tunneling microscopy(STM)tip-induced dehydrogenation to achieve atom-precise engineering of quasi-one-dimensional porphyrin-based Kondo chains (1-7 units) on Au(111). High-resolution STS measurements and low-ene… ▽ More Quantum manipulation of molecular radical spins provides a crucial platform for exploring emergent phenomena in many-body systems. Here, we combine surface-confined synthesis with scanning tunneling microscopy(STM)tip-induced dehydrogenation to achieve atom-precise engineering of quasi-one-dimensional porphyrin-based Kondo chains (1-7 units) on Au(111). High-resolution STS measurements and low-energy effective modeling collectively demonstrate that π-radicals at each fused-porphyrin unit form Kondo singlets screened by conduction electrons. Adjacent singlets develop direct coherent coupling via quantum-state-overlap-enabled electron tunneling. Crucially, chiral symmetry in the effective model governs zero-mode distribution-present in odd-length chains yet absent in even-length chains-which dictates pronounced odd-even quantum effects in STS spectra of finite chains. Furthermore, the number of parallel porphyrin chains non-monotonically tunes the competition between the Kondo effect and spin exchange, showing opposing trends in strength and demonstrating that both wave-function overlap and the SOMO-LUMO gap collectively govern these interactions. This work simultaneously resolves the dimensional dependence of many-body correlations in confined quantum systems and pioneers approaches for quantum-critical manipulation in molecular spin architectures. △ Less

Submitted 14 October, 2025; v1 submitted 12 June, 2025; originally announced June 2025.

$d$-Wave Flat Fermi Surface in Altermagnets Enables Maximum Charge-to-Spin Conversion

Authors: Junwen Lai, Tianye Yu, Peitao Liu, Long Liu, Guozhong Xing, Xing-Qiu Chen, Yan Sun

Abstract: Altermagnets combine antiferromagnetic order with ferromagnet-like spin splitting, a duality that unlocks ultrafast spin-dependent responses. This unique property creates unprecedented opportunities for spin-current generation, overcoming the intrinsic limitations of conventional spin-transfer and spin-orbit torque approaches in magnetic memory technologies. Here, we establish a fundamental relati… ▽ More Altermagnets combine antiferromagnetic order with ferromagnet-like spin splitting, a duality that unlocks ultrafast spin-dependent responses. This unique property creates unprecedented opportunities for spin-current generation, overcoming the intrinsic limitations of conventional spin-transfer and spin-orbit torque approaches in magnetic memory technologies. Here, we establish a fundamental relationship between Fermi surface geometry and time-reversal-odd ($\mathcal{T}$-odd) spin currents in altermagnets through combined model analysis and first-principles calculations. We demonstrate that a $d$-wave altermagnet with a flat Fermi surface can achieve a theoretical upper limit of charge-to-spin conversion efficiency (CSE) of 100%. This mechanism is realized in the newly discovered room-temperature altermagnetic metal KV$_2$O$_2$Se, which exhibits a CSE of $\sim$78% at the charge neutrality point, nearly double that of RuO$_2$, setting a new record for $\mathcal{T}$-odd CSE. Under electron doping, this efficiency further increases to $\sim$98%, approaching the theoretical limit. Our work advances the fundamental understanding of $\mathcal{T}$-odd spin currents via Fermi surface geometry engineering and provides key insights for developing next-generation altermagnet-based memory devices. △ Less

Submitted 9 June, 2025; originally announced June 2025.

Bridging Theory and Experiment in Materials Discovery: Machine-Learning-Assisted Prediction of Synthesizable Structures

Authors: Yu Xin, Peng Liu, Zhuohang Xie, Wenhui Mi, Pengyue Gao, Hong Jian Zhao, Jian Lv, Yanchao Wang, Yanming Ma

Abstract: Even though thermodynamic energy-based crystal structure prediction (CSP) has revolutionized materials discovery, the energy-driven CSP approaches often struggle to identify experimentally realizable metastable materials synthesized through kinetically controlled pathways, creating a critical gap between theoretical predictions and experimental synthesis. Here, we propose a synthesizability-driven… ▽ More Even though thermodynamic energy-based crystal structure prediction (CSP) has revolutionized materials discovery, the energy-driven CSP approaches often struggle to identify experimentally realizable metastable materials synthesized through kinetically controlled pathways, creating a critical gap between theoretical predictions and experimental synthesis. Here, we propose a synthesizability-driven CSP framework that integrates symmetry-guided structure derivation with a Wyckoff encode-based machine-learning model, allowing for the efficient localization of subspaces likely to yield highly synthesizable structures. Within the identified promising subspaces, a structure-based synthesizability evaluation model, fine-tuned using recently synthesized structures to enhance predictive accuracy, is employed in conjunction with ab initio calculations to systematically identify synthesizable candidates. The framework successfully reproduces 13 experimentally known XSe (X = Sc, Ti, Mn, Fe, Ni, Cu, Zn) structures, demonstrating its effectiveness in predicting synthesizable structures. Notably, 92,310 structures are filtered from the 554,054 candidates predicted by GNoME, exhibiting great potential for promising synthesizability. Additionally, eight thermodynamically favorable Hf-X-O (X = Ti, V, and Mn) structures have been identified, among which three HfV$_2$O$_7$ candidates exhibit high synthesizability, presenting viable candidates for experimental realization and potentially associated with experimentally observed temperature-induced phase transitions. This work establishes a data-driven paradigm for machine-learning-assisted inorganic materials synthesis, highlighting its potential to bridge the gap between computational predictions and experimental realization while unlocking new opportunities for the targeted discovery of novel functional materials. △ Less

Submitted 14 May, 2025; originally announced May 2025.

Nonlinear optical response in kagome lattice with inversion symmetry breaking

Authors: Xiangyang Liu, Junwen Lai, Jie Zhan, Tianye Yu, Peitao Liu, Seiji Yunoki, Xing-Qiu Chen, Yan Sun

Abstract: The kagome lattice is a fundamental model structure in condensed matter physics and materials science featuring symmetry-protected flat bands, saddle points, and Dirac points. This structure has emerged as an ideal platform for exploring various quantum physics. By combining effective model analysis and first-principles calculations, we propose that the synergy among inversion symmetry breaking, f… ▽ More The kagome lattice is a fundamental model structure in condensed matter physics and materials science featuring symmetry-protected flat bands, saddle points, and Dirac points. This structure has emerged as an ideal platform for exploring various quantum physics. By combining effective model analysis and first-principles calculations, we propose that the synergy among inversion symmetry breaking, flat bands, and saddle point-related van Hove singularities within the kagome lattice holds significant potential for generating strong second-order nonlinear optical response. This property provides an inspiring insight into the practical application of the kagome-like materials, which is helpful for a comprehensive understanding of kagome lattice-related physics. Moreover, this work offers an alternative approach for designing materials with strong a second-order nonlinear optical response. △ Less

Submitted 13 May, 2025; originally announced May 2025.

A universal scaling law for active diffusion in complex media

Authors: Qun Zhang, Yuxin Tian, Xue Zhang, Xiaoting Yu, Hongwei Zhu, Ning Zheng, Luhui Ning, Ran Ni, Mingcheng Yang, Peng Liu

Abstract: Using granular experiments and computer simulations, we investigate the long-time diffusion of active tracers in a broad class of complex media composed of frozen obstacles of diverse structures. By introducing a dimensionless persistence length $Q = v_d τ_r / d_t$, we propose a modified scaling relation that independently collapses experimental and simulation results across active and passive par… ▽ More Using granular experiments and computer simulations, we investigate the long-time diffusion of active tracers in a broad class of complex media composed of frozen obstacles of diverse structures. By introducing a dimensionless persistence length $Q = v_d τ_r / d_t$, we propose a modified scaling relation that independently collapses experimental and simulation results across active and passive particles, diverse media, and distinct propulsion mechanisms. Our results reveal a universal active diffusion-structure relation that holds across both equilibrium and nonequilibrium regimes, providing a simple predictive framework for active diffusion in complex environments. △ Less

Submitted 4 May, 2025; originally announced May 2025.

Engineering Graphene Nanoribbons via Periodically Embedding Oxygen Atoms

Authors: Yan Zhao, Li-Xia Kang, Yi-Jun Wang, Yi Wu, Guang-Yan Xing, Shi-Wen Li, Jinliang Pan, Nie-Wei Wang, Yin-Ti Ren, Ying Wang, Ya-Cheng Zhu, Xing-Qiang Shi, Mengxi Liu, Xiaohui Qiu, Pei-Nian Liu, Deng-Yuan Li

Abstract: Heteroatom doping is an important method for engineering graphene nanoribbons (GNRs) because of its ability to modify electronic properties by introducing extra electrons or vacancies. However, precisely integrating oxygen atoms into the lattice of GNRs is unexplored, and the resulting electronic properties remain elusive. Here, we achieve the precise embedding of oxygen atoms into the lattice of… ▽ More Heteroatom doping is an important method for engineering graphene nanoribbons (GNRs) because of its ability to modify electronic properties by introducing extra electrons or vacancies. However, precisely integrating oxygen atoms into the lattice of GNRs is unexplored, and the resulting electronic properties remain elusive. Here, we achieve the precise embedding of oxygen atoms into the lattice of GNRs via in situ formation of pyrans, synthesizing two types of oxygen-doped GNRs (O-doped chevron-GNR and O-doped chiral (2,1)-GNR). Using scanning tunneling microscopy, non-contact atomic force microscopy, and density functional theory calculations, the atomic structures and electronic properties of O-doped GNRs are determined, demonstrating that both GNRs are direct bandgap semiconductors with different sensitivities to oxygen dopants. Oxygen dopants have a minor impact on the bandgap of chevron-GNR but a significant effect on the bandgap of chiral (2,1)-GNR, which is attributed to the difference in density of states near the Fermi level between substituted intrinsic carbon atoms and their pristine counterparts. Compared with the pristine chiral (2,1)-GNR, the band structure of O-doped chiral (2,1)-GNR exhibits unexpected band edges transition, which is ascribed to sp2-hybridized oxygen atoms which introduces additional electrons to the conduction band of chiral (2,1)-GNR, leading to the upward shift of Fermi surface. △ Less

Submitted 25 April, 2025; originally announced April 2025.

Floquet control of topological phases and Hall effects in Z2 nodal line semimetals

Authors: Pu Liu, Chaoxi Cui, Lei Li, Runze Li, Dong-Hui Xu, Zhi-Ming Yu

Abstract: Dynamic control of topological properties in materials is central to modern condensed matter physics, and Floquet engineering, utilizing periodic light fields, provides a promising avenue. Here, we use Floquet theory to theoretically study the topological response of a Z2 nodal line semimetal (NLSM) when driven by circularly polarized light (CPL). We demonstrate that the direction of CPL irradiati… ▽ More Dynamic control of topological properties in materials is central to modern condensed matter physics, and Floquet engineering, utilizing periodic light fields, provides a promising avenue. Here, we use Floquet theory to theoretically study the topological response of a Z2 nodal line semimetal (NLSM) when driven by circularly polarized light (CPL). We demonstrate that the direction of CPL irradiation critically dictates the resulting topological phase transitions. Specifically, when light is incident perpendicular to the nodal line plane, increasing the light amplitude induces two successive topological phase transitions: first, from the Z2 NLSM to a vortex NLSM, a rare and intriguing topological state; and second, a transition from the vortex NLSM to a semimetal with a pair of Weyl points (WPs). In stark contrast, irradiation along other directions directly transforms the Z2 nodal line into a pair of WPs. We further investigate the transport properties of the Floquet Z2 NLSM, focusing on the anomalous and planar Hall effects. The anomalous Hall effect exhibits a direction-dependent amplitude variation, deviating from conventional two-band NLSM behavior. Importantly, we reveal a significant and tunable planar Hall effect, a phenomenon largely unexplored in Floquet topological materials, which is highly sensitive to both light amplitude and direction. Our findings not only present a novel route to realize the vortex NLSM, but also establish an efficient way to control Hall transport phenomena in Z2 NLSMs. △ Less

Submitted 27 March, 2025; v1 submitted 25 March, 2025; originally announced March 2025.

Comments: 9 pages, 5 figures

Revisiting the charge-density-wave superlattice of 1$T$-TiSe$_2$

Authors: Wei Wang, Patrick Liu, Lijun Wu, Jing Tao, Genda Gu, Alfred Zong, Yimei Zhu

Abstract: A number of intriguing phenomena, including exciton condensation, orbital ordering, and emergence of chirality, have been proposed to accompany charge-density-wave (CDW) formation in the layered transition metal dichalcogenide 1$T$-TiSe$_2$. Explaining these effects relies on knowledge of the atomic displacement pattern underlying the CDW, yet structural proposals based on spatially-averaging bulk… ▽ More A number of intriguing phenomena, including exciton condensation, orbital ordering, and emergence of chirality, have been proposed to accompany charge-density-wave (CDW) formation in the layered transition metal dichalcogenide 1$T$-TiSe$_2$. Explaining these effects relies on knowledge of the atomic displacement pattern underlying the CDW, yet structural proposals based on spatially-averaging bulk crystal diffraction and surface-dependent scanning tunneling microscopy have remained inconsistent. Here, we revisit the CDW superlattice structure with selected-area electron diffraction, a bulk-sensitive probe capable of capturing sub-micrometer spatial variations while maintaining high momentum resolution. We resolved two distinct, spatially separated CDW phases characterized by different interlayer ordering. In both phases, previously reported atomic displacement patterns fail to account for the observed extinction rules. Instead, our analysis reveals a new superlattice structure, which features a large number of nearly degenerate CDW domains. These findings not only provide a new basis for understanding the gyrotropic electronic order and metastability in 1$T$-TiSe$_2$, they also underscore the importance of bulk-sensitive mesoscopic techniques in investigating materials that host unconventional superlattices. △ Less

Submitted 16 February, 2025; originally announced February 2025.

Electric field control of nonlinear Hall effect in Weyl semimetal TaIrTe4

Authors: Jiaju Yang, Lujun Wei, Yanghui Li, Lina Chen, Wei Niu, Shuo Wang, Feng Li, Ping Liu, Shuang Zhou, Yong Pu

Abstract: The nonlinear Hall effect (NLHE), as an important probe to reveal the symmetry breaking in topological properties of materials, opens up a new dimension for exploring the energy band structure and electron transport mechanism of quantum materials. Current studies mainly focus on the observation of material intrinsic the NLHE or inducing the NLHE response by artificially constructing corrugated/twi… ▽ More The nonlinear Hall effect (NLHE), as an important probe to reveal the symmetry breaking in topological properties of materials, opens up a new dimension for exploring the energy band structure and electron transport mechanism of quantum materials. Current studies mainly focus on the observation of material intrinsic the NLHE or inducing the NLHE response by artificially constructing corrugated/twisted twodimensionalmaterial systems. Notably, the modulation of NLHE signal strength, a core parameter of device performance, has attracted much attention, while theoretical predictions suggest that an applied electric field can achieve the NLHE enhancement through modulation of the Berry curvature dipole (BCD). Here we report effective modulation the magnitude and sign of the NLHE by applying additional constant electric fields of different directions and magnitudes in the semimetal TaIrTe4. The NLHE response strength is enhanced by 168 times compared to the intrinsic one at 4 K when the additional constant electric field of -0.5 kV/cm is applied to the b-axis of TaIrTe4 and the through a.c. current is parallel to the TaIrTe4 a-axis. Scaling law analysis suggests that the enhancement may be the result of the combined effect of the electric field on the intrinsic BCD and disorder scattering effect of TaIrTe4. This work provides a means to study the properties of TaIrTe4, as well as a valuable reference for the study of novel electronic devices. △ Less

Submitted 9 February, 2025; originally announced February 2025.

Comments: 19 pages, 4 figures

Multi-level mechanical modeling and computational design framework for weft knitted fabrics

Authors: Cosima du Pasquier, Sehui Jeong, Pan Liu, Susan Williams, Nour Mnejja, Allison M. Okamura, Skylar Tibbits, Tian Chen

Abstract: This work presents a multi-level modeling and design framework for weft knitted fabrics, beginning with a volumetric finite element analysis capturing their mechanical behavior from fundamental principles. Incorporating yarn-level data, it accurately predicts stress-strain responses, reducing the need for extensive physical testing. A simplified strain energy approach homogenizes the results into… ▽ More This work presents a multi-level modeling and design framework for weft knitted fabrics, beginning with a volumetric finite element analysis capturing their mechanical behavior from fundamental principles. Incorporating yarn-level data, it accurately predicts stress-strain responses, reducing the need for extensive physical testing. A simplified strain energy approach homogenizes the results into three key variables, enabling rapid, accurate predictions in minutes. After validation against experiments, our framework can simulate new knit fabrics without additional tests. In real-world scenarios, fabrics often feature variations in yarn materials or patterns. The framework extends to heterogeneous fabrics, showing that transitions between distinct regions can be captured using simple mechanical analogies: springs in series and parallel. This allows heterogeneous textiles to be treated as idealized patchworks of homogeneous pieces, preserving predictive accuracy. The method is demonstrated by designing and producing a compression sleeve with uniform pressure, illustrating how the framework supports development of knits tailored to specific assistance levels and anatomical features. By combining volumetric finite element analysis, simplified model through homogenization, and controlled material transitions, this approach provides a scalable, high-fidelity path toward next-generation weft knitted fabric design. △ Less

Submitted 21 October, 2025; v1 submitted 13 January, 2025; originally announced January 2025.

Field-free perpendicular magnetization switching of low critical current density at room temperature in TaIrTe4/ferromagnet heterostructures

Authors: Lujun Wei, Pai Liu, Jincheng Peng, Yanghui Li, Lina Chen, Ping Liu, Feng Li, Wei Niu, Fei Huang, Jiaju Yang, Shuang Zhou, Yu Lu, Tianyu Liu, Jiarui Chen, Weihao Wang, Jian Zhang, Jun Du, Yong Pu

Abstract: Spin-orbit torque-induced perpendicular magnetization switching has attracted much attention due to the advantages of nonvolatility, high density, infinite read/write counts, and low power consumption in spintronic applications. To achieve field-free deterministic switching of perpendicular magnetization, additional magnetic field, magnetic layer assistance, or artificially designed structural sym… ▽ More Spin-orbit torque-induced perpendicular magnetization switching has attracted much attention due to the advantages of nonvolatility, high density, infinite read/write counts, and low power consumption in spintronic applications. To achieve field-free deterministic switching of perpendicular magnetization, additional magnetic field, magnetic layer assistance, or artificially designed structural symmetry breaking are usually required, which are not conducive to the high-density integration and application of low-power devices. However, 2D Weyl semimetals with low-symmetry structures have recently been found to generate z-spin-polarized currents, which may induce out-of-plane damping-like torques to their neighboring ferromagnetic layers, and realize deterministic perpendicular magnetization switching at zero magnetic field. In this Letter, we report that current-induced field-free magnetization switching at room temperature can be achieved in a perpendicularly magnetized TaIrTe4/Pt/Co/Pt device, and the critical switching current density can be lowered to be about 2.64*105 Acm-2. This study suggests that TaIrTe4 has great potential for the design of room-temperature efficient spintronic devices. △ Less

Submitted 7 January, 2025; originally announced January 2025.

Interaction-induced inversion of chiral transports

Authors: Li Pan, Qian Liang, Chang-An Yang, Yu Huang, Pengjie Liu, Fanying Xi, Wei Yi, Xiaofan Zhou, Jian-Song Pan

Abstract: We study the chiral transport of interacting bosons in a two-leg flux ladder with on-site interactions. Focusing on the flux-induced chiral current along the two legs, we show that, counter-intuitively, on-site interactions can reverse the direction of the chiral flow. For a Bose-Einstein condensate whose dynamical evolution is driven by the Gross-Pitaevskii equation under the mean-field approxima… ▽ More We study the chiral transport of interacting bosons in a two-leg flux ladder with on-site interactions. Focusing on the flux-induced chiral current along the two legs, we show that, counter-intuitively, on-site interactions can reverse the direction of the chiral flow. For a Bose-Einstein condensate whose dynamical evolution is driven by the Gross-Pitaevskii equation under the mean-field approximation, this reversal can be understood as an interaction-induced dynamic occupation inversion, under which single-particle band with opposing chirality becomes heavily populated in the dynamics. This chirality inversion also persists in the two-body dynamics with strong quantum fluctuations beyond the mean-field regime, as demonstrated through time-dependent density-matrix renormalization group and exact diagonalization analyses. Herein, besides the band-occupation-inversion mechanism, we find that the formation of two-body bound states with opposite chirality contributes significantly to the reversed chiral transport. Our discovery highlights the significance of correlation effects in quantum transport, and can be readily demonstrated using cold atoms. △ Less

Submitted 24 December, 2024; originally announced December 2024.

Comments: 7 pages+3 figures

Magnetism and weak electronic correlations in Kagome metal ScV$_6$Sn$_6$

Authors: Tianye Yu, Junwen Lai, Xiangyang Liu, Peitao Liu, Xing-Qiu Chen, Yan Sun

Abstract: As one class of typical quantum materials, Kagome metals in $A$V$_3$Sb$_5$($A$ = K, Rb, Cs) have attracted extensive attentions due to their interesting physical properties and different quantum phases of charge density wave (CDW), superconductivity and nontrivial topology. Recently, a new CDW phase in ScV$_6$Sn$_6$ was experimentally observed and inspired a wide study of the mechanism of driving… ▽ More As one class of typical quantum materials, Kagome metals in $A$V$_3$Sb$_5$($A$ = K, Rb, Cs) have attracted extensive attentions due to their interesting physical properties and different quantum phases of charge density wave (CDW), superconductivity and nontrivial topology. Recently, a new CDW phase in ScV$_6$Sn$_6$ was experimentally observed and inspired a wide study of the mechanism of driving force. To have a clear understanding of the correlation effect in the CDW phase in ScV$_6$Sn$_6$, we performed a systematic density functional theory plus dynamical mean field theory (DFT + DMFT) calculations. The resulting static local spin susceptibility is nearly independent of temperature, indicating the absence of local moment on atom V, in full agreement with experimental measurements. The mass enhancements of quasiparticles and bandwidth renormalizations near the Fermi level show a weak correlation strength in ScV$_6$Sn$_6$. In addition, the comparable mass enhancements of quasiparticles in ScV$_6$Sn$_6$ with CDW order and YV$_6$Sn$_6$ without CDW phase suggests that electronic correlations corresponding to Fermi surface nesting do not play the dominant role in the formation of CDW order in ScV$_6$Sn$_6$. △ Less

Submitted 17 December, 2024; originally announced December 2024.

Comments: 7 pages, 6 figures

Néel vector-dependent anomalous transport in altermagnetic metal CrSb

Authors: Tianye Yu, Ijaz Shahid, Peitao Liu, Ding-Fu Shao, Xing-Qiu Chen, Yan Sun

Abstract: Altermagnets are predicted to exhibit anomalous transport phenomena, such as the anomalous Hall and Nernst effects, as observed in ferromagnets but with a vanishing net magnetic moment, akin to antiferromagnets. Despite their potential, progress has been limited due to the scarcity of metallic altermagnets. Motivated by the recent discovery of the altermagnetic metal CrSb, we conducted a systemati… ▽ More Altermagnets are predicted to exhibit anomalous transport phenomena, such as the anomalous Hall and Nernst effects, as observed in ferromagnets but with a vanishing net magnetic moment, akin to antiferromagnets. Despite their potential, progress has been limited due to the scarcity of metallic altermagnets. Motivated by the recent discovery of the altermagnetic metal CrSb, we conducted a systematic study of its electrical and thermoelectric transport properties, using first-principles calculations. CrSb exhibits low magnetocrystalline anisotropy energy, enabling the manipulation of the Néel vector in CrSb films through a suitable ferromagnetic substrate. The anomalous Hall and Nernst conductivities reach their maximum when the Néel vector is aligned along $\frac{1}{2}$\textbf{\textit{a}}+\textbf{\textit{b}}. The origins of both conductivities were analyzed in terms of Berry curvature distribution. Our results demonstrate that CrSb provides a good platform for investigating the Néel vector-dependent anomalous transport in altermagnetic metals. △ Less

Submitted 6 May, 2025; v1 submitted 17 December, 2024; originally announced December 2024.

Comments: 7 pages, 6 figures

Exotic properties and manipulation in 2D semimetal Mn2B2(OH)2: a theoretical study

Authors: Pingwei Liu, Dan Liu, Shixin Song, Kang Li, Xueyong Yuan, Jie Guan

Abstract: Most functional materials possess one single outstanding property and are limited to be used for a particular purpose. Instead of integrating materials with different functions into one module, designing materials with controllable multi-functions is more promising for the electronic industry. In this study, we investigate an unexplored alpha-phase of two-dimensional (2D) Mn2B2(OH)2 theoretically.… ▽ More Most functional materials possess one single outstanding property and are limited to be used for a particular purpose. Instead of integrating materials with different functions into one module, designing materials with controllable multi-functions is more promising for the electronic industry. In this study, we investigate an unexplored alpha-phase of two-dimensional (2D) Mn2B2(OH)2 theoretically. Eighteen distinct electrical polarizations, characterized by three different magnitudes and twelve different directions, are found in this phase. The switch of the electrical polarizations is also linked to an observed splitting of band structures between different spin states and the ferroelasticity of the system. The manipulation of these properties can be realized through controlling the alignment of Mn-OH-Mn chains. Additionally, the approximately honeycomb lattice for the atomic layer of boron indicate the potential superconductivity in the system. The diverse and tunable properties make the proposed material as an outstanding candidate for sensing applications at the 2D limit. △ Less

Submitted 6 December, 2024; originally announced December 2024.

Comments: Accepted by Physical Review B

Surface molecular engineering to enable processing of sulfide solid electrolytes in humid ambient air

Authors: Mengchen Liu, Jessica J. Hong, Elias Sebti, Ke Zhou, Shen Wang, Shijie Feng, Tyler Pennebaker, Zeyu Hui, Qiushi Miao, Ershuang Lu, Nimrod Harpak, Sicen Yu, Jianbin Zhou, Jeong Woo Oh, Min-Sang Song, Jian Luo, Raphaële J. Clément, Ping Liu

Abstract: Sulfide solid state electrolytes are promising candidates to realize all solid state batteries due to their superior ionic conductivity and excellent ductility. However, their hypersensitivity to moisture requires processing environments that are not compatible with todays lithium ion battery manufacturing infrastructure. Herein, we present a reversible surface modification strategy that enables t… ▽ More Sulfide solid state electrolytes are promising candidates to realize all solid state batteries due to their superior ionic conductivity and excellent ductility. However, their hypersensitivity to moisture requires processing environments that are not compatible with todays lithium ion battery manufacturing infrastructure. Herein, we present a reversible surface modification strategy that enables the processability of sulfide SSEs under humid ambient air. We demonstrate that a long chain alkyl thiol, undecanethiol, is chemically compatible with the electrolyte with negligible impact on its ion conductivity. Importantly, the thiol modification extends the amount of time that the sulfide SSE can be exposed to air with 33 percent relative humidity with limited degradation of its structure while retaining a conductivity of above 1 mS per cm for up to 2 days, a more than 100 fold improvement in protection time over competing approaches. Experimental and computational results reveal that the thiol group anchors to the SSE surface, while the hydrophobic hydrocarbon tail provides protection by repelling water. The modified Li6PS5Cl SSE maintains its function after exposure to ambient humidity when implemented in a Li0.5In LiNi0.8Co0.1Mn0.1O2 ASSB. The proposed protection strategy based on surface molecular interactions represents a major step forward towards cost competitive and energy efficient sulfide SSE manufacturing for ASSB applications. △ Less

Submitted 5 December, 2024; originally announced December 2024.

Comments: 38 pages, 6 figures

Quantum Delocalization Enables Water Dissociation on Ru(0001)

Authors: Yu Cao, Jiantao Wang, Mingfeng Liu, Yan Liu, Hui Ma, Cesare Franchini, Yan Sun, Georg Kresse, Xing-Qiu Chen, Peitao Liu

Abstract: We revisit the long-standing question of whether water molecules dissociate on the Ru(0001) surface through nanosecond-scale path-integral molecular dynamics simulations on a sizable supercell. This is made possible through the development of an efficient and reliable machine-learning potential with near first-principles accuracy, overcoming the limitations of previous ab initio studies. We show t… ▽ More We revisit the long-standing question of whether water molecules dissociate on the Ru(0001) surface through nanosecond-scale path-integral molecular dynamics simulations on a sizable supercell. This is made possible through the development of an efficient and reliable machine-learning potential with near first-principles accuracy, overcoming the limitations of previous ab initio studies. We show that the quantum delocalization associated with nuclear quantum effects enables rapid and frequent proton transfers between water molecules, thereby facilitating the water dissociation on Ru(0001). This work provides the direct theoretical evidence of water dissociation on Ru(0001), resolving the enduring issue in surface sciences and offering crucial atomistic insights into water-metal interfaces. △ Less

Submitted 19 April, 2025; v1 submitted 30 November, 2024; originally announced December 2024.

Comments: 24 pages, 15 figures (include Supplemental Material)

Journal ref: Phys. Rev. Lett. 134, 178001 (2025)

The Fe-N system: crystal structure prediction, phase stability, and mechanical properties

Authors: Ergen Bao, Jinbin Zhao, Qiang Gao, Ijaz Shahid, Hui Ma, Yixiu Luo, Peitao Liu, Yan Sun, Xing-Qiu Chen

Abstract: Nitriding introduces nitrides into the surface of steels, significantly enhancing the surface me-chanical properties. By combining the variable composition evolutionary algorithm and first-principles calculations based on density functional theory, 50 thermodynamically stable or metastable Fe-N compounds with various stoichiometric ratios were identified, exhibiting also dynamic and mechanical sta… ▽ More Nitriding introduces nitrides into the surface of steels, significantly enhancing the surface me-chanical properties. By combining the variable composition evolutionary algorithm and first-principles calculations based on density functional theory, 50 thermodynamically stable or metastable Fe-N compounds with various stoichiometric ratios were identified, exhibiting also dynamic and mechanical stability. The mechanical properties of these structures were systemati-cally studied, including the bulk modulus, shear modulus, Young's modulus, Poisson's ratio, Pugh's ratio, Cauchy pressure, Klemen parameters, universal elastic anisotropy, Debye tempera-ture, and Vickers hardness. All identified stable and metastable Fe-N compounds were found in the ductile region, with most exhibiting homogeneous elastic properties and isotropic metallic bonding. As the nitrogen concentration increases, their bulk moduli generally increase as well. The Vickers hardness values of Fe-N compounds range from 3.5 to 10.5 GPa, which are signifi-cantly higher than that of pure Fe (2.0 GPa), due to the stronger Fe-N bonds strength. This study provides insights into optimizing and designing Fe-N alloys with tailored mechanical properties. △ Less

Submitted 26 November, 2024; originally announced November 2024.

Comments: 19 pages, 19 figures

Thickness-dependent Topological Phases and Flat Bands in Rhombohedral Multilayer Graphene

Authors: H. B. Xiao, C. Chen, X. Sui, S. H. Zhang, M. Z. Sun, H. Gao, Q. Jiang, Q. Li, L. X. Yang, M. Ye, F. Y. Zhu, M. X. Wang, J. P. Liu, Z. B. Zhang, Z. J. Wang, Y. L. Chen, K. H. Liu, Z. K. Liu

Abstract: Rhombohedral multilayer graphene has emerged as an extraordinary platform for investigating exotic quantum states, such as superconductivity and fractional quantum anomalous Hall effects, mainly due to the existence of topological surface flatbands. Despite extensive research efforts, a systematic spectroscopic investigation on the evolution of its electronic structure from thin layers to bulk rem… ▽ More Rhombohedral multilayer graphene has emerged as an extraordinary platform for investigating exotic quantum states, such as superconductivity and fractional quantum anomalous Hall effects, mainly due to the existence of topological surface flatbands. Despite extensive research efforts, a systematic spectroscopic investigation on the evolution of its electronic structure from thin layers to bulk remains elusive. Using state-of-the-art angle-resolved photoemission spectroscopy with submicron spatial resolution, we directly probe and trace the thickness evolution of the topological electronic structures of rhombohedral multilayer graphene. As the layer number increases, the gapped subbands transform into the 3D Dirac nodes that spirals in the momentum space; while the flatbands are constantly observed around Fermi level, and eventually evolve into the topological drumhead surface states. This unique thickness-dependent topological phase transition can be well captured by the 3D generalization of 1D Su-Schrieffer-Heeger chain in thin layers, to the topological Dirac nodal spiral semimetal in the bulk limit. Our findings establish a solid foundation for exploring the exotic quantum phases with nontrivial topology and correlation effects in rhombohedral multilayer graphene. △ Less

Submitted 25 November, 2024; v1 submitted 18 November, 2024; originally announced November 2024.

Comments: 15 pages, 4 figures, under review. A note added

Journal ref: Science Bulletin (2025)

Efficient moment tensor machine-learning interatomic potential for accurate description of defects in Ni-Al Alloys

Authors: Jiantao Wang, Peitao Liu, Heyu Zhu, Mingfeng Liu, Hui Ma, Yun Chen, Yan Sun, Xing-Qiu Chen

Abstract: Combining the efficiency of semi-empirical potentials with the accuracy of quantum mechanical methods, machine-learning interatomic potentials (MLIPs) have significantly advanced atomistic modeling in computational materials science and chemistry. This necessitates the continual development of MLIP models with improved accuracy and efficiency, which enable long-time scale molecular dynamics simula… ▽ More Combining the efficiency of semi-empirical potentials with the accuracy of quantum mechanical methods, machine-learning interatomic potentials (MLIPs) have significantly advanced atomistic modeling in computational materials science and chemistry. This necessitates the continual development of MLIP models with improved accuracy and efficiency, which enable long-time scale molecular dynamics simulations to unveil the intricate underlying mechanisms that would otherwise remain elusive. Among various existing MLIP models, the moment tensor potential (MTP) model employs a highly descriptive rotationally-covariant moment tensor to describe the local atomic environment, enabling the use of even linear regression for model fitting. Although the current MTP model has achieved state-of-the-art efficiency for similar accuracy, there is still room for optimizing the contraction process of moment tensors. In this work, we propose an effective genetic algorithm based optimization scheme that can significantly reduce the number of independent moment tensor components and intermediate tensor components. This leads to a speedup of nearly one order of magnitude in efficiency and also improved accuracy compared to the traditional MTP model for intricate basis sets. We have applied our improved MTP model to predicting the energetic and dynamical properties of various point and planar defects in Ni-Al alloys, showing overall good performances and in general outperforming the semi-empirical potentials. This work paves the way for fast and accurate atomistic modeling of complex systems and provides a useful tool for modeling defects in Ni-Al alloys. △ Less

Submitted 13 May, 2025; v1 submitted 2 November, 2024; originally announced November 2024.

Comments: 52 pages, 13 figures

Journal ref: Phys. Rev. Materials 9, 053805 (2025)

Origin of the charge density wave state in BaFe$_2$Al$_9$

Authors: Yuping Li, Mingfeng Liu, Jiangxu Li, Jiantao Wang, Junwen Lai, Dongchang He, Ruizhi Qiu, Yan Sun, Xing-Qiu Chen, Peitao Liu

Abstract: Recently, a first-order phase transition associated with charge density wave (CDW) has been observed at low temperatures in intermetallic compound BaFe$_2$Al$_9$. However, this transition is absent in its isostructural sister compound BaCo$_2$Al$_9$. Consequently, an intriguing question arises as to the underlying factors that differentiate BaFe$_2$Al$_9$ from BaCo$_2$Al$_9$ and drive the CDW tran… ▽ More Recently, a first-order phase transition associated with charge density wave (CDW) has been observed at low temperatures in intermetallic compound BaFe$_2$Al$_9$. However, this transition is absent in its isostructural sister compound BaCo$_2$Al$_9$. Consequently, an intriguing question arises as to the underlying factors that differentiate BaFe$_2$Al$_9$ from BaCo$_2$Al$_9$ and drive the CDW transition in BaFe$_2$Al$_9$. Here, we set out to address this question by conducting a comparative \emph{ab initio} study of the electronic structures, lattice dynamics, \textcolor{black}{and electron-phonon interactions} of their high-temperature phases. We find that both compounds are dynamically stable with similar phonon dispersions. The electronic structure calculations reveal that both compounds are nonmagnetic metals; however, they exhibit distinct band structures around the Fermi level. In particular, BaFe$_2$Al$_9$ exhibits a higher density of states at the Fermi level with dominant partially filled Fe-$3d$ states and a more intricate Fermi surface. This leads to an electronic instability of BaFe$_2$Al$_9$ toward the CDW transition, which is manifested by the diverged electronic susceptibility at the CDW wave vector $\mathbf{q}_{\rm CDW}$=(0.5, 0, 0.3), observable in both the real and imaginary parts. Conversely, BaCo$_2$Al$_9$ does not display such behavior, aligning well with experimental observations. Although the electron-phonon interactions in BaFe$_2$Al$_9$ surpass those in BaCo$_2$Al$_9$ by two orders of magnitude, the strength is relatively weak at the CDW wave vector, suggesting that the CDW in BaFe$_2$Al$_9$ is primarily driven by electronic factors. △ Less

Submitted 30 October, 2024; originally announced October 2024.

Comments: 9 pages, 6 figures

Journal ref: Phys. Rev. B 110, 195118 (2024)

Dissipation and dephasing in quantum Hall interferometers

Authors: Peng-Yi Liu, Qing-Feng Sun

Abstract: In recent years, counter-intuitive results have shown that the quantum Hall edge states with topological protection can be dissipative. In this paper, we point out that the non-equilibrium nature of edge states in quantum Hall interferometers leads to inevitable dissipation. We consider a graphene interferometer operating in the integer quantum Hall regime and simulate the inelastic scattering tha… ▽ More In recent years, counter-intuitive results have shown that the quantum Hall edge states with topological protection can be dissipative. In this paper, we point out that the non-equilibrium nature of edge states in quantum Hall interferometers leads to inevitable dissipation. We consider a graphene interferometer operating in the integer quantum Hall regime and simulate the inelastic scattering that causes both dissipation and dephasing in the interferometer using non-equilibrium Green's function and virtual leads. We describe the dissipation process with the numerical results of the spatial distribution of heat generation and the evolution of electron energy distribution. In addition, with the enhancement of dephasing, a competition between Aharonov-Bohm interference and topologically protected quantized Hall plateaus is observed in the oscillations and fluctuations of the Hall resistances. At a suitable dephasing strength, quantum Hall plateaus can be promoted by dephasing. Our results not only give clues for the design of dissipation-free devices but also provide a platform for studying the non-equilibrium relaxation and the dissipation mechanism of topological states. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: 14 pages, 7 figures

Journal ref: Phys. Rev. B 110, 085411 (2024)

Four-terminal graphene-superconductor thermal switch controlled by the superconducting phase difference

Authors: Peng-Yi Liu, Yue Mao, Qing-Feng Sun

Abstract: We propose a superconducting phase-controlled thermal switch based on a four-terminal graphene-superconductor system. By the coupling of two superconducting leads on a zigzag graphene nanoribbon, both the normal-transmission coefficient and the crossed-Andreev-reflection coefficient, which dominate the thermal conductivity of electrons in the graphene nanoribbon, can be well controlled simultaneou… ▽ More We propose a superconducting phase-controlled thermal switch based on a four-terminal graphene-superconductor system. By the coupling of two superconducting leads on a zigzag graphene nanoribbon, both the normal-transmission coefficient and the crossed-Andreev-reflection coefficient, which dominate the thermal conductivity of electrons in the graphene nanoribbon, can be well controlled simultaneously by the phase difference of the superconducting leads. As a result, the thermal conductivity of electrons in the graphene nanoribbon can be tuned and a thermal switching effect appears. Using the nonequilibrium Green's function method, we verify this thermal switching effect numerically. At ambient temperatures less than about one tenth of the superconducting transition temperature, the thermal switching ratio can exceed 2000. The performance of the thermal switch can be regulated by the ambient temperature, and doping or gating can slightly increase the thermal switching ratio. The use of narrower graphene nanoribbons and wider superconducting leads facilitates the obtaining of larger thermal switching ratios. This switching effect of electronic thermal conductance in graphene is expected to be experimentally realized and applied. △ Less

Submitted 29 October, 2024; originally announced October 2024.

Comments: 9 pages, 5 figures

Journal ref: Phys. Rev. Applied 21, 024001 (2024)

Classification of Chern Numbers Based on High-Symmetry Points

Authors: Yu-Hao Wan, Peng-Yi Liu, Qing-Feng Sun

Abstract: The Chern number is a crucial topological invariant for distinguishing the phases of Chern insulators. Here we find that for Chern insulators with inversion symmetry, the Chern number alone is insufficient to fully characterize their topology. Specifically, distinct topological phases can be differentiated based on skyrmions at different high-symmetry points. Interfaces between these topological p… ▽ More The Chern number is a crucial topological invariant for distinguishing the phases of Chern insulators. Here we find that for Chern insulators with inversion symmetry, the Chern number alone is insufficient to fully characterize their topology. Specifically, distinct topological phases can be differentiated based on skyrmions at different high-symmetry points. Interfaces between these topological phases exhibit gapless helical states, which provide counter-propagating transport channels and robust quantized transport. Additionally, we identify topological transitions that do not involve changes in the Chern number but can be characterized by transitions of skyrmions between high-symmetry points. These transitions arise due to the toroidal structure of the two-dimensional Brillouin zone, which is generally applicable to two-dimensional periodic lattice system. Our research introduces new degrees of freedom for controlling topological optical transport and deepens the understanding of Chern insulators with inversion symmetry. △ Less

Submitted 28 September, 2024; originally announced September 2024.

Applications of Chebyshev polynomials and Toeplitz theory to topological metamaterials

Authors: Habib Ammari, Silvio Barandun, Ping Liu

Abstract: We survey the use of Chebyshev polynomials and Toeplitz theory for studying topological metamaterials. We consider both Hermitian and non-Hermitian systems of subwavelength resonators and provide a mathematical framework to explain some spectacular properties of metamaterials. We survey the use of Chebyshev polynomials and Toeplitz theory for studying topological metamaterials. We consider both Hermitian and non-Hermitian systems of subwavelength resonators and provide a mathematical framework to explain some spectacular properties of metamaterials. △ Less

Submitted 23 September, 2024; originally announced September 2024.

MSC Class: 35B34; 47B28; 35P25; 35C20; 81Q12; 15A18; 15B05

Evolution of the Coulomb interactions in correlated transition-metal perovskite oxides from the constrained random phase approximation

Authors: Liang Si, Peitao Liu, Cesare Franchini

Abstract: Determining the strength of electronic correlations of correlated electrons plays important roles in accurately describing the electronic structures and physical properties of transition-metal (TM) perovskite oxides. Here, we study the evolution of electronic interaction parameters as a function of $d$-electron occupancy in an extended class of TM perovskite oxides $AB$O$_3$ ($A$=Sr, Ca, and $B$=3… ▽ More Determining the strength of electronic correlations of correlated electrons plays important roles in accurately describing the electronic structures and physical properties of transition-metal (TM) perovskite oxides. Here, we study the evolution of electronic interaction parameters as a function of $d$-electron occupancy in an extended class of TM perovskite oxides $AB$O$_3$ ($A$=Sr, Ca, and $B$=3$d$-5$d$ TM elements) using the constrained random-phase-approximation method adopting two distinct models: $t_{2g}$-$t_{2g}$ and $d$-$dp$. For Sr$B$O$_3$ with $B$=Fe, Ru, and Ir, the $t_{2g}$-$t_{2g}$ model faces critical challenges, as the low-energy Hamiltonian spanning $t_{2g}$ manifolds is ill-defined. The $t_{2g}$-$t_{2g}$ model suggests that, for early $AB$O$_3$ series ($B$=$d^1$-$d^3$), the bare Coulomb interaction parameters $V$ remain nearly constant due to the competition between extended $t_{2g}$ Wannier orbitals and bandwidth reduction. As the $d$-electron filling increases, both partially screened Coulomb interaction parameters $U$ and fully screened Coulomb interaction parameters $W$ decrease, which are attributed to enhanced $e_g$-$t_{2g}$ and $e_g$-$p$ screenings. In contrast to the $t_{2g}$-$t_{2g}$ model, the $d$-$dp$ model effectively handles both early and late $AB$O$_3$ perovskites and reveals different trends. Specifically, $V$ varies inversely with the spreads of $d$-orbitals. $W$ reaches its minimum at the $d^3$ occupancy due to an interplay between increasing $d$-orbital localization and increasing screening effects. An unusual trend is observed for $U$, with local maxima at both $d^1$ and $d^4$ occupations. This can be understood from two aspects: (1) the increasing full screening effects from $d^1$ to $d^3$ and (2) the strongest $d$-$d$ and the weakest $d$-$p$ screening effects near $d^4$ for Sr$B$O$_3$. △ Less

Submitted 19 August, 2024; originally announced August 2024.

Comments: 21 pages, 10 figures, 3 tables

Atomic-Scale Imaging of Fractional Spinon Quasiparticles in Open-Shell Triangulene Spin-$\frac{1}{2}$ Chains

Authors: Zhangyu Yuan, Xin-Yu Zhang, Yashi Jiang, Xiangjian Qian, Ying Wang, Yufeng Liu, Liang Liu, Xiaoxue Liu, Dandan Guan, Yaoyi Li, Hao Zheng, Canhua Liu, Jinfeng Jia, Mingpu Qin, Pei-Nian Liu, Deng-Yuan Li, Shiyong Wang

Abstract: The emergence of spinon quasiparticles, which carry spin but lack charge, is a hallmark of collective quantum phenomena in low-dimensional quantum spin systems. While the existence of spinons has been demonstrated through scattering spectroscopy in ensemble samples, real-space imaging of these quasiparticles within individual spin chains has remained elusive. In this study, we construct individual… ▽ More The emergence of spinon quasiparticles, which carry spin but lack charge, is a hallmark of collective quantum phenomena in low-dimensional quantum spin systems. While the existence of spinons has been demonstrated through scattering spectroscopy in ensemble samples, real-space imaging of these quasiparticles within individual spin chains has remained elusive. In this study, we construct individual Heisenberg antiferromagnetic spin-$\frac{1}{2}$ chains using open-shell [2]triangulene molecules as building blocks. Each [2]triangulene unit, owing to its sublattice imbalance, hosts a net spin-$\frac{1}{2}$ in accordance with Lieb's theorem, and these spins are antiferromagnetically coupled within covalent chains with a coupling strength of $J = 45$ meV. Through scanning tunneling microscopy and spectroscopy, we probe the spin states, excitation gaps, and their spatial excitation weights within covalent spin chains of varying lengths with atomic precision. Our investigation reveals that the excitation gap decreases as the chain length increases, extrapolating to zero for long chains, consistent with Haldane's gapless prediction. Moreover, inelastic tunneling spectroscopy reveals an m-shaped energy dispersion characteristic of confined spinon quasiparticles in a one-dimensional quantum box. These findings establish a promising strategy for exploring the unique properties of excitation quasiparticles and their broad implications for quantum information. △ Less

Submitted 16 August, 2024; originally announced August 2024.

Topological Charge Quadrupole Protected by Spin-Orbit U(1) Quasi-Symmetry in Antiferromagnet NdBiPt

Authors: Ao Zhang, Xiaobing Chen, Jiayu Li, Pengfei Liu, Yuntian Liu, Qihang Liu

Abstract: The interplay of symmetry and topology in crystal solids has given rise to various elementary excitations as quasiparticles. Among these, those with significant Berry-phase-related transport responses are of particular interest. Here, we predict a new type of quasiparticle called topological charge quadruple (TCQ), which is analogous to a charge quadrupole but consists of two closely-packed pairs… ▽ More The interplay of symmetry and topology in crystal solids has given rise to various elementary excitations as quasiparticles. Among these, those with significant Berry-phase-related transport responses are of particular interest. Here, we predict a new type of quasiparticle called topological charge quadruple (TCQ), which is analogous to a charge quadrupole but consists of two closely-packed pairs of Weyl points in momentum space, specifically in a half-Heusler antiferromagnet NdBiPt. Interestingly, the TCQ is protected by the spin-orbit $U(1)$ quasi-symmetry, rather than any exact crystallographic symmetries. This quasi-symmetry restricts the energy splitting induced by symmetry-lowering perturbations to a second-order effect. Furthermore, the closely located Berry curvature sources and sinks in the TCQ lead to a large Berry curvature dipole, resulting in a significant nonlinear Hall effect. Our work opens an avenue for designing novel quasiparticles using quasi-symmetries and developing materials with enhanced nonlinear responses. △ Less

Submitted 3 February, 2025; v1 submitted 14 August, 2024; originally announced August 2024.

Comments: 4 figures

Journal ref: Newton 1, 100010 (2025)

Generalised Brillouin Zone for Non-Reciprocal Systems

Authors: Habib Ammari, Silvio Barandun, Ping Liu, Alexander Uhlmann

Abstract: Recently, it has been observed that the Floquet-Bloch transform with real quasiperiodicities fails to capture the spectral properties of non-reciprocal systems. The aim of this paper is to introduce the notion of a generalised Brillouin zone by allowing the quasiperiodicities to be complex in order to rectify this. It is proved that this shift of the Brillouin zone into the complex plane accounts… ▽ More Recently, it has been observed that the Floquet-Bloch transform with real quasiperiodicities fails to capture the spectral properties of non-reciprocal systems. The aim of this paper is to introduce the notion of a generalised Brillouin zone by allowing the quasiperiodicities to be complex in order to rectify this. It is proved that this shift of the Brillouin zone into the complex plane accounts for the unidirectional spatial decay of the eigenmodes and leads to correct spectral convergence properties. The results in this paper clarify and prove rigorously how the spectral properties of a finite structure are associated with those of the corresponding semi-infinitely or infinitely periodic lattices and give explicit characterisations of how to extend the Hermitian theory to non-reciprocal settings. Based on our theory, we characterise the generalised Brillouin zone for both open boundary conditions and periodic boundary conditions. Our results are consistent with the physical literature and give explicit generalisations to the $k$-Toeplitz matrix cases. △ Less

Submitted 9 August, 2024; originally announced August 2024.

MSC Class: 35B34; 47B28; 35P25; 35C20; 81Q12; 15A18; 15B05

Ab-initio study of quantum oscillation in altermagnetic and nonmagnetic phases of RuO$_2$

Authors: Yingliang Huang, Junwen Lai, Jie Zhan, Tianye Yu, Rong Chen, Peitao Liu, Xing-Qiu Chen, Yan Sun

Abstract: Altermagnet (AM) is a new proposed magnetic state with collinear antiferromagnetic ground state but presents some transport properties that were only believed to exist in ferromagnets or non-collinear antiferromagnets. To have a comprehensive understanding of the transport properties of AMs, especially from the experimental point of view, a promising altermagnetic metal is crucial. In all the prop… ▽ More Altermagnet (AM) is a new proposed magnetic state with collinear antiferromagnetic ground state but presents some transport properties that were only believed to exist in ferromagnets or non-collinear antiferromagnets. To have a comprehensive understanding of the transport properties of AMs, especially from the experimental point of view, a promising altermagnetic metal is crucial. In all the proposed altermagnetic metals, RuO$_2$ has a special position, since it is the first proposed AM with the largest spin splitting and several important altermagnetism featured experiments were first performed based on it. However, a very recent report based on sensitive muon-spin measurements suggest a super small local magnetization from Ru, i.e. a nonmagnetic ground state in RuO$_2$. Therefore, a determination of the existence of the altermagnetic ground state is the basic starting point for all the previously altermagnetic transport properties in RuO$_2$. In this work, we propose to identify its magnetic ground state from the Fermi surface (FS) via the electronic transport property of quantum oscillation (QO). We systematically analyzed the FSs of RuO$_2$ in both nonmagnetic and altermagnetic states via first principles calculations. Our work should be helpful for future experiments on QO measurements to confirm its ground state by the interplay between transport measurements and computations. △ Less

Submitted 23 September, 2024; v1 submitted 25 July, 2024; originally announced July 2024.

Comments: 11 pages, 6 figures

Electric-field control of the perpendicular magnetization switching in ferroelectric/ferrimagnet heterostructures

Authors: Pengfei Liu, Tao Xu, Qi Liu, Juncai Dong, Ting Lin, Qinhua Zhang, Xiukai Lan, Yu Sheng, Chunyu Wang, Jiajing Pei, Hongxin Yang, Lin Gu, Kaiyou Wang

Abstract: Electric field control of the magnetic state in ferrimagnets holds great promise for developing spintronic devices due to low power consumption. Here, we demonstrate a non-volatile reversal of perpendicular net magnetization in a ferrimagnet by manipulating the electric-field driven polarization within the Pb (Zr0.2Ti0.8) O3 (PZT)/CoGd heterostructure. Electron energy loss spectra and X-ray absorp… ▽ More Electric field control of the magnetic state in ferrimagnets holds great promise for developing spintronic devices due to low power consumption. Here, we demonstrate a non-volatile reversal of perpendicular net magnetization in a ferrimagnet by manipulating the electric-field driven polarization within the Pb (Zr0.2Ti0.8) O3 (PZT)/CoGd heterostructure. Electron energy loss spectra and X-ray absorption spectrum directly verify that the oxygen ion migration at the PZT/CoGd interface associated with reversing the polarization causes the enhanced/reduced oxidation in CoGd. Ab initio calculations further substantiate that the migrated oxygen ions can modulate the relative magnetization of Co/Gd sublattices, facilitating perpendicular net magnetization switching. Our findings offer an approach to effectively control ferrimagnetic net magnetization, holding significant implications for ferrimagnetic spintronic applications. △ Less

Submitted 26 June, 2024; originally announced June 2024.

Comments: 21 pages,4 figures

Electric field tunable non-linear Hall terahertz detector in Dual quantum spin Hall insulator $\text{TaIrTe}_4$

Authors: Junwen Lai, Jie Zhan, Peitao Liu, Xing-Qiu Chen, Yan Sun

Abstract: Nonlinear Hall effect (NHE) can be generated via Berry curvature dipole (BCD) on nonequilibrium Fermi surface in a non-magnetic system without inversion symmetry.To achieve a large BCD, strong local Berry curvatures and their variation with respect to momentum are necessary and hence topological materials with strong inter-band coupling emerge as promising candidates. In this study, we propose a s… ▽ More Nonlinear Hall effect (NHE) can be generated via Berry curvature dipole (BCD) on nonequilibrium Fermi surface in a non-magnetic system without inversion symmetry.To achieve a large BCD, strong local Berry curvatures and their variation with respect to momentum are necessary and hence topological materials with strong inter-band coupling emerge as promising candidates. In this study, we propose a switchable and robust BCD in the newlydiscovered dual quantum spin Hall insulator (QSHI) $\text{TaIrTe}_4$ by applying out-of-plane electric fields. Switchable BCD could be found along with topological phase transitions or insulator-metal transition in the primitive cell and CDW phases of $\text{TaIrTe}_4$ monolayer. This work presents an instructive strategy for achieving a switchable and robust BCD within dual QSHIs, which should be helpful for designing the NHE-based THz radiations detector. △ Less

Submitted 23 June, 2024; originally announced June 2024.

Prediction of dual quantum spin Hall insulator in NbIrTe$_4$ monolayer

Authors: Xiangyang Liu, Junwen Lai, Jie Zhan, Tianye Yu, Wujun Shi, Peitao Liu, Xing-Qiu Chen, Yan Sun

Abstract: Dual quantum spin Hall insulator (QSHI) is a newly discovered topological state in the 2D material TaIrTe$_4$, exhibiting both a traditional $Z_2$ band gap at charge neutrality point and a van Hove singularity (VHS) induced correlated $Z_2$ band gap with weak doping. Inspired by the recent progress in theoretical understanding and experimental measurements, we predicted a promising dual QSHI in th… ▽ More Dual quantum spin Hall insulator (QSHI) is a newly discovered topological state in the 2D material TaIrTe$_4$, exhibiting both a traditional $Z_2$ band gap at charge neutrality point and a van Hove singularity (VHS) induced correlated $Z_2$ band gap with weak doping. Inspired by the recent progress in theoretical understanding and experimental measurements, we predicted a promising dual QSHI in the counterpart material of the NbIrTe4 monolayer by first-principles calculations. In addition to the well-known band inversion at the charge neutrality point, two new band inversions were found after CDW phase transition when the chemical potential is near the VHS, one direct and one indirect $Z_2$ band gap. The VHS-induced non-trivial band gap is around 10 meV, much larger than that from TaIrTe$_4$. Furthermore, since the new generated band gap is mainly dominated by the $4d$ orbitals of Nb, electronic correlation effects should be relatively stronger in NbIrTe$_4$ as compared to TaIrTe$_4$. Therefore, the dual QSHI state in the NbIrTe$_4$ monolayer is expected to be a good platform for investigating the interplay between topology and correlation effects. △ Less

Submitted 3 June, 2024; originally announced June 2024.

Ferroelectricity in oxygen-terminated 2D carbides of lanthanide elements

Authors: Lin Han, Wencong Sun, Pingwei Liu, Xianqing Lin, Dan Liu, David Tomanek

Abstract: We investigate the properties of oxygen-functionalized carbides of lanthanide elements with the composition M2CO2 (M=Gd, Tb,Dy) that form two-dimensional (2D) structures. Our ab initio calculations reveal that oxygen termination turns M2C monolayers into semiconductors with two dynamically stable phases. Of these, the energetically favored alpha-phase becomes ferroelectric, whereas the beta-phase… ▽ More We investigate the properties of oxygen-functionalized carbides of lanthanide elements with the composition M2CO2 (M=Gd, Tb,Dy) that form two-dimensional (2D) structures. Our ab initio calculations reveal that oxygen termination turns M2C monolayers into semiconductors with two dynamically stable phases. Of these, the energetically favored alpha-phase becomes ferroelectric, whereas the beta-phase turns anti-ferroelectric. Applying in-plane biaxial strain may transform one phase into the other, changes the ferroelectric polarization of the alpha-phase in a linear fashion, and modifies the size and nature of the fundamental band gap from direct to indirect. The structure with a direct band gap exhibits in-plane isotropic electronic and optical properties. This previously unexplored class of systems also exhibits excellent photon absorption in the ultraviolet range. △ Less

Submitted 1 June, 2024; originally announced June 2024.

Journal ref: Phys. Rev. Materials 8, 054005 (2024)