Showing 1–50 of 64 results for author: Tao, Z

High harmonic generation light source with polarization selectivity and sub-100-$μ$m beam size for time- and angle-resolved photoemission spectroscopy

Authors: Haoyuan Zhong, Xuanxi Cai, Changhua Bao, Fei Wang, Tianyun Lin, Yudong Chen, Sainan Peng, Lin Tang, Chen Gu, Zhensheng Tao, Hongyun Zhang, Shuyun Zhou

Abstract: High-quality ultrafast light sources are critical for developing advanced time- and angle-resolved photoemission spectroscopy (TrARPES). While the application of high harmonic generation (HHG) light sources in TrARPES has increased significantly over the past decade, the optimization of the HHG probe beam size and selective control of the light polarization, which are important for TrARPES measure… ▽ More High-quality ultrafast light sources are critical for developing advanced time- and angle-resolved photoemission spectroscopy (TrARPES). While the application of high harmonic generation (HHG) light sources in TrARPES has increased significantly over the past decade, the optimization of the HHG probe beam size and selective control of the light polarization, which are important for TrARPES measurements, have been rarely explored. In this work, we report the implementation of high-quality HHG probe source with an optimum beam size down to 57 $μ$m $\times$ 90 $μ$m and selective light polarization control, together with mid-infrared (MIR) pumping source for TrARPES measurements using a 10 kHz amplifier laser. The selective polarization control of the HHG probe source allows to enhance bands with different orbital contributions or symmetries, as demonstrated by experimental data measured on a few representative transition metal dichalcogenide materials (TMDCs) as well as topological insulator Bi$_2$Se$_3$. Furthermore, by combining the HHG probe source with MIR pumping at 2 $μ$m wavelength, TrARPES on a bilayer graphene shows a time resolution of 140 fs, allowing to distinguish two different relaxation processes in graphene. Such high-quality HHG probe source together with the MIR pumping expands the capability of TrARPES in revealing the ultrafast dynamics and light-induced emerging phenomena in quantum materials. △ Less

Submitted 18 October, 2025; originally announced October 2025.

Comments: 15 pages, 5 figures

Journal ref: Ultrafast Sci. 4, 0063 (2024)

Real-Space Quantification of Exciton Localization in Acene Crystals Using Wannier Function Decomposition

Authors: Zui Tao, Jonah B. Haber, Jeffrey B. Neaton

Abstract: We introduce the Wannier function decomposition of excitons (WFDX) method to quantify exciton localization in solids within the ab initio Bethe-Salpeter equation framework. By decomposing each Bloch exciton wavefunction into products of single-particle electron and hole maximally localized Wannier functions, this real-space approach provides well-defined orbital- and spatial- resolved measures of… ▽ More We introduce the Wannier function decomposition of excitons (WFDX) method to quantify exciton localization in solids within the ab initio Bethe-Salpeter equation framework. By decomposing each Bloch exciton wavefunction into products of single-particle electron and hole maximally localized Wannier functions, this real-space approach provides well-defined orbital- and spatial- resolved measures of both Frenkel and charge-transfer excitons at low computational cost. We apply WFDX to excitons in acene crystals, quantifying how the number of rings, the exciton spin state, and the center-of-mass momntum affect spatial localization. Additionally, we show how this real-space representation reflects structural nonsymmorphic symmetries that are hidden in standard reciprocal-space descriptions. We demonstrate how the WFDX framework can be used to efficiently interpolate exciton expansion coefficients in reciprocal-space and outline how it may facilitate evaluation of observables involving position operators, highlighting its potential as a general tool for both analyzing and computing excitonic properties in solids. △ Less

Submitted 7 October, 2025; originally announced October 2025.

A Unified Symmetry-Constrained Framework for Band Inversions in Photonic Crystals with $C_n$ Symmetry

Authors: Ze Tao, Fujun Liu

Abstract: The lack of a unified theoretical framework for characterizing band inversions across different crystal symmetries hinders the rapid development of topological photonic band engineering. To address this issue, we have constructed a framework constrained by symmetry $k \cdot p$ that universally models bands near high-symmetry points for symmetric photonic crystals $C_6$, $C_4$, $C_3$, and $C_2$. Th… ▽ More The lack of a unified theoretical framework for characterizing band inversions across different crystal symmetries hinders the rapid development of topological photonic band engineering. To address this issue, we have constructed a framework constrained by symmetry $k \cdot p$ that universally models bands near high-symmetry points for symmetric photonic crystals $C_6$, $C_4$, $C_3$, and $C_2$. This framework enables a coefficient-free quantitative diagnosis of band topology. We have demonstrated the power of this framework by systematically engineering band inversions. In $C_6$ crystals, we induce a reopening of the linear gap at $Γ$. In $C_4$ systems, mirror symmetry enforces a characteristic quadratic coupling leading to distinct spectral features. Our analysis further reveals that a lone $E$ doublet prevents inversion at the $Γ$ point in $C_3$ symmetry, while $C_2$ symmetry facilitates a unique inversion of $Y$ pointsints with anisotropic gap. This symmetry-first, fit-free approach establishes a direct link between experimental band maps and the extraction of fundamental topological parameters. It offers a universal tool for inversion and coupling-order identification. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Comments: 7 pages, 5 figures

Soliton Transitions Mediated by Skin-Mode Localization and Band Nonreciprocity

Authors: Shanyue Li, Mengying Hu, Jing Lin, Chen Fang, Zhensheng Tao, Kun Ding

Abstract: Solitons, typically resulting from a competition between band dispersion and nonlinearity, occur in lattices featuring the non-Hermitian skin effect as nonlinearity increases, accompanied by a transition in localization from linear skin modes to solitons. However, localization does not disentangle the role of skin modes in the soliton formation from that of band dispersion. Here, in such lattices,… ▽ More Solitons, typically resulting from a competition between band dispersion and nonlinearity, occur in lattices featuring the non-Hermitian skin effect as nonlinearity increases, accompanied by a transition in localization from linear skin modes to solitons. However, localization does not disentangle the role of skin modes in the soliton formation from that of band dispersion. Here, in such lattices, we uncover two distinct soliton phases, skin-mode-assisted solitons (SMASs) and nonreciprocity-dressed solitons (NRDSs). Rooted in fundamentally different mechanisms, SMASs originate from skin effect, while NRDSs stem from band nonreciprocity, each exhibiting unique spatial profiles. Using a stacked Su-Schrieffer-Heeger-like model as a prototype, we delineate the phase diagram of SMASs and NRDSs, each having clear phase boundaries. To interpret them, we formulate a Wannier-function-based nonlinear Hamiltonian, showing that soliton formation depends critically on how skin-mode localization and band nonreciprocity suppress or enhance wave dispersion. For SMASs, skin-mode localization reduces wave broadening at the localization sites, thereby lowering the formation threshold. This soliton phase is observable from edge dynamics and accompanied by a dynamical stability reentrance when transitioning from linear skin modes. In contrast, NRDSs, as well as their thresholds, originate from bulk band nonreciprocity and persist under periodic boundary conditions. Our framework offers predictive tools for characterizing and engineering solitons in experimentally realizable non-Hermitian systems, spanning optics to mechanics. △ Less

Submitted 4 August, 2025; originally announced August 2025.

Comments: 21 pages, 6 figures

Plasmonic detection of Rashba spin-orbit coupling in monolayer transition-metal dichalcogenides

Authors: Y. Li, Z. H. Tao, Y. M. Xiao, W. Xu, Q. N. Li, F. M. Peeters, D. Neilson, M. V. Milosevic

Abstract: Rashba spin-orbit coupling (RSOC) induces strong momentum-dependent spin splitting and plays a crucial role in fields like spintronics and topological photonics. We here theoretically investigate the collective excitations in monolayer transition metal dichalcogenides (ML-TMDs) hosting RSOC, and conceive an approach to precisely quantify the strength of RSOC using plasmons. We determine the electr… ▽ More Rashba spin-orbit coupling (RSOC) induces strong momentum-dependent spin splitting and plays a crucial role in fields like spintronics and topological photonics. We here theoretically investigate the collective excitations in monolayer transition metal dichalcogenides (ML-TMDs) hosting RSOC, and conceive an approach to precisely quantify the strength of RSOC using plasmons. We determine the electron energy loss function (EELF) and plasmon dispersions for n-type ML-TMD from the dynamic dielectric function in the framework of the standard random phase approximation (RPA). In this system, both optical and acoustic plasmon modes are observed in the EELF and plasmon dispersions. Moreover, the plasmonic and spectral properties are tunable by electron density and dependent on RSOC. Crucially, we identify a minimum energy gap between the two plasmon modes to serve as a direct spectral signature of the RSOC strength. These results establish plasmons as a non-invasive, precise, and broadly tunable technique for determining RSOC in TMD van der Waals heterostructures and devices. △ Less

Submitted 13 October, 2025; v1 submitted 1 July, 2025; originally announced July 2025.

Journal ref: Phys. Rev. B 112, L161404(2025)

Emergence of Chern metal in a moiré Kondo lattice

Authors: Wenjin Zhao, Zui Tao, Yichi Zhang, Bowen Shen, Zhongdong Han, Patrick Knüppel, Yihang Zeng, Zhengchao Xia, Kenji Watanabe, Takashi Taniguchi, Jie Shan, Kin Fai Mak

Abstract: A Chern metal is a two-dimensional metallic state of matter carrying chiral edge states. It can emerge as a doped Chern insulator, but theoretical studies have also predicted its emergence near a Kondo breakdown separating a metallic chiral spin liquid and a heavy Fermi liquid in a frustrated lattice. To date, the latter exotic scenario has not been realized. Here, we report the observation of a C… ▽ More A Chern metal is a two-dimensional metallic state of matter carrying chiral edge states. It can emerge as a doped Chern insulator, but theoretical studies have also predicted its emergence near a Kondo breakdown separating a metallic chiral spin liquid and a heavy Fermi liquid in a frustrated lattice. To date, the latter exotic scenario has not been realized. Here, we report the observation of a Chern metal at the onset of the magnetic Kondo breakdown in a frustrated moiré Kondo lattice--angle-aligned MoTe2/WSe2 bilayers. The state is compressible and is manifested by a nearly quantized Hall resistance but a finite longitudinal resistance that arises from a bad metallic bulk. The state also separates an itinerant and a heavy Fermi liquid and appears far away from the band inversion critical point of the material, thus ruling out its origin from simply doping a Chern insulator. We demonstrate the presence of a chiral edge state by nonlocal transport measurements and current-induced quantum anomalous Hall breakdown. Magnetic circular dichroism measurements further reveal a magnetization plateau for the Chern metal before a metamagnetic transition at the Kondo breakdown. Our results open an opportunity for moiré engineering of exotic quantum phases of matter through the close interplay between band topology and Kondo interactions. △ Less

Submitted 16 June, 2025; originally announced June 2025.

Reconstructing the wavefunction of magnetic topological insulators MnBi2Te4 and MnBi4Te7 using spin-resolved photoemission

Authors: Xue Han, Jason Qu, Hengxin Tan, Zicheng Tao, Noah M. Meyer, Patrick S. Kirchmann, Yanfeng Guo, Binghai Yan, Zhi-Xun Shen, Jonathan A. Sobota

Abstract: Despite their importance for exotic quantum effects, the surface electronic structure of magnetic topological insulators MnBi2Te4 and MnBi4Te7 remains poorly understood. Using high-efficiency spin- and angle-resolved photoemission spectroscopy, we directly image the spin-polarization and orbital character of the surface states in both compounds and map our observations onto a model wavefunction to… ▽ More Despite their importance for exotic quantum effects, the surface electronic structure of magnetic topological insulators MnBi2Te4 and MnBi4Te7 remains poorly understood. Using high-efficiency spin- and angle-resolved photoemission spectroscopy, we directly image the spin-polarization and orbital character of the surface states in both compounds and map our observations onto a model wavefunction to describe the complex spin-orbital texture, which solidifies our understanding of the surface band structure by establishing the single-band nature of the most prominent states. Most importantly, our analysis reveals a new mechanism for reducing the magnetic gap of the topological surface states based on the orbital composition of the wavefunction. △ Less

Submitted 3 June, 2025; originally announced June 2025.

Wannier decay and the Thouless conjecture

Authors: Simon Becker, Zhongkai Tao, Mengxuan Yang

Abstract: Non-trivial Chern classes pose an obstruction to the existence of exponentially decaying Wannier functions which provide natural bases for spectral subspaces. For non-trivial Bloch bundles, we obtain decay rates of Wannier functions in dimensions $d=2,3$. For $d=2$, we construct Wannier functions with full asymptotics and optimal decay rate $\mathcal{O}(|x|^{-2})$ as conjectured by Thouless; for… ▽ More Non-trivial Chern classes pose an obstruction to the existence of exponentially decaying Wannier functions which provide natural bases for spectral subspaces. For non-trivial Bloch bundles, we obtain decay rates of Wannier functions in dimensions $d=2,3$. For $d=2$, we construct Wannier functions with full asymptotics and optimal decay rate $\mathcal{O}(|x|^{-2})$ as conjectured by Thouless; for $d=3$, we construct Wannier functions with the uniform decay rate $\mathcal{O}(|x|^{-7/3})$. △ Less

Submitted 4 May, 2025; originally announced May 2025.

Comments: Outsourced from a previous paper, which included a discussion of fragile topology arXiv:2502.03442v2. arXiv admin note: substantial text overlap with arXiv:2502.03442

Evidence of Ultrashort Orbital Transport in Heavy Metals Revealed by Terahertz Emission Spectroscopy

Authors: Tongyang Guan, Jiahao Liu, Wentao Qin, Yongwei Cui, Shunjia Wang, Yizheng Wu, Zhensheng Tao

Abstract: The orbital angular momentum of electrons offers a promising, yet largely unexplored, degree of freedom for ultrafast, energy-efficient information processing. As the foundation of orbitronics, understanding how orbital currents propagate and convert into charge currents is essential - but remains elusive due to the challenge in disentangling orbital and spin dynamics in ultrathin films. Although… ▽ More The orbital angular momentum of electrons offers a promising, yet largely unexplored, degree of freedom for ultrafast, energy-efficient information processing. As the foundation of orbitronics, understanding how orbital currents propagate and convert into charge currents is essential - but remains elusive due to the challenge in disentangling orbital and spin dynamics in ultrathin films. Although orbital currents have been predicted to propagate over long distances in materials, recent theoretical studies argue that lattice symmetry may constrain their mean free paths (MFPs) to the scale of a single atomic layer. In this work, we provide the first direct experimental evidence for ultrashort orbital MFPs in heavy metals (HMs) - W, Ta, Pt - revealed by femtosecond terahertz emission spectroscopy. This is enabled by sub-nanometer-precision control of thin-film thickness using wedge-shaped HM|Ni heterostructures. By employing a multi-component terahertz-emission model, we quantitatively extract the orbital MFPs, consistently finding them shorter than their spin counterparts. Furthermore, control experiments rule out interfacial orbital-to-charge conversion as the dominant mechanism, confirming that the process is governed by the bulk inverse orbital Hall effect. Our findings resolve a central controversy in orbitronics and provide key insights into orbital transport and conversion mechanisms. △ Less

Submitted 7 August, 2025; v1 submitted 21 April, 2025; originally announced April 2025.

Exact quantum critical states with a superconducting quantum processor

Authors: Wenhui Huang, Xin-Chi Zhou, Libo Zhang, Jiawei Zhang, Yuxuan Zhou, Bing-Chen Yao, Zechen Guo, Peisheng Huang, Qixian Li, Yongqi Liang, Yiting Liu, Jiawei Qiu, Daxiong Sun, Xuandong Sun, Zilin Wang, Changrong Xie, Yuzhe Xiong, Xiaohan Yang, Jiajian Zhang, Zihao Zhang, Ji Chu, Weijie Guo, Ji Jiang, Xiayu Linpeng, Wenhui Ren , et al. (7 additional authors not shown)

Abstract: Anderson localization physics features three fundamental types of eigenstates: extended, localized, and critical. Confirming the presence of critical states necessitates either advancing the analysis to the thermodynamic limit or identifying a universal mechanism which can rigorously determine these states. Here we report the unambiguous experimental realization of critical states, governed by a r… ▽ More Anderson localization physics features three fundamental types of eigenstates: extended, localized, and critical. Confirming the presence of critical states necessitates either advancing the analysis to the thermodynamic limit or identifying a universal mechanism which can rigorously determine these states. Here we report the unambiguous experimental realization of critical states, governed by a rigorous mechanism for exact quantum critical states, and further observe a generalized mechanism that quasiperiodic zeros in hopping couplings protect the critical states. Leveraging a superconducting quantum processor with up to 56 qubits, we implement a programmable mosaic model with tunable couplings and on-site potentials. By measuring time-evolved observables, we identify both delocalized dynamics and incommensurately distributed zeros in the couplings, which are the defining features of the critical states. We map the localized-to-critical phase transition and demonstrate that critical states persist until quasiperiodic zeros are removed by strong long-range couplings, highlighting a novel generalized mechanism discovered in this experiment and shown with rigorous theory. Finally, we resolve the energy-dependent transition between localized and critical states, revealing the presence of anomalous mobility edges. △ Less

Submitted 25 March, 2025; v1 submitted 26 February, 2025; originally announced February 2025.

Comments: 6+29 pages, 4+14 figures. Discussions are updated

Observation of Topological Nodal-Ring Phonons in Monolayer Hexagonal Boron Nitride

Authors: Zhiyu Tao, Yani Wang, Shuyi He, Jiade Li, Siwei Xue, Zhibin Su, Jiatao Sun, Hailin Peng, Jiandong Guo, Xuetao Zhu

Abstract: Topological physics has evolved from its initial focus on fermionic systems to the exploration of bosonic systems, particularly phononic excitations in crystalline materials. Two-dimensional (2D) topological phonons emerge as promising candidates for future technological applications. Currently, experimental verification of 2D topological phonons has remained exclusively limited to graphene, a con… ▽ More Topological physics has evolved from its initial focus on fermionic systems to the exploration of bosonic systems, particularly phononic excitations in crystalline materials. Two-dimensional (2D) topological phonons emerge as promising candidates for future technological applications. Currently, experimental verification of 2D topological phonons has remained exclusively limited to graphene, a constraint that hinders their applications in phononic devices. Here, we report experimental evidence of topological phonons in monolayer hexagonal boron nitride using advanced high-resolution electron energy loss spectroscopy. Our high-precision measurements explicitly demonstrate two topological nodal rings in monolayer hexagonal boron nitride, protected by mirror symmetry, expanding the paradigm of 2D topological phonons beyond graphene. This research not only deepens fundamental understanding of 2D topological phonons, but also establishes a phononic device platform based on wide-bandgap insulators, crucial for advancements in electronics and photonics applications. △ Less

Submitted 25 February, 2025; originally announced February 2025.

Comments: 14 pages, 4 figures

Journal ref: Chinese Physics Letters 42 027405 (2025)

Fragile topology on solid grounds: a mathematical perspective

Authors: Simon Becker, Zhongkai Tao, Mengxuan Yang

Abstract: This paper provides a mathematical perspective on fragile topology phenomena in condensed matter physics. In dimension $d \leq 3$, vanishing Chern classes of bundles of Bloch eigenfunctions characterize operators with exponentially localized Wannier functions (these functions form convenient bases of spectrally determined subspaces of $L^2$). However, for systems with additional symmetries, such a… ▽ More This paper provides a mathematical perspective on fragile topology phenomena in condensed matter physics. In dimension $d \leq 3$, vanishing Chern classes of bundles of Bloch eigenfunctions characterize operators with exponentially localized Wannier functions (these functions form convenient bases of spectrally determined subspaces of $L^2$). However, for systems with additional symmetries, such as the $C_{2}T$ (space-time reversal) or the $PT$ (parity-time) symmetry, a set of exponentially localized Wannier functions compatible with such symmetry may not exist. We show that for rank 2 Bloch bundles with such symmetry, non-trivial Euler classes are obstructions to constructing exponentially localized compatible Wannier functions. We also show that this obstruction can be lifted by adding additional Bloch bundles with the symmetry, even though the Stiefel--Whitney class of the total bundle is non-trivial. This allows a construction of exponentially localized Wannier functions compatible with the symmetry and that is referred to as topological fragility. △ Less

Submitted 1 August, 2025; v1 submitted 5 February, 2025; originally announced February 2025.

Comments: v3: minor modifications. Comments are welcome

Ultralow-temperature heat transport evidence for residual density of states in the superconducting state of CsV3Sb5

Authors: C. C. Zhao, L. S. Wang, W. Xia, Q. W. Yin, H. B. Deng, G. W. Liu, J. J. Liu, X. Zhang, J. M. Ni, Y. Y. Huang, C. P. Tu, Z. C. Tao, Z. J. Tu, C. S. Gong, Z. W. Wang, H. C. Lei, Y. F. Guo, X. F. Yang, J. X. Yin, S. Y. Li

Abstract: The V-based kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) host charge density wave (CDW) and a topological nontrivial band structure, thereby provide a great platform to study the interplay of superconductivity (SC), CDW, frustration, and topology. Here, we report ultralow-temperature thermal conductivity measurements on CsV$_3$Sb$_5$ and Ta-doped Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$… ▽ More The V-based kagome superconductors $A$V$_3$Sb$_5$ ($A$ = K, Rb, and Cs) host charge density wave (CDW) and a topological nontrivial band structure, thereby provide a great platform to study the interplay of superconductivity (SC), CDW, frustration, and topology. Here, we report ultralow-temperature thermal conductivity measurements on CsV$_3$Sb$_5$ and Ta-doped Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$ and scanning tunneling microscopy (STM) measurements on CsV$_3$Sb$_5$. The finite residual linear term of thermal conductivity at zero magnetic field suggests the existence of a residual density of states (DOS) in the superconducting state of CsV$_3$Sb$_5$. This is supported by the observation of non-zero conductance at zero bias in STM spectrum at an electronic temperature of 90 mK. However, in Cs(V$_{0.86}$Ta$_{0.14}$)$_3$Sb$_5$, which does not have CDW order, there is no evidence for residual DOS. These results show the importance of CDW order for the residual DOS, and a nodal $s$-wave gap or residual Fermi arc may be the origin of the residual DOS in such an unusual multiband kagome superconductor, CsV$_3$Sb$_5$. △ Less

Submitted 24 December, 2024; originally announced December 2024.

Comments: A small part of the contents overlaps with arXiv:2102.08356

Journal ref: Chinese Physics Letters 41, 127303 (2024)

Chiral propagation of plasmons due to competing anisotropies in a twisted photonic heterostructure

Authors: Ze-Hua Tao, Icaro R. Lavor, Hai-Ming Dong, Andrey Chaves, David Neilson, Milorad V. Milosevic

Abstract: We demonstrate chiral propagation of plasmon polaritons and show it is more efficient and easier to control than the recently observed chiral shear phonon polaritons. We consider plasmon polaritons created in an anisotropic two-dimensional (2D) material, twisted with respect to an anisotropic substrate, to best exploit the competition between anisotropic electron-electron interactions and the anis… ▽ More We demonstrate chiral propagation of plasmon polaritons and show it is more efficient and easier to control than the recently observed chiral shear phonon polaritons. We consider plasmon polaritons created in an anisotropic two-dimensional (2D) material, twisted with respect to an anisotropic substrate, to best exploit the competition between anisotropic electron-electron interactions and the anisotropic electronic structure of the host material. Gate voltage and twist angle are then used for precise control of the chiral plasmon polaritons, overcoming the existing restrictions with chiral phonon polaritons. These findings open up feasible opportunities for efficient and tunable plasmon-based nanophotonics and compact high-performance on-chip optical devices. △ Less

Submitted 17 September, 2024; originally announced September 2024.

Sound Wave Manipulation Based on Valley Acoustic Interferometers

Authors: Wei Zhao, Jia-He Chen, Shu-Guang Cheng, Yong Mao, Xiaojun Zhang, Zhi Tao, Hua Jiang, Zhi Hong Hang

Abstract: Topological acoustics provides new opportunities for materials with unprecedented functions. In this work, we report a design of topological valley acoustic interferometers by Y-shaped valley sonic crystals. By tight-bounding calculation and experimental demonstration, we successfully tune the acoustic energy partition rate by configuring the channel. An analytical theory proposed to explain the t… ▽ More Topological acoustics provides new opportunities for materials with unprecedented functions. In this work, we report a design of topological valley acoustic interferometers by Y-shaped valley sonic crystals. By tight-bounding calculation and experimental demonstration, we successfully tune the acoustic energy partition rate by configuring the channel. An analytical theory proposed to explain the transmission property matches well with experimental observations. An additional π Berry phase is verified to accumulate circling along the shape-independent topological valley acoustic interferometer, unique in the pseudospin half systems. Based on the spectral oscillation originating from the accumulated dynamic phase and π Berry phase, a simplified method to measure acoustic valley interface dispersion is explored, which overcomes the shortcomings of the traditional fast Fourier transform method and improves the measuring efficiency by simply analyzing the peaks and dips of the measured transmission spectrum. Moreover, an effective approach to tuning its transmissions, as well as the spectral line shapes proposed and realized by the local geometry design of the interferometer, exhibits strong tunability under an unchanged physical mechanism. Our work opens an avenue to design future acoustic devices with the function of sound wave manipulation based on the physical mechanism of interference and Berry phase. △ Less

Submitted 11 September, 2024; originally announced September 2024.

Interlayer excitons in double-layer transition metal dichalcogenides quantum dots

Authors: Xiang Liu, Zheng Tao, Wenchen Luo, Tapash Chakraborty

Abstract: Various properties of interlayer excitons in double-layer transition metal dichalcogenides quantum dots are analyzed using a low-energy effective Hamiltonian with Coulomb interaction. We solve the single-particle Hamiltonian with and without a magnetic field analytically, then present the electron-hole pairing features of interlayer exciton by employing the exact diagonalization technique, where t… ▽ More Various properties of interlayer excitons in double-layer transition metal dichalcogenides quantum dots are analyzed using a low-energy effective Hamiltonian with Coulomb interaction. We solve the single-particle Hamiltonian with and without a magnetic field analytically, then present the electron-hole pairing features of interlayer exciton by employing the exact diagonalization technique, where the electron and hole are located in two layers respectively. In a magnetic field, the Landau level gap, as well as the electron-hole separation of an exciton varies non-monotonously as the interlayer distance increases, attributed to the pseudospin-orbit coupling which also leads to the emergence of topological non-trivial pseudospin textures in the exciton states. We examine the influence of different materials in quantum dots stacking on the exciton states, comparing their impact to variations in layer distances and quantum dot sizes. We further explore two interacting interlayer excitons numerically. The binding energy is significantly enhanced by the exchange interaction when the two electrons have different spins. The optical absorption spectra from the ground state to low-lying excited states reveal distinct behaviors for different interlayer excitons, which can be utilized to distinguish the spin of electrons in excitons. Our results highlight the potential for controlling interlayer excitons and applications of optical devices in a magnetic field and tunable layer distance. △ Less

Submitted 14 August, 2024; originally announced August 2024.

Advancing Nonadiabatic Molecular Dynamics Simulations for Solids: Achieving Supreme Accuracy and Efficiency with Machine Learning

Authors: Changwei Zhang, Yang Zhong, Zhi-Guo Tao, Xinming Qing, Honghui Shang, Zhenggang Lan, Oleg V. Prezhdo, Xin-Gao Gong, Weibin Chu, Hongjun Xiang

Abstract: Non-adiabatic molecular dynamics (NAMD) simulations have become an indispensable tool for investigating excited-state dynamics in solids. In this work, we propose a general framework, N$^2$AMD which employs an E(3)-equivariant deep neural Hamiltonian to boost the accuracy and efficiency of NAMD simulations. The preservation of Euclidean symmetry of Hamiltonian enables N$^2$AMD to achieve state-of-… ▽ More Non-adiabatic molecular dynamics (NAMD) simulations have become an indispensable tool for investigating excited-state dynamics in solids. In this work, we propose a general framework, N$^2$AMD which employs an E(3)-equivariant deep neural Hamiltonian to boost the accuracy and efficiency of NAMD simulations. The preservation of Euclidean symmetry of Hamiltonian enables N$^2$AMD to achieve state-of-the-art performance. Distinct from conventional machine learning methods that predict key quantities in NAMD, N$^2$AMD computes these quantities directly with a deep neural Hamiltonian, ensuring supreme accuracy, efficiency, and consistency. Furthermore, N$^2$AMD demonstrates excellent generalizability and enables seamless integration with advanced NAMD techniques and infrastructures. Taking several extensively investigated semiconductors as the prototypical system, we successfully simulate carrier recombination in both pristine and defective systems at large scales where conventional NAMD often significantly underestimates or even qualitatively incorrectly predicts lifetimes. This framework not only boosts the efficiency and precision of NAMD simulations but also opens new avenues to advance materials research. △ Less

Submitted 13 August, 2024; originally announced August 2024.

Comments: 25 pages, 7 figures

Towards Verifying Exact Conditions for Implementations of Density Functional Approximations

Authors: Sameerah Helal, Zhe Tao, Cindy Rubio-González, Francois Gygi, Aditya V. Thakur

Abstract: Density Functional Theory (DFT) is used extensively in the computation of electronic properties of matter, with various applications. Approximating the exchange-correlation (XC) functional is the key to the Kohn-Sham DFT approach, the basis of most DFT calculations. The choice of this density functional approximation (DFA) depends crucially on the particular system under study, which has resulted… ▽ More Density Functional Theory (DFT) is used extensively in the computation of electronic properties of matter, with various applications. Approximating the exchange-correlation (XC) functional is the key to the Kohn-Sham DFT approach, the basis of most DFT calculations. The choice of this density functional approximation (DFA) depends crucially on the particular system under study, which has resulted in the development of hundreds of DFAs. Though the exact density functional is not known, researchers have discovered analytical properties of this exact functional. Furthermore, these exact conditions are used when designing DFAs. We present XCVerifier, the first approach for verifying whether a DFA implementation satisfies the DFT exact conditions. XCVerifier was evaluated on five DFAs from the popular Libxc library and seven exact conditions from recent work. XCVerifier was able to verify or find violations for a majority of the DFA/condition pairs, demonstrating the feasibility of using formal methods to verify DFA implementations. △ Less

Submitted 13 March, 2025; v1 submitted 9 August, 2024; originally announced August 2024.

Comments: Accepted paper at Correctness 2024

Switchable anomalous Hall effect by selective mirror symmetry breaking in a kagome magnet GdMn6Ge6

Authors: Zicheng Tao, Tianye Yu, Jianyang Ding, Zhicheng Jiang, Zhenhai Yu, Wei Xia, Xia Wang, Xuerong Liu, Yulin Chen, Dawei Shen, Yan Sun, Yanfeng Guo

Abstract: The crystal symmetry plays a pivotal role in protecting the nontrivial electronic states in a topological phase. Manipulation of the crystal symmetry and hence the nontrivial topological states would serve as a fertile ground to explore exotic topological properties. Combining experimental and theoretical investigations, we demonstrate herein the flexible switch of nontrivial topological states in… ▽ More The crystal symmetry plays a pivotal role in protecting the nontrivial electronic states in a topological phase. Manipulation of the crystal symmetry and hence the nontrivial topological states would serve as a fertile ground to explore exotic topological properties. Combining experimental and theoretical investigations, we demonstrate herein the flexible switch of nontrivial topological states in the single phase of kagome magnet GdMn6Ge6. The intrinsic anomalous Hall effect caused by distinct Berry curvatures along different crystallographic directions is realized through selectively breaking the mirror symmetries in these directions by external magnetic field, which is fully supported by the first-principles calculations. Our results set an explicit example demonstrating the strong correlation between structure symmetry and nontrivial topological states, as well as the switchable topological properties in a single magnetic topological phase. △ Less

Submitted 6 August, 2024; v1 submitted 4 August, 2024; originally announced August 2024.

Comments: Main Text 15 pages + SI 12 pages, 4 main figures + 11 SI figures + 1 SI table

Dirac cones and magic angles in the Bistritzer--MacDonald TBG Hamiltonian

Authors: Simon Becker, Solomon Quinn, Zhongkai Tao, Alexander Watson, Mengxuan Yang

Abstract: We demonstrate the generic existence of Dirac cones in the full Bistritzer--MacDonald Hamiltonian for twisted bilayer graphene. Its complementary set, when Dirac cones are absent, is the set of magic angles. We show the stability of magic angles obtained in the chiral limit by demonstrating that the perfectly flat bands transform into quadratic band crossings when perturbing away from the chiral l… ▽ More We demonstrate the generic existence of Dirac cones in the full Bistritzer--MacDonald Hamiltonian for twisted bilayer graphene. Its complementary set, when Dirac cones are absent, is the set of magic angles. We show the stability of magic angles obtained in the chiral limit by demonstrating that the perfectly flat bands transform into quadratic band crossings when perturbing away from the chiral limit. Moreover, using the invariance of Euler number, we show that at magic angles there are more band crossings beyond these quadratic band crossings. This is the first result showing the existence of magic angles for the full Bistritzer--MacDonald Hamiltonian and solves Open Problem No.2 proposed in the recent survey arXiv:2310.20642. △ Less

Submitted 8 July, 2024; originally announced July 2024.

Comments: 26 pages; comments are welcome

Ultrasensitive acoustic graphene plasmons in a graphene-transition metal dichalcogenide heterostructure: strong plasmon-phonon coupling and wavelength sensitivity enhanced by a metal screen

Authors: Ícaro R. Lavora, Z. H. Tao, H. M. Dongd, Andrey Chaves, F. M. Peeters, Milorad V. Milosevic

Abstract: Acoustic plasmons in graphene exhibit strong confinement induced by a proximate metal surface and hybridize with phonons of transition metal dichalcogenides (TMDs) when these materials are combined in a van der Waals heterostructure, thus forming screened graphene plasmon-phonon polaritons (SGPPPs), a type of acoustic mode. While SGPPPs are shown to be very sensitive to the dielectric properties o… ▽ More Acoustic plasmons in graphene exhibit strong confinement induced by a proximate metal surface and hybridize with phonons of transition metal dichalcogenides (TMDs) when these materials are combined in a van der Waals heterostructure, thus forming screened graphene plasmon-phonon polaritons (SGPPPs), a type of acoustic mode. While SGPPPs are shown to be very sensitive to the dielectric properties of the environment, enhancing the SGPPPs coupling strength in realistic heterostructures is still challenging. Here we employ the quantum electrostatic heterostructure model, which builds upon the density functional theory calculations for monolayers, to show that the use of a metal as a substrate for graphene-TMD heterostructures (i) vigorously enhances the coupling strength between acoustic plasmons and the TMD phonons, and (ii) markedly improves the sensitivity of the plasmon wavelength on the structural details of the host platform in real space, thus allowing one to use the effect of environmental screening on acoustic plasmons to probe the structure and composition of a van der Waals heterostructure down to the monolayer resolution. △ Less

Submitted 21 June, 2024; originally announced June 2024.

Comments: 21 pages, 7 figures

Mott insulating phase and coherent-incoherent crossover across magnetic phase transition in 2D antiferromagnetic CrSBr

Authors: Fan Wu, Xuefeng Zhang, Yi Chen, Ding Pei, Mengwen Zhan, Zicheng Tao, Cheng Chen, Shipeng Lu, Jingzhi Chen, Shujie Tang, Xia Wang, Yanfeng Guo, Lexian Yang, Yan Zhang, Yulin Chen, Qixi Mi, Gang Li, Zhongkai Liu

Abstract: In two-dimensional van der Waals magnetic materials, the interplay between magnetism and electron correlation can give rise to new ground states and lead to novel transport and optical properties. A fundamental question in these materials is how the electron correlation manifests and interacts with the magnetic orders. In this study, we demonstrate that the recently discovered 2D antiferromagnetic… ▽ More In two-dimensional van der Waals magnetic materials, the interplay between magnetism and electron correlation can give rise to new ground states and lead to novel transport and optical properties. A fundamental question in these materials is how the electron correlation manifests and interacts with the magnetic orders. In this study, we demonstrate that the recently discovered 2D antiferromagnetic material, CrSBr, is a Mott insulator, through the combined use of resonant and temperature-dependent angle-resolved photoemission spectroscopy techiniques, supplemented by dynamical mean-field theory analysis. Intriguingly, we found that as the system transitions from the antiferromagnetic to the paramagnetic phases, its Mott bands undergo a reconfiguration, and a coherent-incoherent crossover, driven by the dissolution of the magnetic order. Our findings reveal a distinctive evolution of band structure associated with magnetic phase transitions, shedding light on the investigation of intricate interplay between correlation and magnetic orders in strongly correlated van der Waals magnetic materials. △ Less

Submitted 30 May, 2024; originally announced May 2024.

Journal ref: SCIENCE CHINA Physics, Mechanics & Astronomy (2025)

Large band-splitting in $g$-wave type altermagnet CrSb

Authors: Jianyang Ding, Zhicheng Jiang, Xiuhua Chen, Zicheng Tao, Zhengtai Liu, Tongrui Li, Jishan Liu, Jianping Sun, Jinguang Cheng, Jiayu Liu, Yichen Yang, Runfeng Zhang, Liwei Deng, Wenchuan Jing, Yu Huang, Yuming Shi, Mao Ye, Shan Qiao, Yilin Wang, Yanfeng Guo, Donglai Feng, Dawei Shen

Abstract: Altermagnetism (AM), a newly discovered magnetic state, ingeniously integrates the properties of ferromagnetism and antiferromagnetism, representing a significant breakthrough in the field of magnetic materials. Despite experimental verification of some typical AM materials, such as MnTe and MnTe$_2$, the pursuit of AM materials that feature larger spin splitting and higher transition temperature… ▽ More Altermagnetism (AM), a newly discovered magnetic state, ingeniously integrates the properties of ferromagnetism and antiferromagnetism, representing a significant breakthrough in the field of magnetic materials. Despite experimental verification of some typical AM materials, such as MnTe and MnTe$_2$, the pursuit of AM materials that feature larger spin splitting and higher transition temperature is still essential. Here, our research focuses on CrSb, which possesses N{é}el temperature of up to 700K and giant spin splitting near the Fermi level ($E_F$). Utilizing high-resolution angle-resolved photoemission spectroscopy and density functional theory calculations, we meticulously map the three-dimensional electronic structure of CrSb. Our photoemission spectroscopic results on both (0001) and (10$\overline{1}$0) cleavages of CrSb collaboratively reveal unprecedented details on AM-induced band splitting, and subsequently pin down its unique bulk $g$-wave symmetry through quantitative analysis of the angular and photon-energy dependence of spin splitting. Moreover, the observed spin splitting reaches the magnitude of 0.93~eV near $E_F$, the most substantial among all confirmed AM materials. This study not only validates the nature of CrSb as a prototype $g$-wave like AM material but also underscores its pivotal role in pioneering applications in spintronics. △ Less

Submitted 15 November, 2024; v1 submitted 21 May, 2024; originally announced May 2024.

Comments: 7 pages, 4 figures

Journal ref: Phys.Rev.Lett.133,206401(2024)

Noncentrosymmetric Nowotny Chimney Ladder Ferromagnet Cr4Ge7 with a High Curie Temperature of ~ 207 K

Authors: Zhenhai Yu, Kaijuan Zhou, Xiaofei Hou, Xuejiao Chen, Zhen Tao, Yunguan Ye, Wei Xia, Zhongyang Li, Jinggeng Zhao, Wei Wu, Ziyi Liu, Xia Wang, Na Yu, Jinguang Cheng, Jianlin Luo, Qiang Zhang, Vladimir Pomjakushin, Zhicheng Zhong, Soh Jian Rui, Xingye Lu, Yanfeng Guo

Abstract: Noncentrosymmetric magnets usually host intriguing magnetic interactions inherent the crystal structure with broken inversion symmetry, which can give rise to rich magnetic behaviors. We report herein the high-pressure synthesis, crystal structure, magnetizations and magnetic structure of a so-called Nowotny chimney ladder compound Cr4Ge7. Our analysis on the powder neutron diffraction data revise… ▽ More Noncentrosymmetric magnets usually host intriguing magnetic interactions inherent the crystal structure with broken inversion symmetry, which can give rise to rich magnetic behaviors. We report herein the high-pressure synthesis, crystal structure, magnetizations and magnetic structure of a so-called Nowotny chimney ladder compound Cr4Ge7. Our analysis on the powder neutron diffraction data revises the crystal structure as a noncentrosymmetric space group (P-4c2, No.116). It exhibits two magnetic orders within the temperature range of 2 - 400 K. The first order at ~ 207 K associated with a small magnetic moment of ~ 0.75 miuB is assigned to a commensurate ferromagnetic structure with a propagation vector k = (0, 0, 0). The weak itinerant ferromagnet nature should be caused by the complex Cr spin orders from different Wyckoff positions. The second order at ~ 18 K is assumed to arise from a competition between the Dzyaloshinskii-Moria and Heisenberg interactions. The results provide an excellent platform for study on intricate interactions between various magnetic exchanges as well as for the exploration of high temperature exotic magnetic properties which host potential applications in next-generation spintronics. △ Less

Submitted 3 March, 2024; originally announced March 2024.

Comments: 21 pages, 5 figures, 2 tables; Supporting Information is not included

Journal ref: Chemistry of Materials, 2024

Unveiling the charge density wave mechanism in vanadium-based Bi-layered kagome metals

Authors: Yi-Chen Yang, Soohyun Cho, Tong-Rui Li, Xiang-Qi Liu, Zheng-Tai Liu, Zhi-Cheng Jiang, Jian-Yang Ding, Wei Xia, Zi-Cheng Tao, Jia-Yu Liu, Wen-Chuan Jing, Yu Huang, Yu-Ming Shi, Soonsang Huh, Takeshi Kondo, Zhe Sun, Ji-Shan Liu, Mao Ye, Yi-Lin Wang, Yan-Feng Guo, Da-Wei Shen

Abstract: The charge density wave (CDW), as a hallmark of vanadium-based kagome superconductor AV3Sb5 (A = K, Rb, Cs), has attracted intensive attention. However, the fundamental controversy regarding the underlying mechanism of CDW therein persists. Recently, the vanadium-based bi-layered kagome metal ScV6Sn6, reported to exhibit a long-range charge order below 94 K, has emerged as a promising candidate to… ▽ More The charge density wave (CDW), as a hallmark of vanadium-based kagome superconductor AV3Sb5 (A = K, Rb, Cs), has attracted intensive attention. However, the fundamental controversy regarding the underlying mechanism of CDW therein persists. Recently, the vanadium-based bi-layered kagome metal ScV6Sn6, reported to exhibit a long-range charge order below 94 K, has emerged as a promising candidate to further clarify this core issue. Here, employing micro-focusing angle-resolved photoemission spectroscopy (μ-ARPES) and first-principles calculations, we systematically studied the unique CDW order in vanadium-based bi-layered kagome metals by comparing ScV6Sn6 with its isostructural counterpart YV6Sn6, which lacks a CDW ground state. Combining ARPES data and the corresponding joint density of states (DOS), we suggest that the VHS nesting mechanism might be invalid in these materials. Besides, in ScV6Sn6, we identified multiple hybridization energy gaps resulting from CDW-induced band folding, along with an anomalous band dispersion, implying a potential electron-phonon coupling driven mechanism underlying the formation of the CDW order. Our finding not only comprehensively maps the electronic structure of V-based bi-layer kagome metals but also provide constructive experimental evidence for the unique origin of CDW in this system. △ Less

Submitted 6 February, 2024; originally announced February 2024.

Comments: 14 pages, 5 figures

Nematic charge-density-wave correlations in FeSe$_{1-x}$S$_{x}$

Authors: Ruixian Liu, Wenliang Zhang, Yuan Wei, Zhen Tao, Teguh C. Asmara, Vladimir N. Strocov, Thorsten Schmitt, Xingye Lu

Abstract: The occurrence of charge-density-wave (CDW) order is a common thread in the phase diagram of cuprate high-transition-temperature ($T_c$) superconductors. In iron-based superconductors (FeSCs), nematic order and fluctuations play a decisive role in driving other emergent orders. CDW order has been observed by scanning tunneling microscopy for various FeSCs such as FeSe thin films, uniaxially strain… ▽ More The occurrence of charge-density-wave (CDW) order is a common thread in the phase diagram of cuprate high-transition-temperature ($T_c$) superconductors. In iron-based superconductors (FeSCs), nematic order and fluctuations play a decisive role in driving other emergent orders. CDW order has been observed by scanning tunneling microscopy for various FeSCs such as FeSe thin films, uniaxially strained LiFeAs, and tetragonal FeSe$_{0.81}$S$_{0.19}$. However, it remains elusive if the CDW in these materials is a bulk phenomenon as well as if and how it intertwines with the electronic nematicity. Using energy-resolved resonant X-ray scattering at the Fe-L$_3$ edge, we report the discovery of a local-strain-induced incommensurate isotropic CDW order in FeSe$_{0.82}$S$_{0.18}$. A highly anisotropic CDW response under uniaxial strain unambiguously manifests that the CDW is directly coupled to the nematicity. Transforming part of Fe$^{2+}$ to Fe$^{3+}$ on the surface of FeSe$_{1-x}$S$_{x}$ reveals that the same isotropic CDW can be induced, enhanced, and stabilized in the whole nematic regime measured ($x=0-0.19$). As Fe$^{3+}$ can create local lattice distortions on the surface, the CDW could arise from the interaction between the local strain around Fe$^{3+}$ and the nematic electron correlations. Our experimental observation of a local-strain-induced CDW gives vital information for understanding the interplay between electron correlations and the electronic nematicity in FeSCs. △ Less

Submitted 21 December, 2023; v1 submitted 19 December, 2023; originally announced December 2023.

Comments: 7 pages, 4 figures

Dephasing of Strong-Field-Driven Floquet States Revealed by Time- and Spectrum-Resolved Quantum-Path Interferometry

Authors: Yaxin Liu, Bingbing Zhu, Shicheng Jiang, Shenyang Huang, Mingyan Luo, Sheng Zhang, Hugen Yan, Yuanbo Zhang, Ruifeng Lu, Zhensheng Tao

Abstract: Floquet engineering, while a powerful tool for ultrafast quantum-state manipulation, faces challenges under strong-field conditions, as recent high harmonic generation studies unveil exceptionally short dephasing times. In this study, using time- and spectrum-resolved quantum-path interferometry, we investigate the dephasing mechanisms of terahertz-driven excitons. Our results reveal a dramatic in… ▽ More Floquet engineering, while a powerful tool for ultrafast quantum-state manipulation, faces challenges under strong-field conditions, as recent high harmonic generation studies unveil exceptionally short dephasing times. In this study, using time- and spectrum-resolved quantum-path interferometry, we investigate the dephasing mechanisms of terahertz-driven excitons. Our results reveal a dramatic increase in exciton dephasing rate beyond a threshold field strength, indicating exciton dissociation as the primary dephasing mechanism. Importantly, we demonstrate long dephasing times of strong-field-dressed excitons, supporting coherent strong-field manipulation of quantum materials. △ Less

Submitted 20 January, 2024; v1 submitted 16 November, 2023; originally announced November 2023.

Mathematical results on the chiral model of twisted bilayer graphene (with an appendix by Mengxuan Yang and Zhongkai Tao)

Authors: Maciej Zworski, Mengxuan Yang, Zhongkai Tao

Abstract: The study of twisted bilayer graphene (TBG) is a hot topic in condensed matter physics with special focus on {\em magic angles} of twisting at which TBG acquires unusual properties. Mathematically, topologically non-trivial flat bands appear at those special angles. The chiral model of TBG pioneered by Tarnopolsky--Kruchkov--Vishwanath has particularly nice mathematical properties and we survey, a… ▽ More The study of twisted bilayer graphene (TBG) is a hot topic in condensed matter physics with special focus on {\em magic angles} of twisting at which TBG acquires unusual properties. Mathematically, topologically non-trivial flat bands appear at those special angles. The chiral model of TBG pioneered by Tarnopolsky--Kruchkov--Vishwanath has particularly nice mathematical properties and we survey, and in some cases, clarify, recent rigorous results which exploit them. △ Less

Submitted 2 November, 2023; v1 submitted 31 October, 2023; originally announced October 2023.

Phase-Modulated Elastic Properties of Two-Dimensional Magnetic FeTe: Hexagonal and Tetragonal Polymorphs

Authors: Yunfei Yu, Mo Cheng, Zicheng Tao, Wuxiao Han, Guoshuai Du, Yanfeng Guo, Jianping Shi, Yabin Chen

Abstract: Two-dimensional (2D) layered magnets, such as iron chalcogenides, have emerged these years as a new family of unconventional superconductor and provided the key insights to understand the phonon-electron interaction and pairing mechanism. Their mechanical properties are of strategic importance for the potential applications in spintronics and optoelectronics. However, there is still lack of effici… ▽ More Two-dimensional (2D) layered magnets, such as iron chalcogenides, have emerged these years as a new family of unconventional superconductor and provided the key insights to understand the phonon-electron interaction and pairing mechanism. Their mechanical properties are of strategic importance for the potential applications in spintronics and optoelectronics. However, there is still lack of efficient approach to tune the elastic modulus despite the extensive studies. Herein, we report the modulated elastic modulus of 2D magnetic FeTe and its thickness-dependence via phase engineering. The grown 2D FeTe by chemical vapor deposition can present various polymorphs, i.e. tetragonal FeTe (t-FeTe, antiferromagnetic) and hexagonal FeTe (h-FeTe, ferromagnetic). The measured Young's modulus of t-FeTe by nanoindentation method showed an obvious thickness-dependence, from 290.9+-9.2 to 113.0+-8.7 GPa when the thicknesses increased from 13.2 to 42.5 nm, respectively. In comparison, the elastic modulus of h-FeTe remains unchanged. Our results could shed light on the efficient modulation of mechanical properties of 2D magnetic materials and pave the avenues for their practical applications in nanodevices. △ Less

Submitted 31 October, 2023; originally announced October 2023.

Comments: 19 pages, 4 figures

Emergence of ferromagnetism at the onset of moiré Kondo breakdown

Authors: Wenjin Zhao, Bowen Shen, Zui Tao, Sunghoon Kim, Patrick Knüppel, Zhongdong Han, Yichi Zhang, Kenji Watanabe, Takashi Taniguchi, Debanjan Chowdhury, Jie Shan, Kin Fai Mak

Abstract: The interaction of a lattice of localized magnetic moments with a sea of conduction electrons in Kondo lattice models induces rich quantum phases of matter, such as Fermi liquids with heavily renormalized electronic quasiparticles, quantum critical non-Fermi liquid metals and unconventional superconductors, among others. The recent demonstration of moiré Kondo lattices has opened the door to inves… ▽ More The interaction of a lattice of localized magnetic moments with a sea of conduction electrons in Kondo lattice models induces rich quantum phases of matter, such as Fermi liquids with heavily renormalized electronic quasiparticles, quantum critical non-Fermi liquid metals and unconventional superconductors, among others. The recent demonstration of moiré Kondo lattices has opened the door to investigate the Kondo problem with continuously tunable parameters. Although a heavy Fermi liquid phase has been identified in moiré Kondo lattices, the magnetic phases and Kondo breakdown transitions remain unexplored. Here we report a density-tuned Kondo destruction in AB-stacked MoTe2/WSe2 moiré bilayers by combining magneto transport and optical studies. As the itinerant carrier density decreases, the Kondo temperature decreases. At a critical density, we observe a heavy Fermi liquid to insulator transition, and a nearly concomitant emergence of ferromagnetic order. The observation is consistent with the scenario of a ferromagnetic Anderson insulator and suppression of the Kondo screening effect. Our results pave the path for inducing other exotic quantum phase transitions in moiré Kondo lattices. △ Less

Submitted 9 October, 2023; originally announced October 2023.

Observation of spin polarons in a frustrated moiré Hubbard system

Authors: Zui Tao, Wenjin Zhao, Bowen Shen, Patrick Knüppel, Kenji Watanabe, Takashi Taniguchi, Jie Shan, Kin Fai Mak

Abstract: The electron's kinetic energy plays a pivotal role in magnetism. While virtual electron hopping promotes antiferromagnetism in an insulator, the real process usually favors ferromagnetism. But in kinetically frustrated systems, such as hole doped triangular lattice Mott insulators, real hopping has been shown to favor antiferromagnetism. Kinetic frustration has also been predicted to induce a new… ▽ More The electron's kinetic energy plays a pivotal role in magnetism. While virtual electron hopping promotes antiferromagnetism in an insulator, the real process usually favors ferromagnetism. But in kinetically frustrated systems, such as hole doped triangular lattice Mott insulators, real hopping has been shown to favor antiferromagnetism. Kinetic frustration has also been predicted to induce a new quasiparticle -- a bound state of the doped hole and a spin flip called a spin polaron -- at intermediate magnetic fields, which could form an unusual metallic state. However, the direct experimental observation of spin polarons has remained elusive. Here we report the observation of spin polarons in triangular lattice MoTe2/WSe2 moiré bilayers by the reflective magnetic circular dichroism measurements. We identify a spin polaron phase at lattice filling factor between 0.8-1 and magnetic field between 2-4 T; it is separated from the fully spin polarized phase by a metamagnetic transition. We determine that the spin polaron is a spin-3/2 particle and its binding energy is commensurate to the kinetic hopping energy. Our results open the door for exploring spin polaron pseudogap metals, spin polaron pairing and other new phenomena in triangular lattice moiré materials. △ Less

Submitted 22 July, 2023; originally announced July 2023.

Nematic spin correlations pervading the phase diagram of FeSe$_{1-x}$S$_{x}$

Authors: Ruixian Liu, Wenliang Zhang, Yuan Wei, Zhen Tao, Teguh C. Asmara, Yi Li, Vladimir N. Strocov, Rong Yu, Qimiao Si, Thorsten Schmitt, Xingye Lu

Abstract: We use resonant inelastic X-ray scattering (RIXS) at the Fe-L$_3$ edge to study the spin excitations of uniaxial-strained and unstrained FeSe$_{1-x}$S$_{x}$ ($0\leq x\leq0.21$) samples. The measurements on unstrained samples reveal dispersive spin excitations in all doping levels, which show only minor doping dependence in energy dispersion, lifetime, and intensity, indicating that high-energy spi… ▽ More We use resonant inelastic X-ray scattering (RIXS) at the Fe-L$_3$ edge to study the spin excitations of uniaxial-strained and unstrained FeSe$_{1-x}$S$_{x}$ ($0\leq x\leq0.21$) samples. The measurements on unstrained samples reveal dispersive spin excitations in all doping levels, which show only minor doping dependence in energy dispersion, lifetime, and intensity, indicating that high-energy spin excitations are only marginally affected by sulfur doping. RIXS measurements on uniaxial-strained samples reveal that the high-energy spin-excitation anisotropy observed previously in FeSe is also present in the doping range $0< x\leq0.21$ of FeSe$_{1-x}$S$_{x}$. The spin-excitation anisotropy persists to a high temperature up to $T>200$ K in $x=0.18$ and reaches a maximum around the nematic quantum critical doping ($x_c\approx0.17$). Since the spin-excitation anisotropy directly reflects the existence of nematic spin correlations, our results indicate that high-energy nematic spin correlations pervade the regime of nematicity in the phase diagram and are enhanced by the nematic quantum criticality. These results emphasize the essential role of spin fluctuations in driving electronic nematicity and open the door for uniaxial strain tuning of spin excitations in quantum materials hosting strong magnetoelastic coupling and electronic nematicity. △ Less

Submitted 16 July, 2023; originally announced July 2023.

Comments: 6 pages, 4 figures, supplemental materials uploaded

Journal ref: Physical Review Letters 132, 016501 (2024)

Observation of the Breakdown of Optical Phonon Splitting in a Two-dimensional Polar Monolayer

Authors: Jiade Li, Li Wang, Zhiyu Tao, Weiliang Zhong, Siwei Xue, Guangyao Miao, Weihua Wang, Jiandong Guo, Xuetao Zhu

Abstract: Phonon splitting of the longitudinal optical and transverse optical modes (LO-TO splitting), a ubiquitous phenomenon in three-dimensional (3D) polar materials, is essential for the formation of the 3D phonon polaritons. Theories predict that the LO-TO splitting will break down in two-dimensional (2D) polar systems, but direct experimental verification is still missing. Here, using monolayer hexago… ▽ More Phonon splitting of the longitudinal optical and transverse optical modes (LO-TO splitting), a ubiquitous phenomenon in three-dimensional (3D) polar materials, is essential for the formation of the 3D phonon polaritons. Theories predict that the LO-TO splitting will break down in two-dimensional (2D) polar systems, but direct experimental verification is still missing. Here, using monolayer hexagonal boron nitride (h-BN) as a prototypical example, we report the direct observation of the breakdown of LO-TO splitting and the finite slope of the LO phonons at the center of the Brillouin zone in 2D polar materials by inelastic electron scattering spectroscopy. Interestingly, the slope of the LO phonon in our measurements is lower than the theoretically predicted value for a freestanding monolayer due to the screening of the Cu foil substrate. This enables the phonon polaritons (PhPs) in monolayer h-BN/Cu foil to exhibit ultra-slow group velocity (~ 5 x 10^-6 c, c is the speed of light) and ultra-high confinement (~ 4000 times smaller wavelength than that of light). Our work reveals the universal law of the LO phonons in 2D polar materials and lays a physical foundation for future research on 2D PhPs. △ Less

Submitted 30 June, 2023; originally announced June 2023.

Journal ref: Nat. Commun. 15, 1938 (2024)

Kagome surface states and weak electronic correlation in vanadium-kagome metals

Authors: Jianyang Ding, Ningning Zhao, Zicheng Tao, Zhe Huang, Zhicheng Jiang, Yichen Yang, Soohyun Cho, Zhengtai Liu, Jishan Liu, Yanfeng Guo, Kai Liu, Zhonghao Liu, Dawei Shen

Abstract: RV6Sn6 (R = Y and lanthanides) with two-dimensional vanadium-kagome surface states is an ideal platform to investigate kagome physics and manipulate the kagome features to realize novel phenomena. Utilizing the micron-scale spatially resolved angle-resolved photoemission spectroscopy and first-principles calculations, we report a systematical study of the electronic structures of RV6Sn6 (R = Gd, T… ▽ More RV6Sn6 (R = Y and lanthanides) with two-dimensional vanadium-kagome surface states is an ideal platform to investigate kagome physics and manipulate the kagome features to realize novel phenomena. Utilizing the micron-scale spatially resolved angle-resolved photoemission spectroscopy and first-principles calculations, we report a systematical study of the electronic structures of RV6Sn6 (R = Gd, Tb, and Lu) on the two cleaved surfaces, i.e., the V- and RSn1-terminated (001) surfaces. The calculated bands without any renormalization match well with the main ARPES dispersive features, indicating the weak electronic correlation in this system. We observe 'W'-like kagome surface states around the Brillouin zone corners showing R-element-dependent intensities, which is probably due to various coupling strengths between V and RSn1 layers. Our finding suggests an avenue for tuning electronic states by interlayer coupling based on two-dimensional kagome lattices. △ Less

Submitted 29 June, 2023; originally announced June 2023.

Photo-accelerated hot carrier transfer at MoS2/WS2:a first-principles study

Authors: Zhi-Guo Tao, Guo-Jun Zhu, Weibin Chu, Xin-Gao Gong, Ji-Hui Yang

Abstract: Charge transfer in type-II heterostructures plays important roles in determining device performance for photovoltaic and photocatalytic applications. However, current theoretical studies of charge transfer process don't consider the effects of operating conditions such as illuminations and yield systemically larger interlayer transfer time of hot electrons in MoS2/WS2 compared to experimental resu… ▽ More Charge transfer in type-II heterostructures plays important roles in determining device performance for photovoltaic and photocatalytic applications. However, current theoretical studies of charge transfer process don't consider the effects of operating conditions such as illuminations and yield systemically larger interlayer transfer time of hot electrons in MoS2/WS2 compared to experimental results. Here in this work, we propose a general picture that, illumination can induce interfacial dipoles in type-II heterostructures, which can accelerate hot carrier transfer by reducing the energy difference between the electronic states in separate materials and enhancing the nonadiabatic couplings. Using the first-principles calculations and the ab-initio nonadiabatic molecular dynamics, we demonstrate this picture using MoS2/WS2 as a prototype. The calculated characteristic time for the interlayer transfer (60 fs) and the overall relaxation (700 fs) processes of hot electrons is in good agreement with the experiments. We further find that illumination mainly affects the ultrafast interlayer transfer process but has little effects on the relatively slow intralayer relaxation process. Therefore, the overall relaxation process of hot electrons has a saturated time with increased illumination strengths. The illumination-accelerated charge transfer is expected to universally exist in type-II heterostructures. △ Less

Submitted 8 May, 2023; originally announced May 2023.

Thickness-dependent magnetic properties in Pt[CoNi]n multilayers with perpendicular magnetic anisotropy

Authors: Chunjie Yan, Lina Chen, Kaiyuan Zhou, Liupeng Yang, Qingwei Fu, Wenqiang Wang, Wen-Cheng Yue, Like Liang, Zui Tao, Jun Du, Yong-Lei Wang, Ronghua Liu

Abstract: We systematically investigated the Ni and Co thickness-dependent perpendicular magnetic anisotropy (PMA) coefficient, magnetic domain structures, and magnetization dynamics of Pt(5 nm)/[Co(t_Co nm)/Ni(t_Ni nm)]5/Pt(1 nm) multilayers by combining the four standard magnetic characterization techniques. The magnetic-related hysteresis loops obtained from the field-dependent magnetization M and anomal… ▽ More We systematically investigated the Ni and Co thickness-dependent perpendicular magnetic anisotropy (PMA) coefficient, magnetic domain structures, and magnetization dynamics of Pt(5 nm)/[Co(t_Co nm)/Ni(t_Ni nm)]5/Pt(1 nm) multilayers by combining the four standard magnetic characterization techniques. The magnetic-related hysteresis loops obtained from the field-dependent magnetization M and anomalous Hall resistivity (AHR) \r{ho}_xy found that the two serial multilayers with t_Co = 0.2 and 0.3 nm have the optimum PMA coefficient K_U well as the highest coercivity H_C at the Ni thickness t_Ni = 0.6 nm. Additionally, the magnetic domain structures obtained by Magneto-optic Kerr effect (MOKE) microscopy also significantly depend on the thickness and K_U of the films. Furthermore, the thickness-dependent linewidth of ferromagnetic resonance is inversely proportional to K_U and H_C, indicating that inhomogeneous magnetic properties dominate the linewidth. However, the intrinsic Gilbert damping constant determined by a linear fitting of frequency-dependent linewidth does not depend on Ni thickness and K_U. Our results could help promote the PMA [Co/Ni] multilayer applications in various spintronic and spin-orbitronic devices. △ Less

Submitted 18 April, 2023; originally announced April 2023.

Comments: 17 pages, 4 figures

Journal ref: Chinese Phys. B 32 (2023) 017503

Excitation and voltage-gated modulation of single-mode dynamics in a planar nano-gap spin Hall nano-oscillator

Authors: Lina Chen, Yu Chen, Zhenyu Gao, Kaiyuan Zhou, Zui Tao, Yong Pu, Tiejun Zhou, Ronghua Liu

Abstract: We experimentally study the dynamical modes excited by current-induced spin-orbit torque and its electrostatic gating effect in a 3-terminal planar nano-gap spin Hall nano-oscillator (SHNO) with a moderate interfacial perpendicular magnetic anisotropy (IPMA). Both quasilinear propagating spin-wave and localized "bullet" modes are achieved and controlled by varying the applied in-plane magnetic fie… ▽ More We experimentally study the dynamical modes excited by current-induced spin-orbit torque and its electrostatic gating effect in a 3-terminal planar nano-gap spin Hall nano-oscillator (SHNO) with a moderate interfacial perpendicular magnetic anisotropy (IPMA). Both quasilinear propagating spin-wave and localized "bullet" modes are achieved and controlled by varying the applied in-plane magnetic field and driving current. The minimum linewidth shows a linear dependence on the actual temperature of the active area, confirming single-mode dynamics based on the nonlinear theory of spin-torque nano-oscillation with a single mode. The observed electrostatic gating tuning oscillation frequency arises from voltage-controlled magnetic anisotropy and threshold current of SHNO via modification of the nonlinear damping and/or the interfacial spin-orbit coupling of the magnetic multilayer. In contrast to previously observed two-mode coexistence degrading the spectral purity in Py/Pt-based SHNOs with a negligible IPMA, a single coherent spin-wave mode with a low driven current can be achieved by selecting the ferromagnet layer with a suitable IPMA because the nonlinear mode coupling can be diminished by bringing in the PMA field to compensate the easy-plane shape anisotropy. Moreover, the simulations demonstrate that the experimentally observed current and gate-voltage modulation of auto-oscillation modes are also closely associated with the nonlinear damping and mode coupling, which are determined by the ellipticity of magnetization precession. The demonstrated nonlinear mode coupling mechanism and electrical control approach of spin-wave modes could provide the clue to facilitate the implementation of the mutual synchronization map for neuromorphic computing applications in SHNO array networks. △ Less

Submitted 11 April, 2023; originally announced April 2023.

Comments: 11 pages, 10 figures

Giant spin Hall effect in AB-stacked MoTe2/WSe2 bilayers

Authors: Zui Tao, Bowen Shen, Wenjin Zhao, Nai Chao Hu, Tingxin Li, Shengwei Jiang, Lizhong Li, Kenji Watanabe, Takashi Taniguchi, Allan H. MacDonald, Jie Shan, Kin Fai Mak

Abstract: The spin Hall effect (SHE), in which electrical current generates transverse spin current, plays an important role in spintronics for the generation and manipulation of spin-polarized electrons. The phenomenon originates from spin-orbit coupling. In general, stronger spin-orbit coupling favors larger SHEs but shorter spin relaxation times and diffusion lengths. To achieve both large SHEs and long-… ▽ More The spin Hall effect (SHE), in which electrical current generates transverse spin current, plays an important role in spintronics for the generation and manipulation of spin-polarized electrons. The phenomenon originates from spin-orbit coupling. In general, stronger spin-orbit coupling favors larger SHEs but shorter spin relaxation times and diffusion lengths. To achieve both large SHEs and long-range spin transport in a single material has remained a challenge. Here we demonstrate a giant intrinsic SHE in AB-stacked MoTe2/WSe2 moiré bilayers by direct magneto optical imaging. Under moderate electrical currents with density < 1 A/m, we observe spin accumulation on transverse sample edges that nearly saturates the spin density. We also demonstrate long-range spin Hall transport and efficient non-local spin accumulation limited only by the device size (about 10 um). The gate dependence shows that the giant SHE occurs only near the Chern insulating state, and at low temperatures, it emerges after the quantum anomalous Hall breakdown. Our results demonstrate moiré engineering of Berry curvature and large SHEs for potential spintronics applications. △ Less

Submitted 22 March, 2023; originally announced March 2023.

Gate-tunable heavy fermions in a moiré Kondo lattice

Authors: Wenjin Zhao, Bowen Shen, Zui Tao, Zhongdong Han, Kaifei Kang, Kenji Watanabe, Takashi Taniguchi, Kin Fai Mak, Jie Shan

Abstract: The Kondo lattice, describing a matrix of local magnetic moments coupled via spin-exchange interactions to itinerant conduction electrons, is a prototype of strongly correlated quantum matter. Traditionally, Kondo lattices are realized in intermetallic compounds containing lanthanide or actinide. The complex electronic structure and limited tunability of both the electron density and exchange inte… ▽ More The Kondo lattice, describing a matrix of local magnetic moments coupled via spin-exchange interactions to itinerant conduction electrons, is a prototype of strongly correlated quantum matter. Traditionally, Kondo lattices are realized in intermetallic compounds containing lanthanide or actinide. The complex electronic structure and limited tunability of both the electron density and exchange interactions in these bulk materials pose significant challenges to study Kondo lattice physics. Here, we report the realization of a synthetic Kondo lattice in AB-stacked MoTe2/WSe2 moiré bilayers, where the MoTe2 layer is tuned to a Mott insulating state, supporting a triangular moiré lattice of local moments, and the WSe2 layer is doped with itinerant conduction carriers. We observe heavy fermions with a large Fermi surface below the Kondo temperature. We also observe destruction of the heavy fermions by an external magnetic field with an abrupt decrease of the Fermi surface size and quasiparticle mass. We further demonstrate widely and continuously gate-tunable Kondo temperatures through either the itinerant carrier density or Kondo interaction. Our study opens the possibility of in-situ access to the rich phase diagram of the Kondo lattice with exotic quantum criticalities in a single device based on semiconductor moiré materials △ Less

Submitted 1 November, 2022; originally announced November 2022.

Enhancement of spin-orbit torque efficiency by tailoring interfacial spin-orbit coupling in Pt-based magnetic multilayers

Authors: Wenqiang Wang, Kaiyuan Zhou, Xiang Zhan, Zui Tao, Qingwei Fu, Like Liang, Zishuang Li, Lina Chen, Chunjie Yan, Haotian Li, Tiejun Zhou, Ronghua Liu

Abstract: We study inserting Co layer thickness-dependent spin transport and spin-orbit torques (SOTs) in the Pt/Co/Py trilayers by spin-torque ferromagnetic resonance. The interfacial perpendicular magnetic anisotropy energy density ($K_s = 2.7~erg/cm^2$), which is dominated by interfacial spin-orbit coupling (ISOC) in the Pt/Co interface, total effective spin-mixing conductance (… ▽ More We study inserting Co layer thickness-dependent spin transport and spin-orbit torques (SOTs) in the Pt/Co/Py trilayers by spin-torque ferromagnetic resonance. The interfacial perpendicular magnetic anisotropy energy density ($K_s = 2.7~erg/cm^2$), which is dominated by interfacial spin-orbit coupling (ISOC) in the Pt/Co interface, total effective spin-mixing conductance ($G_{eff,tot} = 0.42 {\times} 10^{15}~Ω^{-1} m^{-2}$) and two-magnon scattering ($β_{TMS} = 0.46~nm^2$) are first characterized, and the damping-like torque ($ξ_{DL}$ = 0.103) and field-like torque ($ξ_{FL}$ = -0.017) efficiencies are also calculated quantitatively by varying the thickness of the inserting Co layer. The significant enhancement of $ξ_{DL}$ and $ξ_{FL}$ in Pt/Co/Py than Pt/Py bilayer system originates from the interfacial Rashba-Edelstein effect due to the strong ISOC between Co-3d and Pt-5d orbitals at the Pt/Co interface. Additionally, we find a considerable out-of-plane spin polarization SOT, which is ascribed to the spin anomalous Hall effect and possible spin precession effect due to IPMA-induced perpendicular magnetization at the Pt/Co interface. Our results demonstrate that the ISOC of the Pt/Co interface plays a vital role in spin transport and SOTs-generation. Our finds offer an alternative approach to improve the conventional SOTs efficiencies and generate unconventional SOTs with out-of-plane spin polarization to develop low power Pt-based spintronic via tailoring the Pt/FM interface. △ Less

Submitted 6 September, 2022; originally announced September 2022.

Comments: 17 pages, 4 figures

Journal ref: Chin. Phys. B 31, 097504 (2022)

Current-driven magnetization dynamics and their correlation with magnetization configurations in perpendicularly magnetized tunnel junctions

Authors: Kaiyuan Zhou, Lina Chen, Kequn Chi, Qingwei Fu, Zui Tao, Like Liang, Zhenyu Gao, Haotian Li, Hao Meng, Bo Liu, Tiejun Zhou, R. H. Liu

Abstract: We study spin-transfer-torque driven magnetization dynamics of a perpendicular magnetic tunnel junction (MTJ) nanopillar. Based on the combination of spin-torque ferromagnetic resonance and microwave spectroscopy techniques, we demonstrate that the free layer (FL) and the weak pinned reference layer (RL) exhibit distinct dynamic behaviors with opposite frequency vs. field dispersion relations. The… ▽ More We study spin-transfer-torque driven magnetization dynamics of a perpendicular magnetic tunnel junction (MTJ) nanopillar. Based on the combination of spin-torque ferromagnetic resonance and microwave spectroscopy techniques, we demonstrate that the free layer (FL) and the weak pinned reference layer (RL) exhibit distinct dynamic behaviors with opposite frequency vs. field dispersion relations. The FL can support a single coherent spin-wave (SW) mode for both parallel and antiparallel configurations, while the RL exhibits spin-wave excitation only for the antiparallel state. These two SW modes corresponding to the FL and RL coexist at an antiparallel state and exhibit a crossover phenomenon of oscillation frequency with increasing the external magnetic field, which could be helpful in the mutual synchronization of auto-oscillations for SW-based neuromorphic computing. △ Less

Submitted 6 September, 2022; originally announced September 2022.

Comments: 13 pages, 5 figures

Journal ref: J. Appl. Phys. 132, 173906 (2022)

Multiple topological nodal structure in LaSb2 with large linear magnetoresistance

Authors: Y. X. Qiao, Z. C. Tao, F. Y. Wang, Huaiqiang Wang, Z. C. Jiang, Z. T. Liu, Soohyun Cho, F. Y. Zhang, Q. K. Meng, W. Xia, Y. C. Yang, Z. Huang, J. S. Liu, Z. H. Liu, Z. W. Zhu, S. Qiao, Y. F. Guo, Haijun Zhang, Dawei Shen

Abstract: Unconventional fermions in the immensely studied topological semimetals are the source for rich exotic topological properties. Here, using symmetry analysis and first-principles calculations, we propose the coexistence of multiple topological nodal structure in LaSb2, including topological nodal surfaces, nodal lines and in particular eightfold degenerate nodal points, which have been scarcely obs… ▽ More Unconventional fermions in the immensely studied topological semimetals are the source for rich exotic topological properties. Here, using symmetry analysis and first-principles calculations, we propose the coexistence of multiple topological nodal structure in LaSb2, including topological nodal surfaces, nodal lines and in particular eightfold degenerate nodal points, which have been scarcely observed in a single material. Further, utilizing high resolution angle-resolved photoemission spectroscopy in combination with Shubnikov-de Haas quantum oscillations measurements, we confirm the existence of nodal surfaces and eightfold degenerate nodal points in LaSb2, and extract the π Berry phase proving the non-trivial electronic band structure topology therein. The intriguing multiple topological nodal structure might play a crucial role in giving rise to the large linear magnetoresistance. Our work renews the insights into the exotic topological phenomena in LaSb2 and its analogous. △ Less

Submitted 22 August, 2022; originally announced August 2022.

Valley-coherent quantum anomalous Hall state in AB-stacked MoTe2/WSe2 bilayers

Authors: Zui Tao, Bowen Shen, Shengwei Jiang, Tingxin Li, Lizhong Li, Liguo Ma, Wenjin Zhao, Jenny Hu, Kateryna Pistunova, Kenji Watanabe, Takashi Taniguchi, Tony F. Heinz, Kin Fai Mak, Jie Shan

Abstract: Moiré materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed… ▽ More Moiré materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed in both graphene and transition metal dichalcogenide (TMD) moiré materials. It is thought to arise from the interaction-driven valley polarization of the narrow moiré bands. Here, we show surprisingly that the newly discovered QAH state in AB-stacked MoTe2/WSe2 moiré bilayers is not valley-polarized but valley-coherent. The layer- and helicity-resolved optical spectroscopy measurement reveals that the QAH ground state possesses spontaneous spin (valley) polarization aligned (anti-aligned) in two TMD layers. In addition, saturation of the out-of-plane spin polarization in both layers occurs only under high magnetic fields, supporting a canted spin texture. Our results call for a new mechanism for the QAH effect and highlight the potential of TMD moiré materials with strong electronic correlations and spin-orbit interactions for exotic topological states. △ Less

Submitted 15 August, 2022; originally announced August 2022.

Geometric Effect of High-Resolution Electron Energy Loss Spectroscopy on the Identification of Plasmons: An Example of Graphene

Authors: Jiade Li, Zijian Lin, Guangyao Miao, Weiliang Zhong, Siwei Xue, Yi Li, Zhiyu Tao, Weihua Wang, Jiandong Guo, Xuetao Zhu

Abstract: High-resolution electron energy loss spectroscopy (HREELS) is one of the most powerful methods to detect the dispersion of plasmons. However, we find that in the HREELS measurement, the scattering geometric configuration will seriously affect the identification of plasmons. Here, taking graphene as an example, using the HREELS capable of two-dimensional energy-momentum mapping combined with the in… ▽ More High-resolution electron energy loss spectroscopy (HREELS) is one of the most powerful methods to detect the dispersion of plasmons. However, we find that in the HREELS measurement, the scattering geometric configuration will seriously affect the identification of plasmons. Here, taking graphene as an example, using the HREELS capable of two-dimensional energy-momentum mapping combined with the intensity distribution calculations, we visually display the intensity distribution of the scattering geometric factor. We demonstrate that the energy loss peaks from the scattering geometric effect may be misinterpreted as the features of an acoustic plasmon. In any HREELS measurement, it is necessary to evaluate the effect of the scattering geometry quantitatively to identify the intrinsic surface excitations. △ Less

Submitted 16 March, 2022; originally announced March 2022.

Comments: 5 figures

Journal ref: Surface Science 721 122067 (2022)

Air Stable and Layer Dependent Ferromagnetism in Atomically Thin van der Waals CrPS$_{4}$

Authors: Joolee Son, Suhan Son, Pyeongjae Park, Maengsuk Kim, Zui Tao, Juhyun Oh, Taehyeon Lee, Sanghyun Lee, Junghyun Kim, Kaixuan Zhang, Kwanghee Cho, Takashi Kamiyama, Jun Hee Lee, Kin Fai Mak, Jie Shan, Miyoung Kim, Je-Geun Park, Jieun Lee

Abstract: Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI$_{3}$, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of materi… ▽ More Ferromagnetism in two-dimensional materials presents a promising platform for the development of ultrathin spintronic devices with advanced functionalities. Recently discovered ferromagnetic van der Waals crystals such as CrI$_{3}$, readily isolated two-dimensional crystals, are highly tunable through external fields or structural modifications. However, there remains a challenge because of material instability under air exposure. Here, we report the observation of an air stable and layer dependent ferromagnetic (FM) van der Waals crystal, CrPS$_{4}$, using magneto-optic Kerr effect microscopy. In contrast to the antiferromagnetic (AFM) bulk, the FM out-of-plane spin orientation is found in the monolayer crystal. Furthermore, alternating AFM and FM properties observed in even and odd layers suggest robust antiferromagnetic exchange interactions between layers. The observed ferromagnetism in these crystals remains resilient even after the air exposure of about a day, providing possibilities for the practical applications of van der Waals spintronics. △ Less

Submitted 18 October, 2021; originally announced October 2021.

Comments: 22 pages, 5 figures

Weak Antilocalization Effect up to ~ 120 K in the van der Waals Crystal Fe5-xGeTe2 with Near Room Temperature Ferromagnetism

Authors: Zhengxian Li, Kui Huang, Deping Guo, Guodong Ma, Xiaolei Liu, Yueshen Wu, Jian Yuan, Zicheng Tao, Binbin Wang, Xia Wang, Zhiqiang Zou, Na Yu, Geliang Yu, Jiamin Xue, Jun Li, Zhongkai Liu, Wei Ji, Yanfeng Guo

Abstract: The weak antilocalization (WAL) effect is known as a quantum correction to the classical conductivity, which never appeared in two-dimensional magnets. In this work, we reported the observation of a WAL effect in the van der Waals ferromagnet Fe5-xGeTe2 with a Curie temperature Tc ~ 270 K, which can even reach as high as ~ 120 K. The WAL effect could be well described by the Hikami-Larkin-Nagaoka… ▽ More The weak antilocalization (WAL) effect is known as a quantum correction to the classical conductivity, which never appeared in two-dimensional magnets. In this work, we reported the observation of a WAL effect in the van der Waals ferromagnet Fe5-xGeTe2 with a Curie temperature Tc ~ 270 K, which can even reach as high as ~ 120 K. The WAL effect could be well described by the Hikami-Larkin-Nagaoka and Maekawa-Fukuyama theories in the presence of strong spin-orbit coupling (SOC). Moreover, A crossover from a peak to dip behavior around 60 K in both the magnetoresistance and magnetoconductance was observed, which could be ascribed to a rare example of temperature driven Lifshitz transition as indicated by the angle-resolved photoemission spectroscopy measurements and first principles calculations. The reflective magnetic circular dichroism measurements indicate a possible spin reorientation that kills the WAL effect above 120 K. Our findings present a rare example of WAL effect in two-dimensional ferromagnet and also a magnetotransport fingerprint of the strong SOC in Fe5-xGeTe2. The results would be instructive for understanding the interaction Hamiltonian for such high Tc itinerant ferromagnetism as well as be helpful for the design of next-generation room temperature spintronic or twistronic devices. △ Less

Submitted 5 September, 2021; originally announced September 2021.

Comments: main text 32 pages + SI 7 pages;main text 8 figures + SI 5 figures and 3 tables

Spin-excitation anisotropy in the nematic state of detwinned FeSe

Authors: Xingye Lu, Wenliang Zhang, Yi Tseng, Ruixian Liu, Zhen Tao, Eugenio Paris, Panpan Liu, Tong Chen, Vladimir Strocov, Yu Song, Rong Yu, Qimiao Si, Pengcheng Dai, Thorsten Schmitt

Abstract: The origin of the electronic nematicity in FeSe is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at $\mathbf{Q}_1=(1, 0)$ and $\mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected to provide decisive criteria for clarifying this issue. Although a spin-fluctuation anisotropy below… ▽ More The origin of the electronic nematicity in FeSe is one of the most important unresolved puzzles in the study of iron-based superconductors. In both spin- and orbital-nematic models, the intrinsic magnetic excitations at $\mathbf{Q}_1=(1, 0)$ and $\mathbf{Q}_2=(0, 1)$ of twin-free FeSe are expected to provide decisive criteria for clarifying this issue. Although a spin-fluctuation anisotropy below 10 meV between $\mathbf{Q}_1$ and $\mathbf{Q}_2$ has been observed by inelastic neutron scattering around $T_c\sim 9$ K ($<<T_s\sim 90$ K), it remains unclear whether such an anisotropy also persists at higher energies and associates with the nematic transition $T_{\rm s}$. Here we use resonant inelastic x-ray scattering (RIXS) to probe the high-energy magnetic excitations of uniaxial-strain detwinned FeSe and {\BFA}. A prominent anisotropy between the magnetic excitations along the $H$ and $K$ directions is found to persist to $\sim200$ meV in FeSe, which is even more pronounced than the anisotropy of spin waves in {\BFA}. This anisotropy decreases gradually with increasing temperature and finally vanishes at a temperature around the nematic transition temperature $T_{\rm s}$. Our results reveal an unprecedented strong spin-excitation anisotropy with a large energy scale well above the $d_{xz}/d_{yz}$ orbital splitting, suggesting that the nematic phase transition is primarily spin-driven. Moreover, the measured high-energy spin excitations are dispersive and underdamped, which can be understood from a local-moment perspective. Our findings provide the much-needed understanding of the mechanism for the nematicity of FeSe and points to a unified description of the correlation physics across seemingly distinct classes of Fe-based superconductors. △ Less

Submitted 27 December, 2021; v1 submitted 10 August, 2021; originally announced August 2021.

Comments: 15 pages, 5 figures

Possible Dirac quantum spin liquid in a kagome quantum antiferromagnet YCu$_3$(OH)$_6$Br$_2$[Br$_{x}$(OH)$_{1-x}$]

Authors: Zhenyuan Zeng, Xiaoyan Ma, Si Wu, Hai-Feng Li, Zhen Tao, Xingye Lu, Xiao-hui Chen, Jin-Xiao Mi, Shi-Jie Song, Guang-Han Cao, Guangwei Che, Kuo Li, Gang Li, Huiqian Luo, Zi Yang Meng, Shiliang Li

Abstract: We studied the magnetic properties of YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$] ($x$ = 0.33), where Cu$^{2+}$ ions form two-dimensional kagome layers. There is no magnetic order down to 50 mK while the Curie-Weiss temperature is on the order of -100 K. At zero magnetic field, the low-temperature specific heat shows a $T^2$ dependence. Above 2 T, a linear temperature dependence term in specific h… ▽ More We studied the magnetic properties of YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$] ($x$ = 0.33), where Cu$^{2+}$ ions form two-dimensional kagome layers. There is no magnetic order down to 50 mK while the Curie-Weiss temperature is on the order of -100 K. At zero magnetic field, the low-temperature specific heat shows a $T^2$ dependence. Above 2 T, a linear temperature dependence term in specific heat emerges, and the value of $γ= C/T$ increases linearly with the field. Furthermore, the magnetic susceptibility tends to a constant value at $T = 0$. Our results suggest that the magnetic ground state of YCu$_3$(OH)$_6$Br$_2$[Br$_{1-x}$(OH)$_{x}$] is consistent with a Dirac quantum-spin-liquid state with a linearly dispersing spinon strongly coupled to an emergent gauge field, which has long been theoretically proposed as a candidate ground state in the two-dimensional kagome Heisenberg antiferromagnetic system. △ Less

Submitted 15 March, 2022; v1 submitted 25 July, 2021; originally announced July 2021.

Comments: 7 pages, 3 figures

Journal ref: Phys. Rev. B 105, L121109 (2022)

Competing superconductivity and charge-density wave in Kagome metal CsV3Sb5: evidence from their evolutions with sample thickness

Authors: B. Q. Song, X. M. Kong, W. Xia, Q. W. Yin, C. P. Tu, C. C. Zhao, D. Z. Dai, K. Meng, Z. C. Tao, Z. J. Tu, C. S. Gong, H. C. Lei, Y. F. Guo, X. F. Yang, S. Y. Li

Abstract: Recently superconductivity and topological charge-density wave (CDW) were discovered in the Kagome metals $A$V$_3$Sb$_5$ ($A$ = Cs, Rb, and K), which have an ideal Kagome lattice of vanadium. Here we report resistance measurements on thin flakes of CsV$_3$Sb$_5$ to investigate the evolution of superconductivity and CDW with sample thickness. The CDW transition temperature ${\it T}_{\rm CDW}$ decre… ▽ More Recently superconductivity and topological charge-density wave (CDW) were discovered in the Kagome metals $A$V$_3$Sb$_5$ ($A$ = Cs, Rb, and K), which have an ideal Kagome lattice of vanadium. Here we report resistance measurements on thin flakes of CsV$_3$Sb$_5$ to investigate the evolution of superconductivity and CDW with sample thickness. The CDW transition temperature ${\it T}_{\rm CDW}$ decreases from 94 K in bulk to a minimum of 82 K at thickness of 60 nm, then increases to 120 K as the thickness is reduced further to 4.8 nm (about five monolayers). Since the CDW order in CsV$_3$Sb$_5$ is quite three-dimensional (3D) in the bulk sample, the non-monotonic evolution of ${\it T}_{\rm CDW}$ with reducing sample thickness can be explained by a 3D to 2D crossover around 60 nm. Strikingly, the superconducting transition temperature ${\it T}_{\rm c}$ shows an exactly opposite evolution, increasing from 3.64 K in the bulk to a maximum of 4.28 K at thickness of 60 nm, then decreasing to 0.76 K at 4.8 nm. Such exactly opposite evolutions provide strong evidence for competing superconductivity and CDW, which helps us to understand these exotic phases in $A$V$_3$Sb$_5$ Kagome metals. △ Less

Submitted 19 May, 2021; originally announced May 2021.

Comments: 4 pages, 4 figures

Journal ref: Nature Communications 14, 2492 (2023)

Double-dome superconductivity under pressure in the V-based Kagome metals AV3Sb5 (A = Rb and K)

Authors: C. C. Zhu, X. F. Yang, W. Xia, Q. W. Yin, L. S. Wang, C. C. Zhao, D. Z. Dai, C. P. Tu, B. Q. Song, Z. C. Tao, Z. J. Tu, C. S. Gong, H. C. Lei, Y. F. Guo, S. Y. Li

Abstract: We present high-pressure electrical transport measurements on the newly discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have an ideal Kagome lattice of vanadium. Two superconducting domes under pressure are observed in both compounds, as previously observed in their sister compound CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient pressure to th… ▽ More We present high-pressure electrical transport measurements on the newly discovered V-based superconductors $A$V$_3$Sb$_5$ ($A$ = Rb and K), which have an ideal Kagome lattice of vanadium. Two superconducting domes under pressure are observed in both compounds, as previously observed in their sister compound CsV$_3$Sb$_5$. For RbV$_3$Sb$_5$, the $T_c$ increases from 0.93 K at ambient pressure to the maximum of 4.15 K at 0.38 GPa in the first dome. The second superconducting dome has the highest $T_c$ of 1.57 K at 28.8 GPa. KV$_3$Sb$_5$ displays a similar double-dome phase diagram, however, its two maximum $T_c$s are lower, and the $T_c$ drops faster in the second dome than RbV$_3$Sb$_5$. An integrated temperature-pressure phase diagram of $A$V$_3$Sb$_5$ ($A$ = Cs, Rb and K) is constructed, showing that the ionic radius of the intercalated alkali-metal atoms has a significant effect. Our work demonstrates that double-dome superconductivity under pressure is a common feature of these V-based Kagome metals. △ Less

Submitted 29 April, 2021; originally announced April 2021.

Comments: 5 pages, 4 figures

Journal ref: Physical Review B 105, 094507 (2022), Editors' Suggestion