Showing 1–50 of 66 results for author: Zhu, P

Non-Hermitian Exceptional Dynamics in First-Order Heat Transport

Authors: Pengfei Zhu

Abstract: Heat transport exhibits distinct regimes ranging from ballistic propagation to diffusive relaxation, traditionally described by disparate theoretical frameworks. Here, we introduce a unified first-order operator formulation in which temperature and heat flux are treated as a coupled state vector, yielding a minimal dynamical closure of heat transport. The resulting generator is intrinsically non-H… ▽ More Heat transport exhibits distinct regimes ranging from ballistic propagation to diffusive relaxation, traditionally described by disparate theoretical frameworks. Here, we introduce a unified first-order operator formulation in which temperature and heat flux are treated as a coupled state vector, yielding a minimal dynamical closure of heat transport. The resulting generator is intrinsically non-Hermitian and gives rise to a spectral structure governed by an exceptional point that separates overdamped diffusion from underdamped wave-like propagation. In this framework, Fourier law emerges as a singular limit of a hyperbolic dissipative system, while the Cattaneo equation arises naturally as the minimal hydrodynamic closure of kinetic theory. We show that the exceptional point induces nonanalytic spectral transitions, nonmodal transient dynamics, and a breakdown of conventional modal decomposition. The theory further generalizes to anisotropic media, where direction-dependent exceptional surfaces enable intrinsic steering of heat flow. Our results establish a unified non-Hermitian dynamical framework for heat transport and reveal exceptional-point physics as a fundamental organizing principle underlying thermal dynamics across scales. △ Less

Submitted 15 April, 2026; originally announced April 2026.

Identification of sub-angstrom many-body localization in quantum materials by Bragg scattering phase breaking and ultrafast structural dynamics

Authors: Yingpeng Qi, Jianmin Yang, Zhihui Zhou, Qing Xu, Yang Lv, Xiao Zou, Tao Jiang, Pengfei Zhu, Dongxue Chen, Zhenrong Sun, Lin Xie, Dao Xiang, Jiaqing He

Abstract: Defects, fluctuations, degenerate states and correlated interactions facilitate the emergence of exotic properties in condensed matter systems while also inducing atomic-scale local correlated structures that deviate from the average long-range order. Establishing the structure-property relationship from the perspective of these atomic-scale local correlated structures remains ambiguous and contro… ▽ More Defects, fluctuations, degenerate states and correlated interactions facilitate the emergence of exotic properties in condensed matter systems while also inducing atomic-scale local correlated structures that deviate from the average long-range order. Establishing the structure-property relationship from the perspective of these atomic-scale local correlated structures remains ambiguous and controversial due to the lack of direct methods for identifying such local correlated structures. In this work, based on the photoexcited ultrafast structural response, we propose a Bragg scattering phase breaking regime to identify the sub-angstrom local correlated structures in quantum materials. With this regime, we unambiguously identify the many-body-interaction driven local correlated structures with static off-center Ag displacements of 0 to 0.5 angstrom in the low temperature ground state of AgCrSe2. As temperature rising, these static local correlated structures transform to a dynamic state where the thermal fluctuations overwhelm the multiple localized quantum states, signifying the strong anharmonicity of the local structures. The state-of-the-art density functional theory simulation well reproduces the intrinsic many-body-interaction driven local correlated structures. These unique local correlated structures evidence the first many-body localization with topological order characteristic in real material systems and provide a unified scenario for the versatile quantum properties in single crystalline AgCrSe2. Our work not only offers a universal approach to characterize sub-angstrom local correlated structures across a wide range of quantum materials but also deepens our understanding of the fundamental mechanism behind exotic properties from the perspective of atomic-scale local correlated structures. △ Less

Submitted 18 March, 2026; originally announced March 2026.

Direct observation of ultrafast amorphous-amorphous transitions indicated by bond stretching and angle bending in phase-change material GeTe

Authors: Yingpeng Qi, Nianke Chen, Zhihui Zhou, Qing Xu, Yang Lv, Xiao Zou, Tao Jiang, Pengfei Zhu, Min Zhu, Dongxue Chen, Zhenrong Sun, Xianbin Li, Dao Xiang

Abstract: The intrinsic nature of glass states and glass transitions at the atomic scale remain a fundamental open question in condensed-matter physics and materials science. By combining femtosecond electron diffraction with time-dependent density-functional theory molecular dynamics simulations, we directly observe ultrafast amorphous-amorphous transitions in amorphous GeTe, manifested as rapid Ge-Te (Ge)… ▽ More The intrinsic nature of glass states and glass transitions at the atomic scale remain a fundamental open question in condensed-matter physics and materials science. By combining femtosecond electron diffraction with time-dependent density-functional theory molecular dynamics simulations, we directly observe ultrafast amorphous-amorphous transitions in amorphous GeTe, manifested as rapid Ge-Te (Ge) bond stretching within 0.2 ps and subsequent angle bending of the Ge-Te (Ge)-Ge motif on a 0.5-2 ps timescale. Critically, the ultrafast bond stretching is accompanied by localized oscillation modes with the frequency of 3.10 THz, unambiguously signaling the local Peierls-like bonding structure and the flexibility of these polarized bonds. These ultrafast collective atomic motions provide a direct structural origin for the boson peak and pay the way for systematic optimization of relaxation and crystallization kinetics. △ Less

Submitted 18 March, 2026; originally announced March 2026.

Non-Hermitian topological superfluidity in a three-dimensional fermi gas with spin-orbit coupling

Authors: Pingcheng Zhu, Lihong Zhou, Jianxin Zhong

Abstract: The experimental advances in realizing artificial spin-orbit coupling (SOC) and non-Hermitian potentials in ultracold atomic system open a new avenue for exploring their significant roles in quantum many-body physics. Here, we investigate a non-Hermitian, two-component Fermi system in a cubic lattice with Rashba SOC and complex-valued interaction arising from two-body loss. We adopt the non-Hermit… ▽ More The experimental advances in realizing artificial spin-orbit coupling (SOC) and non-Hermitian potentials in ultracold atomic system open a new avenue for exploring their significant roles in quantum many-body physics. Here, we investigate a non-Hermitian, two-component Fermi system in a cubic lattice with Rashba SOC and complex-valued interaction arising from two-body loss. We adopt the non-Hermitian mean field theory and map out the phase diagram at zero temperature. The interplay of dissipation and on-site interaction drives a dissipation-induced phase transition from superfluid (SF) to normal phase (N). Notably, for weak interaction strengths, this leads to a reentrance of the superfluid state. The presence of SOC significantly expands the parameter regime for both the normal phase and the metastable superfluid phase(MSF). Whereas, the Zeeman field can drive the system from a conventional superfluid into a topological superfluid phase(TSF), characterized by a nontrivial topological invariant. These results enrich our knowledge of pairing superfluidity in Fermi systems. △ Less

Submitted 8 January, 2026; v1 submitted 5 January, 2026; originally announced January 2026.

Comments: 7 pages, 4 figures

Strain Response as a Probe of Spinons in Quantum Spin Liquids

Authors: Penghao Zhu, Archisman Panigrahi, Leonid Levitov, Nandini Trivedi

Abstract: Quantum spin liquids (QSLs) host emergent, fractionalized fermionic excitations that are charge-neutral. Identifying clear experimental signatures of these excitations remains a central challenge in the field of strongly correlated systems, as they do not couple to conventional electromagnetic probes. Here, we propose lattice strain as a powerful and tunable probe: Mechanical deformation of the la… ▽ More Quantum spin liquids (QSLs) host emergent, fractionalized fermionic excitations that are charge-neutral. Identifying clear experimental signatures of these excitations remains a central challenge in the field of strongly correlated systems, as they do not couple to conventional electromagnetic probes. Here, we propose lattice strain as a powerful and tunable probe: Mechanical deformation of the lattice generates large pseudomagnetic fields, inducing pseudo-Landau levels that serve as distinctive spectroscopic signatures of these excitations. Using the Kitaev model on the honeycomb lattice, we show that distinct QSL phases exhibit strikingly different strain responses. The semimetallic Kitaev spin liquid and the gapped chiral spin liquid display pronounced Landau quantization and a diamagnetic-like response to strain, whereas the Majorana metal phase shows a paramagnetic-like response without forming Landau levels. These contrasting behaviors provide a direct route to experimentally identifying and distinguishing QSL phases hosting fractionalized excitations. We further outline how local resonant ultrasound spectroscopy can detect the strain-induced resonances associated with these responses, offering a practical pathway towards identifying fractionalized excitations in candidate materials. △ Less

Submitted 2 December, 2025; originally announced December 2025.

Comments: 8+7 pages,5+3 figures

Surface elasticity effect on Plateau-Rayleigh instability in soft solids

Authors: Pingping Zhu, Dun Li, Xiang Yu, Zheng Zhong

Abstract: Soft solids exhibit instability and develop surface undulations due to surface effects, a phenomenon known as the elastic Plateau-Rayleigh (PR) instability, driven by the interplay of surface and bulk elasticity. Previous studies on the PR instability in solids mainly focused on the case of constant surface tension and ignored the effect of surface elasticity. It has been shown by experiments that… ▽ More Soft solids exhibit instability and develop surface undulations due to surface effects, a phenomenon known as the elastic Plateau-Rayleigh (PR) instability, driven by the interplay of surface and bulk elasticity. Previous studies on the PR instability in solids mainly focused on the case of constant surface tension and ignored the effect of surface elasticity. It has been shown by experiments that the surface effects in solid-like materials depend both on the surface tension and surface elasticity, but little is known about the role of the latter in the elasto-capillary instabilities in soft solids. Here, we conduct an in-depth exploration of the effect of surface elasticity on the PR instability in an elastic cylinder by coupling theoretical and numerical methods. We derive an asymptotically consistent one-dimensional (1d) model to characterize the PR instability from three-dimensional (3d) nonlinear bulk-surface elasticity, and develop a new finite-element (FE) scheme for simulating 3d deformations of the bulk-surface system. The initiation and evolution of the PR instability are obtained analytically with the aid of the 1d model. The 1d results are further validated by the 3d FE simulations. By synthesizing the 1d analytic solutions and 3d numerical results, the effects of surface elasticity, surface compressibility, surface tension, axial force and geometrical size on the PR instability are thoroughly elucidated. Our results can be applied to calibrate surface parameters for solid-like materials and develop constitutive models for elastic surfaces. △ Less

Submitted 20 November, 2025; originally announced November 2025.

Comments: 33 pages, 10 figures

In-Plane Field induced Quantized Longitudinal Conductivity in Magnetic Topological Insulators

Authors: Ting-Hsun Yang, Yaochen Li, Peng Zhang, Penghao Zhu, Hung-Yu Yang, Eun Sang Choi, Kaiwei Chen, Wenqiang Cui, Kin Wong, Peng Deng, Gang Qiu, Kang L. Wang

Abstract: We report the discovery of an in plane quantization (IPQ) state in trilayer magnetic topological insulators, characterized by a quantized longitudinal conductivity of e2/h under strong in-plane magnetic fields. This state emerges at a quantum critical point separating quantum anomalous Hall phases tuned by field angle and orientation, directly linking gap-closing behavior to quantized criticality.… ▽ More We report the discovery of an in plane quantization (IPQ) state in trilayer magnetic topological insulators, characterized by a quantized longitudinal conductivity of e2/h under strong in-plane magnetic fields. This state emerges at a quantum critical point separating quantum anomalous Hall phases tuned by field angle and orientation, directly linking gap-closing behavior to quantized criticality. Temperature and gate dependent transport measurements, supported by a self consistent approximation model, reveal that electron hole puddles dominate charge transport in this regime, highlighting the essential role of impurity disorder in stabilizing quantized critical transport. These findings establish a tunable experimental framework that connects gap-closing physics with universal conductivity, offering both microscopic insight into critical transport in magnetic topological insulators and a robust platform for probing quantum criticality in topological systems. △ Less

Submitted 6 November, 2025; originally announced November 2025.

Probing the Critical Point (CritPt) of AI Reasoning: a Frontier Physics Research Benchmark

Authors: Minhui Zhu, Minyang Tian, Xiaocheng Yang, Tianci Zhou, Lifan Yuan, Penghao Zhu, Eli Chertkov, Shengyan Liu, Yufeng Du, Ziming Ji, Indranil Das, Junyi Cao, Yufeng Du, Jiabin Yu, Peixue Wu, Jinchen He, Yifan Su, Yikun Jiang, Yujie Zhang, Chang Liu, Ze-Min Huang, Weizhen Jia, Yunkai Wang, Farshid Jafarpour, Yong Zhao , et al. (39 additional authors not shown)

Abstract: While large language models (LLMs) with reasoning capabilities are progressing rapidly on high-school math competitions and coding, can they reason effectively through complex, open-ended challenges found in frontier physics research? And crucially, what kinds of reasoning tasks do physicists want LLMs to assist with? To address these questions, we present the CritPt (Complex Research using Integr… ▽ More While large language models (LLMs) with reasoning capabilities are progressing rapidly on high-school math competitions and coding, can they reason effectively through complex, open-ended challenges found in frontier physics research? And crucially, what kinds of reasoning tasks do physicists want LLMs to assist with? To address these questions, we present the CritPt (Complex Research using Integrated Thinking - Physics Test, pronounced "critical point"), the first benchmark designed to test LLMs on unpublished, research-level reasoning tasks that broadly covers modern physics research areas, including condensed matter, quantum physics, atomic, molecular & optical physics, astrophysics, high energy physics, mathematical physics, statistical physics, nuclear physics, nonlinear dynamics, fluid dynamics and biophysics. CritPt consists of 71 composite research challenges designed to simulate full-scale research projects at the entry level, which are also decomposed to 190 simpler checkpoint tasks for more fine-grained insights. All problems are newly created by 50+ active physics researchers based on their own research. Every problem is hand-curated to admit a guess-resistant and machine-verifiable answer and is evaluated by an automated grading pipeline heavily customized for advanced physics-specific output formats. We find that while current state-of-the-art LLMs show early promise on isolated checkpoints, they remain far from being able to reliably solve full research-scale challenges: the best average accuracy among base models is only 5.7%, achieved by GPT-5 (high), moderately rising to around 10% when equipped with coding tools. Through the realistic yet standardized evaluation offered by CritPt, we highlight a large disconnect between current model capabilities and realistic physics research demands, offering a foundation to guide the development of scientifically grounded AI tools. △ Less

Submitted 20 November, 2025; v1 submitted 30 September, 2025; originally announced September 2025.

Comments: 39 pages, 6 figures, 6 tables

Transient localization from fractionalization: vanishingly small heat conductivity in gapless quantum magnets

Authors: Shi Feng, Penghao Zhu, Johannes Knolle, Michael Knap

Abstract: Several candidate materials for gapless quantum spin liquids exhibit a vanishing thermal conductivity, which is at odds with theoretical predictions. Here, we show that a suppressed response can arise due to transient localization from fractionalization, even in the absence of extrinsic defects or disorder. Concretely, we consider a Kitaev ladder model in a uniform magnetic field, whose spin degre… ▽ More Several candidate materials for gapless quantum spin liquids exhibit a vanishing thermal conductivity, which is at odds with theoretical predictions. Here, we show that a suppressed response can arise due to transient localization from fractionalization, even in the absence of extrinsic defects or disorder. Concretely, we consider a Kitaev ladder model in a uniform magnetic field, whose spin degrees of freedom fractionalize into visons and spinons. For moderate magnetic fields, visons are heavy and act as quasi-static disorder that induce transient localization of light spinons even in the translation-invariant model and at zero temperature, which strongly suppresses the residual conductivity at finite but low frequencies. At ultralow frequencies the conductivity is restored; however, such scales can be extremely hard to reach in experiments. Our results identify transient localization as a signature of fractionalization and provide a framework for interpreting anomalous transport in gapless spin liquid candidates. △ Less

Submitted 26 September, 2025; v1 submitted 8 September, 2025; originally announced September 2025.

Comments: 7+10 pages, 4+9 figures

Structural contribution to light-induced gap suppression in Ta$_2$NiSe$_5$

Authors: Zijing Chen, Chenhang Xu, Chendi Xie, Weichen Tang, Qiaomei Liu, Dong Wu, Qing Xu, Tao Jiang, Pengfei Zhu, Xiao Zou, Jun Li, Zhiwei Wang, Nanlin Wang, Dong Qian, Alfred Zong, Dao Xiang

Abstract: An excitonic insulator is a material that hosts an exotic ground state, where an energy gap opens due to spontaneous condensation of bound electron-hole pairs. Ta$_2$NiSe$_5$ is a promising candidate for this type of material, but the coexistence of a structural phase transition with the gap opening has led to a long-standing debate regarding the origin of the insulating gap. Here we employ MeV ul… ▽ More An excitonic insulator is a material that hosts an exotic ground state, where an energy gap opens due to spontaneous condensation of bound electron-hole pairs. Ta$_2$NiSe$_5$ is a promising candidate for this type of material, but the coexistence of a structural phase transition with the gap opening has led to a long-standing debate regarding the origin of the insulating gap. Here we employ MeV ultrafast electron diffraction to obtain quantitative insights into the atomic displacements in Ta$_2$NiSe$_5$ following photoexcitation, which has been overlooked in previous time-resolved spectroscopy studies. In conjunction with first-principles calculations using the measured atomic displacements, we find that the structural change can largely account for the photoinduced reduction in the energy gap without considering excitonic effects. Our work illustrates the importance of a quantitative reconstruction of individual atomic pathways during nonequilibrium phase transitions, paving the way for a mechanistic understanding of a diverse array of phase transitions in correlated materials where lattice dynamics can play a pivotal role. △ Less

Submitted 18 August, 2025; v1 submitted 17 August, 2025; originally announced August 2025.

Observation of a dynamic magneto-chiral instability in photoexcited tellurium

Authors: Yijing Huang, Nick Abboud, Yinchuan Lv, Penghao Zhu, Azel Murzabekova, Changjun Lee, Emma A. Pappas, Dominic Petruzzi, Jason Y. Yan, Dipanjan Chauduri, Peter Abbamonte, Daniel P. Shoemaker, Rafael M. Fernandes, Jorge Noronha, Fahad Mahmood

Abstract: In a system of charged chiral fermions driven out of equilibrium, an electric current parallel to the magnetic field can generate a dynamic instability by which electromagnetic waves become amplified. Whether a similar instability can occur in chiral solid-state systems remains an open question. Using time-domain terahertz (THz) emission spectroscopy, we detect signatures of what we dub a ``dynami… ▽ More In a system of charged chiral fermions driven out of equilibrium, an electric current parallel to the magnetic field can generate a dynamic instability by which electromagnetic waves become amplified. Whether a similar instability can occur in chiral solid-state systems remains an open question. Using time-domain terahertz (THz) emission spectroscopy, we detect signatures of what we dub a ``dynamic magneto-chiral instability" in elemental tellurium, a structurally chiral crystal. Upon transient photoexcitation in a moderate external magnetic field, tellurium emits THz radiation consisting of coherent modes that amplify over time. An explanation for this amplification is proposed using a theoretical model based on a dynamic instability of electromagnetic waves interacting with infrared-active oscillators of impurity acceptor states in tellurium to form an amplifying polariton. Our work not only uncovers the presence of a magneto-chiral instability but also highlights its promise for THz-wave amplification in chiral materials. △ Less

Submitted 7 February, 2025; originally announced February 2025.

Comments: Supplementary Information (SI) available as a PDF in the TeX source

Journal ref: Nature Physics 22, 202-208 (2026)

Engineering nonlinear Hall effect in bilayer graphene/black phosphorus heterostructures

Authors: Xing-Guo Ye, Zhen-Tao Zhang, Peng-Fei Zhu, Wen-Zheng Xu, An-Qi Wang, Zhi-Min Liao

Abstract: Two-dimensional van der Waals materials offer a highly tunable platform for generating emergent quantum phenomena through symmetry breaking. Stacking-induced symmetry breaking at interfaces provides an effective method to modulate their electronic properties for functional devices. Here, we strategically stack bilayer graphene with black phosphorus, a low-symmetry semiconductor, to break the symme… ▽ More Two-dimensional van der Waals materials offer a highly tunable platform for generating emergent quantum phenomena through symmetry breaking. Stacking-induced symmetry breaking at interfaces provides an effective method to modulate their electronic properties for functional devices. Here, we strategically stack bilayer graphene with black phosphorus, a low-symmetry semiconductor, to break the symmetries and induce the nonlinear Hall effect (NLHE) that can persist up to room temperature. Intriguingly, it is found the NLHE undergoes sign reversals by varying the electrical displacement field under fixed carrier density. The scaling analysis reveals that the sign reversal of the NLHE is contributed from both the Berry curvature dipole (BCD) and extrinsic scatterings. The displacement field-induced sign reversal of the BCD indicates asymmetric distributions of Berry curvature hot spots across different Fermi pockets in bilayer graphene. Our findings suggest that symmetry engineering of van der Waals heterostructures is promising for room-temperature applications based on nonlinear quantum devices, such as high-frequency rectifiers and wireless charging. △ Less

Submitted 22 January, 2025; originally announced January 2025.

Journal ref: Phys. Rev. B 111, L041403 (2025)

Nonlinear spin and orbital Edelstein effect in WTe2

Authors: Xing-Guo Ye, Peng-Fei Zhu, Wen-Zheng Xu, Tong-Yang Zhao, Zhi-Min Liao

Abstract: In materials with spin-momentum locked spin textures, such as Rashba states and topological surface states, the current-induced shift of the Fermi contour in the k space leads to spin polarization, known as the Edelstein effect, which depends linearly on the applied current. However, its nonlinear counterpart has not yet been discovered. Here, we report the observation of the nonlinear Edelstein e… ▽ More In materials with spin-momentum locked spin textures, such as Rashba states and topological surface states, the current-induced shift of the Fermi contour in the k space leads to spin polarization, known as the Edelstein effect, which depends linearly on the applied current. However, its nonlinear counterpart has not yet been discovered. Here, we report the observation of the nonlinear Edelstein effect in few-layer WTe2. Under a current bias, an out-of-plane magnetization is induced in WTe2, which is electrically probed using an Fe3GeTe2 electrode, a van der Waals ferromagnet with perpendicular magnetic anisotropy. Notably, with an applied ac at frequency ω, an induced magnetization with second-harmonic response at frequency 2ω is observed, and its magnitude demonstrates a quadratic dependence on the applied current, characteristic of the nonlinear Edelstein effect. This phenomenon is well explained by the current-induced orbital magnetization via the Berry connection polarizability tensors in WTe2. The orbital degree of freedom plays the primary role in the observed nonlinear Edelstein effect, that is, the nonlinear orbital Edelstein effect. This can, in turn, give rise to a nonlinear spin Edelstein effect through spin-orbit coupling. △ Less

Submitted 3 December, 2024; originally announced December 2024.

Comments: 29 pages

Journal ref: Physical Review B 110, L201407 (2024)

Facilitating field-free perpendicular magnetization switching with a Berry curvature dipole in a Weyl semimetal

Authors: Dong Li, Xing-Yu Liu, Xing-Guo Ye, Zhen-Cun Pan, Wen-Zheng Xu, Peng-Fei Zhu, An-Qi Wang, Kenji Watanabe, Takashi Taniguchi, Zhi-Min Liao

Abstract: We report the synergy between orbital and spin-orbit torques in WTe2/Fe3GeTe2 heterostructures characterized by a Berry curvature dipole. By applying a current along the a axis in WTe2, we detect an out-of-plane magnetization in the system, which we attribute to nonequilibrium orbital magnetization linked to the Berry curvature dipole based on first-principles calculations, manifesting as the orbi… ▽ More We report the synergy between orbital and spin-orbit torques in WTe2/Fe3GeTe2 heterostructures characterized by a Berry curvature dipole. By applying a current along the a axis in WTe2, we detect an out-of-plane magnetization in the system, which we attribute to nonequilibrium orbital magnetization linked to the Berry curvature dipole based on first-principles calculations, manifesting as the orbital Edelstein effect. This effect generates orbital torques that enable field-free perpendicular magnetization switching. Furthermore, by applying a relatively small current along the a axis and a pulsed current along the b axis in WTe2, we demonstrate controllable field-free magnetization switching of the adjacent Fe3GeTe2 layer, independently manipulating the orbital and spin-orbit torques. Our findings not only enhance the understanding of the collaborative dynamics between these torques but also suggest potential applications in magnetoresistive random-access memory. △ Less

Submitted 3 December, 2024; originally announced December 2024.

Comments: 29 pages

Journal ref: Physical Review B 110, L100409 (2024)

Superconductivity at Pd/Bi$_2$Se$_3$ Interfaces Due to Self-Formed PdBiSe Interlayers

Authors: Kaixuan Fan, Ze Hua, Siyao Gu, Peng Zhu, Guangtong Liu, Hechen Ren, Ruiwen Shao, Zhiwei Wang, Li Lu, Fan Yang

Abstract: Understanding the physical and chemical processes at the interface of metals and topological insulators is crucial for developing the next generation of topological quantum devices. Here we report the discovery of robust superconductivity in Pd/Bi$_2$Se$_3$ bilayers fabricated by sputtering Pd on the surface of Bi$_2$Se$_3$. Through transmission electron microscopy measurements, we identify that t… ▽ More Understanding the physical and chemical processes at the interface of metals and topological insulators is crucial for developing the next generation of topological quantum devices. Here we report the discovery of robust superconductivity in Pd/Bi$_2$Se$_3$ bilayers fabricated by sputtering Pd on the surface of Bi$_2$Se$_3$. Through transmission electron microscopy measurements, we identify that the observed interfacial superconductivity originates from the diffusion of Pd into Bi$_2$Se$_3$. In the diffusion region, Pd chemically reacts with Bi$_2$Se$_3$ and forms a layer of PdBiSe, a known su-perconductor with a bulk transition temperature of 1.5 K. Our work provides a method for in-troducing superconductivity into Bi$_2$Se$_3$, laying the foundation for developing sophisticated Bi$_2$Se$_3$-based topological devices. △ Less

Submitted 1 December, 2024; originally announced December 2024.

Report number: CJQS-2024-014

Journal ref: Materials 2024, 17(22), 5460

Bilayer construction for mixed state phenomena with strong, weak symmetries and symmetry breakings

Authors: Shuangyuan Lu, Penghao Zhu, Yuan-Ming Lu

Abstract: We introduce the bilayer construction, as a specific purification scheme for a general mixed state, where each mixed state has a one-to-one correspondence with a bilayer pure state with two constraints: non-negativity of the bilayer wavefunction; and the presence of an anti-unitary layer-exchange symmetry T. Different from the Choi-Jamiołkowski isomorphism, any mixed state can be realized as the m… ▽ More We introduce the bilayer construction, as a specific purification scheme for a general mixed state, where each mixed state has a one-to-one correspondence with a bilayer pure state with two constraints: non-negativity of the bilayer wavefunction; and the presence of an anti-unitary layer-exchange symmetry T. Different from the Choi-Jamiołkowski isomorphism, any mixed state can be realized as the monolayer reduced density matrix of a bilayer pure state, and its physical properties can be experimentally realized and detected in non-magnetic bilayer 2D materials with a layer-exchange mirror symmetry. We study a variety of mixed state phenomena in the bilayer construction: (1) strong and weak symmetries, their explicit and spontaneous breakings in mixed states can be understood as usual Landau-type symmetry breakings in the bilayer pure state, and their criteria can be derived accordingly; (2) decoherence of a pure state by local errors can be mapped to quantum quench dynamics of the bilayer pure states; (3) mixed symmetry protected topological (SPT) states and mixed state topological orders can be classified, characterized and realized as pure state SPTs and topological orders in the bilayer. We further study examples of strong-to-weak spontaneous symmetry breaking (SWSSB) and their critical scalings at the SWSSB transition in the bilayer construction. △ Less

Submitted 3 December, 2024; v1 submitted 11 November, 2024; originally announced November 2024.

Comments: 18 pages, 1 figure, 1 table, added discussions on the definition of mixed state phases, added details for the examples discussed in section V B

Obstruction to Broken Symmetries in Topological Flat Bands

Authors: Penghao Zhu, Shi Feng, Yuan-Ming Lu

Abstract: Motivated by the abundance of symmetry breaking states in magic-angle twisted bilayer graphene and other two-dimensional materials, we study superconducting (SC) and charge orders in two-dimensional topological flat bands in the strong correlation regime. By relating the half-filled 2D topological flat bands to the surface states of 3D topological insulators in symmetry class AIII, we reveal the t… ▽ More Motivated by the abundance of symmetry breaking states in magic-angle twisted bilayer graphene and other two-dimensional materials, we study superconducting (SC) and charge orders in two-dimensional topological flat bands in the strong correlation regime. By relating the half-filled 2D topological flat bands to the surface states of 3D topological insulators in symmetry class AIII, we reveal the topological obstruction to the formation of gapped SC and inter-valley charge orders without intrinsic topological orders, in the presence of the anti-unitary particle-hole symmetry at half filling. This is a generalization of the Li-Haldane arguments for nodal superconductivity to strongly interacting electrons. In contrast to the $\mathbb{Z}$-valued obstruction derived from the non-interacting band topology, the topological obstruction of interacting electrons in half-filled flat bands has a $\mathbb{Z}_{8}$ classification, depending on the charge (valley) Chern number of the superconducting (inter-valley charge) orders. This is demonstrated by an interacting Hamiltonian for half-filled flat bands with a net Chern number $C=4$, where superconductivity and $\mathbb{Z}_2$ topological order coexist in a gapped ground state with particle-hole symmetry. △ Less

Submitted 12 September, 2024; v1 submitted 26 August, 2024; originally announced August 2024.

Comments: 5+9 pages, 2+1 figures; We added a short discussion about materials in v2

Emergent quantum Majorana metal from a chiral spin liquid

Authors: Penghao Zhu, Shi Feng, Kang Wang, Tao Xiang, Nandini Trivedi

Abstract: We propose a mechanism to explain the emergence of an intermediate gapless spin liquid phase in the antiferromagnetic Kitaev model in an externally applied magnetic field, sandwiched between the well-known gapped chiral spin liquid and the gapped partially polarized phase. We propose that, in moderate fields, $π$-fluxes nucleate in the ground state and trap Majorana zero modes. As these fluxes pro… ▽ More We propose a mechanism to explain the emergence of an intermediate gapless spin liquid phase in the antiferromagnetic Kitaev model in an externally applied magnetic field, sandwiched between the well-known gapped chiral spin liquid and the gapped partially polarized phase. We propose that, in moderate fields, $π$-fluxes nucleate in the ground state and trap Majorana zero modes. As these fluxes proliferate with increasing field, the Majorana zero modes overlap creating an emergent $\mathbb{Z}_2$ quantum Majorana metallic state with a `Fermi surface' at zero energy. We further show that the Majorana spectral function captures the dynamical spin and dimer correlations obtained by the infinite Projected Entangled Pair States method, thereby validating our variational approach. △ Less

Submitted 12 March, 2025; v1 submitted 20 May, 2024; originally announced May 2024.

Comments: 10+12 pages, 4+7 figures

Journal ref: Nat Commun 16, 2420 (2025)

Two Toy Spin Chain Models of Decoherence

Authors: P. C. E. Stamp Zhen Zhu

Abstract: We solve for the decoherence dynamics of two models in which a simple qubit or Central Spin couples to a bath of spins; the bath is made from a chain of spins. In model 1, the bath spins are Ising spins; in Model 2, they are coupled by transverse spin-spin interactions, and the chain supports spin waves. We look at (i) the case where the Hamiltonian is static, with a constant system/bath coupling,… ▽ More We solve for the decoherence dynamics of two models in which a simple qubit or Central Spin couples to a bath of spins; the bath is made from a chain of spins. In model 1, the bath spins are Ising spins; in Model 2, they are coupled by transverse spin-spin interactions, and the chain supports spin waves. We look at (i) the case where the Hamiltonian is static, with a constant system/bath coupling, and (ii) where this coupling varies in time. △ Less

Submitted 6 May, 2024; originally announced May 2024.

Comments: 9 page, 4 figures

Dipole-Obstructed Cooper Pairing: Theory and Application to $j=3/2$ Superconductors

Authors: Penghao Zhu, Rui-Xing Zhang

Abstract: Like electrons, Cooper pairs can carry a monopole charge if the pairing electrons come from two or more Fermi surfaces with different Chern numbers. In such an instance, a superconductor is necessarily nodal due to an inherent topological pairing obstruction. In this work, we show that a similar obstruction is also possible when there is only one Fermi surface involved in the pairing process. By d… ▽ More Like electrons, Cooper pairs can carry a monopole charge if the pairing electrons come from two or more Fermi surfaces with different Chern numbers. In such an instance, a superconductor is necessarily nodal due to an inherent topological pairing obstruction. In this work, we show that a similar obstruction is also possible when there is only one Fermi surface involved in the pairing process. By developing a Chern-vorticity theorem, we have identified a class of Fermi surfaces with a quantized dipolar Berry flux pattern, where all intra-Fermi-surface Cooper pairings are ``dipole-obstructed" and nodal. As a real-world application, we find that the dipole obstruction plays a crucial role in stabilizing the superconducting nodal structure for $j=3/2$ half-Heusler compounds. △ Less

Submitted 30 April, 2024; originally announced May 2024.

Comments: 6+9 pages, 3+5 figures

Fractionalization Signatures in the Dynamics of Quantum Spin Liquids

Authors: Kang Wang, Shi Feng, Penghao Zhu, Runze Chi, Hai-Jun Liao, Nandini Trivedi, Tao Xiang

Abstract: We investigate the signatures of fractionalization in quantum spin liquids by studying different phases of the Kitaev honeycomb model in the presence of an out-of-plane magnetic field through which the model becomes non-integrable. Using the infinite projected entangled pair states (iPEPS) ansatz, along with analytical calculations and exact diagonalization, we calculate dynamical signatures of fr… ▽ More We investigate the signatures of fractionalization in quantum spin liquids by studying different phases of the Kitaev honeycomb model in the presence of an out-of-plane magnetic field through which the model becomes non-integrable. Using the infinite projected entangled pair states (iPEPS) ansatz, along with analytical calculations and exact diagonalization, we calculate dynamical signatures of fractionalized particles through spin-spin and dimer-dimer correlations. Our analysis demonstrates the ability of these correlations to discern distinct fractionalized quantum sectors, namely Majorana fermions and the emergent $Z_2$ fluxes, in both the chiral spin liquid (CSL) phase under weak field and the emergent intermediate gapless phase (IGP) under moderate field. Importantly, our calculation reveals the nature of IGP observed at moderate fields, a region of ongoing debate, indicating that this phase is a Majorana metal induced by strong flux fluctuations. △ Less

Submitted 12 March, 2025; v1 submitted 18 March, 2024; originally announced March 2024.

Comments: 5+10 pages, 4+12 figures

Journal ref: PhysRevB.111.L100402 (2025) Editors' Suggestion

New three-dimensional dispersion in the type-II Dirac semimetals PtTe$_2$ and PdTe$_2$ revealed through Angle Resolved Photoemission Spectroscopy

Authors: Ivan Pelayo, Derek Bergner, Archibald J. Williams, Jiayuwen Qi, Penghao Zhu, Mahfuzun Nabi, Warren L. B. Huey, Luca Moreschini, Ziling Deng, Jonathan Denlinger, Alessandra Lanzara, Yuan-Ming Lu, Wolfgang Windl, Joshua Goldberger, Claudia Ojeda-Aristizabal

Abstract: PtTe$_2$ and PdTe$_2$ are among the first transition metal dichalcogenides that were predicted to host type-II Dirac fermions, exotic particles prohibited in free space. These materials are layered and air-stable, which makes them top candidates for technological applications that take advantage of their anisotropic magnetotransport properties. Here, we provide a detailed characterization of the e… ▽ More PtTe$_2$ and PdTe$_2$ are among the first transition metal dichalcogenides that were predicted to host type-II Dirac fermions, exotic particles prohibited in free space. These materials are layered and air-stable, which makes them top candidates for technological applications that take advantage of their anisotropic magnetotransport properties. Here, we provide a detailed characterization of the electronic structure of PtTe$_2$ and PdTe$_2$ using Angle Resolved Photoemission Spectroscopy (ARPES) and Density Functional Theory (DFT) calculations, unveiling a new three-dimensional dispersion in these materials. Through the use of circularly polarized light, we report a different behavior of such dispersion in PdTe$_2$ compared to PtTe$_2$, that we relate to a symmetry analysis of the dipole matrix element. Such analysis reveals a link between the observed circular dichroism and the different momentum-dependent terms in the dispersion of these two compounds, despite their close similarity in crystal structure. Additionally, our data shows a clear difference in the circular dichroic signal for the type-II Dirac cones characteristic of these materials, compared to their topologically protected surface states. Our work provides a useful reference for the ARPES characterization of other transition metal dichalcogenides with topological properties and illustrates the use of circular dichroism as a guide to identify the topological character of two otherwise equivalent band dispersions, and to recognize different attributes in the band structure of similar materials. △ Less

Submitted 16 May, 2024; v1 submitted 23 December, 2023; originally announced December 2023.

Comments: 14 pages, 9 figures

Charge-density wave transition in magnetic topological semimetal EuAl$_4$

Authors: R. Yang, C. C. Le, P. Zhu, Z. W. Wang, T. Shang, Y. M. Dai, J. P. Hu, M. Dressel

Abstract: The interplay among topology, charge-density wave (CDW), and magnetism can give rise to a plethora of exotic quantum phenomena. Recently, a group of magnetic topological semimetals with tetragonal lattices and CDW order were found to exhibit anomalous magnetic instability, helical spin ordering, and the presence of skyrmions. However, the underlying mechanism responsible for these observations rem… ▽ More The interplay among topology, charge-density wave (CDW), and magnetism can give rise to a plethora of exotic quantum phenomena. Recently, a group of magnetic topological semimetals with tetragonal lattices and CDW order were found to exhibit anomalous magnetic instability, helical spin ordering, and the presence of skyrmions. However, the underlying mechanism responsible for these observations remains unclear. Here, we conducted a comprehensive investigation into the impact of CDW on the topological and magnetic properties of EuAl$_4$ using optical spectroscopy and the first-principles calculations. Through optical spectroscopy, we observed a partial gap (60~meV) on the Fermi surface and an enhanced mid-infrared absorption around 0.4~eV after the CDW transition. Magneto-optical spectroscopy and the first-principles calculations proved that, by affecting the band structure, the CDW order frustrates the antiferromagnetic interactions but strengthened the ferromagnetic ones, which can destabilize the magnetism. With lower symmetry in the CDW ordered state, carriers from the Weyl bands will mediate the anisotropic magnetic interactions promoting the formation of chiral spin textures. Conversely, without the CDW order, the counterpart EuGa$_4$ shows robust collinear antiferromagnetic order. Our findings uncover the pivotal role played by CDW order in arousing intricate magnetism in topological materials and provide valuable insights into controlling topological and magnetic properties through the manipulation of CDW orders. △ Less

Submitted 27 November, 2023; originally announced November 2023.

Comments: 8 pages, 4 figures

Report number: RIKEN-iTHEMS-Report-24

Pressure-induced Superconductivity and Topological Quantum Phase Transitions in the Topological Semimetal ZrTe2

Authors: Shihao Zhu, Juefei Wu, Peng Zhu, Cuiying Pei, Qi Wang, Donghan Jia, Xinyu Wang, Yi Zhao, Lingling Gao, Changhua Li, Weizheng Cao, Mingxin Zhang, Lili Zhang, Mingtao Li, Huiyang Gou, Wenge Yang, Jian Sun, Yulin Chen, Zhiwei Wang, Yugui Yao, Yanpeng Qi

Abstract: Topological transition metal dichalcogenides (TMDCs) have attracted much attention due to its potential applications in spintronics and quantum computations. In this work, we systematically investigate the structural and electronic properties of topological TMDCs candidate ZrTe2 under high pressure. A pressure-induced Lifshitz transition is evidenced by the change of charge carrier type as well as… ▽ More Topological transition metal dichalcogenides (TMDCs) have attracted much attention due to its potential applications in spintronics and quantum computations. In this work, we systematically investigate the structural and electronic properties of topological TMDCs candidate ZrTe2 under high pressure. A pressure-induced Lifshitz transition is evidenced by the change of charge carrier type as well as the Fermi surface. Superconductivity was observed at around 8.3 GPa without structural phase transition. A typical dome-shape phase diagram is obtained with the maximum Tc of 5.6 K for ZrTe2. Furthermore, our theoretical calculations suggest the presence of multiple pressure-induced topological quantum phase transitions, which coexists with emergence of superconductivity. The results demonstrate that ZrTe2 with nontrivial topology of electronic states display new ground states upon compression. △ Less

Submitted 19 October, 2023; originally announced October 2023.

Comments: Accepted for publication in Advanced Science 2023

Journal ref: Advanced Science 2023,10, 2301332

Observation of strong attenuation within the photonic band gap of multiconnected networks

Authors: Pengbo Zhu, Runkai Chen, Xiangbo Yang, Yanglong Fan, Huada Lian, Zhen-Yu Wang

Abstract: We theoretically and experimentally study a photonic band gap (PBG) material made of coaxial cables. The coaxial cables are waveguides for the electromagnetic waves and provide paths for direct wave interference within the material. Using multiconnected coaxial cables to form a unit cell, we realize PBGs via (i) direct interference between the waveguides within each cell and (ii) scattering among… ▽ More We theoretically and experimentally study a photonic band gap (PBG) material made of coaxial cables. The coaxial cables are waveguides for the electromagnetic waves and provide paths for direct wave interference within the material. Using multiconnected coaxial cables to form a unit cell, we realize PBGs via (i) direct interference between the waveguides within each cell and (ii) scattering among different cells. We systematically investigate the transmission of EM waves in our PBG materials and discuss the mechanism of band gap formation. We observe experimentally for the first time the wide band gap with strong attenuation caused by direct destructive interference. △ Less

Submitted 28 September, 2023; originally announced October 2023.

Stripe charge order driven manipulation of Majorana bound states in 2M-WS2 topological superconductor

Authors: Xuemin Fan, Xiaoqi Sun, Penghao Zhu, Yuqiang Fang, Yongkang Ju, Yonghao Yuan, Fuqiang Huang, Taylor L. Hughes, Peizhe Tang, Qi-Kun Xue, Wei Li

Abstract: Majorana bound states (MBSs) are building blocks for topological quantum computing. They can be generated via the combination of electronic topology and superconductivity. To achieve logic operations via Majorana braiding, positional control of the MBS must be established. To this end, exotic co-existing phases or collective modes in an intrinsic topological superconductor can provide a tuning kno… ▽ More Majorana bound states (MBSs) are building blocks for topological quantum computing. They can be generated via the combination of electronic topology and superconductivity. To achieve logic operations via Majorana braiding, positional control of the MBS must be established. To this end, exotic co-existing phases or collective modes in an intrinsic topological superconductor can provide a tuning knob to manipulate the MBS. Here we report the observation of a striped surface charge order coexisting with superconductivity and its controllable tuning of the MBS in the topological superconductor 2M-WS2 using low-temperature scanning tunneling microscopy. By applying an out-of-plane magnetic field, we observe that MBS is absent in vortices in the region with strong stripe order. This is in contrast to adjacent underlaying layers without charge order where vortex-bound MBSs are observed. Via theoretical simulations, we show that the surface stripe order does not destroy the bulk topology, but it can effectively modify the spatial distribution of MBS, i.e., it pushes them downward away from the 2M-WS2 surface. Our findings demonstrate that the interplay of charge order and topological superconductivity can be used to manipulate the positions of the MBS, and to explore of new states of matter. △ Less

Submitted 30 August, 2023; originally announced August 2023.

Comments: 20 pages, 4 figures

Anomalous shift and optical vorticity in the steady photovoltaic current

Authors: Penghao Zhu, A. Alexandradinata

Abstract: Steady illumination of a non-centrosymmetric semiconductor results in a bulk photovoltaic current, which is contributed by real-space displacements (`shifts') of charged quasiparticles as they transit between Bloch states. The shift induced by interband excitation via absorption of photons has received the prevailing attention. However, this excitation-induced shift can be far outweighed ($\ll$) b… ▽ More Steady illumination of a non-centrosymmetric semiconductor results in a bulk photovoltaic current, which is contributed by real-space displacements (`shifts') of charged quasiparticles as they transit between Bloch states. The shift induced by interband excitation via absorption of photons has received the prevailing attention. However, this excitation-induced shift can be far outweighed ($\ll$) by the shift induced by intraband relaxation, or by the shift induced by radiative recombination of electron-hole pairs. This outweighing ($\ll$) is attributed to (i) time-reversal-symmetric, intraband Berry curvature, which results in an anomalous shift of quasiparticles as they scatter with phonons, as well as to (ii) topological singularities in the interband Berry phase (`optical vortices'), which makes the photovoltaic current extraordinarily sensitive to the linear polarization vector of the light source. Both (i-ii) potentially lead to nonlinear conductivities of order $mAV^{-2}$, without finetuning of the incident radiation frequency, band gap, or joint density of states. A case study of BiTeI showcases the anomalous shift and optical vorticity in a realistic material. △ Less

Submitted 9 August, 2025; v1 submitted 16 August, 2023; originally announced August 2023.

Comments: 22+55 pages, 10+9 Figures; We touched up a few derivations, presented a more nuanced discussion of potential solar cell applications in v2; We presented a Chern-vorticity theorem that relates optical vortices and Chern numbers, and a case study on BiTeI in v3; In v4, we presented more discussion about photovoltaic effect in BiTeI

Journal ref: Phys. Rev. B 110, 115108 (2024)

Delicate Topology of Luttinger Semimetal

Authors: Penghao Zhu, Rui-Xing Zhang

Abstract: Recent advances in delicate topology have expanded the classification of topological bands, but its presence in solid-state materials remains elusive. Here we show that delicate topology naturally emerges in the Luttinger-Kohn model that describes many semiconductors and semimetals. In particular, the Luttinger semimetal is found to be a quantum critical point leading to a quantized jump of an int… ▽ More Recent advances in delicate topology have expanded the classification of topological bands, but its presence in solid-state materials remains elusive. Here we show that delicate topology naturally emerges in the Luttinger-Kohn model that describes many semiconductors and semimetals. In particular, the Luttinger semimetal is found to be a quantum critical point leading to a quantized jump of an integer-valued delicate topological invariant. Away from this criticality, we have identified new types of electronic insulators and semimetals with intertwined stable and delicate topologies. They all carry gapless surface states that transform anomalously under rotation symmetry. Our work provides a starting point for exploring delicate topological phenomena in quantum materials. △ Less

Submitted 10 August, 2023; originally announced August 2023.

Comments: 10 + 6 pages, 4 + 5 figures

Spin-momentum Locking and Topological Vector Charge Response with Conserved Spin

Authors: Yoonseok Hwang, Penghao Zhu, Taylor L. Hughes

Abstract: Spin-momentum locking plays a fundamental role in spintronics and, more broadly, is an important concept in condensed matter physics. In 2D and 3D, spin-momentum locking typically does not allow spin-conservation because the spin-1/2 operators of electrons anticommute. Instead, here we study spin-momentum locking terms with conserved, commuting pseudospins built from a combination of spin and orbi… ▽ More Spin-momentum locking plays a fundamental role in spintronics and, more broadly, is an important concept in condensed matter physics. In 2D and 3D, spin-momentum locking typically does not allow spin-conservation because the spin-1/2 operators of electrons anticommute. Instead, here we study spin-momentum locking terms with conserved, commuting pseudospins built from a combination of spin and orbitals. We find that 2D spin-momentum locking terms with conserved pseudospins generally lead to linearly dispersing modes at low-energy with anomalous charge and pseudospin currents. To cure the anomaly we show that such anomalous modes can be realized on the surface of a 3D Weyl semimetal (or an associated weak topological insulator) with a nonzero mixed spin-momentum quadrupole moment, which is determined by the momentum location and pseudospin eigenvalues of Weyl points at the Fermi level. Crucially, this mixed quadrupole moment captures a mixed pseudospin-charge bulk response that cancels the anomaly of surface modes, and can generate a giant 3D spin Hall effect, among other phenomena. △ Less

Submitted 4 February, 2026; v1 submitted 6 April, 2023; originally announced April 2023.

Comments: 8+8 pages;3+3 figures

Journal ref: Phys. Rev. B 113, 075105 (2026)

Control over Berry Curvature Dipole with Electric Field in WTe2

Authors: Xing-Guo Ye, Huiying Liu, Peng-Fei Zhu, Wen-Zheng Xu, Shengyuan A. Yang, Nianze Shang, Kaihui Liu, Zhi-Min Liao

Abstract: Berry curvature dipole plays an important role in various nonlinear quantum phenomena. However, the maximum symmetry allowed for nonzero Berry curvature dipole in the transport plane is a single mirror line, which strongly limits its effects in materials. Here, via probing the nonlinear Hall effect, we demonstrate the generation of Berry curvature dipole by applied dc electric field in WTe2, which… ▽ More Berry curvature dipole plays an important role in various nonlinear quantum phenomena. However, the maximum symmetry allowed for nonzero Berry curvature dipole in the transport plane is a single mirror line, which strongly limits its effects in materials. Here, via probing the nonlinear Hall effect, we demonstrate the generation of Berry curvature dipole by applied dc electric field in WTe2, which is used to break the symmetry constraint. A linear dependence between the dipole moment of Berry curvature and the dc electric field is observed. The polarization direction of the Berry curvature is controlled by the relative orientation of the electric field and crystal axis, which can be further reversed by changing the polarity of the dc field. Our Letter provides a route to generate and control Berry curvature dipole in broad material systems and to facilitate the development of nonlinear quantum devices. △ Less

Submitted 5 January, 2023; originally announced January 2023.

Journal ref: Phys. Rev. Lett. 130, 016301 (2023)

Pressure-Induced Superconductivity in Topological Heterostructure (PbSe)5(Bi2Se3)6

Authors: Cuiying Pei, Peng Zhu, Bingtan Li, Yi Zhao, Lingling Gao, Changhua Li, Shihao Zhu, Qinghua Zhang, Tianping Ying, Lin Gu, Bo Gao, Huiyang Gou, Yansun Yao, Jian Sun, Hanyu Liu, Yulin Chen, Zhiwei Wang, Yugui Yao, Yanpeng Qi

Abstract: Recently, the natural heterostructure of (PbSe)5(Bi2Se3)6 has been theoretically predicted and experimentally confirmed as a topological insulator. In this work, we induce superconductivity in (PbSe)5(Bi2Se3)6 by implementing high pressure. As increasing pressure up to 10 GPa, superconductivity with Tc ~ 4.6 K suddenly appears, followed by an abrupt decrease. Remarkably, upon further compression a… ▽ More Recently, the natural heterostructure of (PbSe)5(Bi2Se3)6 has been theoretically predicted and experimentally confirmed as a topological insulator. In this work, we induce superconductivity in (PbSe)5(Bi2Se3)6 by implementing high pressure. As increasing pressure up to 10 GPa, superconductivity with Tc ~ 4.6 K suddenly appears, followed by an abrupt decrease. Remarkably, upon further compression above 30 GPa, a new superconducting state arises, where pressure raises the Tc to an unsaturated 6.0 K within the limit of our research. Combining XRD and Raman spectroscopies, we suggest that the emergence of two distinct superconducting states occurs concurrently with the pressure-induced structural transition in this topological heterostructure (PbSe)5(Bi2Se3)6. △ Less

Submitted 3 January, 2023; originally announced January 2023.

Comments: 16 pages,5 figures, 1 table

Pressured-induced superconductivity extending across the topological phase transition in thallium-based topological materials TlBi(S1-xSex)2

Authors: Cuiying Pei, Peihao Huang, Peng Zhu, Linlin Liu, Qi Wang, Yi Zhao, Lingling Gao, Changhua Li, Weizheng Cao, Jian Lv, Xiang Li, Zhiwei Wang, Yugui Yao, Binghai Yan, Claudia Felser, Yulin Chen, Hanyu Liu, Yanpeng Qi

Abstract: The coexistence of superconductivity and topology holds the potential to realize exotic quantum states of matter. Here we report that superconductivity induced by high pressure in three thallium-based materials, covering the phase transition from a normal insulator (TlBiS2) to a topological insulator (TlBiSe2) through a Dirac semimetal (TlBiSeS). By increasing the pressure up to 60 GPa, we observe… ▽ More The coexistence of superconductivity and topology holds the potential to realize exotic quantum states of matter. Here we report that superconductivity induced by high pressure in three thallium-based materials, covering the phase transition from a normal insulator (TlBiS2) to a topological insulator (TlBiSe2) through a Dirac semimetal (TlBiSeS). By increasing the pressure up to 60 GPa, we observe superconductivity phase diagrams with maximal Tc values at 6.0-8.1 K. Our density-functional theory calculations reveal topological surface states in superconductivity phases for all three compounds. Our study paves the path to explore topological superconductivity and topological phase transitions. △ Less

Submitted 18 November, 2022; originally announced November 2022.

Comments: 39 pages, 5+22 figures

Journal ref: Cell Reports Physical Science 3, 101094 (2022)

Ultrafast formation of topological defects in a 2D charge density wave

Authors: Yun Cheng, Alfred Zong, Lijun Wu, Qingping Meng, Wei Xia, Fengfeng Qi, Pengfei Zhu, Xiao Zou, Tao Jiang, Yanfeng Guo, Jasper van Wezel, Anshul Kogar, Michael W. Zuerch, Jie Zhang, Yimei Zhu, Dao Xiang

Abstract: Topological defects play a key role in nonequilibrium phase transitions, ranging from birth of the early universe to quantum critical behavior of ultracold atoms. In solids, transient defects are known to generate a variety of hidden orders not accessible in equilibrium, but how defects are formed at the nanometer lengthscale and femtosecond timescale remains unknown. Here, we employ an intense la… ▽ More Topological defects play a key role in nonequilibrium phase transitions, ranging from birth of the early universe to quantum critical behavior of ultracold atoms. In solids, transient defects are known to generate a variety of hidden orders not accessible in equilibrium, but how defects are formed at the nanometer lengthscale and femtosecond timescale remains unknown. Here, we employ an intense laser pulse to create topological defects in a 2D charge density wave, and track their morphology and dynamics with ultrafast electron diffraction. Leveraging its high temporal resolution and sensitivity in detecting weak diffuse signals, we discover a dual-stage growth of 1D domain walls within 1 ps, a process not dictated by the order parameter amplitude but instead mediated by a nonthermal population of longitudinal optical phonons. Our work provides a framework for ultrafast engineering of topological defects based on selective excitation of collective modes, opening new avenues for dynamical control of nonequilibrium phases in correlated materials. △ Less

Submitted 10 November, 2022; originally announced November 2022.

Journal ref: Nature Physics 20, 54 (2024)

Scattering theory of delicate topological insulators

Authors: Penghao Zhu, Jiho Noh, Yingkai Liu, Taylor L. Hughes

Abstract: We study the scattering theory of delicate topological insulators (TIs), which are novel topological phases beyond the paradigm of the tenfold way, topological quantum chemistry, and the symmetry indicator method. We demonstrate that the phase of the reflection amplitude can probe the delicate topology by capturing a characteristic feature of a delicate TI. This feature is the returning Thouless p… ▽ More We study the scattering theory of delicate topological insulators (TIs), which are novel topological phases beyond the paradigm of the tenfold way, topological quantum chemistry, and the symmetry indicator method. We demonstrate that the phase of the reflection amplitude can probe the delicate topology by capturing a characteristic feature of a delicate TI. This feature is the returning Thouless pump, where an integer number of charges are pumped forward and backward in the first and second half of the adiabatic cycle respectively. As a byproduct of our analysis we show that requiring additional symmetries can stable the boundary states of a delicate TI beyond the conventional requirement of a sharply defined surface. Furthermore, we propose a photonic crystal experiment to implement a delicate TI and measure its reflection phase which reveals the delicate topology. △ Less

Submitted 5 October, 2022; originally announced October 2022.

Comments: 10+5 pages; 7+4 figures

$\mathbb{Z}_{2}$ Spin Hopf Insulator: Helical Hinge States and Returning Thouless Pump

Authors: Penghao Zhu, A. Alexandradinata, Taylor L. Hughes

Abstract: We introduce a time-reversal-symmetric analog of the Hopf insulator that we call a spin Hopf insulator. The spin Hopf insulator harbors nontrivial Kane-Mele $\Z_2$ invariants on its surfaces, and is the first example of a nonmagnetic delicate topological insulator with spin-orbit coupling. We show that the Kane-Mele $\Z_2$ topology on the surface is generically unstable, but can be stabilized by t… ▽ More We introduce a time-reversal-symmetric analog of the Hopf insulator that we call a spin Hopf insulator. The spin Hopf insulator harbors nontrivial Kane-Mele $\Z_2$ invariants on its surfaces, and is the first example of a nonmagnetic delicate topological insulator with spin-orbit coupling. We show that the Kane-Mele $\Z_2$ topology on the surface is generically unstable, but can be stabilized by the addition of a composition of the particle hole and spatial inversion symmetry. Such a symmetry not only protects the surface $\Z_2$ invariant, but also protects gapless helical hinge states on the spin Hopf insulator. Furthermore, we show that in the presence of four-fold rotational symmetry, the spin Hopf insulator exhibits a returning Thouless pump, as well as surface states on sharp boundary terminations. △ Less

Submitted 9 April, 2023; v1 submitted 17 August, 2022; originally announced August 2022.

Comments: 9+4 pages, 6+1 figures

Journal ref: Phys. Rev. B 107, 115159 (2023)

Orbital polarization and third-order anomalous Hall effect in WTe2

Authors: Xing-Guo Ye, Peng-Fei Zhu, Wen-Zheng Xu, Zhihao Zang, Yu Ye, Zhi-Min Liao

Abstract: The anomalous Hall effect (AHE) has been extended into the nonlinear regime, where the Hall voltage shows higher-order response to the applied current. Nevertheless, the microscopic mechanism of the nonlinear AHE remains unclear. Here we report the orbital polarization and its induced third-order AHE in few-layer WTe2 flakes. Through angle-dependent electric measurements, it is found that the thir… ▽ More The anomalous Hall effect (AHE) has been extended into the nonlinear regime, where the Hall voltage shows higher-order response to the applied current. Nevertheless, the microscopic mechanism of the nonlinear AHE remains unclear. Here we report the orbital polarization and its induced third-order AHE in few-layer WTe2 flakes. Through angle-dependent electric measurements, it is found that the third-order AHE is quite consistent with the electric field induced polarization of orbital magnetic moment caused by the Berry connection polarizability tensor, which is further directly detected by polar reflective magnetic circular dichroism spectroscopy. The microscopic mechanisms of third-order AHE are analyzed through the scaling law, that is, the opposite orbital magnetic moments (up or down) deflect to opposite directions driven by electric field induced Berry curvature, forming the intrinsic contribution; driven by the Magnus effect of the self-rotating Bloch electrons, the opposite orbital magnetic moments are scattered towards opposite transverse directions, resulting in the skew scattering. △ Less

Submitted 16 July, 2022; originally announced July 2022.

Journal ref: Phys. Rev. B 106, 045414 (2022)

Higher rank chirality and non-Hermitian skin effect in a topolectrical circuit

Authors: Penghao Zhu, Xiao-Qi Sun, Taylor L. Hughes, Gaurav Bahl

Abstract: While chirality imbalances are forbidden in conventional lattice systems, non-Hermiticity can effectively avoid the chiral-doubling theorem to facilitate 1D chiral dynamics. Indeed, such systems support unbalanced unidirectional flows that can lead to the localization of an extensive number of states at the boundary, known as the non-Hermitian skin effect (NHSE). Recently, a generalized (rank-2) c… ▽ More While chirality imbalances are forbidden in conventional lattice systems, non-Hermiticity can effectively avoid the chiral-doubling theorem to facilitate 1D chiral dynamics. Indeed, such systems support unbalanced unidirectional flows that can lead to the localization of an extensive number of states at the boundary, known as the non-Hermitian skin effect (NHSE). Recently, a generalized (rank-2) chirality describing a 2D robust gapless mode with dispersion $ω=k_{x}k_{y}$ has been introduced in crystalline systems. Here we demonstrate that rank-2 chirality imbalances can be established in a non-Hermitian (NH) lattice system leading to momentum-resolved chiral dynamics, and a rank-2 NHSE where there are both edge- and corner-localized skin modes. We then experimentally test this phenomenology in a 2-dimensional topolectric circuit that implements a NH Hamiltonian with a long-lived rank-2 chiral mode. Using impedance measurements, we confirm the rank-2 NHSE in this system, and its manifestation in the predicted skin modes and a highly unusual momentum-position locking response. Our investigation demonstrates a circuit-based path to exploring higher-rank chiral physics, with potential applications in systems where momentum resolution is necessary, e.g., in beamformers and non-reciprocal devices. △ Less

Submitted 5 July, 2022; originally announced July 2022.

Comments: 12+8 pages, 5+6 figures

Proximity-induced superconducting gap in the intrinsic magnetic topological insulator MnBi2Te4

Authors: Wen-Zheng Xu, Chun-Guang Chu, Zhen-Cun Pan, Jing-Jing Chen, An-Qi Wang, Zhen-Bing Tan, Peng-Fei Zhu, Xing-Guo Ye, Da-Peng Yu, Zhi-Min Liao

Abstract: We report magnetotransport measurements in the NbN/ magnetic topological insulator MnBi2Te4 (MBT)/ NbN junction at low temperature. At 10 mK, the nonlinear current-voltage characteristic of the junction shows a tunneling behavior, indicating the existence of interfacial potential barriers within the heterostructure. Under an out of plane perpendicular magnetic field, a transition from negative to… ▽ More We report magnetotransport measurements in the NbN/ magnetic topological insulator MnBi2Te4 (MBT)/ NbN junction at low temperature. At 10 mK, the nonlinear current-voltage characteristic of the junction shows a tunneling behavior, indicating the existence of interfacial potential barriers within the heterostructure. Under an out of plane perpendicular magnetic field, a transition from negative to positive magnetoresistance (MR) is found when increasing the bias voltage. A proximity-induced superconducting gap is estimated to be 0.1meV by a pair of differential resistance dips. Moreover, the induced gap is enhanced by gradually tuning the Fermi level toward the charge neutral point by a back gate voltage, which is ascribed to the increased transport contribution of the topological surface states in MBT. Intriguingly, the induced gap exhibits an anomalous magnetic field assisted enhancement, which may originate from the spin orbit coupling and magnetic order of MBT. Our results reveal the interplay between magnetism and superconductivity in MBT, paving the way for further studies on topological superconductivity and chiral Majorana edge modes in quantum anomalous Hall insulator/superconductor hybrid systems. △ Less

Submitted 9 June, 2022; originally announced June 2022.

Comments: 9 pages, 8 figures

Journal ref: Phys. Rev. B 105, 184515 (2022)

Light-induced dimension crossover in 1T-TiSe$_2$ dictated by excitonic correlations

Authors: Yun Cheng, Alfred Zong, Jun Li, Wei Xia, Shaofeng Duan, Wenxuan Zhao, Yidian Li, Fengfeng Qi, Jun Wu, Lingrong Zhao, Pengfei Zhu, Xiao Zou, Tao Jiang, Yanfeng Guo, Lexian Yang, Dong Qian, Wentao Zhang, Anshul Kogar, Michael W. Zuerch, Dao Xiang, Jie Zhang

Abstract: In low-dimensional systems with strong electronic correlations, the application of an ultrashort laser pulse often yields novel phases that are otherwise inaccessible. The central challenge in understanding such phenomena is to determine how dimensionality and many-body correlations together govern the pathway of a non-adiabatic transition. To this end, we examine a layered compound, 1T-TiSe$_2$,… ▽ More In low-dimensional systems with strong electronic correlations, the application of an ultrashort laser pulse often yields novel phases that are otherwise inaccessible. The central challenge in understanding such phenomena is to determine how dimensionality and many-body correlations together govern the pathway of a non-adiabatic transition. To this end, we examine a layered compound, 1T-TiSe$_2$, whose three-dimensional charge-density-wave (3D CDW) state also features exciton condensation due to strong electron-hole interactions. We find that photoexcitation suppresses the equilibrium 3D CDW while creating a nonequilibrium 2D CDW. Remarkably, the dimension reduction does not occur unless bound electron-hole pairs are broken. This relation suggests that excitonic correlations maintain the out-of-plane CDW coherence, settling a long-standing debate over their role in the CDW transition. Our findings demonstrate how optical manipulation of electronic interaction enables one to control the dimensionality of a broken-symmetry order, paving the way for realizing other emergent states in strongly correlated systems. △ Less

Submitted 19 February, 2022; originally announced February 2022.

Journal ref: Nature Communications 13, 963 (2022)

Orbit-transfer torque driven field-free switching of perpendicular magnetization

Authors: Xing-Guo Ye, Peng-Fei Zhu, Wen-Zheng Xu, Nianze Shang, Kaihui Liu, Zhi-Min Liao

Abstract: The reversal of perpendicular magnetization (PM) by electric control is crucial for high-density integration of low-power magnetic random-access memory (MRAM). Although the spin-transfer torque (STT) and spin-orbit torque (SOT) technologies have been used to switch the magnetization of a free layer with perpendicular magnetic anisotropy, the former has limited endurance because of the high current… ▽ More The reversal of perpendicular magnetization (PM) by electric control is crucial for high-density integration of low-power magnetic random-access memory (MRAM). Although the spin-transfer torque (STT) and spin-orbit torque (SOT) technologies have been used to switch the magnetization of a free layer with perpendicular magnetic anisotropy, the former has limited endurance because of the high current density directly through the junction, while the latter requires an external magnetic field or unconventional configuration to break the symmetry. Here we propose and realize the orbit-transfer torque (OTT), that is, exerting torque on the magnetization using the orbital magnetic moments, and thus demonstrate a new strategy for current-driven PM reversal without external magnetic field. The perpendicular polarization of orbital magnetic moments is generated by a direct current in a few-layer WTe2 due to the existence of nonzero Berry curvature dipole, and the polarization direction can be switched by changing the current polarity. Guided by this principle, we construct the WTe2/Fe3GeTe2 heterostructures, where the OTT driven field-free deterministic switching of PM is achieved. △ Less

Submitted 15 February, 2022; originally announced February 2022.

Journal ref: Chin. Phys. Lett. 39, 037303 (2022)

Multi-gap topology of the Wilson loop operator in mirror symmetric insulators

Authors: Penghao Zhu, Taylor L. Hughes, Xiao-Qi Sun

Abstract: We study the multi-gap topology of the periodic spectra of Wilson loop operators (WLOs) in mirror symmetric insulators. We develop two topological invariants each associated with a mirror-invariant gap in the Wilson loop spectrum. We propose that both topological invariants in combination determine the general higher-order bulk-boundary correspondence in 2D mirror symmetric, boundary-obstructed to… ▽ More We study the multi-gap topology of the periodic spectra of Wilson loop operators (WLOs) in mirror symmetric insulators. We develop two topological invariants each associated with a mirror-invariant gap in the Wilson loop spectrum. We propose that both topological invariants in combination determine the general higher-order bulk-boundary correspondence in 2D mirror symmetric, boundary-obstructed topological insulators. Finally, we demonstrate that these new multi-gap topological invariants apply to anomalous cases beyond those captured by the nested Wilson loop, and we subsequently develop an understanding of the correlation between WLOs along two orthogonal directions. △ Less

Submitted 28 September, 2021; originally announced September 2021.

Comments: 5+4 pages; 5+2 figures

Strain-dependent resistance and giant gauge factor in monolayer WSe2

Authors: Mao-Sen Qin, Xing-Guo Ye, Peng-Fei Zhu, Wen-Zheng Xu, Jing Liang, Kaihui Liu, Zhi-Min Liao

Abstract: We report the strong dependence of resistance on uniaxial strain in monolayer WSe2 at various temperatures, where the gauge factor can reach as large as 2400. The observation of strain-dependent resistance and giant gauge factor is attributed to the emergence of nonzero Berry curvature dipole. Upon increasing strain, Berry curvature dipole can generate net orbital magnetization, which would introd… ▽ More We report the strong dependence of resistance on uniaxial strain in monolayer WSe2 at various temperatures, where the gauge factor can reach as large as 2400. The observation of strain-dependent resistance and giant gauge factor is attributed to the emergence of nonzero Berry curvature dipole. Upon increasing strain, Berry curvature dipole can generate net orbital magnetization, which would introduce additional magnetic scattering, decreasing the mobility and thus conductivity. Our work demonstrates the strain engineering of Berry curvature and thus the transport properties, making monolayer WSe2 potential for the application in the high-performance flexible and transparent electronics. △ Less

Submitted 7 September, 2021; originally announced September 2021.

Comments: 15 pages, 4 figures

Journal ref: Chin. Phys. B 30, 097203 (2021)

Mechanical Cloak via Data-Driven Aperiodic Metamaterial Design

Authors: Liwei Wang, Jagannadh Boddapati, Ke Liu, Ping Zhu, Chiara Daraio, Wei Chen

Abstract: Mechanical cloaks are materials engineered to manipulate the elastic response around objects to make them indistinguishable from their homogeneous surroundings. Typically, methods based on material-parameter transformations are used to design optical, thermal and electric cloaks. However, they are not applicable in designing mechanical cloaks, since continuum-mechanics equations are not form-invar… ▽ More Mechanical cloaks are materials engineered to manipulate the elastic response around objects to make them indistinguishable from their homogeneous surroundings. Typically, methods based on material-parameter transformations are used to design optical, thermal and electric cloaks. However, they are not applicable in designing mechanical cloaks, since continuum-mechanics equations are not form-invariant under general coordinate transformations. As a result, existing design methods for mechanical cloaks have so far been limited to a narrow selection of voids with simple shapes. To address this challenge, we present a systematic, data-driven design approach to create mechanical cloaks composed of aperiodic metamaterials using a large pre-computed unit cell database. Our method is flexible to allow the design of cloaks with various boundary conditions, multiple loadings, different shapes and numbers of voids, and different homogeneous surroundings. It enables a concurrent optimization of both topology and properties distribution of the cloak. Compared to conventional fixed-shape solutions, this results in an overall better cloaking performance, and offers unparalleled versatility. Experimental measurements on 3D-printed structures further confirm the validity of the proposed approach. Our research illustrates the benefits of data-driven approaches in quickly responding to new design scenarios and resolving the computational challenge associated with multiscale designs of functional structures. It could be generalized to accommodate other applications that require heterogeneous property distribution, such as soft robots and implants design. △ Less

Submitted 6 December, 2021; v1 submitted 27 July, 2021; originally announced July 2021.

Comments: Under review

Evidence for Higher order topology in Bi and Bi$_{0.92}$Sb$_{0.08}$

Authors: Leena Aggarwal, Penghao Zhu, Taylor L. Hughes, Vidya Madhavan

Abstract: Higher order topological insulators (HOTIs) are a new class of topological materials which host protected states at the corners or hinges of a crystal. HOTIs provide an intriguing alternative platform for helical and chiral edge states and Majorana modes, but there are very few known materials in this class. Recent studies have proposed Bi as a potential HOTI, however, its topological classificati… ▽ More Higher order topological insulators (HOTIs) are a new class of topological materials which host protected states at the corners or hinges of a crystal. HOTIs provide an intriguing alternative platform for helical and chiral edge states and Majorana modes, but there are very few known materials in this class. Recent studies have proposed Bi as a potential HOTI, however, its topological classification is not yet well accepted. In this work, we show that the (110) facets of Bi and BiSb alloys can be used to unequivocally establish the topology of these systems. Bi and Bi$_{0.92}$Sb$_{0.08}$ (110) films were grown on silicon substrates using molecular beam epitaxy and studied by scanning tunneling spectroscopy. The surfaces manifest rectangular islands which show localized hinge states on three out of the four edges, consistent with the theory for the HOTI phase. This establishes Bi and Bi$_{0.92}$Sb$_{0.08}$ as HOTIs, and raises questions about the topological classification of the full family of Bi$_{x}$Sb$_{1-x}$ alloys. △ Less

Submitted 23 July, 2021; v1 submitted 1 July, 2021; originally announced July 2021.

Journal ref: Nature Communications,2021

Electronic nature of chiral charge order in the kagome superconductor CsV3Sb5

Authors: Zhiwei Wang, Yu-Xiao Jiang, Jia-Xin Yin, Yongkai Li, Guan-Yong Wang, Hai-Li Huang, Shen Shao, Jinjin Liu, Peng Zhu, Nana Shumiya, Md Shafayat Hossain, Hongxiong Liu, Youguo Shi, Junxi Duan, Xiang Li, Guoqing Chang, Pengcheng Dai, Zijin Ye, Gang Xu, Yanchao Wang, Hao Zheng, Jinfeng Jia, M. Zahid Hasan, Yugui Yao

Abstract: Kagome superconductors with Tc up to 7K have been discovered over 40 years. Recently, unconventional chiral charge order has been reported in kagome superconductor KV3Sb5, with an ordering temperature of one order of magnitude higher than the TC. However, the chirality of the charge order has not been reported in the cousin kagome superconductor CsV3Sb5, and the electronic nature of the chirality… ▽ More Kagome superconductors with Tc up to 7K have been discovered over 40 years. Recently, unconventional chiral charge order has been reported in kagome superconductor KV3Sb5, with an ordering temperature of one order of magnitude higher than the TC. However, the chirality of the charge order has not been reported in the cousin kagome superconductor CsV3Sb5, and the electronic nature of the chirality remains elusive. In this letter, we report the observation of electronic chiral charge order in CsV3Sb5 via scanning tunneling microscopy (STM). We observe a 2x2 charge modulation and a 1x4 superlattice in both topographic data and tunneling spectroscopy. 2x2 charge modulation is highly anticipated as a charge order by fundamental kagome lattice models at van Hove filling, and is shown to exhibit intrinsic chirality. We find that the 1x4 superlattices forms various small domain walls, and can be a surface effect as supported by our first-principles calculations. Crucially, we find that the amplitude of the energy gap opened by the charge order exhibits real space modulations, and features 2x2 wave vectors with chirality, highlighting the electronic nature of the chiral charge order. STM study at 0.4K reveals a superconducting energy gap with a gap size 2Δ=0.85meV, which estimates a moderate superconductivity coupling strength with 2Δ/kBTc=3.9. When further applying a c-axis magnetic field, vortex core bound states are observed within this gap, indicative of clean-limit superconductivity. △ Less

Submitted 25 August, 2021; v1 submitted 10 May, 2021; originally announced May 2021.

Journal ref: Phys. Rev. B 104, 075148 (2021)

Geometric Response and Disclination-Induced Skin Effects in Non-Hermitian Systems

Authors: Xiao-Qi Sun, Penghao Zhu, Taylor L. Hughes

Abstract: We study the geometric response of three-dimensional non-Hermitian crystalline systems with nontrivial point-gap topology. For systems with fourfold rotation symmetry, we show that in the presence of disclination lines with a total Frank angle which is an integer multiple of $2π$, there can be nontrivial one-dimensional point-gap topology along the direction of the disclination lines. This results… ▽ More We study the geometric response of three-dimensional non-Hermitian crystalline systems with nontrivial point-gap topology. For systems with fourfold rotation symmetry, we show that in the presence of disclination lines with a total Frank angle which is an integer multiple of $2π$, there can be nontrivial one-dimensional point-gap topology along the direction of the disclination lines. This results in disclination-induced non-Hermitian skin effects. By doubling a non-Hermitian Hamiltonian to a Hermitian three-dimensional chiral topological insulator, we show that the disclination-induced skin modes are zero modes of the effective surface Dirac fermion(s) in the presence of a pseudomagnetic flux induced by disclinations. Furthermore, we find that our results have a field theoretic description, and the corresponding geometric response actions (e.g., the Euclidean Wen-Zee action) enrich the topological field theory of non-Hermitian systems. △ Less

Submitted 10 August, 2021; v1 submitted 10 February, 2021; originally announced February 2021.

Comments: 10 pages including the Supplemental Material; close to the published version

Journal ref: Phys. Rev. Lett. 127, 066401 (2021)

Strain Tunable Berry Curvature Dipole, Orbital Magnetization and Nonlinear Hall Effect in WSe2 Monolayer

Authors: Mao-Sen Qin, Peng-Fei Zhu, Xing-Guo Ye, Wen-Zheng Xu, Zhen-Hao Song, Jing Liang, Kaihui Liu, Zhi-Min Liao

Abstract: The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent K and K' valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization… ▽ More The electronic topology is generally related to the Berry curvature, which can induce the anomalous Hall effect in time-reversal symmetry breaking systems. Intrinsic monolayer transition metal dichalcogenides possesses two nonequivalent K and K' valleys, having Berry curvatures with opposite signs, and thus vanishing anomalous Hall effect in this system. Here we report the experimental realization of asymmetrical distribution of Berry curvature in a single valley in monolayer WSe2 through applying uniaxial strain to break C3v symmetry. As a result, although the Berry curvature itself is still opposite in K and K' valleys, the two valleys would contribute equally to nonzero Berry curvature dipole. Upon applying electric field, the emergent Berry curvature dipole would lead to an out-of-plane orbital magnetization, which further induces an anomalous Hall effect with a linear response to E^2, known as nonlinear Hall effect. We show the strain modulated transport properties of nonlinear Hall effect in monolayer WSe2 with moderate hole-doping by gating. The second-harmonic Hall signals show quadratic dependence on electric field, and the corresponding orbital magnetization per current density can reach as large as 60. In contrast to the conventional Rashba-Edelstein effect with in-plane spin polarization, such current-induced orbital magnetization is along the out-of-plane direction, thus promising for high-efficient electrical switching of perpendicular magnetization. △ Less

Submitted 6 July, 2021; v1 submitted 6 December, 2020; originally announced December 2020.

Journal ref: Chin. Phys. Lett. 38, 017301 (2021)

Quantized surface magnetism and higher-order topology: Application to the Hopf insulator

Authors: Penghao Zhu, Taylor L. Hughes, A. Alexandradinata

Abstract: We identify topological aspects of the subextensive magnetic moment contributed by the surfaces of a three-dimensional crystallite -- assumed to be insulating in the bulk as well as on all surface facets, with trivial Chern invariants in the bulk. The geometric component of this subextensive moment is given by its derivative with respect to the chemical potential, at zero temperature and zero fiel… ▽ More We identify topological aspects of the subextensive magnetic moment contributed by the surfaces of a three-dimensional crystallite -- assumed to be insulating in the bulk as well as on all surface facets, with trivial Chern invariants in the bulk. The geometric component of this subextensive moment is given by its derivative with respect to the chemical potential, at zero temperature and zero field, per unit surface area, and hence corresponds to the surface magnetic compressibility. The sum of the surface compressibilities contributed by two opposite facets of a cube-shaped crystallite is quantized to an integer multiple of the fundamental constant $e/h c$; this integer is in one-to-one correspondence with the net chirality of hinge modes on the surface of the crystallite, manifesting a link with higher-order topology. The contribution by a single facet to the magnetic compressibility need not be quantized to integers; however, symmetry and/or Hilbert-space constraints can fix the single-facet compressibility to half-integer multiples of $e/hc$, as will be exemplified by the Hopf insulator. △ Less

Submitted 13 January, 2021; v1 submitted 17 September, 2020; originally announced September 2020.

Comments: 11+5 pages, 7 figures

Journal ref: Phys. Rev. B 103, 014417 (2021)

Achieving 50 femtosecond resolution in MeV ultrafast electron diffraction with a double bend achromat compressor

Authors: Fengfeng Qi, Zhuoran Ma, Lingrong Zhao, Yun Cheng, Wenxiang Jiang, Chao Lu, Tao Jiang, Dong Qian, Zhe Wang, Wentao Zhang, Pengfei Zhu, Xiao Zou, Weishi Wan, Dao Xiang, Jie Zhang

Abstract: We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio-frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispers… ▽ More We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio-frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispersion where electrons at the bunch tail with lower energies follow shorter paths and thus catch up with the bunch head, leading to longitudinal bunch compression. We show that with optimized parameter sets, the whole beam path from the electron source to the compression point can be made isochronous such that the time of flight for the electron beam is immune to the fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch length and arrival time jitter for a 20 fC beam after bunch compression are measured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such an ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond (FWHM) resolution in MeV ultrafast electron diffraction where lattice oscillation at 2.6 THz corresponding to Bismuth A1g mode is clearly observed without correcting both the short-term timing jitter and long-term timing drift. Furthermore, oscillating weak diffuse scattering signal related to phonon coupling and decay is also clearly resolved thanks to the improved temporal resolution and increased electron flux. We expect that this technique will have a strong impact in emerging ultrashort electron beam based facilities and applications. △ Less

Submitted 18 March, 2020; originally announced March 2020.

Comments: Accepted for publication in Physical Review Leters

Identifying Higher Order Topology and Fractional Corner Charge Using Entanglement Spectra

Authors: Penghao Zhu, Kieran Loehr, Taylor L. Hughes

Abstract: We study the entanglement spectrum (ES) of two-dimensional $C_{n}$-symmetric second-order topological insulators (TIs). We show that some characteristic higher order topological observables, e.g., the filling anomaly and its associated fractional corner charge, can be determined from the ES of atomic and fragile TIs. By constructing the relationship between the configuration of Wannier orbitals an… ▽ More We study the entanglement spectrum (ES) of two-dimensional $C_{n}$-symmetric second-order topological insulators (TIs). We show that some characteristic higher order topological observables, e.g., the filling anomaly and its associated fractional corner charge, can be determined from the ES of atomic and fragile TIs. By constructing the relationship between the configuration of Wannier orbitals and the number of protected in-gap states in the ES for different symmetric cuts in real space, we express the fractional corner charge in terms of the number of protected in-gap states of the ES. We show that our formula is robust in the presence of electron-electron interactions as long as the interactions preserve $C_{n}$ rotation symmetry and charge-conservation symmetry. Moreover, we discuss the possible signatures higher order topology in the many-body ES. Our methods allow the identification of some classes of higher order topology without requiring the usage of nested Wilson loops or nested entanglement spectra. △ Less

Submitted 20 March, 2020; v1 submitted 22 October, 2019; originally announced October 2019.

Comments: 11+4 pages, 8 figures in the main text, 3 figures in appendices

Journal ref: Phys. Rev. B 101, 115140 (2020)