Showing 1–50 of 93 results for author: Ye, L

Dominant in-plane anomalous Hall effect in a monoclinic room-temperature ferromagnet

Authors: Guoxin Zheng, Arjyama Bordoloi, Mingjun Fan, Shunsuke Kitou, Hiraku Saito, Taro Nakajima, Sobhit Singh, Takashi Kurumaji, Linda Ye

Abstract: Ferromagnetic metals are characterized by enhanced dissipationless transverse transport responses via the anomalous Hall effect, offering a route towards magnetic sensing and spintronic readout functionalities. In most ferromagnets, the anomalous Hall current is constrained to lie in the plane perpendicular to the magnetization (or applied magnetic field). Recently, it has been recognized that sel… ▽ More Ferromagnetic metals are characterized by enhanced dissipationless transverse transport responses via the anomalous Hall effect, offering a route towards magnetic sensing and spintronic readout functionalities. In most ferromagnets, the anomalous Hall current is constrained to lie in the plane perpendicular to the magnetization (or applied magnetic field). Recently, it has been recognized that selected symmetries can also permit a Hall response in a traditionally forbidden configuration, where the Hall current lies in the same plane as the magnetization, realizing an in-plane anomalous Hall effect. Reported realizations of this effect, however, are typically much weaker than the conventional Hall response in the same material. Here, through engineering specific crystallographic mirror symmetry-breaking, we realize a strongly enhanced in-plane anomalous Hall response in monoclinic Cr3Te4 with room-temperature ferromagnetism. Remarkably, the in-plane anomalous Hall signal exceeds the out-of-plane response by a factor of five, with which we demonstrate a unique in-plane field and current sensing functionality. Combined with density functional theory calculations, our results establish low-crystalline-symmetry ferromagnets with near-Fermi-level Weyl points as a practical platform for symmetry-engineered Hall responses, and point to a route towards room-temperature, geometry-flexible sensing devices. △ Less

Submitted 10 June, 2026; v1 submitted 8 June, 2026; originally announced June 2026.

Layer-Resolved Nonlinear Optics in Finite-Thickness Two-Dimensional Systems

Authors: Liangting Ye, Chengzhi Wu, Zeyu Jiang, Bing Huang

Abstract: Nonlinear optical (NLO) responses in two-dimensional quantum-confined systems are typically described within bulk-based frameworks as macroscopic spatial averages. In finite-thickness van der Waals multilayers directly relevant to nanoscale devices, this picture substantially breaks down. Here, we establish a general symmetry-based framework for classifying second-order NLO responses in multilayer… ▽ More Nonlinear optical (NLO) responses in two-dimensional quantum-confined systems are typically described within bulk-based frameworks as macroscopic spatial averages. In finite-thickness van der Waals multilayers directly relevant to nanoscale devices, this picture substantially breaks down. Here, we establish a general symmetry-based framework for classifying second-order NLO responses in multilayers. We reveal a layer-resolved organization into skin, weak-skin, and hidden effects governed by local symmetry and stacking order. First-principles calculations for both nonmagnetic and spin-polarized systems confirm our predictions, demonstrating that stacking alone suffices to dramatically reshape both the spatial pattern and magnitude of the NLO response, a phenomenon not explainable within standard bulk theory. Our results establish stacking geometry as an effective knob for engineering surface-selective NLO responses in layered materials. △ Less

Submitted 1 June, 2026; originally announced June 2026.

Comments: 9 pages, 5 figures

Slow spin-lattice relaxation dynamics in YbVO4 revealed by extended thermal impedance spectroscopy from AC susceptibility and AC magnetocaloric measurements

Authors: Yuntian Li, Jiayi Hu, Dominic Petruzzi, Linda Ye, Mark P. Zic, Arkady Shekhter, Ian R. Fisher

Abstract: Alternating (AC) magnetic fields can induce not only an alternating magnetization in materials, but also an alternating temperature via the magnetocaloric effect. The latter effect is typically neglected when performing AC susceptibility measurements, but consideration of both effects on an equal footing is necessary in order to reliably distinguish between internal and external causes of magnetic… ▽ More Alternating (AC) magnetic fields can induce not only an alternating magnetization in materials, but also an alternating temperature via the magnetocaloric effect. The latter effect is typically neglected when performing AC susceptibility measurements, but consideration of both effects on an equal footing is necessary in order to reliably distinguish between internal and external causes of magnetic response and accurately extract quantitative information about relaxation processes. In order to address this, we have developed a method to measure the AC magnetocaloric effect that is compatible with AC susceptibility measurements, and also a framework to analyze these data in combination. We demonstrate the efficacy of this approach using YbVO4, a material for which strong single-ion anisotropy leads to slow spin-lattice relaxation at low temperatures via a phonon bottleneck effect. We report AC magnetic susceptibility and AC magnetocaloric effect measurements for this material as a function of field and frequency at a temperature of 3 K. We analyze the data using a discretized thermal model, and extract the field-dependence of the intrinsic spin-lattice relaxation rate. This demonstration experiment illustrates a general approach to quantitatively address multiple measured quantities in driven systems using a unified thermal circuit analysis. The thermal analysis methods presented in this report can be extended to study other magnetic, dielectric, and elastic materials exhibiting a complex response to an external driving field in the presence of internal and external relaxation, particularly when an energy dissipation process is within an accessible frequency regime. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Elastoresistivity Signatures of Nematic Fluctuations in Layered Antiferromagnet CoTa3S6

Authors: Tao Lu, Zili Feng, Mengxing Ye, Takashi Kurumaji, Linda Ye

Abstract: Nematic phases that break rotational symmetry are widely observed in quantum materials, and clarifying their origin and relationship with other symmetry-breaking phases remains an important but challenging task. In this work, we investigate nematic fluctuations in CoTa$_3$S$_6$ using elastoresistivity experiments to resolve the nature of the proposed nematic phase intertwined with collinear and no… ▽ More Nematic phases that break rotational symmetry are widely observed in quantum materials, and clarifying their origin and relationship with other symmetry-breaking phases remains an important but challenging task. In this work, we investigate nematic fluctuations in CoTa$_3$S$_6$ using elastoresistivity experiments to resolve the nature of the proposed nematic phase intertwined with collinear and non-coplanar antiferromagnetic orders. We observe a divergence-like antisymmetric elastoresistivity that rapidly develops below the stripe antiferromagnetic transition, consistent with a distinct nematic degree of freedom coupled to the magnetic order. While nematic fluctuations are strongly modulated by an external out-of-plane magnetic field and the onset temperature of resistivity anisotropy shows pronounced strain dependence, the antiferromagnetic transition temperatures remain nearly unchanged under either magnetic field or strain. Additionally, complementary magnetoresistance measurements reveal characteristic signatures of three-state nematicity in a hexagonal system. Our findings demonstrate CoTa$_3$S$_6$ as a unique case of intertwined nematic and AFM orders with distinct origins. △ Less

Submitted 13 March, 2026; originally announced March 2026.

Comments: 10 pages, 4 figures

Tunable decoupling of coexisting magnetic orders in Co$_{1/3}$TaS$_2$

Authors: Yining Hu, Zili Feng, Takashi Kurumaji, Linda Ye, Chunyu Mark Guo, Philip J. W. Moll

Abstract: In multiferroics, new physical responses and functionalities emerge when symmetry-distinct order parameters couple. This conventionally occurs when lattice and magnetic degrees of freedom order independently in a material. Here, we report an all-magnetic analogue of multiferroic behavior in the antiferromagnet Co$_{1/3}$TaS$_2$, where topological scalar spin chirality and nematicity coexist on the… ▽ More In multiferroics, new physical responses and functionalities emerge when symmetry-distinct order parameters couple. This conventionally occurs when lattice and magnetic degrees of freedom order independently in a material. Here, we report an all-magnetic analogue of multiferroic behavior in the antiferromagnet Co$_{1/3}$TaS$_2$, where topological scalar spin chirality and nematicity coexist on the same spin lattice. While the chiral spin texture generates an anomalous Hall effect (AHE), the nematic order breaks threefold rotational symmetry and dominates longitudinal transport. Crucially, in zero field these symmetry-distinct orders merely coexist yet magnetic fields induce strong coupling between them, thus realizing a new type of multiferroic bebhavior via tuning of the coupling itself instead of direct manipulation of secondary orders. In sub-domain sized devices with achiral geometry, we demonstrate that nonreciprocal transport serves as a symmetry-based probe of the global spin chirality, co-aligned with the strong topological AHE of the system. In Co$_{1/3}$TaS$_2$ the topological Hall state inherits a large resistance anomaly via chiral-nematic coupling, thus our results showcase how hybrid magnetic orders can achieve advanced functionalities by merging symmetry-forbidden material responses. △ Less

Submitted 12 March, 2026; originally announced March 2026.

Comments: 25 pages, 13 figures

Two-dimensional Intrinsic Janus Structures: Design Principle and Anomalous Nonlinear Optics

Authors: Yang Li, Chengzhi Wu, Xuelian Sun, Liangting Ye, Yirui Lu, Hai-Qing Lin, Wenhui Duan, Bing Huang

Abstract: Two-dimensional Janus structures have garnered rapidly growing attention across multidisciplinary fields. However, despite extensive theoretical and experimental efforts, a principle for designing intrinsic Janus materials remains elusive. Here, we propose a first-principles alloy theory based on cluster expansion, incorporating a strong repulsive interaction of a cation-mediated anion-pair cluste… ▽ More Two-dimensional Janus structures have garnered rapidly growing attention across multidisciplinary fields. However, despite extensive theoretical and experimental efforts, a principle for designing intrinsic Janus materials remains elusive. Here, we propose a first-principles alloy theory based on cluster expansion, incorporating a strong repulsive interaction of a cation-mediated anion-pair cluster and refined short-range cluster-cluster competitions, to unravel the formation mechanism of intrinsic Janus structures with a distorted 1T phase among numerous competing phases. Our theory not only explains why intrinsic Janus structures are accidentally observed in RhSeCl and BiTeI which are composed of alloyed elements from different groups, but also accurately predicts a wide range of 1T-like intrinsic Janus materials that are ready for synthesis. Intriguingly, as demonstrated in the case of RhSeCl, we reveal that intrinsic Janus materials can exhibit anomalous second-harmonic generation (SHG) with a distinct quantum geometric effect, originating from strong lattice and chemical-potential mirror asymmetry. Furthermore, a novel skin effect unexpectedly emerges in finite-thickness RhSeCl, accompanied by a hidden SHG effect within the bulk region. Our theory paves the way for the ab initio design of intrinsic Janus materials, significantly accelerating progress in Janus science. △ Less

Submitted 16 January, 2026; originally announced January 2026.

Comments: 23 pages, 4 figures

Journal ref: Newton, 2, 100368 (2026)

Multi-AI Agent Framework Reveals the "Oxide Gatekeeper" in Aluminum Nanoparticle Oxidation

Authors: Yiming Lu, Tingyu Lu, Di Zhang, Lili Ye, Hao Li

Abstract: Aluminum nanoparticles (ANPs) are among the most energy-dense solid fuels, yet the atomic mechanisms governing their transition from passivated particles to explosive reactants remain elusive. This stems from a fundamental computational bottleneck: ab initio methods offer quantum accuracy but are restricted to small spatiotemporal scales (< 500 atoms, picoseconds), while empirical force fields lac… ▽ More Aluminum nanoparticles (ANPs) are among the most energy-dense solid fuels, yet the atomic mechanisms governing their transition from passivated particles to explosive reactants remain elusive. This stems from a fundamental computational bottleneck: ab initio methods offer quantum accuracy but are restricted to small spatiotemporal scales (< 500 atoms, picoseconds), while empirical force fields lack the reactive fidelity required for complex combustion environments. Herein, we bridge this gap by employing a "human-in-the-loop" closed-loop framework where self-auditing AI Agents validate the evolution of a machine learning potential (MLP). By acting as scientific sentinels that visualize hidden model artifacts for human decision-making, this collaborative cycle ensures quantum mechanical accuracy while exhibiting near-linear scalability to million-atom systems and accessing nanosecond timescales (energy RMSE: 1.2 meV/atom, force RMSE: 0.126 eV/Angstrom). Strikingly, our simulations reveal a temperature-regulated dual-mode oxidation mechanism: at moderate temperatures, the oxide shell acts as a dynamic "gatekeeper," regulating oxidation through a "breathing mode" of transient nanochannels; above a critical threshold, a "rupture mode" unleashes catastrophic shell failure and explosive combustion. Importantly, we resolve a decades-old controversy by demonstrating that aluminum cation outward diffusion, rather than oxygen transport, dominates mass transfer across all temperature regimes, with diffusion coefficients consistently exceeding those of oxygen by 2-3 orders of magnitude. These discoveries establish a unified atomic-scale framework for energetic nanomaterial design, enabling the precision engineering of ignition sensitivity and energy release rates through intelligent computational design. △ Less

Submitted 27 December, 2025; originally announced December 2025.

Comments: 21 pages, 7 figures

Dimensional hierarchy of topological bound states in the continuum

Authors: Shunda Yin, Zhenyu Wang, Liping Ye, Hailong He, Manzhu Ke, Weiyin Deng, Jiuyang Lu, Zhengyou Liu

Abstract: Bound states in the continuum (BICs), with the ability of trapping and manipulating waves within the radiation continuum, have gained significant attention for their potential applications in optics and acoustics. However, challenges arise in reducing wave leakage and noise from fabrication imperfections. The emergence of robust wave manipulations based on topological BICs (TBICs) offers promising… ▽ More Bound states in the continuum (BICs), with the ability of trapping and manipulating waves within the radiation continuum, have gained significant attention for their potential applications in optics and acoustics. However, challenges arise in reducing wave leakage and noise from fabrication imperfections. The emergence of robust wave manipulations based on topological BICs (TBICs) offers promising solutions. Traditionally, TBICs of different dimensions are observed separately in distinct systems. Here, we report the experimental discovery of the coexistence of two-dimensional surface TBICs and one-dimensional hinge TBICs in a single three-dimensional phononic crystal system. Such an unprecedented dimensional hierarchy of TBICs is triggered by the mechanism of separability and protected by the valley Chern numbers. Notably, these TBICs inherit dispersive propagation characteristics from valley topology and can propagate robustly against defects without leakage. Our findings offer an efficient approach to multidimensional TBICs and can be applied in designing highly efficient acoustic devices for wave trapping and manipulation in multidimensional environments. △ Less

Submitted 29 August, 2025; originally announced September 2025.

Comments: 17 pages, 7 figures

Nonvolatile Nematic Order Manipulated by Strain and Magnetic Field in a Layered Antiferromagnet

Authors: Zili Feng, Weihang Lu, Tao Lu, Fangyan Liu, Joseph R. Sheeran, Mengxing Ye, Jing Xia, Takashi Kurumaji, Linda Ye

Abstract: The operation mechanism of nematic liquid crystals lies in the control of their optical properties by the orientation of underlying nematic directors. In analogy, electronic nematicity refers to a state whose electronic properties spontaneously break rotation symmetries of the host crystalline lattice, leading to anisotropic electronic properties. In this work, we demonstrate that the layered anti… ▽ More The operation mechanism of nematic liquid crystals lies in the control of their optical properties by the orientation of underlying nematic directors. In analogy, electronic nematicity refers to a state whose electronic properties spontaneously break rotation symmetries of the host crystalline lattice, leading to anisotropic electronic properties. In this work, we demonstrate that the layered antiferromagnet CoTa$_3$S$_6$ exhibits a switchable nematic order, evidenced by the emergence of both resistivity anisotropy and optical birefringence. This nematic state sets in at a temperature $T^*$ distinct from that of the antiferromagnetic transitions in the system, indicating a separate symmetry-breaking mechanism. The nematic order can be manipulated either by an in-plane rotation symmetry-breaking strain or in-plane magnetic field, with the latter exhibiting a pronounced non-volatile memory effect. Remarkably, we find that the broken three-fold rotation symmetry in electronic transport is restored with a moderate out-of-plane field. We hypothesize that the nematicity is of electronic origin and emerges from instabilities associated with van Hove singularities. The resulting phase diagram points to an intertwined interplay between the electronic nematicity and the proposed underlying collinear and non-coplanar spin orders. Our findings establish CoTa$_3$S$_6$ as a versatile antiferromagnetic platform with highly tunable functionalities arising from the breaking of rotational, time-reversal, and inversion symmetries. △ Less

Submitted 7 July, 2025; originally announced July 2025.

Comments: 11 pages, 4 figures

Criticality and magnetic phases of Ising Shastry-Sutherland candidate holmium tetraboride

Authors: Guga Khundzakishvili, Bishnu P. Belbase, Pravin Mahendran, Kevin Zhang, Hanjing Xu, Eliana Stoyanoff, Joseph G. Checkelsky, Yaohua Liu, Linda Ye, Arnab Banerjee

Abstract: Frustrated magnetic systems arising in geometrically constrained lattices represent rich platforms for exploring unconventional phases of matter, including fractional magnetization plateaus, incommensurate orders, and complex domain dynamics. However, determining the microscopic spin configurations that stabilize such phases is a key challenge, especially when in-plane and out-of-plane spin compon… ▽ More Frustrated magnetic systems arising in geometrically constrained lattices represent rich platforms for exploring unconventional phases of matter, including fractional magnetization plateaus, incommensurate orders, and complex domain dynamics. However, determining the microscopic spin configurations that stabilize such phases is a key challenge, especially when in-plane and out-of-plane spin components coexist and compete. Here, we combine neutron scattering and magnetic susceptibility experiments with simulations to investigate the emergence of field-induced fractional plateaus and the related criticality in a frustrated magnet holmium tetraboride (HoB4) that represents the family of rare earth tetraborides that crystalize in a Shastry-Sutherland lattice in the ab plane. We focus on the interplay between classical and quantum criticality near phase boundaries as well as the role of material defects in the stabilization of the ordered phases. We find that simulations using classical annealing can explain certain observed features in the experimental Laue diffraction and the origin of multiple magnetization plateaus. Our results show that defects and out of plane interactions play an important role and can guide the route towards resolving microscopic spin textures in highly frustrated magnets. △ Less

Submitted 22 April, 2025; originally announced April 2025.

Realization of discretized response in rare-earth vanadates accessed by AC susceptibility and magnetocaloric methods

Authors: Yuntian Li, Linda Ye, Mark P. Zic, Arkady Shekhter, Ian R. Fisher

Abstract: This report presents a new technique to probe the quantitative dynamical response of the magnetic field induced heating/cooling process in rare-earth vanadium materials. The approach combines AC susceptibility and AC caloric measurements to reveal the intrinsic timescale associated with the magnetic relaxation process of rare-earth ions at low temperatures. Utilizing the well-known crystal field e… ▽ More This report presents a new technique to probe the quantitative dynamical response of the magnetic field induced heating/cooling process in rare-earth vanadium materials. The approach combines AC susceptibility and AC caloric measurements to reveal the intrinsic timescale associated with the magnetic relaxation process of rare-earth ions at low temperatures. Utilizing the well-known crystal field effect in YbVO4, we prove and demonstrate a discretized thermal analysis through a common spin-lattice relaxation phenomenon. The demonstration experiment presented in this study provides a general approach to quantitatively address multiple measured quantities in one unified discretized thermal circuit analysis. It can be extended to study other magnetic, dielectric, and elastic materials exhibiting a complex response to an external driving field in the presence of intrinsic interactions and fluctuations, particularly when an energy dissipation process is within an accessible frequency regime. △ Less

Submitted 14 May, 2025; v1 submitted 22 February, 2025; originally announced February 2025.

Electronic Structure of Kramers Nodal-Line Semimetal YAuGe and Anomalous Hall Effect Induced by Magnetic Rare-Earth Substitution

Authors: Takashi Kurumaji, Jorge I. Facio, Natsuki Mitsuishi, Shusaku Imajo, Masaki Gen, Motoi Kimata, Linda Ye, David Graf, Masato Sakano, Miho Kitamura, Kohei Yamagami, Kyoko Ishizaka, Koichi Kindo, Taka-hisa Arima

Abstract: Nodal-line semimetals are a class of topological materials hosting one dimensional lines of band degeneracy. Kramers nodal-line (KNL) metals/semimetals have recently been theoretically recognized as a class of topological states inherent to all non-centrosymmetric achiral crystal lattices. We investigate the electronic structure of candidate KNL semimetal YAuGe by angle-resolved photoemission spec… ▽ More Nodal-line semimetals are a class of topological materials hosting one dimensional lines of band degeneracy. Kramers nodal-line (KNL) metals/semimetals have recently been theoretically recognized as a class of topological states inherent to all non-centrosymmetric achiral crystal lattices. We investigate the electronic structure of candidate KNL semimetal YAuGe by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillations as well as by density functional theory (DFT) calculations. DFT has revealed that YAuGe hosts KNLs on the G-A-L-M plane of the Brillouin zone, that are protected by the time reversal and mirror-inversion symmetries. Through ARPES and quantum oscillations we identify signatures of hole bands enclosing the G point, and the observed splitting of quantum oscillation frequency with angle is attributed to spin-orbit-coupling-induced band splitting away from the KNLs. Furthermore, we show that the degeneracy of the nodal lines along the G-A line is lifted by the time-reversal-symmetry breaking when the Y is substituted by magnetic R ions (R = rare earth). This becomes a source of Berry curvature and contributes to the anomalous Hall effect in magnetic RAuGe. These findings establish RAuGe as a new class of KNL semimetals offering significant potential for engineering of anomalous magnetotransport properties via magnetic rare-earth substitution. △ Less

Submitted 9 May, 2025; v1 submitted 15 January, 2025; originally announced January 2025.

Comments: Main: 8 pages; 4 figures, Supplementary: 7 pages; 9 figures; 3 tables

Journal ref: Advanced Science (2025)

Measurements of the quantum geometric tensor in solids

Authors: Mingu Kang, Sunje Kim, Yuting Qian, Paul M. Neves, Linda Ye, Junseo Jung, Denny Puntel, Federico Mazzola, Shiang Fang, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Jun Fuji, Ivana Vobornik, Jae-Hoon Park, Joseph G. Checkelsky, Bohm-Jung Yang, Riccardo Comin

Abstract: Understanding the geometric properties of quantum states and their implications in fundamental physical phenomena is at the core of modern physics. The Quantum Geometric Tensor (QGT) is a central physical object in this regard, encoding complete information about the geometry of the quantum state. The imaginary part of the QGT is the well-known Berry curvature, which plays a fundamental role in th… ▽ More Understanding the geometric properties of quantum states and their implications in fundamental physical phenomena is at the core of modern physics. The Quantum Geometric Tensor (QGT) is a central physical object in this regard, encoding complete information about the geometry of the quantum state. The imaginary part of the QGT is the well-known Berry curvature, which plays a fundamental role in the topological magnetoelectric and optoelectronic phenomena. The real part of the QGT is the quantum metric, whose importance has come to prominence very recently, giving rise to a new set of quantum geometric phenomena, such as anomalous Landau levels, flat band superfluidity, excitonic Lamb shifts, and nonlinear Hall effect. Despite the central importance of the QGT, its experimental measurements have been restricted only to artificial two-level systems. In this work, we develop a framework to measure the QGT (both quantum metric and Berry curvature) in crystalline solids using polarization-, spin-, and angle-resolved photoemission spectroscopy. Using this framework, we demonstrate the effective reconstruction of the QGT in solids in the archetype kagome metal CoSn, which hosts topological flat bands. The key idea is to introduce another geometrical tensor, the quasi-QGT, whose components, the band Drude weight and orbital angular momentum, are experimentally accessible and can be used for extracting the QGT. Establishing such a momentum- and energy-resolved spectroscopic probe of the QGT is poised to significantly advance our understanding of quantum geometric responses in a wide range of crystalline systems. △ Less

Submitted 23 December, 2024; originally announced December 2024.

Journal ref: Nat. Phys. (2024)

Distinct amplitude mode dynamics upon resonant and off-resonant excitation across the charge density wave energy gap in LaTe3 investigated by time- and angle-resolved photoemission spectroscopy

Authors: Kecheng Liu, Takeshi Suzuki, Yigui Zhong, Teruto Kanai, Jiro Itatani, Linda Ye, Maya Martinez, Anisha Singh, Ian R. Fisher, Uwe Bovensiepen, Kozo Okazaki

Abstract: Non-equilibrium states generated by ultrafast laser pulses are characterized by specific phenomena that are not accessible in static measurements. Previous time- and angle-resolved photoemission spectroscopy (TARPES) studies on rare-earth tritelluride materials have revealed the laser-driven melting of the charge density wave order as well as its collective amplitude mode excitation. Variation of… ▽ More Non-equilibrium states generated by ultrafast laser pulses are characterized by specific phenomena that are not accessible in static measurements. Previous time- and angle-resolved photoemission spectroscopy (TARPES) studies on rare-earth tritelluride materials have revealed the laser-driven melting of the charge density wave order as well as its collective amplitude mode excitation. Variation of the excess energy deposited by optical pumping in the material promises pathways to control the dynamic material response. To this end, we use an optical parametric amplifier to generate a tunable pump photon energy. Studying LaTe3 we compare the dynamics driven by pumping resonantly across the charge density wave energy gap with the effect of pumping at a twice higher photon energy in a TARPES pump-probe experiment. We clearly identify a pump photon energy dependent behavior. At the larger pump photon energy, the excess electronic energy generates lattice heating mediated by e-ph coupling and softening of the amplitude mode frequency from 3 to 2 THz. Remarkably, the resonant pumping across the CDW gap results in a time-independent amplitude mode frequency. We conclude that the resonant excitation across the energy gap excites the amplitude mode selectively while additional electronic excess energy deposited at higher pump photon energy modifies the crystal properties transiently by incoherent dissipative processes. △ Less

Submitted 17 October, 2025; v1 submitted 18 December, 2024; originally announced December 2024.

Comments: 14 pages, 4 figures

Journal ref: J. Phys. Soc. Jpn. 94, 104706 (2025)

A Novel Energy-Efficient Salicide-Enhanced Tunnel Device Technology Based on 300mm Foundry Platform Towards AIoT Applications

Authors: Kaifeng Wang, Qianqian Huang, Yongqin Wu, Ye Ren, Renjie Wei, Zhixuan Wang, Libo Yang, Fangxing Zhang, Kexing Geng, Yiqing Li, Mengxuan Yang, Jin Luo, Ying Liu, Kai Zheng, Jin Kang, Le Ye, Lining Zhang, Weihai Bu, Ru Huang

Abstract: This work demonstrates a novel energy-efficient tunnel FET (TFET)-CMOS hybrid foundry platform for ultralow-power AIoT applications. By utilizing the proposed monolithic integration process, the novel complementary n and p-type Si TFET technology with dopant segregated source junction and self-aligned drain underlap design is successfully integrated into a 300mm CMOS baseline process without CMOS… ▽ More This work demonstrates a novel energy-efficient tunnel FET (TFET)-CMOS hybrid foundry platform for ultralow-power AIoT applications. By utilizing the proposed monolithic integration process, the novel complementary n and p-type Si TFET technology with dopant segregated source junction and self-aligned drain underlap design is successfully integrated into a 300mm CMOS baseline process without CMOS performance penalty and any new materials, experimentally demonstrating the large Ion and record high Ion/Ioff ratio of 10^7 among TFETs by industry-manufacturers. The device performance and variability are also co-optimized for high-volume production. Further circuit-level implementations are presented based on the calibrated compact model. The proposed TFET-CMOS hybrid logic and SRAM topologies show significant energy efficiency improvement with comparable operation speed compared with standard CMOS circuits, indicating its great potential for power-constraint AIoT applications. △ Less

Submitted 16 October, 2024; originally announced October 2024.

Realization of giant elastocaloric cooling at cryogenic temperatures in TmVO$_4$ via a strain load/unload technique

Authors: Mark P. Zic, Linda Ye, Maya H. Martinez, Ian R. Fisher

Abstract: The adiabatic elastocaloric effect relates changes in the strain that a material experiences to resulting changes in its temperature. While elastocaloric materials have been utilized for cooling in room temperature applications, the use of such materials for cryogenic cooling remains relatively unexplored. Here, we use a strain load/unload technique at low temperatures, similar to those employed a… ▽ More The adiabatic elastocaloric effect relates changes in the strain that a material experiences to resulting changes in its temperature. While elastocaloric materials have been utilized for cooling in room temperature applications, the use of such materials for cryogenic cooling remains relatively unexplored. Here, we use a strain load/unload technique at low temperatures, similar to those employed at room-temperature, to demonstrate a large cooling effect in TmVO$_4$. For strain changes of $1.8 \cdot 10^{-3}$, the inferred cooling reaches approximately 50% of the material's starting temperature at 5 K, justifying the moniker "giant". Beyond establishing the suitability of this class of material for cryogenic elastocaloric cooling, these measurements also provide additional insight to the entropy landscape in the material as a function of strain and temperature, including the behavior proximate to the quadrupolar phase transition. △ Less

Submitted 10 September, 2024; originally announced September 2024.

Controlling nonergodicity in quantum many-body systems by reinforcement learning

Authors: Li-Li Ye, Ying-Cheng Lai

Abstract: Finding optimal control strategies to suppress quantum thermalization for arbitrarily initial states, the so-called quantum nonergodicity control, is important for quantum information science and technologies. Previous control methods largely relied on theoretical model of the target quantum system, but invertible model approximations and inaccuracies can lead to control failures. We develop a mod… ▽ More Finding optimal control strategies to suppress quantum thermalization for arbitrarily initial states, the so-called quantum nonergodicity control, is important for quantum information science and technologies. Previous control methods largely relied on theoretical model of the target quantum system, but invertible model approximations and inaccuracies can lead to control failures. We develop a model-free and deep-reinforcement learning (DRL) framework for quantum nonergodicity control. It is a machine-learning method with the unique focus on balancing exploration and exploitation strategies to maximize the cumulative rewards so as to preserve the initial memory in the time-dependent nonergodic metrics over a long stretch of time. We use the paradigmatic one-dimensional tilted Fermi-Hubbard system to demonstrate that the DRL agent can efficiently learn the quantum many-body system solely through the interactions with the environment. The optimal policy obtained by the DRL provides broader control scenarios for managing nonergodicity in the phase diagram as compared to, e.g., the specific protocol for Wannier-Stark localization. The continuous control protocols and observations are experimentally feasible. The model-free nature of DRL and its versatile search space for control functions render promising nonergodicity control in more complex quantum many-body systems. △ Less

Submitted 4 March, 2025; v1 submitted 21 August, 2024; originally announced August 2024.

Comments: 20 pages, 9 figures

Journal ref: Phys. Rev. Research 7, 013256, 2025

Metamagnetic multiband Hall effect in Ising antiferromagnet ErGa$_2$

Authors: Takashi Kurumaji, Shiang Fang, Linda Ye, Shunsuke Kitou, Joseph G. Checkelsky

Abstract: Frustrated rare-earth-based intermetallics provide a promising platform for emergent magnetotransport properties through exchange coupling between conduction electrons and localized rare-earth magnetic moments. Metamagnetism, the abrupt change of magnetization under an external magnetic field, is a signature of first-order magnetic phase transitions; recently, metamagnetic transitions in frustrate… ▽ More Frustrated rare-earth-based intermetallics provide a promising platform for emergent magnetotransport properties through exchange coupling between conduction electrons and localized rare-earth magnetic moments. Metamagnetism, the abrupt change of magnetization under an external magnetic field, is a signature of first-order magnetic phase transitions; recently, metamagnetic transitions in frustrated rare earths intermetallics have attracted interest for their accompanying nontrivial spin structures (e.g. skyrmions) and associated non-linear and topological Hall effects. Here, we present metamagnetism-induced Hall anomalies in single-crystalline ErGa$_2$, which recalls features arising from the topological Hall effect but wherein the strong Ising type anisotropy of Er moments prohibit noncoplanar spin structures. We show that the observed anomalies are neither due to anomalous Hall effect nor topological Hall effect, instead, can be accounted for via 4f-5d interactions which produce a band-dependent mobility modulation. This leads to a pronounced multiband Hall response across the magnetization process -- a metamagnetic multiband Hall effect that resembles a topological-Hall-like response but without nontrivial origins. The present findings may be of general relevance in itinerant metamagnetic systems regardless of coplanar/non-coplanar nature of spins and are important for the accurate identification of Hall signals due to emergent magnetic fields. △ Less

Submitted 28 May, 2024; originally announced May 2024.

Comments: Main 15 pages, 4 figures, Supple 22 pages, 9 figures, 1 Table

Journal ref: PNAS, 121 e2318411121 (2024)

Deep-learning design of graphene metasurfaces for quantum control and Dirac electron holography

Authors: Chen-Di Han, Li-Li Ye, Zin Lin, Vassilios Kovanis, Ying-Cheng Lai

Abstract: Metasurfaces are sub-wavelength patterned layers for controlling waves in physical systems. In optics, meta-surfaces are created by materials with different dielectric constants and are capable of unconventional functionalities. We develop a deep-learning framework for Dirac-material metasurface design for controlling electronic waves. The metasurface is a configuration of circular graphene quantu… ▽ More Metasurfaces are sub-wavelength patterned layers for controlling waves in physical systems. In optics, meta-surfaces are created by materials with different dielectric constants and are capable of unconventional functionalities. We develop a deep-learning framework for Dirac-material metasurface design for controlling electronic waves. The metasurface is a configuration of circular graphene quantum dots, each created by an electric potential. Employing deep convolutional neural networks, we show that the original scattering wave can be reconstructed with fidelity over 95$\%$, suggesting the feasibility of Dirac electron holography. Additional applications such as plane wave generation, designing broadband, and multi-functionality graphene metasurface systems are illustrated. △ Less

Submitted 1 May, 2024; originally announced May 2024.

Comments: 13 pages, 9 figures

Valley edge states as bound states in the continuum

Authors: Shunda Yin, Liping Ye, Hailong He, Xueqin Huang, Manzhu Ke, Weiyin Deng, Jiuyang Lu, Zhengyou Liu

Abstract: Bound states in the continuum (BICs) are spatially localized states with energy embedded in the continuum spectrum of extended states. The combination of BICs physics and nontrivial band topology theory giving rise to topological BICs, which are robust against disorders and meanwhile of the merit of conventional BICs, is attracting wide attention recently. Here, we report valley edge states as top… ▽ More Bound states in the continuum (BICs) are spatially localized states with energy embedded in the continuum spectrum of extended states. The combination of BICs physics and nontrivial band topology theory giving rise to topological BICs, which are robust against disorders and meanwhile of the merit of conventional BICs, is attracting wide attention recently. Here, we report valley edge states as topological BICs, which appear at domain wall between two distinct valley topological phases. The robustness of such BICs is demonstrated. The simulations and experiments show great agreement. Our findings of valley related topological BICs shed light on both BICs and valley physics, and may foster innovative applications of topological acoustic devices. △ Less

Submitted 8 April, 2024; originally announced April 2024.

Comments: A revised version has been accepted by Science Bulletin

Elastocaloric evidence for a multicomponent superconductor stabilized within the nematic state in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

Authors: Sayak Ghosh, Matthias S. Ikeda, Anzumaan R. Chakraborty, Thanapat Worasaran, Florian Theuss, Luciano B. Peralta, P. M. Lozano, Jong-Woo Kim, Philip J. Ryan, Linda Ye, Aharon Kapitulnik, Steven A. Kivelson, B. J. Ramshaw, Rafael M. Fernandes, Ian R. Fisher

Abstract: The iron-based high-$T_c$ superconductors exhibit rich phase diagrams with intertwined phases, including magnetism, nematicity and superconductivity. The superconducting $T_c$ in many of these materials is maximized in the regime of strong nematic fluctuations, making the role of nematicity in influencing the superconductivity a topic of intense research. Here, we use the AC elastocaloric effect (… ▽ More The iron-based high-$T_c$ superconductors exhibit rich phase diagrams with intertwined phases, including magnetism, nematicity and superconductivity. The superconducting $T_c$ in many of these materials is maximized in the regime of strong nematic fluctuations, making the role of nematicity in influencing the superconductivity a topic of intense research. Here, we use the AC elastocaloric effect (ECE) to map out the phase diagram of Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ near optimal doping. The ECE signature at $T_c$ on the overdoped side, where superconductivity condenses without any nematic order, is quantitatively consistent with other thermodynamic probes that indicate a single-component superconducting state. In contrast, on the slightly underdoped side, where superconductivity condenses within the nematic phase, ECE reveals a second thermodynamic transition proximate to and below $T_c$. We rule out magnetism and re-entrant tetragonality as the origin of this transition, and find that our observations strongly suggest a phase transition into a multicomponent superconducting state. This implies the existence of a sub-dominant pairing instability that competes strongly with the dominant $s^\pm$ instability. Our results thus motivate a re-examination of the pairing state and its interplay with nematicity in this extensively studied iron-based superconductor, while also demonstrating the power of ECE in uncovering strain-tuned phase diagrams of quantum materials. △ Less

Submitted 27 February, 2024; originally announced February 2024.

Disorder-induced local strain distribution in Y-substituted TmVO4

Authors: Yuntian Li, Mark P. Zic, Linda Ye, W. Joe Meese, Pierre Massat, Yanbing Zhu, Rafael M. Fernandes, Ian R. Fisher

Abstract: We report an investigation of the effect of substitution of Y for Tm in $Tm_{1-x}Y_xVO4$ via low-temperature heat capacity measurements, with the yttrium content $x$ varying from $0$ to $0.997$. Because the Tm ions support a local quadrupolar (nematic) moment, they act as reporters of the local strain state in the material, with the splitting of the ion's non-Kramers crystal field groundstate prop… ▽ More We report an investigation of the effect of substitution of Y for Tm in $Tm_{1-x}Y_xVO4$ via low-temperature heat capacity measurements, with the yttrium content $x$ varying from $0$ to $0.997$. Because the Tm ions support a local quadrupolar (nematic) moment, they act as reporters of the local strain state in the material, with the splitting of the ion's non-Kramers crystal field groundstate proportional to the quadrature sum of the in-plane tetragonal symmetry-breaking transverse and longitudinal strains experienced by each ion individually. Analysis of the heat capacity therefore provides detailed insights into the distribution of local strains that arise as a consequence of the chemical substitution. These local strains suppress long-range quadrupole order for $x>0.22$, and result in a broad Schottky-like feature for higher concentrations. Heat capacity data are compared to expectations for a distribution of uncorrelated (random) strains. For dilute Tm concentrations, the heat capacity cannot be accounted for by randomly distributed strains, demonstrating the presence of significant strain correlations between sites. For intermediate Tm concentrations, these correlations must still exist, but the data cannot be distinguished from that which would be obtained from a 2D Gaussian distribution. The cross-over between these limits is discussed in terms of the interplay of key length scales in the substituted material. The central result of this work, that local strains arising from chemical substitution are not uncorrelated, has implications for the range of validity of theoretical models based on random effective fields that are used to describe such chemically substituted materials, particularly when electronic nematic correlations are present. △ Less

Submitted 26 February, 2024; originally announced February 2024.

An experimental scheme for determining the Berry phase in two-dimensional quantum materials with a flat band

Authors: Li-Li Ye, Cheng-Zhen Wang, Ying-Cheng Lai

Abstract: Experimentally feasible methods to determine the Berry phase, a fundamental quantity characterizing a quantum material, are often needed in applications. We develop an approach to detecting the Berry phase by using a class of two-dimensional (2D) Dirac materials with a flat band, the $α$-$\mathcal{T}_3$ lattices. The properties of this class of quantum materials are controlled by a single paramete… ▽ More Experimentally feasible methods to determine the Berry phase, a fundamental quantity characterizing a quantum material, are often needed in applications. We develop an approach to detecting the Berry phase by using a class of two-dimensional (2D) Dirac materials with a flat band, the $α$-$\mathcal{T}_3$ lattices. The properties of this class of quantum materials are controlled by a single parameter $0 \le α\le 1$, where the left and right endpoints correspond to graphene with pseudospin-1/2 and the dice lattice with pseudospin-1 Dirac-Weyl quasiparticles, respectively, and each specific value of $α$ represents a material with a unique Berry phase. Applying a constant electric field to the $α$-$\mathcal{T}_3$ lattice, we calculate the resulting electric current and find a one-to-one correspondence between the current and the Berry phase in both the linear and nonlinear response regimes. In the linear (Kubo) regime, the main physics is the Zitterbewegung effect. In the nonlinear regime, the Schwinger mechanism dominates. Beyond the nonlinear regime, Bloch-Zener oscillations can arise. Measuring the current thus provides an effective and experimentally feasible way to determine the Berry phase for this spectrum of 2D quantum materials. △ Less

Submitted 21 August, 2024; v1 submitted 23 February, 2024; originally announced February 2024.

Comments: 12 pages, 9 figures

Spin-dependent edge states in two-dimensional Dirac materials with a flat band

Authors: Li-Li Ye, Chen-Di Han, Ying-Cheng Lai

Abstract: The phenomenon of spin-dependent quantum scattering in two-dimensional (2D) pseudospin-1/2 Dirac materials leading to a relativistic quantum chimera was recently uncovered. We investigate spin-dependent Dirac electron optics in 2D pseudospin-1 Dirac materials, where the energy-band structure consists of a pair of Dirac cones and a flat band. In particular, with a suitable combination of external e… ▽ More The phenomenon of spin-dependent quantum scattering in two-dimensional (2D) pseudospin-1/2 Dirac materials leading to a relativistic quantum chimera was recently uncovered. We investigate spin-dependent Dirac electron optics in 2D pseudospin-1 Dirac materials, where the energy-band structure consists of a pair of Dirac cones and a flat band. In particular, with a suitable combination of external electric fields and a magnetic exchange field, electrons with a specific spin orientation (e.g., spin-down) can be trapped in a class of long-lived edge modes, generating resonant scattering. The spin-dependent edge states are a unique feature of flat-band Dirac materials and have no classical correspondence. However, electrons with the opposite spin (i.e., spin up) undergo conventional quantum scattering with a classical correspondence, which can be understood in the framework of Dirac electron optics. A consequence is that the spin-down electrons produce a large scattering probability with broad scattering angle distribution in both near- and far-field regions, while the spin-up electrons display the opposite behavior. Such characteristically different behaviors of the electrons with opposite spins lead to spin polarization that can be as high as nearly 100%. △ Less

Submitted 21 August, 2024; v1 submitted 21 February, 2024; originally announced February 2024.

Comments: 18 pages, 9 figures, journal

Journal ref: Phys. Rev. B 108 (2023), 235404

Irregular Bloch Zener oscillations in two-dimensional flat-band Dirac materials

Authors: Li-Li Ye, Ying-Cheng Lai

Abstract: When a static electrical field is applied to a two-dimensional (2D) Dirac material, Landau-Zener transition (LZT) and Bloch-Zener oscillations can occur. Employing alpha-T3 lattices as a paradigm for a broad class of 2D Dirac materials, we uncover two phenomena. First, due to the arbitrarily small energy gaps near a Dirac point that make it more likely for LZTs to occur than in other regions of th… ▽ More When a static electrical field is applied to a two-dimensional (2D) Dirac material, Landau-Zener transition (LZT) and Bloch-Zener oscillations can occur. Employing alpha-T3 lattices as a paradigm for a broad class of 2D Dirac materials, we uncover two phenomena. First, due to the arbitrarily small energy gaps near a Dirac point that make it more likely for LZTs to occur than in other regions of the Brillouin zone, the distribution of differential LZT probability in the momentum space can form a complicated morphological pattern. Second, a change in the LZT morphology as induced by a mutual switching of the two distinct Dirac points can lead to irregular Bloch-Zener oscillations characterized by a non-smooth behavior in the time evolution of the electrical current density associated with the oscillation. These phenomena are due to mixed interference of quantum states in multiple bands modulated by the geometric and dynamic phases. We demonstrate that the adiabatic-impulse model describing Landau-Zener-Stuckelberg interferometry can be exploited to calculate the phases, due to the equivalence between the alpha-T3 lattice subject to a constant electrical field and strongly periodically driven two- or three-level systems. The degree of irregularity of Bloch-Zener oscillations can be harnessed by selecting the morphology pattern, which is potentially experimentally realizable. △ Less

Submitted 21 August, 2024; v1 submitted 21 February, 2024; originally announced February 2024.

Comments: 18 pages, 11 figures, journal

Journal ref: Phys. Rev. B 107 (2023), 165422

Optical properties of two dimensional Dirac Weyl materials with a flatband

Authors: Li-Li Ye, Chen-Di Han, Ying-Cheng Lai

Abstract: The emergence of a flat band in Dirac-Weyl materials offers new possibilities for electronic transitions, leading to stronger interaction with light. As a result, the optical conductivity can be significantly enhanced in these flat-band materials as compared with graphene, making them potentially better candidates for optical sensing and modulation. Recently, a comprehensive theory for the optical… ▽ More The emergence of a flat band in Dirac-Weyl materials offers new possibilities for electronic transitions, leading to stronger interaction with light. As a result, the optical conductivity can be significantly enhanced in these flat-band materials as compared with graphene, making them potentially better candidates for optical sensing and modulation. Recently, a comprehensive theory for the optical conductivity of a spectrum of flat-band Dirac-Weyl materials has been developed, with explicit formulas for both the real and imaginary parts of the conductivity derived through two independent approaches. This Perspective offers a review of the development. An understanding of the optical properties of the flat-band Dirac-Weyl materials paves the way for optical device applications in the terahertz-frequency domain. △ Less

Submitted 21 February, 2024; originally announced February 2024.

Comments: 8 pages, 3 figures, journal

Journal ref: Appl. Phys. Lett. 124, 060501 (2024)

Theory of Electron Spin Resonance Spectroscopy in Scanning Tunneling Microscope

Authors: Lyuzhou Ye, Xiao Zheng, Xin Xu

Abstract: The integration of scanning tunneling microscopy (STM) and electron spin resonance (ESR) spectroscopy has emerged as a powerful and innovative tool for discerning spin excitations and spin-spin interactions within atoms and molecules adsorbed on surfaces. However, the origin of the STM-ESR signal and the underlying mechanisms that govern the essential features of the measured spectra have remained… ▽ More The integration of scanning tunneling microscopy (STM) and electron spin resonance (ESR) spectroscopy has emerged as a powerful and innovative tool for discerning spin excitations and spin-spin interactions within atoms and molecules adsorbed on surfaces. However, the origin of the STM-ESR signal and the underlying mechanisms that govern the essential features of the measured spectra have remained elusive, thereby significantly impeding the future development of the STM-ESR approach. Here, we construct a model to carry out precise numerical simulations of STM-ESR spectra for a single hydrogenated Ti adatom and a hydrogenated Ti dimer, achieving excellent agreement with experimental observations. We further develop an analytic theory that elucidates the fundamental origin of the signal as well as the essential features in the measured spectra. These new theoretical developments establish a solid foundation for the on-demand detection and manipulation of atomic-scale spin states, with promising implications for cutting-edge applications in spin sensing, quantum information, and quantum computing. △ Less

Submitted 31 October, 2024; v1 submitted 2 February, 2024; originally announced February 2024.

Comments: 6 pages, 4 figures

Observation of Higher Order Nodal Line Semimetal in Phononic Crystals

Authors: Qiyun Ma, Zhenhang Pu, Liping Ye, Jiuyang Lu, Xueqin Huang, Manzhu Ke, Hailong He, Weiyin Deng, Zhengyou Liu

Abstract: Higher-order topological insulators and semimetals, which generalize the conventional bulk-boundary correspondence, have attracted extensive research interest. Among them, higher-order Weyl semimetals feature two-fold linear crossing points in three-dimensional (3D) momentum space, 2D Fermi-arc surface states, and 1D hinge states. Higher-order nodal-point semimetals possessing Weyl points or Dirac… ▽ More Higher-order topological insulators and semimetals, which generalize the conventional bulk-boundary correspondence, have attracted extensive research interest. Among them, higher-order Weyl semimetals feature two-fold linear crossing points in three-dimensional (3D) momentum space, 2D Fermi-arc surface states, and 1D hinge states. Higher-order nodal-point semimetals possessing Weyl points or Dirac points have been implemented. However, higher-order nodal-line or nodal-surface semimetals remain to be further explored in experiments in spite of many previous theoretical efforts. In this work, we realize a second-order nodal-line semimetal in 3D phononic crystals. The bulk nodal lines, 2D drumhead surface states guaranteed by Zak phases, and 1D flat hinge states attributed to kz-dependent quadrupole moments, are observed in simulations and experiments. Our findings of nondispersive surface and hinge states may promote applications in acoustic sensing and energy harvesting. △ Less

Submitted 12 January, 2024; v1 submitted 9 January, 2024; originally announced January 2024.

Comments: accepted for publication in PRL

HEOM-QUICK2: a general-purpose simulator for fermionic many-body open quantum systems -- An Update

Authors: Daochi Zhang, Lyuzhou Ye, Jiaan Cao, Yao Wang, Rui-Xue Xu, Xiao Zheng, YiJing Yan

Abstract: Many-body open quantum systems (OQS) have a profound impact on various subdisciplines of physics, chemistry, and biology. Thus, the development of a computer program capable of accurately, efficiently, and versatilely simulating many-body OQS is highly desirable. In recent years, we have focused on the advancement of numerical algorithms based on the fermionic hierarchical equations of motion (HEO… ▽ More Many-body open quantum systems (OQS) have a profound impact on various subdisciplines of physics, chemistry, and biology. Thus, the development of a computer program capable of accurately, efficiently, and versatilely simulating many-body OQS is highly desirable. In recent years, we have focused on the advancement of numerical algorithms based on the fermionic hierarchical equations of motion (HEOM) theory. Being in-principle exact, this approach allows for the precise characterization of many-body correlations, non-Markovian memory, and non-equilibrium thermodynamic conditions. These efforts now lead to the establishment of a new computer program, HEOM for QUantum Impurity with a Correlated Kernel, version 2 (HEOM-QUICK2), which, to the best of our knowledge, is currently the only general-purpose simulator for fermionic many-body OQS. Compared with version 1, the HEOM-QUICK2 program features more efficient solvers for stationary states, more accurate treatment of non-Markovian memory, and improved numerical stability for long-time dissipative dynamics. Integrated with quantum chemistry software, HEOM-QUICK2 has become a valuable theoretical tool for the precise simulation of realistic many-body OQS, particularly the single atomic or molecular junctions. Furthermore, the unprecedented precision achieved by HEOM-QUICK2 enables accurate simulation of low-energy spin excitations and coherent spin relaxation. The unique usefulness of HEOM-QUICK2 is demonstrated through several examples of strongly correlated quantum impurity systems under non-equilibrium conditions. Thus, the new HEOM-QUICK2 program offers a powerful and comprehensive tool for studying many-body OQS with exotic quantum phenomena and exploring applications in various disciplines. △ Less

Submitted 15 November, 2024; v1 submitted 3 January, 2024; originally announced January 2024.

Comments: 67 pages; 13 figures

Thermal transport measurements of the charge density wave transition in CsV$_3$Sb$_5$

Authors: Erik D. Kountz, Chaitanya R. Murthy, Dong Chen, Linda Ye, Mark Zic, Claudia Felser, Ian R. Fisher, Steven A. Kivelson, Aharon Kapitulnik

Abstract: We study thermalization and thermal transport in single crystals of CsV$_3$Sb$_5$ through the CDW transition by directly measuring thermal diffusivity ($D$), thermal conductivity ($κ$), resistivity ($ρ$), and specific heat ($c$). Commensurate with previous reports, we observe a sharp, narrow anomaly in specific heat associated with a first order transition that results in a CDW state below… ▽ More We study thermalization and thermal transport in single crystals of CsV$_3$Sb$_5$ through the CDW transition by directly measuring thermal diffusivity ($D$), thermal conductivity ($κ$), resistivity ($ρ$), and specific heat ($c$). Commensurate with previous reports, we observe a sharp, narrow anomaly in specific heat associated with a first order transition that results in a CDW state below $\sim94$ K. While a corresponding sharp anomaly in thermal diffusivity is also observed, resistivity and thermal conductivity only exhibit small steps at the transition, where the feature is sharp for resistivity and broader for thermal conductivity. Scrutinizing the thermal Einstein relation $κ=cD$, we find that this relation is satisfied in the entire temperature range, except in a narrow range around the transition. The Wiedemann-Franz law seems to work outside the critical region as well. Below the transition and persisting below the two-phase regime we find strong resemblance between the resistivity anomaly and the specific heat, which may point to a secondary electronic order parameter that emerges continuously below the transition. △ Less

Submitted 27 December, 2023; originally announced December 2023.

Comments: 11 pages, 6 figures

Measurement of the magnetic octupole susceptibility of PrV2Al20

Authors: Linda Ye, Matthew E. Sorensen, Maja D. Bachmann, Ian R. Fisher

Abstract: In the electromagnetic multipole expansion, magnetic octupoles are the subsequent order of magnetic multipoles allowed in centrosymmetric systems, following the more commonly observed magnetic dipoles. As order parameters in condensed matter systems, magnetic octupoles have been experimentally elusive. In particular, the lack of simple external fields that directly couple to them makes their exper… ▽ More In the electromagnetic multipole expansion, magnetic octupoles are the subsequent order of magnetic multipoles allowed in centrosymmetric systems, following the more commonly observed magnetic dipoles. As order parameters in condensed matter systems, magnetic octupoles have been experimentally elusive. In particular, the lack of simple external fields that directly couple to them makes their experimental detection challenging. Here, we demonstrate a methodology for probing the magnetic octupole susceptibility using a product of magnetic field $H_i$ and shear strain $ε_{jk}$ to couple to the octupolar fluctuations, while using an adiabatic elastocaloric effect to probe the response to this composite effective field. We observe a Curie-Weiss behavior in the obtained octupolar susceptibility of \ce{PrV2Al20} up to temperatures approximately forty times the putative octupole ordering temperature. Our results demonstrate the presence of magnetic octupole fluctuations in the particular material system, and more broadly highlight how anisotropic strain can be combined with magnetic fields to formulate a versatile probe to observe otherwise elusive emergent `hidden' electronic orders. △ Less

Submitted 8 September, 2023; originally announced September 2023.

Comments: 7 pages, 3 figures

Giant elastocaloric effect at low temperatures in TmVO$_4$ and implications for cryogenic cooling

Authors: Mark P. Zic, Matthias S. Ikeda, Pierre Massat, Patrick M. Hollister, Linda Ye, Elliott W. Rosenberg, Joshua A. W. Straquadine, Brad J. Ramshaw, Ian R. Fisher

Abstract: Adiabatic decompression of para-quadrupolar materials has significant potential as a cryogenic cooling technology. We focus on TmVO$_4$, an archetypal material that undergoes a continuous phase transition to a ferroquadrupole-ordered state at 2.15 K. Above the phase transition, each Tm ion contributes an entropy of $k_B \ln{2}$ due to the degeneracy of the crystal electric field groundstate. Owing… ▽ More Adiabatic decompression of para-quadrupolar materials has significant potential as a cryogenic cooling technology. We focus on TmVO$_4$, an archetypal material that undergoes a continuous phase transition to a ferroquadrupole-ordered state at 2.15 K. Above the phase transition, each Tm ion contributes an entropy of $k_B \ln{2}$ due to the degeneracy of the crystal electric field groundstate. Owing to the large magnetoelastic coupling, which is a prerequisite for a material to undergo a phase transition via the cooperative Jahn-Teller effect, this level splitting, and hence the entropy, can be readily tuned by externally-induced strain. Using a dynamic technique in which the strain is rapidly oscillated, we measure the adiabatic elastocaloric coefficient of single-crystal TmVO$_4$, and thus experimentally obtain the entropy landscape as a function of strain and temperature. The measurement confirms the suitability of this class of materials for cryogenic cooling applications, and provides insight to the dynamic quadrupole strain susceptibility. △ Less

Submitted 29 August, 2023; originally announced August 2023.

Large anomalous Hall effect in single crystals of the kagome Weyl ferromagnet Fe$_3$Sn

Authors: Bishnu P. Belbase, Linda Ye, Bishnu Karki, Jorge I. Facio, Jhih-Shih You, Joseph G. Checkelsky, Jeroen van den Brink, Madhav Prasad Ghimire

Abstract: The material class of kagome metals has rapidly grown and has been established as a field to explore the interplay between electronic topology and magnetism. In this work, we report a combined theoretical and experimental study of the anomalous Hall effect of the ferromagnetic kagome metal Fe$_3$Sn. The compound orders magnetically at 725 K and presents an easy-plane anisotropy. Hall measurements… ▽ More The material class of kagome metals has rapidly grown and has been established as a field to explore the interplay between electronic topology and magnetism. In this work, we report a combined theoretical and experimental study of the anomalous Hall effect of the ferromagnetic kagome metal Fe$_3$Sn. The compound orders magnetically at 725 K and presents an easy-plane anisotropy. Hall measurements in single crystals below room temperature yield an anomalous Hall conductivity $σ_{xy}\sim500\,(Ω\textrm{cm})^{-1}$, which is found to depend weakly on temperature. This value is in good agreement with the band-intrinsic contribution obtained by density-functional calculations. Our calculations also yield the correct magnetic anisotropy energy and predict the existence of Weyl nodes near the Fermi energy. △ Less

Submitted 28 August, 2023; originally announced August 2023.

Comments: 9 pages and 9 figures including supplement

Journal ref: Phys. Rev. B 108, 075164 (2023)

Acoustic Higher-Order Weyl Semimetal with Bound Hinge States in the Continuum

Authors: Zhenhang Pu, Hailong He, Licheng Luo, Qiyun Ma, Liping Ye, Manzhu Ke, Zhengyou Liu

Abstract: Higher-order topological phases have raised widespread interest in recent years with the occurrence of the topological boundary states of dimension two or more less than that of the system bulk. The higher-order topological states have been verified in gapped phases, in a wide variety of systems, such as photonic and acoustic systems, and recently also observed in gapless semimetal phase, such as… ▽ More Higher-order topological phases have raised widespread interest in recent years with the occurrence of the topological boundary states of dimension two or more less than that of the system bulk. The higher-order topological states have been verified in gapped phases, in a wide variety of systems, such as photonic and acoustic systems, and recently also observed in gapless semimetal phase, such as Weyl and Dirac phases, in systems alike. The higher-order topology is signaled by the hinge states emerging in the common bandgaps of the bulk states and the surface states. In this Letter, we report our first prediction and observation of a new type of hinge states, the bound hinge states in the continuum (BHICs) bulk band, in a higher-order Weyl semimetal implemented in phononic crystal. In contrast to the hinge state in gap, which is characterized by the bulk polarization, the BHIC is identified by the nontrivial surface polarization. The finding of the topological BHICs broadens our insight to the topological states, and may stimulate similar researches in other systems such as electronic, photonic, and cold atoms systems. Our work may pave the way toward high-Q acoustic devices in application. △ Less

Submitted 18 March, 2023; originally announced March 2023.

Journal ref: Physical Review Letters 130, 116103 (2023)

Crystal Net Catalog of Model Flat Band Materials

Authors: Paul M. Neves, Joshua P. Wakefield, Shiang Fang, Haimi Nguyen, Linda Ye, Joseph G. Checkelsky

Abstract: Flat band systems are currently under intense investigation in quantum materials, optical lattices, and metamaterials. These efforts are motivated by potential realization of strongly correlated phenomena enabled by frustration-induced flat band dispersions; identification of candidate platforms plays an important role in these efforts. Here, we develop a high-throughput materials search for bulk… ▽ More Flat band systems are currently under intense investigation in quantum materials, optical lattices, and metamaterials. These efforts are motivated by potential realization of strongly correlated phenomena enabled by frustration-induced flat band dispersions; identification of candidate platforms plays an important role in these efforts. Here, we develop a high-throughput materials search for bulk crystalline flat bands by automated construction of uniform-hopping near-neighbor tight binding models. We show that this approach captures many of the essential features relevant to identifying flat band lattice motifs in candidate materials in a computationally inexpensive manner. We apply this algorithm to 139,367 materials in the Materials Project database and identify 63,076 materials that host at least one flat band elemental sublattice. We further categorize these candidate systems into at least 31,635 unique flat band crystal nets and identify candidates of interest from both lattice and band structure perspectives. This work expands the number of known flat band lattices that exist in physically realizable crystal structures and classifies the majority of these systems by the underlying lattice, providing new insights for familiar (e.g. kagome, pyrochlore, Lieb, and dice) as well as previously unknown motifs. △ Less

Submitted 4 March, 2023; originally announced March 2023.

Comments: 29 pages, 8 figures, 2 tables

Unusual Nonlinear Optical Responses in Layered Ferroelectric Niobium Oxide Dihalides: Origin and Manipulation

Authors: Liangting Ye, Wenju Zhou, Dajian Huang, Xiao Jiang, Donghan Jia, Qiangbing Guo, Dequan Jiang, Yonggang Wang, Xiaoqiang Wu, Yang Li, Huiyang Gou, Bing Huang

Abstract: Realization of large and highly tunable second-order nonlinear optical (NLO) responses, e.g., second-harmonic generation (SHG) and bulk photovoltaic effect (BPVE), is critical for developing modern optical and optoelectronic devices. Very recently, the two-dimensional van der Waals ferroelectric NbOX2 (X = Cl, Br or I) are discovered to exhibit unusually large and anisotropic SHG. However, the phy… ▽ More Realization of large and highly tunable second-order nonlinear optical (NLO) responses, e.g., second-harmonic generation (SHG) and bulk photovoltaic effect (BPVE), is critical for developing modern optical and optoelectronic devices. Very recently, the two-dimensional van der Waals ferroelectric NbOX2 (X = Cl, Br or I) are discovered to exhibit unusually large and anisotropic SHG. However, the physical origin and possible tunability of NLO responses in NbOX2 remain to be unclear. In this article, we reveal that the large SHG in NbOCl2 is dominated by the synergy between large transition dipole moment and band-nesting-induced large intensity of electron-hole pairs. Remarkably, the NbOCl2 can exhibit dramatically different strain-dependent BPVE under different polarized light, originating from the interesting light-polarization-dependent orbital transition. Importantly, we successfully achieve a reversible ferroelectric-to-antiferroelectric phase transition via controlling ambient temperature or external pressure, accompanied by the greatly tunable NLO responses. Furthermore, we discover that the evolutions of SHG and BPVE in NbOX2 with variable X obey different rules. Our study provides a deep understanding on the novel NLO physics in NbOX2 and establishes great external-field tunability for device applications. △ Less

Submitted 24 February, 2023; originally announced February 2023.

Spin wavepackets in the Kagome ferromagnet Fe$_3$Sn$_2$: propagation and precursors

Authors: Changmin Lee, Yue Sun, Linda Ye, Sumedh Rathi, Kevin Wang, Yuan-Ming Lu, Joel Moore, Joseph G. Checkelsky, Joseph Orenstein

Abstract: The propagation of spin waves in magnetically ordered systems has emerged as a potential means to shuttle quantum information over large distances. Conventionally, the arrival time of a spin wavepacket at a distance, $d$, is assumed to be determined by its group velocity, $v_g$. He we report time-resolved optical measurements of wavepacket propagation in the Kagome ferromagnet Fe$_3$Sn$_2$ that de… ▽ More The propagation of spin waves in magnetically ordered systems has emerged as a potential means to shuttle quantum information over large distances. Conventionally, the arrival time of a spin wavepacket at a distance, $d$, is assumed to be determined by its group velocity, $v_g$. He we report time-resolved optical measurements of wavepacket propagation in the Kagome ferromagnet Fe$_3$Sn$_2$ that demonstrate the arrival of spin information at times significantly less than $d/v_g$. We show that this spin wave "precursor" originates from the interaction of light with the unusual spectrum of magnetostatic modes in Fe$_3$Sn$_2$. Related effects may have far-reaching consequences toward realizing long-range, ultrafast spin wave transport in both ferromagnetic and antiferromagnetic systems. △ Less

Submitted 5 December, 2022; originally announced December 2022.

Comments: 17+12 pages, 5+4 figures

Journal ref: PNAS 120, e2220589120 (2023)

Metal-insulator phase separation in KTaO3-based two-dimensional electron gas

Authors: Jinlei Zhang, Jiayong Zhang, Dapeng Cui, Li Ye, Shuainan Gong, Zhichao Wang, Zhenping Wu, Chunlan Ma, Ju Gao, Yuanyuan Zhao, Yucheng Jiang

Abstract: Electronic phase separation (EPS) originates from an incomplete transformation between electronic phases, causing the inhomogeneous spatial distribution of electronic properties. In the system of two-dimensional electron gas (2DEG), the EPS is usually identified based on a percolative metal-to-superconductor transition. Here, we report a metal-insulator transition (MIT) in KTaO3-based 2DEG with th… ▽ More Electronic phase separation (EPS) originates from an incomplete transformation between electronic phases, causing the inhomogeneous spatial distribution of electronic properties. In the system of two-dimensional electron gas (2DEG), the EPS is usually identified based on a percolative metal-to-superconductor transition. Here, we report a metal-insulator transition (MIT) in KTaO3-based 2DEG with the width of conductive channel decreasing into micrometer scale. Hysteretic resistance-temperature relations are observed due to the competition between metallic and insulating phases, which is tunable by magnetic field. Such a size-dependent MIT effect is attributed to the coexistence and separation of metallic and insulating phases. Combining density functional theory calculation, we propose a theoretical model to simulate the dynamic process of the EPS using the percolation theory, demonstrating the mechanism of size-dependent MIT. Our work suggests a clear and simple 2DEG platform to achieve the spatial coexistence of metallic and insulating phases. △ Less

Submitted 25 November, 2022; originally announced November 2022.

Elastocaloric signatures of symmetric and antisymmetric strain-tuning of quadrupolar and magnetic phases in DyB2C2

Authors: Linda Ye, Yue Sun, Veronika Sunko, Joaquin F. Rodriguez-Nieva, Matthias S. Ikeda, Thanapat Worasaran, Matthew E. Sorensen, Maja D. Bachmann, Joseph Orenstein, Ian R. Fisher

Abstract: The adiabatic elastocaloric effect measures the temperature change of given systems with strain and probes the entropic landscape in the temperature-strain space. In this study we demonstrate that the DC bias strain-dependence of AC elastocaloric effect can be used to decompose the latter into contributions from symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking… ▽ More The adiabatic elastocaloric effect measures the temperature change of given systems with strain and probes the entropic landscape in the temperature-strain space. In this study we demonstrate that the DC bias strain-dependence of AC elastocaloric effect can be used to decompose the latter into contributions from symmetric (rotation-symmetry-preserving) and antisymmetric (rotation-symmetry-breaking) strains, using a tetragonal f-electron system DyB2C2--whose antiferroquadrupolar order locally breaks four-fold rotational site symmetries while globally remaining tetragonal--as a showcase example. We capture the strain evolution of the quadrupolar and magnetic phase transitions in the system using both singularities in the elastocaloric coefficient and its jump at the transitions, and the latter we show follows a modified Ehrenfest relation. We find that antisymmetric strain couples to the underlying order parameter in a bi-quadratic manner in the antiferroquadrupolar (AFQ) phase but in a linear-quadratic manner in the canted antiferromagnetic (CAFM) phase; the contrast is attributed to a preserved (broken) tetragonal symmetry in the AFQ (CAFM) phase, respectively. The broken tetragonal symmetry in the CAFM phase is further supported by elastocaloric strain-hysteresis and observation of two sets of domains with mutually perpendicular principal axes in optical birefringence. Additionally, when the quadrupolar moments are ordered in a staggered fashion, we uncover an elastocaloric response that reflects a quadratic increase of entropy with antisymmetric strain, analogous to the role magnetic field plays for Ising antiferromagnets by promoting pseudospin flips. Our results show that AC elastocaloric effect is a compact and incisive thermodynamic probe into the coupling between electronic degrees of freedom and strain, which can potentially be applied to broader classes of quantum materials. △ Less

Submitted 22 November, 2022; originally announced November 2022.

Comments: 10 pages, 7 figures

Journal ref: Proc. Natl. Acad. Sci. 120 (35), e2302800120 (2023)

Kondo cooling in quantum impurity systems

Authors: Xiangzhong Zeng, Lyuzhou Ye, Long Cao, Rui-Xue Xu, Xiao Zheng, Massimiliano Di Ventra

Abstract: The Peltier effect is the reverse phenomenon of the Seebeck effect, and has been observed experimentally in nanoscale junctions. However, despite its promising applications in local cooling of nanoelectronic devices, the role of strong electron correlations on such a phenomenon is still unclear. Here, by analyzing the thermoelectric properties of quantum impurity systems out of equilibrium, we unv… ▽ More The Peltier effect is the reverse phenomenon of the Seebeck effect, and has been observed experimentally in nanoscale junctions. However, despite its promising applications in local cooling of nanoelectronic devices, the role of strong electron correlations on such a phenomenon is still unclear. Here, by analyzing the thermoelectric properties of quantum impurity systems out of equilibrium, we unveil the essential role of electron-electron interactions and quantum resonant states in Peltier cooling, leading to the prediction of the Kondo cooling phenomenon. The existence of such Kondo cooling is validated by a reverse heat current and a lowered local temperature in a model junction. The discovery of this unconventional Peltier cooling offers a new approach toward nano-refrigeration, and highlights the unique role of strong electron correlations in nonequilibrium quantum systems. △ Less

Submitted 15 March, 2022; originally announced March 2022.

Comments: 6 pages, 4 figures

Field-dependent Shubnikov-de Haas oscillations in ferromagnetic Weyl semimetal Co3Sn2S2

Authors: Linda Ye, Jorge I. Facio, Madhav P. Ghimire, Mun K. Chan, Jhih-Shih You, David C. Bell, Manuel Richter, Jeroen van den Brink, Joseph G. Checkelsky

Abstract: We report a study of Shubnikov-de Haas oscillations in high quality single crystals of ferromagnetic Weyl semimetal Co$_3$Sn$_2$S$_2$. The Fermi surfaces resolved in our experiments are three-dimensional and reflect an underlying trigonal crystallographic symmetry. Combined with density functional theoretical calculations, we identify that the majority of the Fermi surfaces in the system -- of bot… ▽ More We report a study of Shubnikov-de Haas oscillations in high quality single crystals of ferromagnetic Weyl semimetal Co$_3$Sn$_2$S$_2$. The Fermi surfaces resolved in our experiments are three-dimensional and reflect an underlying trigonal crystallographic symmetry. Combined with density functional theoretical calculations, we identify that the majority of the Fermi surfaces in the system -- of both electron and hole nature -- arise from the strong energy dispersion of the (spin-orbit gapped) mirror-protected nodal rings. We observe that an in-plane magnetic field induces a continuous evolution of Fermi surfaces, in contrast to field perpendicular to the kagome lattice planes which has little effect. Viewed alongside the easy-axis anisotropy of the system, our observation reveals an evolution of the electronic structure of Co$_3$Sn$_2$S$_2$ -- including the Weyl points -- with the ferromagnetic moment orientation. Through the case study of Co$_3$Sn$_2$S$_2$, our results provide concrete experimental evidence of an anisotropic interplay via spin-orbit coupling between the magnetic degrees of freedom and electronic band singularities, which has long been expected in semimetallic and metallic magnetic topological systems. △ Less

Submitted 8 March, 2022; originally announced March 2022.

Comments: 6 pages, 3 figures

Snapshots of a light-induced metastable hidden phase driven by the collapse of charge order

Authors: Frank Y. Gao, Zhuquan Zhang, Zhiyuan Sun, Linda Ye, Yu-Hsiang Cheng, Zi-Jie Liu, Joseph G. Checkelsky, Edoardo Baldini, Keith A. Nelson

Abstract: Nonequilibrium hidden states, both transient and long-lived, provide a unique window into thermally inaccessible regimes of strong coupling between microscopic degrees of freedom in quantum materials. Understanding the physical origin of these states is of both fundamental and practical significance, allowing the exploration of far-from-equilibrium thermodynamics and the development of optoelectro… ▽ More Nonequilibrium hidden states, both transient and long-lived, provide a unique window into thermally inaccessible regimes of strong coupling between microscopic degrees of freedom in quantum materials. Understanding the physical origin of these states is of both fundamental and practical significance, allowing the exploration of far-from-equilibrium thermodynamics and the development of optoelectronic devices with on-demand photoresponses. However, mapping the ultrafast formation of a long-lived hidden phase remains a long-standing challenge in physics since the initial state of the system is not recovered rapidly and conventional pump-probe methods are thus not applicable. Here, using a suite of state-of-the-art single-shot spectroscopy techniques, we present a direct ultrafast visualization of the photoinduced phase transition to both transient and long-lived hidden states in an electronic crystal, 1T-TaS2. Capturing the dynamics of this complex phase transformation in a single-shot fashion demonstrates a commonality in microscopic pathways, driven by the collapse of charge order, that the system undergoes to enter the hidden state and provides unambiguous spectral fingerprints that distinguish such state from thermally accessible phases. We present a theory of fluctuation-dominated process that explains both the dynamics and the nature of the metastable state. Our results settle the debate around the origin of this elusive metastable state and pave the way for the discovery of new quantum phases of matter. △ Less

Submitted 13 February, 2022; v1 submitted 21 December, 2021; originally announced December 2021.

Field-Free Deterministic Writing of SOT-MTJ by Unipolar Current

Authors: Tengzhi Yang, Meiyin Yang, Lei Zhao, Jianfeng Gao, Qingyi Xiang, Wenjing Li, Feilong Luo, Li Ye, Jun Luo

Abstract: We propose a manufacturable solution for field-free writing of perpendicular SOT-MTJ without sacrificing integration density on a 200 mm wafer. The field-free writing operation can be achieved by unipolar current pulses via engineering the interlayer exchange coupling in SOT-MTJ thin films. The proposed device can reach a high writing speed of up to 1 ns and work properly at temperature of 100 C.… ▽ More We propose a manufacturable solution for field-free writing of perpendicular SOT-MTJ without sacrificing integration density on a 200 mm wafer. The field-free writing operation can be achieved by unipolar current pulses via engineering the interlayer exchange coupling in SOT-MTJ thin films. The proposed device can reach a high writing speed of up to 1 ns and work properly at temperature of 100 C. The deterministic writing of SOT-MTJ by unipolar current offers an effective approach for high density SOT-MRAM integration. △ Less

Submitted 27 November, 2021; v1 submitted 23 November, 2021; originally announced November 2021.

Comments: 4 pages, 3 figures

Plasmon-Exciton Coupling Effect on Plasmon Damping

Authors: Lulu Ye, Weidong Zhang, Aiqin Hu, Hai Lin, Jinglin Tang, Yunkun Wang, Chenxinyu Pan, Pan Wang, Xin Guo, Limin Tong, Yunan Gao, Qihuang Gong, Guowei Lu

Abstract: Plasmon decay via the surface or interface is a critical process for practical energy conversion and plasmonic catalysis. However, the relationship between plasmon damping and the coupling between the plasmon and 2D materials is still unclear. The spectral splitting due to plasmon-exciton interaction impedes the conventional single-particle method to evaluate the plasmon damping rate by the spectr… ▽ More Plasmon decay via the surface or interface is a critical process for practical energy conversion and plasmonic catalysis. However, the relationship between plasmon damping and the coupling between the plasmon and 2D materials is still unclear. The spectral splitting due to plasmon-exciton interaction impedes the conventional single-particle method to evaluate the plasmon damping rate by the spectral linewidth directly. Here, we investigated the interaction between a single gold nanorod (GNR) and 2D materials using the single-particle spectroscopy method assisted with in situ nanomanipulation technique by comparing scattering intensity and linewidth together. Our approach allows us to indisputably identify that the plasmon-exciton coupling in the GNR-WSe2 hybrid would induce plasmon damping. We can also isolate the contribution between the charge transfer channel and resonant energy transfer channel for the plasmon decay in the GNR-graphene hybrid by comparing that with thin hBN layers as an intermediate medium to block the charge transfer. We find out that the contact layer between the GNR and 2D materials contributes most of the interfacial plasmon damping. These findings contribute to a deep understanding of interfacial excitonic effects on the plasmon and 2D materials hybrid. △ Less

Submitted 17 July, 2021; originally announced July 2021.

A flat band-induced correlated kagome metal

Authors: Linda Ye, Shiang Fang, Min Gu Kang, Josef Kaufmann, Yonghun Lee, Jonathan Denlinger, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Efthimios Kaxiras, David C. Bell, Oleg Janson, Riccardo Comin, Joseph G. Checkelsky

Abstract: The notion of an electronic flat band refers to a collectively degenerate set of quantum mechanical eigenstates in periodic solids. The vanishing kinetic energy of flat bands relative to the electron-electron interaction is expected to result in a variety of many-body quantum phases of matter. Despite intense theoretical interest, systematic design and experimental realization of such flat band-dr… ▽ More The notion of an electronic flat band refers to a collectively degenerate set of quantum mechanical eigenstates in periodic solids. The vanishing kinetic energy of flat bands relative to the electron-electron interaction is expected to result in a variety of many-body quantum phases of matter. Despite intense theoretical interest, systematic design and experimental realization of such flat band-driven correlated states in natural crystals have remained a challenge. Here we report the realization of a partially filled flat band in a new single crystalline kagome metal Ni$_3$In. This flat band is found to arise from the Ni $3d$-orbital wave functions localized at triangular motifs within the kagome lattice plane, where an underlying destructive interference among hopping paths flattens the dispersion. We observe unusual metallic and thermodynamic responses suggestive of the presence of local fluctuating magnetic moments originating from the flat band states, which together with non-Fermi liquid behavior indicate proximity to quantum criticality. These results demonstrate a lattice and orbital engineering approach to designing flat band-based many-body phenomena that may be applied to integrate correlation with topology and as a novel means to construct quantum criticality. △ Less

Submitted 20 June, 2021; originally announced June 2021.

Comments: 18 pages, 4 figures

Journal ref: Nature Physics, 20, 610-614 (2024)

Spin-flop led peculiar behavior of temperature-dependent anomalous Hall effect in Hf/Gd-Fe-Co

Authors: Ramesh Chandra Bhatt, Lin-Xiu Ye, Ngo Trong Hai, Jong-Ching Wu, Te-ho Wu

Abstract: Here we investigate the temperature dependence of anomalous Hall effect in Hf/GdFeCo/MgO sheet film and Hall bar device. The magnetic compensation temperature ($T_{comp}$) for the sheet film and device is found to be ~240 K and ~118 K, respectively. In sheet film, spin-flopping is witnessed at a considerably lower field, 0.6 T, close to $T_{comp}$. The AHE hysteresis loops in the sheet film have a… ▽ More Here we investigate the temperature dependence of anomalous Hall effect in Hf/GdFeCo/MgO sheet film and Hall bar device. The magnetic compensation temperature ($T_{comp}$) for the sheet film and device is found to be ~240 K and ~118 K, respectively. In sheet film, spin-flopping is witnessed at a considerably lower field, 0.6 T, close to $T_{comp}$. The AHE hysteresis loops in the sheet film have a single loop whereas in the Hall bar device, hystereses consist of triple loops are observed just above the Tcomp. Moreover, the temperature-dependent anomalous Hall resistance ($R_\mathrm{AHE}$) responds unusually when a perpendicular magnetic field is applied while recording the $R_\mathrm{AHE}$. The zero-field $R_\mathrm{AHE}$ scan suggests the Hall signal generates solely from the FeCo moment. However, the behavior of 3 T-field $R_\mathrm{AHE}$ scan in which the $R_\mathrm{AHE}$ drops close to zero near the $T_{comp}$ seems to be following the net magnetization response of the device, is explained by considering the low field spin-flopping around the compensation temperature. The results presented here give important insight to understand the complex AHE behavior of ferrimagnets for their spintronic applications. △ Less

Submitted 21 May, 2021; v1 submitted 12 April, 2021; originally announced April 2021.

Comments: 18 pages, 5 figures

Topological dislocation modes in three-dimensional acoustic topological insulators

Authors: Liping Ye, Chunyin Qiu, Meng Xiao, Tianzi Li, Juan Du, Manzhu Ke, Zhengyou Liu

Abstract: Dislocations are ubiquitous in three-dimensional solid-state materials. The interplay of such real space topology with the emergent band topology defined in reciprocal space gives rise to gapless helical modes bound to the line defects. This is known as bulk-dislocation correspondence, in contrast to the conventional bulk-boundary correspondence featuring topological states at boundaries. However,… ▽ More Dislocations are ubiquitous in three-dimensional solid-state materials. The interplay of such real space topology with the emergent band topology defined in reciprocal space gives rise to gapless helical modes bound to the line defects. This is known as bulk-dislocation correspondence, in contrast to the conventional bulk-boundary correspondence featuring topological states at boundaries. However, to date rare compelling experimental evidences are presented for this intriguing topological observable, owing to the presence of various challenges in solid-state systems. Here, using a three-dimensional acoustic topological insulator with precisely controllable dislocations, we report an unambiguous experimental evidence for the long-desired bulk-dislocation correspondence, through directly measuring the gapless dispersion of the one-dimensional topological dislocation modes. Remarkably, as revealed in our further experiments, the pseudospin-locked dislocation modes can be unidirectionally guided in an arbitrarily-shaped dislocation path. The peculiar topological dislocation transport, expected in a variety of classical wave systems, can provide unprecedented controllability over wave propagations. △ Less

Submitted 8 April, 2021; originally announced April 2021.

Ferromagnetic helical nodal line and Kane-Mele spin-orbit coupling in kagome metal Fe3Sn2

Authors: Shiang Fang, Linda Ye, Madhav Prasad Ghimire, Min Gu Kang, Junwei Liu, Liang Fu, Manuel Richter, Jeroen van den Brink, Efthimios Kaxiras, Riccardo Comin, Joseph G. Checkelsky

Abstract: The two-dimensional kagome lattice hosts Dirac fermions at its Brillouin zone corners K and K', analogous to the honeycomb lattice. In the density functional theory electronic structure of ferromagnetic kagome metal Fe$_3$Sn$_2$, without spin-orbit coupling we identify two energetically split helical nodal lines winding along $z$ in the vicinity of K and K' resulting from the trigonal stacking of… ▽ More The two-dimensional kagome lattice hosts Dirac fermions at its Brillouin zone corners K and K', analogous to the honeycomb lattice. In the density functional theory electronic structure of ferromagnetic kagome metal Fe$_3$Sn$_2$, without spin-orbit coupling we identify two energetically split helical nodal lines winding along $z$ in the vicinity of K and K' resulting from the trigonal stacking of the kagome layers. We find that hopping across A-A stacking introduces a layer splitting in energy while that across A-B stacking controls the momentum space amplitude of the helical nodal lines. The effect of spin-orbit coupling is found to resemble that of a Kane-Mele term, where the nodal lines can either be fully gapped to quasi-two-dimensional massive Dirac fermions, or remain gapless at discrete Weyl points depending on the ferromagnetic moment orientation. Aside from numerically establishing Fe$_3$Sn$_2$ as a model Dirac kagome metal, our results provide insights into materials design of topological phases from the lattice point of view, where paradigmatic low dimensional lattice models often find realizations in crystalline materials with three-dimensional stacking. △ Less

Submitted 15 March, 2021; originally announced March 2021.

Comments: 46 pages, 17 figures

Journal ref: Phys. Rev. B 105, 035107 (2022)

Local phase delay effect on the asymmetric spectroscopy of plasmon-exciton coupling systems

Authors: Aiqin Hu, Weidong Zhang, Lulu Ye, Ying Gu, Zhaohang Xue, Hai Lin, Jinglin Tang, Qihuang Gong, Guowei Lu

Abstract: The phase delay of a local electric field, being well-known in plasmonic nanostructures, has seldom been investigated to modulate the plasmon-exciton interaction. Here, with the single-particle spectroscopy method, we experimentally investigate the phase effect in plasmon-exciton coupling systems consisting of monolayer WSe2 and an individual gold nanorod. The local plasmon phase delay is tuned by… ▽ More The phase delay of a local electric field, being well-known in plasmonic nanostructures, has seldom been investigated to modulate the plasmon-exciton interaction. Here, with the single-particle spectroscopy method, we experimentally investigate the phase effect in plasmon-exciton coupling systems consisting of monolayer WSe2 and an individual gold nanorod. The local plasmon phase delay is tuned by adopting various nanorods with different resonant energies respective to the exciton. We find that the local plasmon phase delay between the excitons and the plasmonic modes is as equally essential as the amplitude. The phase delay modulates the plasmon-exciton coupling considerably, resulting in an asymmetric spectral line-shape due to the interference behavior. There is an excellent agreement for the phase delay between the numerically calculated near-field phase distribution and the experimental results. The local phase delay can act as an effective way to modulate the properties of plexcitonic coupling at the nanoscale, which may have potential applications in nanoscale sensing, solar energy devices, and enhancing nonlinear processes. △ Less

Submitted 8 March, 2021; originally announced March 2021.

Sense current dependent coercivity and magnetization relaxation in Gd-Fe-Co Hall bar

Authors: Ramesh Chandra Bhatt, Chun-Ming Liao, Lin-Xiu Ye, Ngo Trong Hai, Jong-Ching Wu, Te-ho Wu

Abstract: The understanding of the characteristics of a magnetic layer in a different environment is crucial for any spintronics application. Before practical applications, thorough scrutiny of such devices is compulsory. Here we study such a potential Hall device of MgO-capped Hf/GdFeCo bilayer (FeCo-rich) for magnetization relaxation around nucleation fields at different voltage probe line widths and dc s… ▽ More The understanding of the characteristics of a magnetic layer in a different environment is crucial for any spintronics application. Before practical applications, thorough scrutiny of such devices is compulsory. Here we study such a potential Hall device of MgO-capped Hf/GdFeCo bilayer (FeCo-rich) for magnetization relaxation around nucleation fields at different voltage probe line widths and dc sensing currents. The device is characterized by anomalous Hall measurements in transverse and longitudinal Hall geometries for two different probe widths A (5 micrometer) and B (1 micrometer). The coercivities of the Hall loops (\r{ho}xy-H and Rxx-H) drop with increasing the sense current for both the probes. For probe B, the sharp and large drop in coercivity (\r{ho}xy-H loops) at comparatively lower sensing currents is observed, which is attributed to the negligible current shunting and presence of pinning site at B caused by the patterning process. The average domain wall velocities at various sensing currents for probe B are found to be smaller than probe A, from the transverse and longitudinal Hall geometry magnetization relaxation measurements, which agrees with pinning sites and Joule heating effect at probe B. The notch position in the pattern and the longitudinal Hall resistance curve peak shape suggest the domain wall propagation direction from probe B to probe A in the current channel. This study highlights the domain wall propagation at different nucleation fields, sensing currents, and the Hall probe aspect ratios. △ Less

Submitted 25 November, 2020; originally announced November 2020.

Comments: 18 pages, 6 figures