Showing 1–50 of 79 results for author: Ding, S

Transport-Noise Witnesses of Electronic Multipartite Entanglement

Authors: Shuhan Ding, Prakash Sharma, Zecheng Shen, Jiang-Xiazi Lin, Sergei Urazhdin, Yao Wang

Abstract: Entanglement among particles is a defining feature of strongly correlated quantum materials, distinguishing them from conventional metals and semiconductors. The ability to certify intrinsic entanglement among interacting electrons in solid-state materials is important not only for classifying quantum states of matter, but also for developing material-based quantum technologies. Here, we introduce… ▽ More Entanglement among particles is a defining feature of strongly correlated quantum materials, distinguishing them from conventional metals and semiconductors. The ability to certify intrinsic entanglement among interacting electrons in solid-state materials is important not only for classifying quantum states of matter, but also for developing material-based quantum technologies. Here, we introduce a transport-based protocol for witnessing multipartite entangled electronic states, based on the equilibrium noise spectrum as an experimentally accessible observable. The appropriately integrated, symmetrized, and projected current noise obeys an upper bound that can be derived from microscopic model parameters and is invariant with respect to the choice of electronic basis. We benchmark this framework in several paradigmatic systems, including twisted bilayer graphene, twisted bilayer MoTe$_2$, and Hubbard models, certifying entanglement in the fractional Chern insulating state. The method extends recently developed scattering-based entanglement witnesses to ultralow-temperature materials, where conventional spectroscopic probes are inaccessible but candidate entangled states are expected to arise. △ Less

Submitted 16 August, 2026; originally announced August 2026.

Comments: 9 pages, 4 figures

Fractional Chern insulators in alternating twisted multilayer MoTe$_{2}$

Authors: Xi-Hang Feng, Shi-Ping Ding, Xiang-Jian Hou, Ying-Hai Wu, Jin-Hua Gao

Abstract: We study strongly correlated many-body states in alternating twisted trilayer and tetralayer MoTe$_{2}$. By sliding the top layer with respect to others and applying a perpendicular electric field, a variety of band structures can be realized. In many cases, the topmost hole band has unity Chern number and its quantum geometric properties can be tuned to some extent. Exact diagonalizations suggest… ▽ More We study strongly correlated many-body states in alternating twisted trilayer and tetralayer MoTe$_{2}$. By sliding the top layer with respect to others and applying a perpendicular electric field, a variety of band structures can be realized. In many cases, the topmost hole band has unity Chern number and its quantum geometric properties can be tuned to some extent. Exact diagonalizations suggest that fractional Chern insulators are stabilized in certain parameter regimes but not in some regimes even when the band is topological. This contrast is attributed primarily to different quantum geometries as quantified by the trace condition. Our results demonstrate that sliding can serve as a useful knob for probing many-body states in moiré systems. △ Less

Submitted 15 July, 2026; originally announced July 2026.

Comments: 9 pages, 10 figures

High-mobility inertial domain walls driven by spin-transfer torque in a ferrimagnetic spinel oxide

Authors: Mingxing Wu, Shilei Ding, Laura van Schie, Shenghao Cai, Yuhao Qiu, Ao Du, Alexander E. Kossak, Rui Wu, Christian L. Degen, Xuegang Chen, Pietro Gambardella

Abstract: Efficient electrical manipulation of domain walls is key to developing magnetic devices with fast switching capabilities and low energy consumption. Here we demonstrate Bloch-type domain wall velocities exceeding 1 km s$^{-1}$ in the single-layer ferrimagnetic spinel oxide NiCo$_2$O$_4$ induced by spin-transfer torque at a current density of $2 \times 10^{11}$ A m$^{-2}$. This exceptional domain w… ▽ More Efficient electrical manipulation of domain walls is key to developing magnetic devices with fast switching capabilities and low energy consumption. Here we demonstrate Bloch-type domain wall velocities exceeding 1 km s$^{-1}$ in the single-layer ferrimagnetic spinel oxide NiCo$_2$O$_4$ induced by spin-transfer torque at a current density of $2 \times 10^{11}$ A m$^{-2}$. This exceptional domain wall mobility is attributed to the combination of giant nonadiabatic spin-transfer torque, low magnetization, and high spin polarization. Additionally, we report a pronounced domain wall inertia effect in this ferrimagnet due to the large nonadiabaticity of the torque. The characteristic time for domain wall acceleration and deceleration is $\sim 1$ ns, shorter than that reported for typical ferromagnets. Our findings highlight the potential of spinel oxides as a promising platform for engineering high-performance domain wall devices that take advantage of ultrafast ferrimagnetic dynamics. △ Less

Submitted 26 May, 2026; originally announced May 2026.

Comments: 27 pages, 5 figures

Journal ref: Nat Commun 17, 4672 (2026)

Tunneling spectra of $\mathrm{TaO}_x$ junctions for van der Waals superconductors

Authors: Yixuan Niu, Jun Cheng, Shiji Ding, Zhongxin Guo, Shang Wang, Chenglong Li, Meining Zhang, Peng Cai

Abstract: Tunneling spectroscopy and its evolution are crucial for elucidating the intricate electronic structure and emergent phenomena in quantum materials.Nevertheless, high-quality measurements -- specifically those tracking evolution across temperature and external fields -- remain a formidable challenge. We have fabricated a high-quality $\mathrm{TaO}_x$-based planar tunneling junction by using magnet… ▽ More Tunneling spectroscopy and its evolution are crucial for elucidating the intricate electronic structure and emergent phenomena in quantum materials.Nevertheless, high-quality measurements -- specifically those tracking evolution across temperature and external fields -- remain a formidable challenge. We have fabricated a high-quality $\mathrm{TaO}_x$-based planar tunneling junction by using magnetron sputtering for van der Waals (vdW) superconductors. Using the vdW superconductor $\mathrm{Bi}_2\mathrm{Sr}_2\mathrm{CaCu}_2\mathrm{O}_{8+δ}$ (Bi2212) as a benchmark, this platform yields high-quality tunneling spectra, reproducing the electronic signatures obtained from scanning tunneling spectra acquired from atomically clean surfaces under ultra-high vacuum conditions. This architecture enables high-precision spectroscopy across extensive temperature and magnetic field ranges, offering a universal strategy for probing the electronic structures of diverse two-dimensional systems and facilitating future explorations of material properties. △ Less

Submitted 20 May, 2026; originally announced May 2026.

Observation of field-odd and field-free superconducting diode effects in $\mathrm{Mo}_2\mathrm{C}$ nanoflakes

Authors: Wei Gao, Kaixuan Fan, Menghan Li, Jinhao Cheng, Peng Zhu, Qing Zhang, Shuaishuai Ding, Wenping Hu, Fan Yang, Dechao Geng, Hechen Ren

Abstract: The superconducting diode effect (SDE) enables nonreciprocal supercurrent flow, holding immense potential for ultra-low-power quantum electronics. Intrinsic SDE typically requires materials with inherent symmetry breakings. Here, we report the discovery of SDE in chemical vapor deposition-grown molybdenum carbide ($\mathrm{Mo}_2\mathrm{C}$) nanoflakes, a material traditionally considered centrosym… ▽ More The superconducting diode effect (SDE) enables nonreciprocal supercurrent flow, holding immense potential for ultra-low-power quantum electronics. Intrinsic SDE typically requires materials with inherent symmetry breakings. Here, we report the discovery of SDE in chemical vapor deposition-grown molybdenum carbide ($\mathrm{Mo}_2\mathrm{C}$) nanoflakes, a material traditionally considered centrosymmetric. Strikingly, this system uniquely hosts both field-odd and field-free SDEs. Transport measurements reveal a field-odd SDE with tunable efficiency exceeding 40% at 4 K under a perpendicular in-plane magnetic field. In a separate sample, a robust field-free SDE persists under zero-field and field-coolings. Out-of-plane field sweeps confirm the intrinsic nature of these phenomena. We propose that domain-boundary supercurrents or charge density wave-like orders drive this unexpected combination of symmetry breakings. Our findings establish air-stable $\mathrm{Mo}_2\mathrm{C}$ as an ideal platform for nonreciprocal superconducting electronics operating at liquid-helium temperatures, expanding the search for SDE into nominally centrosymmetric superconductors. △ Less

Submitted 4 May, 2026; v1 submitted 21 April, 2026; originally announced April 2026.

Programmable, Spontaneous Superlattice Memory in a Monolayer Topological Insulator

Authors: Jian Tang, Thomas Siyuan Ding, Shuhan Ding, Jiangxu Li, Changjiang Yi, Tianxing Tang, Zumeng Huang, Xuehao Wu, Zhiheng Huang, Birender Singh, Tiema Qian, Vsevolod Belosevich, Mingyang Guo, Anyuan Gao, Nikolai Peshcherenko, Zhe Sun, Mohamed Shehabeldin, Kenji Watanabe, Takashi Taniguchi, Abhay N. Pasupathy, Claudia Felser, Kenneth S. Burch, Ni Ni, Yao Wang, Yang Zhang , et al. (2 additional authors not shown)

Abstract: Memory is a foundational concept across disciplines, from neurobiology and electronics to artificial intelligence and quantum gravity. In materials, memory effects typically arise from ferroic orders, such as ferroelectricity and ferromagnetism, where information is stored in charge or spin degrees of freedom. Here, we report a surprising discovery of a nonvolatile superlattice memory effect in mo… ▽ More Memory is a foundational concept across disciplines, from neurobiology and electronics to artificial intelligence and quantum gravity. In materials, memory effects typically arise from ferroic orders, such as ferroelectricity and ferromagnetism, where information is stored in charge or spin degrees of freedom. Here, we report a surprising discovery of a nonvolatile superlattice memory effect in monolayer TaIrTe4, a dual quantum spin Hall insulator, where information is encoded through sharply contrasting lattice periodicities. In particular, in a pristine monolayer, we observe the spontaneous emergence of a long-period superlattice that can be programmed ON and OFF in a nonvolatile manner by electrostatic tuning of low-energy electronic states. This switching toggles the system between two structural configurations with unit cell areas differing by nearly two orders of magnitude. Mechanistically, our results reveal two independent and distinct instabilities, one in the lattice and the other in the QSH electrons, which are coupled, leading to electrostatic control of lattice configurations with nonvolatile memory. This finding is enabled by combining linear and nonlinear transport measurements, Raman spectroscopy, and scanning tunneling microscopy, which probe complementary aspects of the underlying orders. Remarkably, this nonvolatile memory effect stabilizes a spontaneous superlattice with a periodicity on the few-nanometer scale that remains robust across a wide doping range, persists over days, and survives above 70 K. Combined with the QSH topology, this stability offers a promising route to nonvolatile memory control of topological flat bands and their filling enabled quantum states. Our preliminary data indeed show the emergence of new insulating states at fractional superlattice fillings, which can be clearly switched ON and OFF together with the superlattice. △ Less

Submitted 19 March, 2026; originally announced March 2026.

Comments: 33 pages, 16 figures, submitted version

Generating unconventional spin-orbit torques with patterned phase gradients in tungsten thin films

Authors: Lauren J. Riddiford, Anne Flechsig, Shilei Ding, Emir Karadza, Niklas Kercher, Tobias Goldenberger, Elisabeth Müller, Pietro Gambardella, Laura J. Heyderman, Aleš Hrabec

Abstract: A key aim in spintronics is to achieve current-induced magnetization switching via spin-orbit torques without external magnetic fields. For this, the focus of recent work has been on introducing controlled lateral gradients across ferromagnet/heavy-metal devices, giving variations in thickness, composition, or interface quality. However, the small gradients achievable with common growth techniques… ▽ More A key aim in spintronics is to achieve current-induced magnetization switching via spin-orbit torques without external magnetic fields. For this, the focus of recent work has been on introducing controlled lateral gradients across ferromagnet/heavy-metal devices, giving variations in thickness, composition, or interface quality. However, the small gradients achievable with common growth techniques limit both the impact of this approach and understanding of the underlying physical mechanisms. Here, spin-orbit torques are patterned on a mesoscopic length scale in tungsten thin films using direct-write laser annealing. Through transmission electron microscopy, resistivity, and second harmonic measurements, the continuous transformation of the crystalline phase of W films from the highly spin-orbit coupled, high resistivity $β$ phase to the minimally spin-orbit coupled, low resistivity $α$ phase is tracked with increasing laser fluence. Gradients with different steepness are patterned in the tungsten phase to create spin-orbit torque channels and, when interfaced with CoFeB, tungsten wires with a sufficiently strong gradient can switch the magnetization without an applied magnetic field. Therefore, exploiting the unique microstructure of mixed-phase W allows precise control of the local electronic current density and direction, as well as local spin-orbit torque efficiency, providing a new avenue for the design of efficient spintronic devices. △ Less

Submitted 4 January, 2026; originally announced January 2026.

Comments: 26 pages, 4 figures, 7 supplementary figures

Probing spatially resolved spin density correlations with trapped excitons

Authors: Shanshan Ding, Jose Antonio Valerrama Botia, Aleksi Julku, Zhigang Wu, G. M. Bruun

Abstract: The rapidly growing class of atomically thin and tunable van der Waals materials is intensely investigated both in the context of fundamental science and for new technologies. There is in this connection a widespread need for new ways to probe the electronic properties of these layered materials, since their two-dimensional (2D) character make conventional probes less efficient. Here, we show how… ▽ More The rapidly growing class of atomically thin and tunable van der Waals materials is intensely investigated both in the context of fundamental science and for new technologies. There is in this connection a widespread need for new ways to probe the electronic properties of these layered materials, since their two-dimensional (2D) character make conventional probes less efficient. Here, we show how excitons trapped in a moiré lattice can be used as an optical probe for spatially resolved electron spin density correlations in such materials. The electrons in the material of interest virtually tunnel to the moiré lattice where they scatter on the excitons after which they tunnel back. This gives rise to an effective spin-dependent and spatially localised potential felt by the electrons, which in turn leads to energy shifts that can be measured spectroscopically in the exciton spectrum. Using second order perturbation theory combined with a solution to the exciton-electron scattering problem, we show that the electrons mediate an interaction between two excitons resulting in an energy shift proportional to their two-point spin density-density correlation function evaluated at the exciton positions. We then discuss two specific applications of our setup. First, we show that quantum phase transitions between different in-plane anti-ferromagnetic orders in a 2D lattice give rise to large and measurable shifts in the exciton spectrum in the critical regions. Second, we analyse how different pairing symmetries of superconducting phases can be probed. This demonstrates that our scheme opens up new ways to probe electron spin density correlations, which is a key property of many quantum phases predicted to exist in the new 2D materials. △ Less

Submitted 16 December, 2025; originally announced December 2025.

Comments: main text: 7 pages, 6 figures

Journal ref: Phys. Rev. B 113, 165428 (2026)

Unidirectional magnetoresistance driven by nonequilibrium antiferromagnetic magnons

Authors: Xue He, Hans Gløckner Giil, Caiqiong Xu, Jicheng Wang, Arne Brataas, Jinbo Yang, Yanglong Hou, Rui Wu, Shilei Ding

Abstract: Magnetoresistive effects are typically symmetric under magnetization reversal. However, nonlinear spin transport can give rise to unidirectional magnetoresistance in systems with strong spin-orbit interaction and broken inversion symmetry. Here, we demonstrate that the nonequilibrium magnon accumulation characterized by a finite magnon chemical potential can lead to a large and robust magnonic uni… ▽ More Magnetoresistive effects are typically symmetric under magnetization reversal. However, nonlinear spin transport can give rise to unidirectional magnetoresistance in systems with strong spin-orbit interaction and broken inversion symmetry. Here, we demonstrate that the nonequilibrium magnon accumulation characterized by a finite magnon chemical potential can lead to a large and robust magnonic unidirectional spin Hall magnetoresistance (USMR) in the weakly coupled van der Waals antiferromagnet CrPS4 in contact with Pt. Unlike conventional magnonic USMR driven by magnetization fluctuations, this effect persists under strong magnetic fields and low temperatures, with a pronounced peak near the spin-flip transition. The magnitude of magnonic USMR in CrPS4/Pt exceeds that of YIG/Pt by more than two orders of magnitude and surpasses the electrical USMR in metallic Ta/Co bilayers by a factor of two. The observed field and temperature dependence indicates that spin transport is dominated by magnon chemical potential gradients rather than thermal- or fluctuation-driven magnon generation. These findings establish a new mechanism for nonlinear magnetoresistance in antiferromagnetic van der Waals heterostructures and open a route to magnon-based antiferromagnetic spintronic functionalities in two-terminal device geometries. △ Less

Submitted 28 January, 2026; v1 submitted 12 December, 2025; originally announced December 2025.

Comments: 21 pages, 4 figures

Witnessing Spin-Orbital Entanglement using Resonant Inelastic X-Ray Scattering

Authors: Zecheng Shen, Shuhan Ding, Zijun Zhao, Francesco A. Evangelista, Yao Wang

Abstract: Entanglement plays a central role in quantum technologies, yet its characterization and control in materials remain challenging. Recent developments in spectrum-based entanglement witnesses have enabled new strategies for quantifying many-body entanglement in macroscopic materials. Here, we develop a protocol for detecting spin-orbital entanglement using experiment-accessible resonant inelastic x-… ▽ More Entanglement plays a central role in quantum technologies, yet its characterization and control in materials remain challenging. Recent developments in spectrum-based entanglement witnesses have enabled new strategies for quantifying many-body entanglement in macroscopic materials. Here, we develop a protocol for detecting spin-orbital entanglement using experiment-accessible resonant inelastic x-ray scattering (RIXS). Central to our approach is the construction of a Hermitian generator from experimentally measurable spectra, which allows us to compute the quantum Fisher information (QFI) available in spin--orbital systems. The resulting QFI provides upper bounds for $k$-producible states and thus serves as a robust witness of spin-orbital entanglement. To account for realistic experimental limitations, we further extend our framework to include relaxed QFI bounds applicable to measurements lacking full polarization resolution. △ Less

Submitted 14 June, 2026; v1 submitted 7 December, 2025; originally announced December 2025.

Comments: 6 pages, 3 figures

Journal ref: Phys. Rev. Lett. 136, 116504 (2026)

Asymmetric quantum Hall effect and diminished $ν=0$ longitudinal resistance in graphene/InSe heterostructures

Authors: Wenxue He, Shijin Li, Jinhao Cheng, Yingpeng Zhang, Kaixuan Fan, Jiabo Liu, Shuaishuai Ding, Wenping Hu, Fan Yang, Chen Wang, Qing-Feng Sun, Hechen Ren

Abstract: We investigate quantum transport in graphene/InSe heterostructures and find major asymmetries in the longitudinal resistance ($R_{xx}$) and vanishing $R_{xx}$ peaks at high magnetic fields, particularly at the charge-neutrality point. Our Landauer-Buttiker analysis and numerical simulations show that a monotonically varying density gradient combined with a full equilibration mechanism can explain… ▽ More We investigate quantum transport in graphene/InSe heterostructures and find major asymmetries in the longitudinal resistance ($R_{xx}$) and vanishing $R_{xx}$ peaks at high magnetic fields, particularly at the charge-neutrality point. Our Landauer-Buttiker analysis and numerical simulations show that a monotonically varying density gradient combined with a full equilibration mechanism can explain these phenomena. Our results also suggest the presence of trivial long-range chiral edge current and offer a broadly applicable way to engineer transport properties in quantum Hall systems. △ Less

Submitted 27 November, 2025; originally announced November 2025.

Collective Magnetic Excitations in a Photo-excited Electron-doped Cuprate Superconductor

Authors: Daniel Jost, Jiarui Li, Jordyn Hales, Jonathan Sobota, Giacomo Merzoni, Leonardo Martinelli, Shuhan Ding, Kejun Xu, Justine Schlappa, Andreas Scherz, Robert Carley, Benjamin E. Van Kuiken, Teguh C. Asmara, Le Phuong Hoang, Laurent Mercadier, Sergii Parchenko, Martin Teichmann, Patrick S. Kirchmann, Giacomo Ghiringhelli, Brian Moritz, Zhi-Xun Shen, Thomas P. Devereaux, Yao Wang, Wei-Sheng Lee

Abstract: Elucidating the microscopic behavior of cuprates under ultrafast photoexcitation offers critical insights into their highly correlated out-of-equilibrium states. Although quasiparticle dynamics have been investigated extensively, the behavior of collective magnetic excitations remains comparatively unexplored. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-… ▽ More Elucidating the microscopic behavior of cuprates under ultrafast photoexcitation offers critical insights into their highly correlated out-of-equilibrium states. Although quasiparticle dynamics have been investigated extensively, the behavior of collective magnetic excitations remains comparatively unexplored. Here, we use time-resolved resonant inelastic X-ray scattering (trRIXS) at the Cu $L_3$-edge to track the collective magnetic excitations (paramagnons) in an optimally electron-doped cuprate driven out-of-equilibrium by a femtosecond pump laser pulse. Upon pumping, we observed an anti-Stokes signal associated with paramagnon generation, which modifies the paramagnon dispersion near the zone center, although the bandwidth remained unchanged. Moreover, the spectral weight exhibits a momentum-dependent variation across the Brillouin zone. The light-driven boost of the paramagnon population and the resulting spectral-weight transfer could provide new leverage to manipulate the properties of cuprates. △ Less

Submitted 14 July, 2026; v1 submitted 26 November, 2025; originally announced November 2025.

Comments: Accepted in Physical Review Letters

Acoustic orbital Hall effect and orbital pumping in light-metal-ferromagnet bilayers

Authors: Mingxing Wu, Shilei Ding, Hiroki Matsumoto, Pietro Gambardella

Abstract: Orbital currents provide a new degree of freedom for controlling magnetism, yet their interaction with lattice dynamics remains largely unexplored. Here we report a systematic investigation of the acoustic orbital Hall effect in light metals such as Ti and Cr, where surface acoustic waves generate orbital currents through phonon-orbital coupling. The acoustic orbital current in Ti exhibits higher… ▽ More Orbital currents provide a new degree of freedom for controlling magnetism, yet their interaction with lattice dynamics remains largely unexplored. Here we report a systematic investigation of the acoustic orbital Hall effect in light metals such as Ti and Cr, where surface acoustic waves generate orbital currents through phonon-orbital coupling. The acoustic orbital current in Ti exhibits higher efficiency and longer diffusion length compared to the acoustic spin current in Pt. The sign and magnitude of the rectified acoustic voltages in nonmagnetic (Ti, Cr)/ferromagnetic (Ni, Co, Fe$_x$Co$_{1-x}$) bilayers are determined by the product of orbital-to-spin conversion and magnetoelastic coupling efficiencies of the ferromagnet. Additionally, we find evidence for acoustic orbital pumping, whereby the excitation of ferromagnetic resonance by surface acoustic waves injects an orbital current from the ferromagnet into the nonmagnet. These results establish lattice dynamics as an efficient driver of orbital transport, opening opportunities for low-dissipation orbitronic devices that harness and sense phonons. △ Less

Submitted 4 November, 2025; originally announced November 2025.

Comments: 18 pages, 9 figures

Sliding-tuned Quantum Geometry in Moiré Systems: Nonlinear Hall Effect and Quantum Metric Control

Authors: Shi-Ping Ding, Miao Liang, Tian-Le Wu, Meng-Hao Wu, Jing-Tao Lü, Jin-Hua Gao, X. C. Xie

Abstract: Sliding is a ubiquitous phenomenon in moiré systems, but its direct influence on moiré bands, especially in multi-twist moiré systems, has been largely overlooked to date. Here, we theoretically show that sliding provides a unique pathway to engineer the quantum geometry (Berry curvature and quantum metric) of moiré bands, exhibiting distinct advantages over conventional strategies. Specifically,… ▽ More Sliding is a ubiquitous phenomenon in moiré systems, but its direct influence on moiré bands, especially in multi-twist moiré systems, has been largely overlooked to date. Here, we theoretically show that sliding provides a unique pathway to engineer the quantum geometry (Berry curvature and quantum metric) of moiré bands, exhibiting distinct advantages over conventional strategies. Specifically, we first suggest alternating twisted trilayer $\mathrm{MoTe_2}$ (AT3L-$\mathrm{MoTe_2}$) and chirally twisted triple bilayer graphene (CT3BLG) as two ideal paradigmatic systems for probing sliding-engineered quantum geometric phenomena. Then, two sliding-induced exotic quantum geometry phenomena are predicted: (1) an intrinsic nonlinear Hall effect via sliding-produced non-zero Berry curvature dipole, with CT3BLG as an ideal platform; (2) significant quantum metric modulation in AT3L-$\mathrm{MoTe_2}$, enabling tests of quantum geometric criteria for fractional Chern insulating state (FCIS). Our work establishes sliding as a new degree of freedom for manipulating quantum geometry of moiré bands, which emerges as a signature phenomenon of multi-twist moiré systems. △ Less

Submitted 10 September, 2025; originally announced September 2025.

Comments: 6 pages, 4 figures

Theory of Localized States in Quasiperiodic Lattices

Authors: Jin-Rong Chen, Xin-Yu Guo, Shi-Ping Ding, Tian-Le Wu, Miao Liang, Jin-Hua Gao, X. C. Xie

Abstract: The physics of localized states in quasiperiodic lattices has been extensively studied for decades, but still lacks an comprehensive theoretical framework. Recently, we developed a incommensurate energy band (IEB) theory, which extends the concept of energy bands to quasiperiodic systems lacking translational symmetry, thereby achieving a breakthrough in elucidating extended states. Here, we demon… ▽ More The physics of localized states in quasiperiodic lattices has been extensively studied for decades, but still lacks an comprehensive theoretical framework. Recently, we developed a incommensurate energy band (IEB) theory, which extends the concept of energy bands to quasiperiodic systems lacking translational symmetry, thereby achieving a breakthrough in elucidating extended states. Here, we demonstrate that, due to the inherent duality between momentum and real space, the IEB theory also offers a comprehensive framework for elucidating localized states. Specifically, via a so-called spiral (module) mapping, the energy spectrum of localized states can be represented as a function defined on a compact circular manifold-akin to the Brillouin zone-whose form resembles conventional energy bands. These localized state energy bands (LSEBs) fully characterize all the properties of the localized states. Moreover, we show that quasiperiodic systems with mobility edges exhibit a unique hybrid band structure: the IEB for extended states (momentum space) and LSEB for localized states (real space), separated by mobility edges. Our theory thus establishes a comprehensive framework for analyzing the localized states in quasiperiodic lattices. △ Less

Submitted 7 September, 2025; originally announced September 2025.

Comments: 6 pages, 3 figures

Ultracoherent self-assembled diamond nanomechanics reveals superfluid dynamics

Authors: Guanhao Huang, Chang Jin, Sophie Weiyi Ding, Chaoshen Zhang, Aaron M. Day, Tobias Elbs, Neil Sinclair, Sukhad Dnyanesh Joshi, Rodrick Kuate Defo, Bertrand I. Halperin, Evelyn Hu, Marko Lončar

Abstract: From gravitational-wave detection, protein force microscopy, to exploration of quantum-classical boundaries, many anticipated discoveries in fundamental science require improving measurement sensitivity limits. Through the fluctuation-dissipation theorem, mechanical dissipation sets the acoustic noise for this limit. Yet, even in high-purity crystals, the microscopic mechanisms responsible for the… ▽ More From gravitational-wave detection, protein force microscopy, to exploration of quantum-classical boundaries, many anticipated discoveries in fundamental science require improving measurement sensitivity limits. Through the fluctuation-dissipation theorem, mechanical dissipation sets the acoustic noise for this limit. Yet, even in high-purity crystals, the microscopic mechanisms responsible for the acoustic loss remain poorly understood. Tension-induced dissipation dilution offers a route to ultralow acoustic loss, but is challenging to implement in crystalline materials including single-crystal diamond. Here we realize a strain-engineered diamond nanomechanical platform using a liquid-assisted van der Waals self-assembly process that harnesses intrinsic surface forces to apply tensile stress exceeding 1 GPa. At cryogenic temperatures these resonators achieve quality factors beyond 10 billion (intrinsic material quality factors beyond 100 million). This exceptional coherence turns them into a sensitive probe for residual dissipation, elucidating three distinct two-level-system channels and one topological dissipation channel from a surface superfluid helium film. Our work shows how advancing mechanical coherence opens access to new regimes of physics in hybrid quantum systems, precision metrology, and condensed-matter physics. △ Less

Submitted 12 February, 2026; v1 submitted 1 July, 2025; originally announced July 2025.

Comments: 35 pages, 16 figures

Interaction-Driven Topological Transitions in Monolayer TaIrTe$_4$

Authors: Jiangxu Li, Jian Tang, Louis Primeau, Thomas Siyuan Ding, Rahul Soni, Tiema Qian, Kenji Watanabe, Takashi Taniguchi, Ni Ni, Adrian Del Maestro, Qiong Ma, Yang Zhang

Abstract: Discovering materials that combine topological phenomena with correlated electron behavior is a central pursuit in quantum materials research. Monolayer TaIrTe$_4$ has recently emerged as a promising platform in this context, hosting robust quantum spin Hall insulator (QSHI) phases both within a single-particle gap and within a correlation-induced gap arising from van Hove singularities (vHSs), ac… ▽ More Discovering materials that combine topological phenomena with correlated electron behavior is a central pursuit in quantum materials research. Monolayer TaIrTe$_4$ has recently emerged as a promising platform in this context, hosting robust quantum spin Hall insulator (QSHI) phases both within a single-particle gap and within a correlation-induced gap arising from van Hove singularities (vHSs), accessed via electrostatic doping. Its intrinsic monolayer nature offers exceptional tunability and the potential to realize a versatile array of interaction-driven topological phases. In this work, we combine theory and experiment to map the phase landscape of monolayer TaIrTe$_4$. Using Hartree-Fock calculations, we investigate the interaction-driven phase diagram near the vHSs under commensurate filling conditions. By systematically tuning the dielectric screening and strain, we uncover a rich set of ground states--including QSHI, trivial insulator, higher-order topological insulator, and metallic phase--among which are interaction-driven topological phase transitions. Experimentally, we perform both local and nonlocal transport measurements across a broad set of devices, which--due to unavoidable strain variations during fabrication-realize several phases consistent with theoretical predictions. Together, our results lay the groundwork for understanding correlation-driven topological phenomena in TaIrTe$_4$ and open new directions for engineering exotic quantum phases in low-dimensional materials beyond the limitations of moiré superlattices. △ Less

Submitted 23 June, 2025; originally announced June 2025.

Comments: 12+18 pages, 8+14 figures

Orbital Pumping in Ferrimagnetic Insulators

Authors: Hanchen Wang, Min-Gu Kang, Davit Petrosyan, Shilei Ding, Richard Schlitz, Lauren J. Riddiford, William Legrand, Pietro Gambardella

Abstract: We report the detection of pure orbital currents generated by both coherent and thermal magnons in the magnetic insulator Bi-doped yttrium iron garnet (BiYIG). The pumping of orbital and spin currents is jointly investigated in nano-devices made of naturally oxidized Cu, pure Cu, Pt, and Cr. The absence of charge conduction in BiYIG and the negligible spin-to-charge conversion of oxidized Cu allow… ▽ More We report the detection of pure orbital currents generated by both coherent and thermal magnons in the magnetic insulator Bi-doped yttrium iron garnet (BiYIG). The pumping of orbital and spin currents is jointly investigated in nano-devices made of naturally oxidized Cu, pure Cu, Pt, and Cr. The absence of charge conduction in BiYIG and the negligible spin-to-charge conversion of oxidized Cu allows us to disambiguate the orbital current contribution. Comparative measurements on YIG and BiYIG show that the origin of the orbital pumping in BiYIG/oxidized Cu is the dynamics of the orbital magnetization in the magnetic insulator. In Cr, the pumping signal is dominated by the negative spin Hall effect rather than the positive orbital Hall effect, indicating that orbital currents represent a minority of the total angular momentum current pumped from the magnetic insulator. Our results also evidence that improving the interfacial transparency significantly enhances pumping efficiencies not only for spin, but also for orbital currents. △ Less

Submitted 13 June, 2025; originally announced June 2025.

Journal ref: Physical Review Letters 134, 126701 (2025)

Quantum spin Hall effects in van der Waals materials

Authors: Jian Tang, Thomas Siyuan Ding, Chengdong Wang, Ning Mao, Vsevolod Belosevich, Yang Zhang, Xiaofeng Qian, Qiong Ma

Abstract: The quantum spin Hall (QSH) effect, first predicted in graphene by Kane and Mele in 2004, has emerged as a prototypical platform for exploring spin-orbit coupling, topology, and electronic interactions. Initially realized experimentally in quantum wells exhibiting characteristic QSH signatures, the field has since expanded with the discovery of van der Waals (vdW) materials. This review focuses on… ▽ More The quantum spin Hall (QSH) effect, first predicted in graphene by Kane and Mele in 2004, has emerged as a prototypical platform for exploring spin-orbit coupling, topology, and electronic interactions. Initially realized experimentally in quantum wells exhibiting characteristic QSH signatures, the field has since expanded with the discovery of van der Waals (vdW) materials. This review focuses on vdW systems, which offer unique advantages: their exposed surfaces enable a combination of surface-sensitive spectroscopic and microscopic tools for comprehensive detection of the QSH state; mechanical stacking with other vdW layers facilitates symmetry engineering and proximity effects; and moiré engineering introduces layer skyrmion topological phases and strong correlation effects. We highlight two monolayer families, 1T$^\prime$-MX$_2$ and MM$^\prime$X$_4$, represented by WTe$_2$ and TaIrTe$_4$, respectively. These materials exhibit QSH phases intertwined with or in close proximity to other quantum phases, such as excitonic insulators, charge density waves, and superconductivity. Their low crystal symmetry and topology enable rich quantum geometrical responses, ranging from nonlinear Hall effects to circular photogalvanic effects. We also discuss moiré systems, which combine topology with flatband physics and enhanced correlations, driving spontaneous symmetry breaking and transitions from QSH to quantum anomalous Hall (QAH) states. Remarkably, fractionalized QAH and QSH states have recently been observed in moiré systems, significantly advancing the field of condensed matter physics. Finally, we explore emerging applications of QSH and derived materials, such as using nonlinear Hall effects for quantum rectification in microwave energy harvesting and harnessing fractional anomalous states for topological quantum computing. △ Less

Submitted 23 May, 2025; originally announced May 2025.

Comments: 52 pages; 12 figures; Invited review, comments are welcome

Reconfigurable Room Temperature Exchange Bias through Néel Order Switching in van der Waals Heterostructures

Authors: Jicheng Wang, Shilei Ding, Bei Ding, Zhipeng Hou, Licong Peng, Yilan Jiang, Fengshan Zheng, Zhaochu Luo, Yu Ye, Jinbo Yang, Yanglong Hou, Rui Wu

Abstract: Exchange bias effect plays a crucial role in modern magnetic memory technology. Recently, van der Waals magnetic materials have emerged and shown potential in spintronic devices at atomic scale. Owing to their tunable physical properties and the flexibility in fabrication, the van der Waals heterostructures offer more possibilities for investigating potential mechanisms of the exchange bias effect… ▽ More Exchange bias effect plays a crucial role in modern magnetic memory technology. Recently, van der Waals magnetic materials have emerged and shown potential in spintronic devices at atomic scale. Owing to their tunable physical properties and the flexibility in fabrication, the van der Waals heterostructures offer more possibilities for investigating potential mechanisms of the exchange bias effect. However, due to low magnetic ordering temperatures for most van der Waals magnets, to establish exchange bias in van der Waals antiferromagnet/ferromagnet heterostructures at room temperature is challenging. In this study, we fabricate (Fe$_{0.56}$Co$_{0.44}$)$_{5}$GeTe$_{2}$(FCGT)/Fe$_{3}$GaTe$_{2}$(FGaT) heterostructures with magnetic ordering temperatures of each component well above room temperature to achieve a room temperature exchange bias effect. It is found that the sign and magnitude of the exchange bias field can be efficiently controlled by manipulating the Néel order of FCGT with magnetic field. The manipulation of Néel order shows significant magnetic field dependence. A strong pre-set field induces a switch in the Néel order of FCGT, which aligns the interfacial magnetization at the FCGT/FGaT interface, leading to robust exchange bias, as revealed by both transport measurements and macro-spin model calculations. Our findings demonstrate the intrinsic manipulation and switchable of room-temperature exchange bias in all-van der Waals heterostructures and further promote the development of novel two-dimensional spintronic devices. △ Less

Submitted 7 May, 2025; originally announced May 2025.

Comments: 24 pages, 4 figures

Indirect Tunneling Enabled Spontaneous Time-Reversal Symmetry Breaking and Josephson Diode Effect in TiN/Al$_2$O$_3$/Hf$_{0.8}$Zr$_{0.2}$O$_2$/Nb tunnel junctions

Authors: Shaoqing Ding, Jinyuan Yao, Zhen Bi, Quyen Tran, Bangzhi Liu, Qi Li, Susan Trolier-McKinstry, Thomas N. Jackson, Ying Liu

Abstract: Josephson diode (JD) effect found in Josephson tunnel junctions (JTJs) has attracted a great deal of attention due to its importance for developing superconducting circuitry based quantum technologies. So far, the highly desirable electrical control of the JD effect has not been demonstrated in any JTJ prepared by techniques used in semiconductor industry. We report the fabrication of JTJs featuri… ▽ More Josephson diode (JD) effect found in Josephson tunnel junctions (JTJs) has attracted a great deal of attention due to its importance for developing superconducting circuitry based quantum technologies. So far, the highly desirable electrical control of the JD effect has not been demonstrated in any JTJ prepared by techniques used in semiconductor industry. We report the fabrication of JTJs featuring a composite tunnel barrier of Al$_2$O$_3$ and Hf$_{\mathrm{0.8}}$Zr$_\mathrm{0.2}$O$_2$ prepared by complementary-metal-oxide-semiconductor (CMOS) compatible atomic layer deposition (ALD). These JTJs were found to show the JD effect in nominally zero magnetic fields with the nonreciprocity controllable using an electric training current, yielding a surprisingly large diode efficiency not achieved previously. The quasiparticle tunneling, through which the Josephson coupling in a JTJ is established, was found to show no nonreciprocity. We attribute these observations to the simultaneous presence of positive and negative Josephson couplings, with the latter originating from indirect tunneling. The resulted spontaneous time-reversal symmetry breaking and the double-minima washboard potential for the ensemble averaged phase difference in the resistively and capacitively shunted junction (RCSJ) model are shown to fully account for the experimentally observed JD effect. △ Less

Submitted 12 December, 2025; v1 submitted 23 April, 2025; originally announced April 2025.

Tailoring spin reorientation and magnetic interaction for room-temperature spintronics in Tb-Doped SmFeO3 single crystal

Authors: Mingzhu Xue, Xin Li, Shilei Ding, Qixin Li, Wenhao Di, Anhua Wu, Bin He, Shishen Yan, Wenyun Yang, Jinbo Yang

Abstract: Selective doping with different R-site ions in rare-earth perovskite RFeO3 compounds offers an effective way to achieve atomic-scale tuning of the complex exchange interactions. In this study, the spin reorientation temperature of Tb-doped SmFeO3 (Sm0.7Tb0.3FeO3) single crystal is lowered to approximately 350 K, making it more suitable for room-temperature applications. Notably, the magnetic compe… ▽ More Selective doping with different R-site ions in rare-earth perovskite RFeO3 compounds offers an effective way to achieve atomic-scale tuning of the complex exchange interactions. In this study, the spin reorientation temperature of Tb-doped SmFeO3 (Sm0.7Tb0.3FeO3) single crystal is lowered to approximately 350 K, making it more suitable for room-temperature applications. Notably, the magnetic compensation point is absent at low temperatures, and both R3+ and Fe3+ ion moments can be fully saturated under high magnetic fields, suggesting that Tb3+ doping drives the R3+ and Fe3+ sublattices toward ferromagnetic coupling. Moreover, the hysteresis loop along the a-axis transitions from a double triangle shape below the spin reorientation temperature to a rectangular shape above the spin reorientation temperature, and the nucleation field exhibits a strong dependence on both the measurement temperature and the maximum applied magnetic field. Above results can be explained by a modified two-domain model with mean field correction. The results of magnetic domain measurements indicate that the emergence of the double-triangular hysteresis loop is jointly determined by domain wall motion and the nonlinear response of the parasitic magnetic moment along the a-axis. These findings provide valuable insights and new materials for advancing the use of RFeO3 compounds in spintronics. △ Less

Submitted 29 March, 2025; originally announced March 2025.

Journal ref: Appl. Phys. Lett. 127, 012403 (2025)

Sustainable wafer-scale integration of epitaxial ZnO on silicon for piezoelectric devices

Authors: D. Sanchez-Fuentes, R. Desgarceaux, A. Rahal, L. Garcia, S. Bousri, S. Ding, N. Camara, F. Pascal, R. Garcia-Bermejo, N. Guillaume, G. Ardila, J. Gazquez, C. Magen, S Plana-Ruiz, C. Guasch, A. Carretero-Genevrier

Abstract: To sustainably support the ongoing energetic transition, we need functional metal oxides capable of converting energy, and produce storage, and sensing devices. However, these materials suffer from a high economic cost of manufacturing, and their production in a sustainable way is, to date, a milestone. Additionally, the technical challenges, such as scalability and integration of silicon for indu… ▽ More To sustainably support the ongoing energetic transition, we need functional metal oxides capable of converting energy, and produce storage, and sensing devices. However, these materials suffer from a high economic cost of manufacturing, and their production in a sustainable way is, to date, a milestone. Additionally, the technical challenges, such as scalability and integration of silicon for industrial processing using microelectronic technologies, impose strict conditions for the entire materials process. In this work, we engineer α-quartz virtual substrates up to 4 inches facilitating the large-scale and sustainable integration of multifunctional epitaxial ZnO metal oxide microwire films on silicon. These materials are exclusively manufactured on silicon using solution chemistry, providing single-chip solutions that can meet strict economic constraints for developing sustainable devices at a lower cost. Through this integrative technology, we demonstrate the microfabrication of epitaxial (110)ZnO/(100)α-quartz/(100)silicon piezoelectric membrane resonators at the wafer-scale with potential applications in energy conversion and sensing. We combined four dimensional (4D)-STEM diffraction technology and Piezoelectric Force Microscopy (PFM) to establish a correlation between out of plane crystalline strain and piezoelectric response in epitaxial (110) ZnO at the microscale. Finally, we proved the fabrication of 800 nm thick (110) ZnO suspended membranes that can be transferred to flexible substrates, making them suitable for flexible devices. △ Less

Submitted 18 March, 2025; originally announced March 2025.

Comments: 40, 8 figures

Interaction-induced reentrance of Bose glass and quench dynamics of Bose gases in twisted bilayer and quasicrystal optical lattices

Authors: Shi-Hao Ding, Li-Jun Lang, Qizhong Zhu, Liang He

Abstract: We investigate the ground-state and dynamical properties of ultracold Bose gases in optical lattices with a quasicrystal structure, inspired by recent experiments on twisted bilayer and quasicrystalline optical lattices. The interplay between on-site repulsive interactions and the quasiperiodic potential leads to rich physics. At low filling factors, increasing the interaction strength induces a d… ▽ More We investigate the ground-state and dynamical properties of ultracold Bose gases in optical lattices with a quasicrystal structure, inspired by recent experiments on twisted bilayer and quasicrystalline optical lattices. The interplay between on-site repulsive interactions and the quasiperiodic potential leads to rich physics. At low filling factors, increasing the interaction strength induces a delocalization effect that transforms a Bose-glass (BG) phase-characterized by disconnected superfluid (SF) regions-into a robust SF phase with a percolated network of SF clusters. This transition is quantitatively identified via the percolation probability. At higher filling factors, we uncover a reentrant behavior: with increasing interaction, the system first changes from BG to SF, but further strengthening reverses the trend, restoring the BG phase. This reentrance originates from an interaction-driven rearrangement of particles, where a percolated SF network fragments into isolated SF islands as repulsion dominates. The quench dynamics show distinct transient features: intraphase quenches cause minor variations in the percolation probability and the inverse participation ratio (IPR), while interphase quenches produce strong responses. In particular, an SF-to-BG quench exhibits an abrupt loss of global SF connectivity, whereas a BG-to-SF quench shows oscillatory percolation and a gradual IPR decrease, stabilizing the SF phase. These results elucidate the competition between quasiperiodicity and interactions in ultracold Bose gases and offer insights relevant to current experiments with twisted bilayer and quasicrystal optical lattices. △ Less

Submitted 4 November, 2025; v1 submitted 5 March, 2025; originally announced March 2025.

Comments: 11 pages, 5 figures

Journal ref: Phys. Rev. A 112, 033322 (2025)

Induced Interactions and Bipolarons in Spin-Orbit Coupled Bose-Einstein Condensates

Authors: Zhe Yang, Shanshan Ding, Qizhong Zhu

Abstract: Impurities immersed in a Bose-Einstein condensate (BEC) can interact indirectly through the exchange of Bogoliubov excitations. These impurities, which form dressed quasiparticles known as Bose polarons due to their interaction with the BEC, can pair up to form a bound state called bipolarons, via an induced interaction. Previous studies on induced interactions have primarily focused on cases with… ▽ More Impurities immersed in a Bose-Einstein condensate (BEC) can interact indirectly through the exchange of Bogoliubov excitations. These impurities, which form dressed quasiparticles known as Bose polarons due to their interaction with the BEC, can pair up to form a bound state called bipolarons, via an induced interaction. Previous studies on induced interactions have primarily focused on cases with an isotropic excitation spectrum. In this work, we investigate the properties of induced interactions and bipolarons mediated by anisotropic Bogoliubov excitations using field theory. Taking a BEC with spin-orbit coupling as an example, we show that the induced interaction becomes anisotropic. Notably, a double-minima feature appears in the induced interaction in momentum space due to the exchange of roton excitations. Additionally, we calculate the binding energy and wave functions of these bipolarons induced by anisotropic interactions. Unlike previously studied bipolarons formed through the exchange of isotropic phonon excitations, we identify a new type of bipolarons whose wave functions feature a double-peak structure under strong impurity-boson interactions. Our work extends the theory of induced interactions from isotropic to anisotropic systems, and reveals the novel features in both the induced interactions and bipolarons arising from BEC with an unconventional excitation spectrum. △ Less

Submitted 27 January, 2025; originally announced January 2025.

Comments: 7 pages, 6 figures. Comments are welcome

Magnetic Orbital Hall Effect in Altermagnet RuO$_2$

Authors: Badsha Sekh, Hasibur Rahaman, Shilei Ding, Pinkesh Kumar Mishra, Ramu Maddu, Tianli Jin, Subhakanta Das, S. N. Piramanayagam

Abstract: Orbital angular momentum provides an alternative channel for current-induced magnetization switching beyond conventional spin--orbit coupling. While orbital Hall effects have been observed in several nonmagnetic materials, their manifestation in symmetry-compensated magnetic systems remains unexplored. Here, we report experimental evidence for a magnetic orbital Hall effect in RuO$_2$. In RuO$_2$(… ▽ More Orbital angular momentum provides an alternative channel for current-induced magnetization switching beyond conventional spin--orbit coupling. While orbital Hall effects have been observed in several nonmagnetic materials, their manifestation in symmetry-compensated magnetic systems remains unexplored. Here, we report experimental evidence for a magnetic orbital Hall effect in RuO$_2$. In RuO$_2$(101)/Pt/Co heterostructures, we observe a pronounced unconventional torque characterized by a large out-of-plane component, strong crystalline anisotropy, and deterministic field-free switching of a perpendicular ferromagnet over a wide range of RuO$_2$ thicknesses. The torque exhibits a non-monotonic dependence on Pt thickness, reaching a maximum at 1.5~nm, and displays a long-range RuO$_2$ thickness ($t_{\mathrm{RuO}_2}$) dependence that saturates for $t_{\mathrm{RuO}_2}>100~\mathrm{nm}$. These features cannot be reconciled with conventional spin-current mechanisms. Rather, they indicate a magnetic orbital Hall effect in RuO$_2$ that could originate from exchange-induced momentum-dependent band splitting and its interplay with spin--orbit and crystal-field coupling, with the generated orbital current converted into torque in Pt. Our findings establish altermagnets as intrinsic sources of orbital currents and extend orbitronics to symmetry-compensated magnetic systems. △ Less

Submitted 13 July, 2026; v1 submitted 21 January, 2025; originally announced January 2025.

Comments: 13 pages, 3 figures

Observation of Anderson localization transitions in a two-dimensional conjugated metal-organic framework

Authors: Jinhao Cheng, Chen Wang, Wenxue He, Jiaojiao Wang, Yifan Pang, Fan Yang, Shuaishuai Ding, Hechen Ren, Wenping Hu

Abstract: Anderson localization transitions are a universal quantum phenomenon sensitive to the disorder and dimensionality of electronic systems. Over the past decades, this intriguing topic has inspired overwhelmingly more theoretical studies than experimental verifications due to the difficulty of controlling a material's disorder or dimensionality without modifying its fundamental electronic properties.… ▽ More Anderson localization transitions are a universal quantum phenomenon sensitive to the disorder and dimensionality of electronic systems. Over the past decades, this intriguing topic has inspired overwhelmingly more theoretical studies than experimental verifications due to the difficulty of controlling a material's disorder or dimensionality without modifying its fundamental electronic properties. Organic crystals with their rich disorders would be terrific playgrounds to investigate such disorder-driven phase transitions except for their low conductivities which usually prohibit low-temperature measurements. Here, we conduct systematic transport experiments in mesoscopic devices made with copper benzenehexathiol thin films across a wide range of thicknesses. We find metal-insulator transitions both among three-dimensional samples with different disorder strengths and between three-dimensional and quasi-two-dimensional samples. Temperature-dependence analysis of the conductivities corroborates the dimensionality crossover. Moreover, our theoretical modeling provides a basis for understanding both types of metal-insulator transitions within the framework of Anderson localization transitions. Our findings establish for the first time that organic crystals such as conductive metal-organic frameworks can exhibit such quantum interference effects. With organic materials' versatile chemical designs and crystalline structures, our work opens new avenues to search for novel quantum phenomena in organic material platforms. △ Less

Submitted 30 October, 2024; originally announced October 2024.

Mitigation of Gilbert Damping in the CoFe/CuOx Orbital Torque System

Authors: Shilei Ding, Hanchen Wang, William Legrand, Paul Noël, Pietro Gambardella

Abstract: Charge-spin interconversion processes underpin the generation of spin-orbit torques in magnetic/nonmagnetic bilayers. However, efficient sources of spin currents such as 5d metals are also efficient spin sinks, resulting in a large increase of magnetic damping. Here we show that a partially-oxidized 3d metal can generate a strong orbital torque without a significant increase in damping. Measuremen… ▽ More Charge-spin interconversion processes underpin the generation of spin-orbit torques in magnetic/nonmagnetic bilayers. However, efficient sources of spin currents such as 5d metals are also efficient spin sinks, resulting in a large increase of magnetic damping. Here we show that a partially-oxidized 3d metal can generate a strong orbital torque without a significant increase in damping. Measurements of the torque efficiency ξ and Gilbert damping α in CoFe/CuOx and CoFe/Pt indicate that ξ is comparable. The increase in damping relative to a single CoFe layer is Δα<0.002 in CoFe/CuOx and Δα ~ 0.005 - 0.02 in CoFe/Pt, depending on CoFe thickness. We ascribe the nonreciprocal relationship between Δα and ξ in CoFe/CuOx to the small orbital-to-spin current ratio generated by magnetic resonance in CoFe and the lack of an efficient spin sink in CuOx. Our findings provide new perspectives on the efficient excitation of magnetization dynamics via the orbital torque. △ Less

Submitted 22 August, 2024; originally announced August 2024.

Journal ref: Nano Letters 2024 24 (33), 10251-10257

Orbital Torque in Rare-Earth Transition-Metal Ferrimagnets

Authors: Shilei Ding, Min-Gu Kang, William Legrand, Pietro Gambardella

Abstract: Orbital currents have recently emerged as a promising tool to achieve electrical control of the magnetization in thin-film ferromagnets. Efficient orbital-to-spin conversion is required in order to torque the magnetization. Here we show that the injection of an orbital current in a ferrimagnetic GdyCo100-y alloy generates strong orbital torques whose sign and magnitude can be tuned by changing the… ▽ More Orbital currents have recently emerged as a promising tool to achieve electrical control of the magnetization in thin-film ferromagnets. Efficient orbital-to-spin conversion is required in order to torque the magnetization. Here we show that the injection of an orbital current in a ferrimagnetic GdyCo100-y alloy generates strong orbital torques whose sign and magnitude can be tuned by changing the Gd content and temperature. The effective spin-orbital Hall angle reaches up to -0.25 in a GdyCo100-y/CuOx bilayer compared to +0.03 in Co/CuOx and +0.13 in GdyCo100-y/Pt. This behavior is attributed to the local orbital-to-spin conversion taking place at the Gd sites, which is about five times stronger and of the opposite sign relative to Co. Furthermore, we observe a manyfold increase in the net orbital torque at low temperature, which we attribute to the improved conversion efficiency following the magnetic ordering of the Gd and Co sublattices. △ Less

Submitted 28 June, 2024; originally announced June 2024.

Moiré flat bands in alternating twisted $\mathrm{MoTe_2}$ multilayer

Authors: Miao Liang, Shi-Ping Ding, Ming Wu, Chen Zhao, Jin-Hua Gao

Abstract: The long-awaited fractional quantum anomalous Hall (FQAH) effect recently has been observed in the twisted $\mathrm{MoTe_2}$ homobilayers, causing a great sensation. Here, we theoretically investigate the moiré band structures of a closely related system, the alternating twisted multilayer $\mathrm{MoTe_2}$ (ATML-$\mathrm{MoTe_2}$), where the adjacent layers have opposite twist angles. We illustra… ▽ More The long-awaited fractional quantum anomalous Hall (FQAH) effect recently has been observed in the twisted $\mathrm{MoTe_2}$ homobilayers, causing a great sensation. Here, we theoretically investigate the moiré band structures of a closely related system, the alternating twisted multilayer $\mathrm{MoTe_2}$ (ATML-$\mathrm{MoTe_2}$), where the adjacent layers have opposite twist angles. We illustrate that such ATML-$\mathrm{MoTe_2}$ is a very unique moiré system, exhibiting multiple topological flat bands highly controllable by the layer number and twist angle, which is not only an ideal platform to simulate Hubbard model, but also may host FQAH states. Specifically, an N-layer ATML-$\mathrm{MoTe_2}$ ($N \geq 3$) always possesses $N-2$ topological flat bands near Fermi energy $E_f$, which has an odd-even dependent decomposition rule to understand the behaviors of the moiré flat bands. We predict three intriguing examples: (1) The AT3L-$\mathrm{MoTe_2}$ ($N=3$) has one isolated moiré flat band, which corresponds to a triangular lattice Hubbard model, resembling the twisted TMD heterobilayers. (2) The AT4L-$\mathrm{MoTe_2}$ ($N=4$) has two topological flat bands that are very similar to the twisted $\mathrm{MoTe_2}$ homobilayers, implying the possible existence of FQAH states. (3) When $N>4$, the giant density of states (DOS) induced by the multiple moiré flat bands may induce exotic correlated states. △ Less

Submitted 27 May, 2024; originally announced May 2024.

Comments: 11 pages, 5 figures

Mixed polytype/polymorph formation and its effects on the electronic properties in InSe films grown by molecular beam epitaxy on GaAs(111)B

Authors: Maria Hilse, Justin Rodriguez, Jennifer Gray, Jinyuan Yao, Shaoqing Ding, Derrick Shao Heng Liu, Mo Li, Joshua Young, Ying Liu, Roman Engel-Herbert

Abstract: The top-down synthesis of inherently ferroelectric semiconductors and their integration with traditional material platforms have the potential to enable new low power logic devices, and to harness the bulk photoelectric effect for more efficient photovoltaic cells. InSe is a layered van der Waals compound exhibiting multiple polytypes, with semiconducting gamma-InSe revealing a non-centrosymmetric… ▽ More The top-down synthesis of inherently ferroelectric semiconductors and their integration with traditional material platforms have the potential to enable new low power logic devices, and to harness the bulk photoelectric effect for more efficient photovoltaic cells. InSe is a layered van der Waals compound exhibiting multiple polytypes, with semiconducting gamma-InSe revealing a non-centrosymmetric space group and showing a high carrier mobility at room temperature. Here we report the growth of InSe films on close to lattice matched semi-insulating GaAs(111)B substrates by molecular beam epitaxy (MBE). Excellent nucleation behavior resulted in the growth of smooth, single phase InSe films. The dominant polytype determined from X-ray diffraction was the targeted gamma-InSe, however Raman spectroscopy revealed spatial variations in the overall low-intensity non-centrosymmetric vibration modes. Transmission electron microscopy uncovered the presence of the three bulk polytypes beta, gamma, and epsilon-InSe coexisting in the films arranging in nanosized domains. The different polytypes can be interpreted as sequences of stacking faults and rotational twin boundaries of gamma-InSe made from individual non-centrosymmetric Se-In-In-Se layers with P-6m2 symmetry. A second, centrosymmetric Se-In-In-Se layer polymorph was identified with P-3m symmetry, which is typically not present in InSe bulk phases. First principles calculations revealed small formation energy differences between the InSe polymorphs and polytypes, yet sizeable differences in their electronic properties. Nanoscale domain sizes of varying polytypes thus resulted in sizeable electronic disorder in the grown films that dominated the electronic transport properties. Our results indicate that bottom-up thin film synthesis is a viable synthesis route towards stabilization of InSe polytypes not present in the bulk. △ Less

Submitted 18 April, 2024; originally announced April 2024.

Sampling Electronic Fock States using Determinant Quantum Monte Carlo

Authors: Shuhan Ding, Shaozhi Li, Yao Wang

Abstract: Analog quantum simulation based on ultracold atoms in optical lattices has catalyzed significant breakthroughs in the study of quantum many-body systems. These simulations rely on the statistical sampling of electronic Fock states, which are not easily accessible in classical algorithms. In this work, we modify the determinant quantum Monte Carlo by integrating a Fock-state update mechanism alongs… ▽ More Analog quantum simulation based on ultracold atoms in optical lattices has catalyzed significant breakthroughs in the study of quantum many-body systems. These simulations rely on the statistical sampling of electronic Fock states, which are not easily accessible in classical algorithms. In this work, we modify the determinant quantum Monte Carlo by integrating a Fock-state update mechanism alongside the auxiliary field. This method enables efficient sampling of Fock-state configurations. The Fock-state restrictive sampling scheme further enables the pre-selection of multiple ensembles at no additional computational cost, thereby broadening the scope of simulation to more general systems and models. Employing this method, we analyze static correlations of the Hubbard model up to the fourth order and achieve quantitative agreement with cold-atom experiments. The simulations of dynamical spectroscopies of the Hubbard and Kondo-lattice models further demonstrate the reliability and advantage of this method. △ Less

Submitted 7 February, 2025; v1 submitted 27 March, 2024; originally announced March 2024.

Comments: 11 pages, 6 figures

Journal ref: Commun. Phys. 8, 48 (2025)

Observation of the dual quantum spin Hall insulator by density-tuned correlations in a van der Waals monolayer

Authors: Jian Tang, Thomas Siyuan Ding, Hongyu Chen, Anyuan Gao, Tiema Qian, Zumeng Huang, Zhe Sun, Xin Han, Alex Strasser, Jiangxu Li, Michael Geiwitz, Mohamed Shehabeldin, Vsevolod Belosevich, Zihan Wang, Yiping Wang, Kenji Watanabe, Takashi Taniguchi, David C. Bell, Ziqiang Wang, Liang Fu, Yang Zhang, Xiaofeng Qian, Kenneth S. Burch, Youguo Shi, Ni Ni , et al. (3 additional authors not shown)

Abstract: The convergence of topology and correlations represents a highly coveted realm in the pursuit of novel quantum states of matter. Introducing electron correlations to a quantum spin Hall (QSH) insulator can lead to the emergence of a fractional topological insulator and other exotic time-reversal-symmetric topological order, not possible in quantum Hall and Chern insulator systems. However, the QSH… ▽ More The convergence of topology and correlations represents a highly coveted realm in the pursuit of novel quantum states of matter. Introducing electron correlations to a quantum spin Hall (QSH) insulator can lead to the emergence of a fractional topological insulator and other exotic time-reversal-symmetric topological order, not possible in quantum Hall and Chern insulator systems. However, the QSH insulator with quantized edge conductance remains rare, let alone that with significant correlations. In this work, we report a novel dual QSH insulator within the intrinsic monolayer crystal of TaIrTe4, arising from the interplay of its single-particle topology and density-tuned electron correlations. At charge neutrality, monolayer TaIrTe4 demonstrates the QSH insulator that aligns with single-particle band structure calculations, manifesting enhanced nonlocal transport and quantized helical edge conductance. Interestingly, upon introducing electrons from charge neutrality, TaIrTe4 only shows metallic behavior in a small range of charge densities but quickly goes into a new insulating state, entirely unexpected based on TaIrTe4's single-particle band structure. This insulating state could arise from a strong electronic instability near the van Hove singularities (VHS), likely leading to a charge density wave (CDW). Remarkably, within this correlated insulating gap, we observe a resurgence of the QSH state, marked by the revival of nonlocal transport and quantized helical edge conduction. Our observation of helical edge conduction in a CDW gap could bridge spin physics and charge orders. The discovery of a dual QSH insulator introduces a new method for creating topological flat minibands via CDW superlattices, which offer a promising platform for exploring time-reversal-symmetric fractional phases and electromagnetism. △ Less

Submitted 23 March, 2024; originally announced March 2024.

Comments: 23 pages, 15 figures, submitted version

Yu-Shiba-Rusinov states in the s-wave superconducting kagome Hubbard model: Self-consistent Bogoliubov-de Gennes calculations

Authors: Shuaibo Ding, Yunfei Bai, A. A. Bulekov, Wenhui Zhang, A. A. Shanenko, Yajiang Chen

Abstract: Significant research has recently been conducted into the Yu-Shiba-Rusinov (YSR) states in kagome superconductors through theoretical modeling and experimental investigations. However, additional efforts are still needed to further understand the local superconductivity near magnetic impurities in the kagome lattice and clarify how relevant quantities depend on the interaction strength $J$ between… ▽ More Significant research has recently been conducted into the Yu-Shiba-Rusinov (YSR) states in kagome superconductors through theoretical modeling and experimental investigations. However, additional efforts are still needed to further understand the local superconductivity near magnetic impurities in the kagome lattice and clarify how relevant quantities depend on the interaction strength $J$ between such impurities and electrons. In this study, we explore a self-consistent numerical solution of the Bogoliubov-de Gennes equations for an $s$-wave superconducting kagome model with a single classical magnetic impurity. Our study reveals that with increasing $J$, the local pair potential is systematically depressed in the vicinity of the impurity, similar to previous results obtained for the square and triangular lattices. Moreover, when further increasing $J$, the system undergoes a first-order phase transition with the appearance of stable and metastable states, reflecting the presence of the hysteresis loop in the pertinent quantities. As a consequence of this transition, the minimal energy of the stable YSR state is nonzero at any $J$, contrary to the expectations based on the assumption of a constant pair potential. A distinctive feature of the kagome lattice is that characteristics of the first-order transition are very sensitive to the position of the chemical potential within the kagome energy spectrum. △ Less

Submitted 30 August, 2024; v1 submitted 10 March, 2024; originally announced March 2024.

Comments: 7 pages, 4 figures

Journal ref: J. Phys. Chem. Lett. 15, 9084-9091 (2024)

Exciton interacting with a moiré lattice: Polarons, strings, and optical probing of spin correlations

Authors: Aleksi Julku, Shanshan Ding, Georg M. Bruun

Abstract: We develop a general theory for how an exciton in an atomically thin transition metal dichacogenide (TMD) monolayer couples to spin and charge correlations in an adjacent moire lattice created by a TMD bi-layer. Virtual tunneling of charge carriers, assumed for concreteness to be holes, between the moire lattice and the monolayer combined with the presence of bound hole-exciton states, i.e. trions… ▽ More We develop a general theory for how an exciton in an atomically thin transition metal dichacogenide (TMD) monolayer couples to spin and charge correlations in an adjacent moire lattice created by a TMD bi-layer. Virtual tunneling of charge carriers, assumed for concreteness to be holes, between the moire lattice and the monolayer combined with the presence of bound hole-exciton states, i.e. trions, give rise to an effective interaction between the moire holes and the exciton. In addition to the Umklapp scattering, we show that this interaction is spin-dependent and therefore couples the exciton to the spin correlations of the moire holes, which may be in- as well as out-of-plane. We then use our theory to examine two specific examples where the moire holes form in-plane ferromagnetic or anti-ferromagnetic order. In both cases, the exciton creates spin waves in the moire lattice, which we analyse by using a self-consistent Born approximation that includes such processes to infinite order. We show that the competition between magnetic order and exciton motion leads to the formation of a well-defined quasiparticle consisting of the exciton surrounded by a cloud of magnetic frustration in the moire lattice sites below. For the anti-ferromagnet, we furthermore demonstrate the presence of the elusive geometric string excitations and discuss how they can be observed via their smoking gun energy dependence on the spin-spin coupling, which can be tuned by varying the twist angle of the moire bi-layer. All these phenomena have clear signatures in the exciton spectrum, and as such our results illustrate that excitons are promising probes providing optical access to the spin correlations of new quantum phases predicted to exist in TMD materials. △ Less

Submitted 2 August, 2024; v1 submitted 6 November, 2023; originally announced November 2023.

Comments: 21 pages, 15 figures

Journal ref: Phys. Rev. Res. 6, 033119 (2024)

Ferroelectric Schottky diodes of CuInP$_2$S$_6$ nanosheet

Authors: Jinyuan Yao, Yongtao Liu, Shaoqing Ding, Yanglin Zhu, Zhiqiang Mao, Sergei V. Kalinin, Ying Liu

Abstract: Ferroelectricity in van der Waals (vdW) layered material has attracted a great deal of interest recently. CuInP$_2$S$_6$ (CIPS), the only vdW layered material whose ferroelectricity in the bulk was demonstrated by direct polarization measurements, was shown to remain ferroelectric down to a thickness of a few nanometers. However, its ferroelectric properties have just started to be explored in the… ▽ More Ferroelectricity in van der Waals (vdW) layered material has attracted a great deal of interest recently. CuInP$_2$S$_6$ (CIPS), the only vdW layered material whose ferroelectricity in the bulk was demonstrated by direct polarization measurements, was shown to remain ferroelectric down to a thickness of a few nanometers. However, its ferroelectric properties have just started to be explored in the context of potential device applications. We report here the preparation and measurements of metal-ferroelectric semiconductor-metal heterostructures using nanosheets of CIPS obtained by mechanical exfoliation. Four bias voltage and polarization dependent resistive states were observed in the current-voltage characteristics, which we attribute to the formation of ferroelectric Schottky diode, along with switching behavior. △ Less

Submitted 18 September, 2023; originally announced September 2023.

Comments: 19 pages, 8 figures

Anomalous excitonic phase diagram in band-gap-tuned Ta2Ni(Se,S)5

Authors: Cheng Chen, Weichen Tang, Xiang Chen, Zhibo Kang, Shuhan Ding, Kirsty Scott, Siqi Wang, Zhenglu Li, Jacob P. C. Ruff, Makoto Hashimoto, Dong-Hui Lu, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Eduardo H. da Silva Neto, Robert J. Birgeneau, Yulin Chen, Steven G. Louie, Yao Wang, Yu He

Abstract: During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators -- an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to… ▽ More During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an important route to realize bulk excitonic insulators -- an insulating ground state distinct from single-particle band insulators. How this route manifests from weak to strong coupling is not clear. In this work, using angle-resolved photoemission spectroscopy (ARPES) and high-resolution synchrotron x-ray diffraction (XRD), we investigate the broken symmetry state across the semimetal-to-semiconductor transition in a leading bulk excitonic insulator candidate system Ta2Ni(Se,S)5. A broken symmetry phase is found to be continuously suppressed from the semimetal side to the semiconductor side, contradicting the anticipated maximal excitonic instability around the Lifshitz transition. Bolstered by first-principles and model calculations, we find strong interband electron-phonon coupling to play a crucial role in the enhanced symmetry breaking on the semimetal side of the phase diagram. Our results not only provide insight into the longstanding debate of the nature of intertwined orders in Ta2NiSe5, but also establish a basis for exploring band-gap-tuned structural and electronic instabilities in strongly coupled systems. △ Less

Submitted 13 September, 2023; originally announced September 2023.

Comments: 27 pages, 4 + 9 figures

Journal ref: Nat Commun 14, 7512 (2023)

A unique van Hove singularity in kagome superconductor CsV$_{3-x}$Ta$_x$Sb$_5$ with enhanced superconductivity

Authors: Yang Luo, Yulei Han, Jinjin Liu, Hui Chen, Zihao Huang, Linwei Huai, Hongyu Li, Bingqian Wang, Jianchang Shen, Shuhan Ding, Zeyu Li, Shuting Peng, Zhiyuan Wei, Yu Miao, Xiupeng Sun, Zhipeng Ou, Ziji Xiang, Makoto Hashimoto, Donghui Lu, Yugui Yao, Haitao Yang, Xianhui Chen, Hong-Jun Gao, Zhenhua Qiao, Zhiwei Wang , et al. (1 additional authors not shown)

Abstract: Van Hove singularity (VHS) has been considered as a driving source for unconventional superconductivity. A VHS in two-dimensional (2D) materials consists of a saddle point connecting electron-like and hole-like bands. In a rare case, when a VHS appears at Fermi level, both electron-like and hole-like conduction can coexist, giving rise to an enhanced density of states as well as an attractive comp… ▽ More Van Hove singularity (VHS) has been considered as a driving source for unconventional superconductivity. A VHS in two-dimensional (2D) materials consists of a saddle point connecting electron-like and hole-like bands. In a rare case, when a VHS appears at Fermi level, both electron-like and hole-like conduction can coexist, giving rise to an enhanced density of states as well as an attractive component of Coulomb interaction for unconventional electronic pairing. However, this van Hove scenario is often destroyed by an incorrect chemical potential or competing instabilities. Here, by using angle-resolved photoemission measurements, we report the observation of a VHS perfectly aligned with the Fermi level in a kagome superconductor CsV$_{3-x}$Ta$_x$Sb$_5$ (x~0.4), in which a record-high superconducting transition temperature is achieved among all the current variants of AV$_3$Sb$_5$ (A=Cs, Rb, K) at ambient pressure. Doping dependent measurements reveal the important role of van Hove scenario in boosting superconductivity, and spectroscopic-imaging scanning tunneling microscopy measurements indicate a distinct superconducting state in this system. △ Less

Submitted 3 July, 2023; originally announced July 2023.

Comments: 20 pages, 4 figures

Journal ref: Nature Communications 14, 3819 (2023)

Atypical sliding and Moire ferroelectricity in pure multilayer graphene

Authors: Liu Yang, Shiping Ding, Jinhua Gao, Menghao Wu

Abstract: Most non-ferroelectric two-dimensional materials can be endowed with so-called sliding ferroelectricity via non-equivalent homo-bilayer stacking, which is not applicable to mono-element systems like pure graphene bilayer with inversion symmetry at any sliding vector. Herein we show first-principles evidence that multilayer graphene with N>3 can all be ferroelectric, where the polarizations of pola… ▽ More Most non-ferroelectric two-dimensional materials can be endowed with so-called sliding ferroelectricity via non-equivalent homo-bilayer stacking, which is not applicable to mono-element systems like pure graphene bilayer with inversion symmetry at any sliding vector. Herein we show first-principles evidence that multilayer graphene with N>3 can all be ferroelectric, where the polarizations of polar states stem from the symmetry breaking in stacking configurations of across-layer instead of adjacent-layer, which are electrically switchable via interlayer sliding. The non-polar states can also be electrically driven to polar states via sliding, all nearly degenerate in energy, and more diverse states with distinct polarizations will emerge in more layers. In contrast to the ferroelectric Moire domains with opposite polarization directions in twisted bilayers reported previously, the Moire pattern in some multilayer graphene systems (e.g., twisted monolayer-trilayer graphene) possess nonzero net polarizations with domains of the same direction separated by non-polar regions, which can be electrically reversed upon interlayer sliding. The distinct Moire bands of two polar states should facilitate electrical detection of such sliding Moire ferroelectricity during switching. △ Less

Submitted 12 May, 2023; originally announced May 2023.

Superconductivity in graphite intercalation compounds with sodium

Authors: Chun-Mei Hao, Xing Li, Artem R. Oganov, Jingyu Hou, Shicong Ding, Yanfeng Ge, Lin Wang, Xiao Dong, Hui-Tian Wang, Guochun Yang, Xiang-Feng Zhou, Yongjun Tian

Abstract: The discovery of superconductivity in CaC6 with a critical temperature (Tc) of 11.5 K reignites much interest in exploring high-temperature superconductivity in graphite intercalation compounds (GICs). Here we identify a GIC NaC4, discovered by ab initio evolutionary structure search, as a superconductor with a computed Tc of 41.2 K at 5 GPa. This value is eight times higher than that of the synth… ▽ More The discovery of superconductivity in CaC6 with a critical temperature (Tc) of 11.5 K reignites much interest in exploring high-temperature superconductivity in graphite intercalation compounds (GICs). Here we identify a GIC NaC4, discovered by ab initio evolutionary structure search, as a superconductor with a computed Tc of 41.2 K at 5 GPa. This value is eight times higher than that of the synthesized GIC NaC2 and possesses the highest Tc among available GICs. The remarkable superconductivity of GIC NaC4 mainly arises from the coupling of π electrons in graphene with the low-frequency vibrations involving both Na and C atoms. These findings suggest that Na-GICs may hold great promise as high-Tc superconductors. △ Less

Submitted 5 July, 2023; v1 submitted 30 April, 2023; originally announced May 2023.

Comments: 4 figures

Surface structure and multigap superconductivity of V3Si (111) revealed by scanning tunneling microscopy

Authors: Shuyue Ding, Dongming Zhao, Tianxing Jiang, Haitao Wang, Donglai Feng, Tong Zhang

Abstract: V3Si, a classical silicide superconductor with relatively high TC (~16 K), is promising for constructing silicon-based superconducting devices and hetero-structures. However, real space characterization on its surfaces and superconducting properties are still limited. Here we report the first low-temperature scanning tunnelling microscopy (STM) study on cleaned V3Si (111) single crystal surface. W… ▽ More V3Si, a classical silicide superconductor with relatively high TC (~16 K), is promising for constructing silicon-based superconducting devices and hetero-structures. However, real space characterization on its surfaces and superconducting properties are still limited. Here we report the first low-temperature scanning tunnelling microscopy (STM) study on cleaned V3Si (111) single crystal surface. We observed a r3 by r3 superstructure which displays mirror symmetry between adjacent terraces, indicating the surface is V-terminated and reconstructed. The tunneling spectrum shows full superconducting gap with double pairs of coherence peaks, but has a relatively small gap size with comparing to bulk TC. Impurity induced in-gap state is absent on surface defects but present on introduced magnetic adatoms. Upon applying magnetic field, a hexagonal vortex lattice is visualized. Interestingly, the vortex size is found to be field dependent, and the coherence length measured from single vortex at low field is significantly larger than estimated value from bulk H_c2. These results reflect V3Si is a multi-band, s- wave superconductor. △ Less

Submitted 21 April, 2023; originally announced April 2023.

Journal ref: Quantum Front 2, 3 (2023)

Strong Inter-valley Electron-Phonon Coupling in Magic-Angle Twisted Bilayer Graphene

Authors: Cheng Chen, Kevin P. Nuckolls, Shuhan Ding, Wangqian Miao, Dillon Wong, Myungchul Oh, Ryan L. Lee, Shanmei He, Cheng Peng, Ding Pei, Yiwei Li, Chenyue Hao, Haoran Yan, Hanbo Xiao, Han Gao, Qiao Li, Shihao Zhang, Jianpeng Liu, Lin He, Kenji Watanabe, Takashi Taniguchi, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Chu Li , et al. (9 additional authors not shown)

Abstract: The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked enormous research interest. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms, the origin of its superconductivity remains elusive. Here, utilizing angle-resolved photoemission spectroscopy with micrometer spatial resolutio… ▽ More The unusual properties of superconductivity in magic-angle twisted bilayer graphene (MATBG) have sparked enormous research interest. However, despite the dedication of intensive experimental efforts and the proposal of several possible pairing mechanisms, the origin of its superconductivity remains elusive. Here, utilizing angle-resolved photoemission spectroscopy with micrometer spatial resolution, we have revealed flat band replicas in superconducting MATBG, where MATBG is unaligned with its hexagonal boron nitride (hBN) substrate11. These replicas exhibit uniform energy spacing, approximately 150 +- 15 meV apart, indicative of strong electron-boson coupling. Strikingly, these replicas are absent in non-superconducting twisted bilayer graphene (TBG) systems, either when MATBG is aligned to hBN or when TBG deviates from the magic angle. Calculations suggest that the formation of these flat band replicas in superconducting MATBG are attributed to the strong coupling between flat band electrons and an optical phonon mode at the graphene K point, facilitated by inter-valley scattering. These findings, although do not necessarily put electron phonon coupling as the main driving force for the superconductivity in MATBG, unravel the unique electronic structure inherent in superconducting MATBG, thereby providing crucial information for understanding the unusual electronic landscape from which the superconductivity is derived. △ Less

Submitted 12 December, 2024; v1 submitted 26 March, 2023; originally announced March 2023.

Comments: 17 pages, 4 figures

Journal ref: Nature 636, 342 (2024)

Superconductivity in Li8Au electride

Authors: Xiaohua Zhang, Yansun Yao, Shicong Ding, Aitor Bergara, Fei Li, Yong Liu, Xiang-Feng Zhou, Guochun Yang

Abstract: Located at crystal voids, interstitial anion electrons (IAEs) have diverse topologies, which may be tuned to achieve new properties. Elucidating the role of IAEs in electron-phonon coupling (EPC), and using it to design new electride superconductors, leads to the current prediction of superconducting Li8Au at high pressure. We suggest that the occurence of high-temperature superconductivity in ele… ▽ More Located at crystal voids, interstitial anion electrons (IAEs) have diverse topologies, which may be tuned to achieve new properties. Elucidating the role of IAEs in electron-phonon coupling (EPC), and using it to design new electride superconductors, leads to the current prediction of superconducting Li8Au at high pressure. We suggest that the occurence of high-temperature superconductivity in electrides requires high-symmetry structures with hydrogen-like cages, an electron acceptor element to balance charges, and isolated IAEs coupled with medium-frequency vibrations. The uniquely designed Li8Au electride has a NaCl-type (B1) lattice, with atomic Au and cubic Li8 cages as bases. Isolated IAEs are formed at the cage centers, with extra charges taken up by Au. These octahedrally coordinated IAEs have a p-orbital-like attribute and are strongly coupled with atomic vibrations in the Li8 cages. The strong EPC in Li8Au results in a calculated Tc of 73.1 K at 250 GPa, which is the highest Tc reported to date for all the electrides. A slight substitutional Pt doping can enhance the Tc of Li8Au to exceed liquid nitrogen temperature. △ Less

Submitted 14 February, 2023; originally announced February 2023.

Comments: 7 Pages, 3 Figures

Mirror symmetry decomposition in double-twisted multilayer graphene systems

Authors: Shi-Ping Ding, Miao Liang, Zhen Ma, Jing-Tao Lü, Jin-Hua Gao

Abstract: Due to the observed superconductivity, the alternating twisted trilayer graphene (ATTLG) has drawn great research interest very recently, in which three monolayer graphene (MLG) are stacked in alternating twist way. If one or several of the MLG in ATTLG are replaced by a multilayer graphene, we get a double twisted multilayer graphene (DTMLG). In this work, we theoretically illustrate that, if the… ▽ More Due to the observed superconductivity, the alternating twisted trilayer graphene (ATTLG) has drawn great research interest very recently, in which three monolayer graphene (MLG) are stacked in alternating twist way. If one or several of the MLG in ATTLG are replaced by a multilayer graphene, we get a double twisted multilayer graphene (DTMLG). In this work, we theoretically illustrate that, if the DTMLG has a mirror symmetry along z direction like the ATTLG, there exists a mirror symmetry decomposition (MSD), by which the DTMLG can be exactly decoupled into two subsystems with opposite parity. The two subsystems are either a twisted multilayer graphene (single twist) or a multilayer graphene, depending on the stacking configuration. Such MSD can give a clear interpretation about all the novel features of the moiré band structures of DTMLG, e.g. the fourfold degenerate flat bands and the enlarged magic angle. Meanwhile, in such DTMLG, the parity becomes a new degree of freedom of the electrons, so that we can define a parity resolved Chern number for the moiré flat bands. More importantly, the MSD implies that all the novel correlated phases in the twisted multilayer graphene should also exist in the corresponding DTMLGs, since they have the exact same Hamiltonian in form. Specifically, according to the MSD, we predict that the superconductivity should exist in the (1+3+1)-DTMLG. △ Less

Submitted 27 December, 2022; originally announced December 2022.

Comments: 12 pages, 6 figures

Polarons and bipolarons in a two-dimensional square lattice

Authors: Shanshan Ding, G. A. Domínguez-Castro, Aleksi Julku, Arturo Camacho-Guardian, Georg M. Bruun

Abstract: Quasiparticles and their interactions are a key part of our understanding of quantum many-body systems. Quantum simulation experiments with cold atoms have in recent years advanced our understanding of isolated quasiparticles, but so far they have provided limited information regarding their interactions and possible bound states. Here, we show how exploring mobile impurities immersed in a Bose-Ei… ▽ More Quasiparticles and their interactions are a key part of our understanding of quantum many-body systems. Quantum simulation experiments with cold atoms have in recent years advanced our understanding of isolated quasiparticles, but so far they have provided limited information regarding their interactions and possible bound states. Here, we show how exploring mobile impurities immersed in a Bose-Einstein condensate (BEC) in a two-dimensional lattice can address this problem. First, the spectral properties of individual impurities are examined, and in addition to the attractive and repulsive polarons known from continuum gases, we identify a new kind of quasiparticle stable for repulsive boson-impurity interactions. The spatial properties of polarons are calculated showing that there is an increased density of bosons at the site of the impurity both for repulsive and attractive interactions. We then derive an effective Schrödinger equation describing two polarons interacting via the exchange of density oscillations in the BEC, which takes into account strong impurity-boson two-body correlations. Using this, we show that the attractive nature of the effective interaction between two polarons combined with the two-dimensionality of the lattice leads to the formation of bound states -- i.e. bipolarons. The wave functions of the bipolarons are examined showing that the ground state is symmetric under particle exchange and therefore relevant for bosonic impurities, whereas the first excited state is doubly degenerate and odd under particle exchange making it relevant for fermionic impurities. Our results show that quantum gas microscopy in optical lattices is a promising platform to explore the spatial properties of polarons as well as to finally observe the elusive bipolarons. △ Less

Submitted 21 January, 2023; v1 submitted 1 December, 2022; originally announced December 2022.

Comments: Submission to SciPost

Journal ref: SciPost Phys. 14, 143 (2023)

Unidirectional orbital magnetoresistance in light metal/ferromagnet bilayers

Authors: Shilei Ding, Paul Noël, Gunasheel Kauwtilyaa Krishnaswamy, Pietro Gambardella

Abstract: We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbita… ▽ More We report the observation of a unidirectional magnetoresistance (UMR) that originates from the nonequilibrium orbital momentum induced by an electric current in a naturally oxidized Cu/Co bilayer. The orbital-UMR scales with the torque efficiency due to the orbital Rashba-Edelstein effect upon changing the Co thickness and temperature, reflecting their common origin. We attribute the UMR to orbital-dependent electron scattering and orbital-to-spin conversion in the ferromagnetic layer. In contrast to the spin-current induced UMR, the magnon contribution to the orbital-UMR is absent in thin Co layers, which we ascribe to the lack of coupling between low energy magnons and orbital current. The magnon contribution to the UMR emerges in Co layers thicker than about 5 nm, which is comparable to the orbital-to-spin conversion length. Our results provide insight into orbital-to-spin momentum transfer processes relevant for the optimization of spintronic devices based on light metals and orbital transport. △ Less

Submitted 19 September, 2022; originally announced September 2022.

Comments: 12 pages, 3 figures

Journal ref: Phys. Rev. Research 4, L032041 (2022)

Observation of robust zero-energy state and enhanced superconducting gap in a tri-layer heterostructure of MnTe/Bi2Te3/Fe(Te, Se)

Authors: Shuyue Ding, Chen Chen, Zhipeng Cao, Di Wang, Yongqiang Pan, Ran Tao, Dongming Zhao, Yining Hu, Tianxing Jiang, Yajun Yan, Zhixiang Shi, Xiangang Wan, Donglai Feng, Tong Zhang

Abstract: The interface between magnetic material and superconductors has long been predicted to host unconventional superconductivity, such as spin-triplet pairing and topological nontrivial pairing state, particularly when spin-orbital coupling (SOC) is incorporated. To identify these novel pairing states, fabricating homogenous heterostructures which contain such various properties are preferred, but oft… ▽ More The interface between magnetic material and superconductors has long been predicted to host unconventional superconductivity, such as spin-triplet pairing and topological nontrivial pairing state, particularly when spin-orbital coupling (SOC) is incorporated. To identify these novel pairing states, fabricating homogenous heterostructures which contain such various properties are preferred, but often challenging. Here we synthesized a tri-layer type van-der Waals heterostructure of MnTe/Bi2Te3/Fe(Te, Se), which combined s-wave superconductivity, thickness dependent magnetism and strong SOC. Via low-temperature scanning tunneling microscopy (STM), we observed robust zero-energy states with notably nontrivial properties and an enhanced superconducting gap size on single unit-cell (UC) MnTe surface. In contrast, no zero-energy state was observed on 2UC MnTe. First-principle calculations further suggest the 1UC MnTe has large interfacial Dzyaloshinskii-Moriya interaction (DMI) and a frustrated AFM state, which could promote non-collinear spin textures. It thus provides a promising platform for exploring topological nontrivial superconductivity. △ Less

Submitted 15 September, 2022; originally announced September 2022.

Comments: 33 pages, supplementary materials included

Journal ref: Sci. Adv. 8, eabq4578 (2022)

Large-Scale Integrated Flexible Tactile Sensor Array for Sensitive Smart Robotic Touch

Authors: Zhenxuan Zhao, Jianshi Tang, Jian Yuan, Yijun Li, Yuan Dai, Jian Yao, Qingtian Zhang, Sanchuan Ding, Tingyu Li, Ruirui Zhang, Yu Zheng, Zhengyou Zhang, Song Qiu, Qingwen Li, Bin Gao, Ning Deng, He Qian, Fei Xing, Zheng You, Huaqiang Wu

Abstract: In the long pursuit of smart robotics, it has been envisioned to empower robots with human-like senses, especially vision and touch. While tremendous progress has been made in image sensors and computer vision over the past decades, the tactile sense abilities are lagging behind due to the lack of large-scale flexible tactile sensor array with high sensitivity, high spatial resolution, and fast re… ▽ More In the long pursuit of smart robotics, it has been envisioned to empower robots with human-like senses, especially vision and touch. While tremendous progress has been made in image sensors and computer vision over the past decades, the tactile sense abilities are lagging behind due to the lack of large-scale flexible tactile sensor array with high sensitivity, high spatial resolution, and fast response. In this work, we have demonstrated a 64x64 flexible tactile sensor array with a record-high spatial resolution of 0.9 mm (equivalently 28.2 pixels per inch), by integrating a high-performance piezoresistive film (PRF) with a large-area active matrix of carbon nanotube thin-film transistors. PRF with self-formed microstructures exhibited high pressure-sensitivity of ~385 kPa-1 for MWCNTs concentration of 6%, while the 14% one exhibited fast response time of ~3 ms, good linearity, broad detection range beyond 1400 kPa, and excellent cyclability over 3000 cycles. Using this fully integrated tactile sensor array, the footprint maps of an artificial honeybee were clearly identified. Furthermore, we hardware-implemented a smart tactile system by integrating the PRF-based sensor array with a memristor-based computing-in-memory chip to record and recognize handwritten digits and Chinese calligraphy, achieving high classification accuracies of 98.8% and 97.3% in hardware, respectively. The integration of sensor networks with deep learning hardware may enable edge or near-sensor computing with significantly reduced power consumption and latency. Our work could pave the road to building large-scale intelligent sensor networks for next-generation smart robotics. △ Less

Submitted 3 November, 2022; v1 submitted 23 August, 2022; originally announced August 2022.

Comments: Correction in Methods: The weight ratio of TPU:DMF was set to be 1:5

Journal ref: ACS Nano 2022, 16, 16784

Prevailing charge order in overdoped cuprates beyond the superconducting dome

Authors: Qizhi Li, Hsiao-Yu Huang, Tianshuang Ren, Eugen Weschke, Lele Ju, Changwei Zou, Shilong Zhang, Qingzheng Qiu, Jiarui Liu, Shuhan Ding, Amol Singh, Oleksandr Prokhnenko, Di-Jing Huang, Ilya Esterlis, Yao Wang, Yanwu Xie, Yingying Peng

Abstract: The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu $L_3$ edge and O $K$ edge resonant x-ray scattering, we reveal a charge order (CO) in overdoped La$_{2-x}$Sr$_x$CuO$_4$ (0.35 $\leq$ x $\leq$ 0.6) beyond the superconducting dome. This CO… ▽ More The extremely overdoped cuprates are generally considered to be Fermi liquid metals without exotic orders, whereas the underdoped cuprates harbor intertwined states. Contrary to this conventional wisdom, using Cu $L_3$ edge and O $K$ edge resonant x-ray scattering, we reveal a charge order (CO) in overdoped La$_{2-x}$Sr$_x$CuO$_4$ (0.35 $\leq$ x $\leq$ 0.6) beyond the superconducting dome. This CO has a periodicity of $\sim$ 6 lattice units with correlation lengths of $\sim 3 - 20$ lattice units. It shows similar in-plane momentum and polarization dependence and dispersive excitations as the CO of underdoped cuprates, but its maximum intensity differs along the c-direction and persists up to 300 K. This CO cannot be explained by either the Fermi surface instability or the doped Hubbard model and its origin remains to be understood. Our results suggest that CO is prevailing in the overdoped metallic regime and superconductivity emerges out of the CO phase upon decreasing hole carriers. △ Less

Submitted 17 October, 2022; v1 submitted 18 August, 2022; originally announced August 2022.

Comments: 6 pages, 4 figures

Journal ref: Physical Review Letters 131, 116002 (2023)

Role of electron-phonon coupling in excitonic insulator candidate Ta2NiSe5

Authors: Cheng Chen, Xiang Chen, Weichen Tang, Zhenglu Li, Siqi Wang, Shuhan Ding, Zhibo Kang, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Makoto Hashimoto, Donghui Lu, Jacob P. C. Ruff, Steven G. Louie, Robert Birgeneau, Yulin Chen, Yao Wang, Yu He

Abstract: Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and ``condense'' into a new insulating ground state -- the so-called excitonic insulator -- when the energy of electron-hole Coulomb attraction exceeds the band gap. In the presence of electron-phonon coupling, a periodic lattice distortion often concomitantly occurs with this exciton cond… ▽ More Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and ``condense'' into a new insulating ground state -- the so-called excitonic insulator -- when the energy of electron-hole Coulomb attraction exceeds the band gap. In the presence of electron-phonon coupling, a periodic lattice distortion often concomitantly occurs with this exciton condensation. However, similar structural transition can also be induced by electron-phonon coupling itself, therefore hindering the clean identification of bulk excitonic insulators based on reductionistic reasoning (e.g. which instability is the ``driving force'' of the phase transition). Using high-resolution synchrotron x-ray diffraction and angle-resolved photoemission spectroscopy techniques, we identify key electron-phonon coupling effects in a leading excitonic insulator candidate Ta2NiSe5. These include an extensive unidirectional lattice fluctuation and an electronic pseudogap in the normal state, as well as a negative electronic compressibility in the charge-doped broken-symmetry state. In combination with first principles and model calculations, we determine a minimal lattice model and the corresponding interaction parameters that capture the experimental observations. More importantly, we show how the Coulomb and electron-phonon coupling effects can be separated on the level of lattice model, and demonstrate a general framework beyond the reductionist approach in the investigation of correlated systems with intertwined orders. △ Less

Submitted 10 April, 2023; v1 submitted 13 March, 2022; originally announced March 2022.

Comments: 18 pages, 12 + 4 figures

Journal ref: Phys. Rev. Research 5, 043089 (2023)