arXiv subjects

Lizhong Li

Publications and source records attributed to Lizhong Li.

15 recordsLinked to original sources

arXiv2026

Emergence of a symmetry-broken Chern insulator near a moiré Kondo breakdown

Moiré semiconductors built on angle-aligned transition metal dichalcogenide (TMD) heterobilayers provide a physical realization of the Kondo lattice model, in which one TMD layer is prepared in a Mott insulating state supporting a lattice of local magnetic moments and the other layer in a metallic state supporting itinerant carriers. The artificial Kondo lattice enables the exploration of exotic states of matter near a continuously tunable Kondo breakdown. Here we report the emergence of a symmetry-broken Chern insulator at a moiré hole filling factor 4/3 in angle-aligned MoTe2/WSe2 moiré bilayers, which realize a chiral Kondo lattice. The symmetry-broken Chern insulator, which exhibits integer quantized Hall conductance at a fractional moiré filling, breaks the translational symmetry of the lattice spontaneously; it also appears only near a magnetic field-induced Kondo breakdown in the mixed-valence regime of the material. We further demonstrate that the magnetic field required to induce the Kondo breakdown and to stabilize the symmetry-broken Chern insulator is twist angle dependent. The results present new opportunities for exploring the subtle interplay between topology and Kondo interactions in moiré semiconductors.

cond-mat.mes-hall↗

arXiv2024

Tuning exciton emission via ferroelectric polarization at a heterogeneous interface between a monolayer transition metal dichalcogenide and a perovskite oxide membrane

We demonstrate the integration of a thin BaTiO$_3$ (BTO) membrane with monolayer MoSe$_2$ in a dual gate device that enables in-situ manipulation of the BTO ferroelectric polarization with a voltage pulse. While two-dimensional (2D) transition metal dichalcogenides (TMDs) offer remarkable adaptability, their hybrid integration with other families of functional materials beyond the realm of 2D materials has been challenging. Released functional oxide membranes offer a solution for 2D/3D integration via stacking. 2D TMD excitons can serve as a local probe of the ferroelectric polarization in BTO at a heterogeneous interface. Using photoluminescence (PL) of MoSe$_2$ excitons to optically readout the doping level, we find that the relative population of charge carriers in MoSe$_2$ depends sensitively on the ferroelectric polarization. This finding points to a promising avenue for future-generations versatile sensing devices with high sensitivity, fast read-out, and diverse applicability for advanced signal processing.

cond-mat.mtrl-sci↗

arXiv2023

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

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

cond-mat.mes-hall↗

arXiv2022

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

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

cond-mat.mes-hall↗

arXiv2022

Realization of the Haldane Chern insulator in a moiré lattice

The Chern insulator displays a quantized Hall effect without Landau levels. In a landmark paper in 1988, Haldane showed that a Chern insulator could be realized through complex next-nearest-neighbor hopping in a honeycomb lattice. Despite its profound impact on the field of topological physics and recent implementation in cold-atom experiments, the Haldane model has remained elusive in solid-state materials. Here, we report the experimental realization of a Haldane Chern insulator in AB-stacked MoTe2/WSe2 moiré bilayers, which form a honeycomb moiré lattice with two sublattices residing in different layers. We show that the moiré bilayer filled with two charge particles per unit cell is a quantum spin Hall (QSH) insulator with a tunable charge gap. Under a small out-of-plane magnetic field, it becomes a Chern insulator with Chern number c=1 from magneto-transport studies. The results are qualitatively captured by a generalized Kane-Mele tight-binding Hamiltonian. The Zeeman field splits the QSH insulator into two halves of opposite valley--one with a positive and the other a negative moiré band gap. Our study highlights the unique potential of semiconductor moiré materials in engineering topological lattice Hamiltonians.

cond-mat.mes-hall↗

arXiv2022

Intrinsic spin Hall torque in a moire Chern magnet

In spin torque magnetic memories, electrically actuated spin currents are used to switch a magnetic bit. Typically, these require a multilayer geometry including both a free ferromagnetic layer and a second layer providing spin injection. For example, spin may be injected by a nonmagnetic layer exhibiting a large spin Hall effect, a phenomenon known as spin-orbit torque. Here, we demonstrate a spin-orbit torque magnetic bit in a single two-dimensional system with intrinsic magnetism and strong Berry curvature. We study AB-stacked MoTe2/WSe2, which hosts a magnetic Chern insulator at a carrier density of one hole per moire superlattice site. We observe hysteretic switching of the resistivity as a function of applied current. Magnetic imaging using a superconducting quantum interference device reveals that current switches correspond to reversals of individual magnetic domains. The real space pattern of domain reversals aligns precisely with spin accumulation measured near the high-Berry curvature Hubbard band edges. This suggests that intrinsic spin- or valley-Hall torques drive the observed current-driven magnetic switching in both MoTe2/WSe2 and other moire materials. The switching current density of 10^3 Amps per square centimeter is significantly less than reported in other platforms paving the way for efficient control of magnetic order.

cond-mat.mes-hall↗

arXiv2022

Frustrated magnetic interactions in a Wigner-Mott insulator

Two-dimensional semiconductor moiré materials have emerged as a highly controllable platform to simulate and explore quantum condensed matter. Compared to real solids, electrons in semiconductor moiré materials are less strongly attracted to the moiré lattice sites, making the nonlocal contributions to the magnetic interactions as important as the Anderson super-exchange. It provides a unique platform to study the effects of competing magnetic interactions. Here, we report the observation of strongly frustrated magnetic interactions in a Wigner-Mott insulating state at 2/3 filling of the moiré lattice in angle-aligned WSe2/WS2 heterobilayers. Magneto-optical measurements show that the net exchange interaction is antiferromagnetic for filling factors below 1 with a strong suppression at 2/3 filling. The suppression is lifted upon screening of the long-range Coulomb interactions and melting of the Wigner-Mott insulator by a nearby metallic gate. The results can be qualitatively captured by a honeycomb-lattice spin model with an antiferromagnetic nearest-neighbor coupling and a ferromagnetic second-neighbor coupling. Our study establishes semiconductor moiré materials as a model system for the lattice-spin physics and frustrated magnetism.

cond-mat.mtrl-sci↗

arXiv2021

Quantum anomalous Hall effect from intertwined moiré bands

Electron correlation and topology are two central threads of modern condensed matter physics. Semiconductor moiré materials provide a highly tunable platform for studies of electron correlation. Correlation-driven phenomena, including the Mott insulator, generalized Wigner crystals, stripe phases and continuous Mott transition, have been demonstrated. However, nontrivial band topology has remained elusive. Here we report the observation of a quantum anomalous Hall (QAH) effect in AB-stacked MoTe2/WSe2 moiré heterobilayers. Unlike in the AA-stacked structures, an out-of-plane electric field controls not only the bandwidth but also the band topology by intertwining moiré bands centered at different high-symmetry stacking sites. At half band filling, corresponding to one particle per moiré unit cell, we observe quantized Hall resistance, h/e2 (with h and e denoting the Planck's constant and electron charge, respectively), and vanishing longitudinal resistance at zero magnetic field. The electric-field-induced topological phase transition from a Mott insulator to a QAH insulator precedes an insulator-to-metal transition; contrary to most known topological phase transitions, it is not accompanied by a bulk charge gap closure. Our study paves the path for discovery of a wealth of emergent phenomena arising from the combined influence of strong correlation and topology in semiconductor moiré materials.

cond-mat.mes-hall↗

arXiv2021

Continuous Mott transition in semiconductor moiré superlattices

The evolution of a Landau Fermi liquid into a nonmagnetic Mott insulator with increasing electronic interactions is one of the most puzzling quantum phase transitions in physics. The vicinity of the transition is believed to host exotic states of matter such as quantum spin liquids, exciton condensates and unconventional superconductivity. Semiconductor moiré materials realize a highly controllable Hubbard model simulator on a triangular lattice, providing a unique opportunity to drive a metal-insulator transition (MIT) via continuous tuning of the electronic interactions. Here, by electrically tuning the effective interaction strength in MoTe2/WSe2 moiré superlattices, we observe a continuous MIT at a fixed filling of one electron per unit cell. The existence of quantum criticality is supported by the scaling behavior of the resistance, a continuously vanishing charge-gap as the critical point is approached from the insulating side, and a diverging quasiparticle effective mass from the metallic side. We also observe a smooth evolution of the low-temperature magnetic susceptibility across the MIT and find no evidence of long-range magnetic order down to ~ 5% of the Curie-Weiss temperature. The results signal an abundance of low-energy spinful excitations on the insulating side that is further corroborated by the presence of the Pomeranchuk effect on the metallic side. Our results are consistent with the universal critical theory of a continuous MIT from a Landau Fermi liquid to a nonmagnetic Mott insulator in two dimensions.

cond-mat.str-el↗

arXiv2020

Direct observation of distinct minibands in moiré superlattices

Moiré superlattices comprised of stacked two-dimensional materials present a versatile platform for engineering and investigating new emergent quantum states of matter. At present, the vast majority of investigated systems have long moiré wavelengths, but investigating these effects at shorter, incommensurate wavelengths, and at higher energy scales, remains a challenge. Here, we employ angle-resolved photoemission spectroscopy (ARPES) with sub-micron spatial resolution to investigate a series of different moiré superlattices which span a wide range of wavelengths, from a short moiré wavelength of 0.5 nm for a graphene/WSe2 (g/WSe2) heterostructure, to a much longer wavelength of 8 nm for a WS2/WSe2 heterostructure. We observe the formation of minibands with distinct dispersions formed by the moiré potential in both systems. Finally, we discover that the WS2/WSe2 heterostructure can imprint a surprisingly large moiré potential on a third, separate layer of graphene (g/WS2/WSe2), suggesting a new avenue for engineering moiré superlattices in two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2020

Creation of moiré bands in a monolayer semiconductor by spatially periodic dielectric screening

Moiré superlattices of two-dimensional van der Waals materials have emerged as a powerful platform for designing electronic band structures and discovering emergent physical phenomena. A key concept involves the creation of long-wavelength periodic potential and moiré bands in a crystal through interlayer hybridization when two materials are overlaid. Here we demonstrate a new approach based on spatially periodic dielectric screening to create moiré bands in a monolayer semiconductor. It relies on reduced dielectric screening of the Coulomb interactions in monolayer semiconductors and their environmental dielectric-dependent electronic band structure. We observe optical transitions between moiré bands in monolayer WSe$_{2}$ when it is placed close to small angle-misaligned graphene on hexagonal boron nitride. The moiré bands are a result of long-range Coulomb interactions, strongly gate-tunable, and can have versatile superlattice symmetries independent of the crystal lattice of the host material. Our result also demonstrates that monolayer semiconductors are sensitive local dielectric sensors.

cond-mat.mtrl-sci↗

arXiv2020

Gate-tunable spin waves in antiferromagnetic atomic bilayers

The emergence of two-dimensional (2D) layered magnetic materials has opened an exciting playground for both fundamental studies of magnetism in 2D and explorations of spinbased applications. Remarkable properties, including spin filtering in magnetic tunnel junctions and gate control of magnetic states, have recently been demonstrated in 2D magnetic materials. While these studies focus on the static properties, dynamic magnetic properties such as excitation and control of spin waves have remained elusive. Here we excite spin waves and probe their dynamics in antiferromagnetic CrI3 bilayers by employing an ultrafast optical pump/magneto-optical Kerr probe technique. We identify sub-terahertz magnetic resonances under an in-plane magnetic field, from which we determine the anisotropy and interlayer exchange fields and the spin damping rates. We further show tuning of antiferromagnetic resonances by tens of gigahertz through electrostatic gating. Our results shed light on magnetic excitations and spin dynamics in 2D magnetic materials, and demonstrate their unique potential for applications in ultrafast data storage and processing.

cond-mat.mes-hall↗

arXiv2019

WSe2/WS2 moiré superlattices: a new Hubbard model simulator

The Hubbard model, first formulated by physicist John Hubbard in the 1960s, is a simple theoretical model of interacting quantum particles in a lattice. The model is thought to capture the essential physics of high-temperature superconductors, magnetic insulators, and other complex emergent quantum many-body ground states. Although the Hubbard model is greatly simplified as a representation of most real materials, it has nevertheless proved difficult to solve accurately except in the one-dimensional case. Physical realization of the Hubbard model in two or three dimensions, which can act as quantum simulators, therefore have a vital role to play in solving the strong-correlation puzzle. Here we obtain a quantum phase diagram of the two-dimensional triangular lattice Hubbard model by studying angle-aligned WSe2/WS2 bilayers, which form moiré superlattices because of the difference in lattice constant between the two two-dimensional materials. We probe both charge and magnetic properties of the system by measuring the dependence of optical response on out-of-plane magnetic field, and on gate-tuned carrier density. At half filling of the first hole moiré superlattice band, we observe a Mott insulating state with antiferromagnetic Curie-Weiss behavior as expected for a Hubbard model in the strong interaction regime. Past half filling, our experiment suggests an antiferromagnetic to paramagnetic quantum phase transition near 0.6 filling. Our results establish a new solid-state platform based on moiré superlattices which can be used to simulate outstanding problems in strong correlation physics that are manifested by triangular lattice Hubbard models.

cond-mat.mtrl-sci↗

arXiv2018

Spin transistor built on 2D van der Waals heterostructures

Spin transistors (whose on-off operation is achieved by electric-field-controlled spin orientation 1), if realized, can revolutionize modern electronics through the implementation of a faster and a more energy-efficient performance as well as non-volatile data storage 2, 3. The original proposal by Datta and Das 1 that relies on electric-field-controlled spin precession in a semiconductor channel faces significant challenges including inefficient spin injection, spin relaxation and spread of the spin precession angle 4, 5. Recent demonstration of electric-field switching of magnetic order 6-8 and spin filtering 9-12 in two-dimensional magnetic insulator CrI3 has inspired a new operational principle for spin transistors. Here we demonstrate spin field-effect transistors based on dual-gated graphene/CrI3 tunnel junctions. These devices show an ambipolar transistor behavior and tunnel magnetoresistance widely tunable by gating when the CrI3 magnetic tunnel barrier undergoes an antiferromagnetic-ferromagnetic spin-flip transition. Under a constant magnetic bias in the vicinity of the spin-flip transition, the gate voltage can repeatedly alter the device between a high and a low conductance state with a large hysteresis. This new spin transistor concept based on the electric-field-controlled spin-flip transition in the magnetic tunnel barrier is immune to interface imperfections and allows spin injection, control and detection in a single device.

cond-mat.mes-hall↗

arXiv2018

Controlling magnetism in 2D CrI3 by electrostatic doping

The atomic thickness of two-dimensional (2D) materials provides a unique opportunity to control material properties and engineer new functionalities by electrostatic doping. Electrostatic doping has been demonstrated to tune the electrical and optical properties of 2D materials in a wide range, as well as to drive the electronic phase transitions. The recent discovery of atomically thin magnetic insulators has opened up the prospect of electrical control of magnetism and new devices with unprecedented performance. Here we demonstrate control of the magnetic properties of monolayer and bilayer CrI3 by electrostatic doping using a dual-gate field-effect device structure. In monolayer CrI3, doping significantly modifies the saturation magnetization, coercive force and Curie temperature, showing strengthened (weakened) magnetic order with hole (electron) doping. Remarkably, in bilayer CrI3 doping drastically changes the interlayer magnetic order, causing a transition from an antiferromagnetic ground state in the pristine form to a ferromagnetic ground state above a critical electron density. The result reveals a strongly doping-dependent interlayer exchange coupling, which enables robust switching of magnetization in bilayer CrI3 by small gate voltages.

cond-mat.mtrl-sci↗