arXiv subjects

Zhongdong Han

Publications and source records attributed to Zhongdong Han.

13 recordsLinked to original sources

arXiv2026

Stacking-order-dependent electronic properties of MoTe2/WSe2 moiré bilayers

Transition metal dichalcogenide (TMD) moiré bilayers have realized a wide range of strongly correlated and topological phenomena. The physics in these materials is often sensitive to the interlayer stacking order. Polarization-resolved optical second harmonic generation (SHG) is the most used technique for stacking order characterization but unverified for most heterobilayers. Here we calibrate the optical SHG for angle-aligned MoTe2/WSe2 bilayers by the scanning transmission electron microscopy (STEM). We directly compare the transport and magnetic properties and the electronic phase diagram for two distinct stacking orders. With the calibrated stacking order assignment, we clarify the interpretation of earlier results, including the nature of the Chern insulator, mechanism of an electric-field-tuned metal-insulator transition at half band filling, and the Kondo lattice physics. Our work provides a consistent picture of the relation between the stacking order and the electronic properties of MoTe2/WSe2 moiré bilayers.

cond-mat.mes-hall↗

arXiv2025

Thermoelectricity of moiré heavy fermions in MoTe2/WSe2 bilayers

Tunable Kondo lattice and heavy fermion physics have been recently reported in moiré materials, but most of the studies have focused on the electrical and magnetic properties. Quantitative thermoelectric measurements, which can reveal entropic information of the heavy fermions, have yet to be achieved. Here, we report a comprehensive thermoelectric study on the moiré heavy fermion phase realized in hole-doped angle-aligned MoTe2/WSe2 bilayers. By electrically gating the material to the Kondo lattice region of the phase diagram, we observe a sign change in the Seebeck coefficient near the Kondo coherence temperature, where the heavy fermion phase with an electron-like Fermi surface evolves into an itinerant Fermi liquid with a hole-like Fermi surface. We compare the results with the semiclassical Mott relation and discuss the observed discrepancies. In addition to the thermal dissociation of Kondo singlets in the heavy Fermi liquid, a sign change accompanied by a strong peak in the Seebeck coefficient is also observed near a Zeeman breakdown of the Kondo singlets, signaling an entropy accumulation. Our results provide entropic information on both the formation and breakdown of heavy fermions in moiré semiconductors.

cond-mat.str-el↗

arXiv2025

Quantum oscillations between excitonic and quantum spin Hall insulators in moiré WSe2

Quantum spin Hall insulators (QSHIs) and excitonic insulators (EIs) are prototypical topological and correlated states of matter, respectively. The topological phase transition between the two has attracted much theoretical interest but experimental studies have been hindered by the availability of tunable materials that can access such a transition. Here, by utilizing the interaction-enhanced g-factor and the flat moiré bands in twisted bilayer WSe2 (tWSe2), we realize tunable electron-like and hole-like Landau levels (LLs) in the opposite valleys of tWSe2 under a perpendicular magnetic field. At half-band-filling, which corresponds to electron-hole charge neutrality, periodic oscillations between QSHIs (for fully filled LLs) and EIs (for half-filled LLs) are observed due to the interplay between the cyclotron energy and the intervalley correlation; QSHIs with up to four pairs of helical edge states can be resolved. We further analyze the effect of Fermi surface nesting on the stability of EIs via electric field-tuning of the moiré band structure. Our results demonstrate a novel QSHI-to-EI topological phase transition and provide a comprehensive understanding of the fermiology of tWSe2.

cond-mat.mes-hall↗

arXiv2025

Simulating high-temperature superconductivity in moiré WSe2

The emergence of high transition temperature (Tc) superconductivity in strongly correlated materials remains a major unsolved problem in physics. High-Tc materials, such as cuprates, are generally complex and not easily tunable, making theoretical modelling difficult. Although the Hubbard model--a simple theoretical model of interacting electrons on a lattice--is believed to capture the essential physics of high-Tc materials, obtaining accurate solutions of the model, especially in the relevant regime of moderate correlation, is challenging. The recent demonstration of robust superconductivity in moiré WSe2, whose low-energy electronic bands can be described by the Hubbard model and are highly tunable, presents a new platform for tackling the high-Tc problem. Here, we tune moiré WSe2 bilayers to the moderate correlation regime through the twist angle and map the phase diagram around one hole per moiré unit cell (v = 1) by electrostatic gating and electrical transport and magneto-optical measurements. We observe a range of high-Tc phenomenology, including an antiferromagnetic insulator at v = 1, superconducting domes upon electron and hole doping, and unusual metallic states at elevated temperatures including strange metallicity. The highest Tc occurs adjacent to the Mott transition, reaching about 6% of the effective Fermi temperature. Our results establish a new material system based on transition metal dichalcogenide (TMD) moiré superlattices that can be used to study high-Tc superconductivity in a highly controllable manner and beyond.

cond-mat.supr-con↗

arXiv2025

Evidence of topological Kondo insulating state in MoTe2/WSe2 moiré bilayers

Topological Kondo insulators (TKIs) are topologically protected insulating states induced not by single-particle band inversions, but by the Kondo interaction between itinerant electrons and a lattice of local magnetic moments. Although experiments have suggested the emergence of three-dimensional (3D) TKIs in the rare earth compound SmB6, its two-dimensional (2D) counterpart has not been demonstrated to date. Here we report experimental evidence of a TKI in angle-aligned MoTe2/WSe2 moiré bilayers, which support a Kondo lattice with topologically nontrivial Kondo interactions. We prepare in a dual-gated device a triangular lattice Mott insulator in the MoTe2 layer Kondo-coupled to a half-filled itinerant band in the WSe2 layer. Combined transport and compressibility measurements show that the prepared state supports metallic transport at high temperatures and, at low temperatures, an insulating bulk with conducting helical edge states protected by spin-Sz conservation. The presence of Kondo singlets is further evidenced by their breakdown at high magnetic fields. Such behaviors are in stark contrast to the simple metallic state when the Mott insulator in the MoTe2 layer is depleted by gating. Our results open the door for exploring tunable topological Kondo physics in moiré materials.

cond-mat.mes-hall↗

arXiv2025

Emergence of Chern metal in a moiré Kondo lattice

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

cond-mat.str-el↗

arXiv2024

Correlated states controlled by tunable van Hove singularity in moiré WSe2

Twisted bilayers of transition metal dichalcogenide semiconductors have enabled the discovery of superconductivity, ferromagnetism, correlated insulators and a series of new topological phases of matter. However, the connection between these electronic phases and the underlying band structure singularities in these materials has remained largely unexplored. Here, combining the magnetic circular dichroism and electronic compressibility measurements, we investigate the influence of a van Hove singularity on the correlated phases in bilayer WSe2 with twist angle between 2-3 degrees. We demonstrate stabilizing the Stoner ferromagnetism below moiré lattice filling one and Chern insulators at filling one by tuning the van Hove singularity cross the Fermi level using the electric and magnetic fields. The experimental observations are supported by the continuum model band structure calculations. Our results highlight the prospect of engineering the electronic phases by tunable van Hove singularities.

cond-mat.str-el↗

arXiv2024

Unconventional superconductivity in twisted bilayer WSe2

Moiré materials have enabled the realization of flat electron bands and quantum phases that are driven by strong correlations associated with flat bands. Superconductivity has been observed, but solely, in graphene moiré materials. The absence of robust superconductivity in moiré materials beyond graphene, such as semiconductor moiré materials, has remained a mystery and challenged our current understanding of superconductivity in flat bands. Here, we report the observation of robust superconductivity in 3.65-degree twisted bilayer WSe2 which hosts a honeycomb moiré lattice. Superconductivity emerges at half-band filling and under small sublattice potential differences, where the moiré band is a flat Chern band. The optimal superconducting transition temperature is about 220 mK and constitutes 2% of the effective Fermi temperature; the latter is comparable to the value in high-temperature cuprate superconductors and suggests strong pairing. The superconductor borders on two distinct metals below and above half-band filling; it undergoes a continuous transition to a correlated insulator by tuning the sublattice potential difference. The observed superconductivity on the verge of Coulomb-induced charge localization suggests roots in strong electron correlations.

cond-mat.mes-hall↗

arXiv2024

Magneto-Induced Topological Phase Transition in Inverted InAs/GaSb Bilayers

We report a magneto-induced topological phase transition in inverted InAs/GaSb bilayers from a quantum spin Hall insulator to a normal insulator. We utilize a dual-gated Corbino device in which the degree of band inversion, or equivalently the electron and hole densities, can be continuously tuned. We observe a topological phase transition around the magnetic field where a band crossing occurs, that is accompanied by a bulk-gap closure characterized by a bulk conductance peak (BCP). In another set of experiments, we study the transition under a tilted magnetic field (tilt angle $θ$). We observe the characteristic magneto-conductance around BCP as a function of $θ$, which dramatically depends on the density of the bilayers. In a relatively deep-inversion (hence a higher density) regime, where the electron-hole hybridization dominates the excitonic interaction, the BCP grows with $θ$. On the contrary, in a shallowly-inverted (a lower density) regime, where the excitonic interaction dominates the hybridization, the BCP is suppressed indicating a smooth crossover without a gap closure. This suggests the existence of a low-density, correlated insulator with spontaneous symmetry breaking near the critical point. Our highly controllable electron-hole system offers an ideal platform to study interacting topological states as proposed by recent theories.

cond-mat.mes-hall↗

arXiv2023

Emergence of ferromagnetism at the onset of moiré Kondo breakdown

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

cond-mat.str-el↗

arXiv2022

Gate-tunable heavy fermions in a moiré Kondo lattice

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

cond-mat.str-el↗

arXiv2019

Anomalous Conductance Oscillations in the Hybridization Gap of InAs/GaSb Quantum Wells

We observe the magnetic oscillation of electric conductance in the two-dimensional InAs/GaSb quantum spin Hall insulator. Its insulating bulk origin is unambiguously demonstrated by the antiphase oscillations of the conductance and the resistance. Characteristically, the in-gap oscillation frequency is higher than the Shubnikov-de Haas oscillation close to the conduction band edge in the metallic regime. The temperature dependence shows both thermal activation and smearing effects, which cannot be described by the Lifshitz-Kosevich theory. A two-band Bernevig-Hughes-Zhang model with a large quasiparticle self-energy in the insulating regime is proposed to capture the main properties of the in-gap oscillations.

cond-mat.mes-hall↗

arXiv2017

Tuning Edge States in Strained-Layer InAs/GaInSb Quantum Spin Hall Insulators

We report on a class of quantum spin Hall insulators (QSHIs) in strained-layer InAs/GaInSb quantum wells, in which the bulk gaps are enhanced by up to five folds as compared to the binary InAs/GaSb QSHI. Remarkably, with consequently increasing edge velocity, the edge conductance at zero and applied magnetic fields manifests time reversal symmetry (TRS) -protected properties consistent with Z2 topological insulator. The InAs/GaInSb bilayers offer a much sought-after platform for future studies and applications of the QSHI.

cond-mat.mes-hall↗