arXiv subjects

Yiyu Xia

Publications and source records attributed to Yiyu Xia.

6 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

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↗

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↗