Kin Fai Mak

A Chern metal is a two-dimensional metallic state of matter carrying chiral edge states. We report the observation of a Chern metal at the onset of the magnetic Kondo breakdown in a frustrated moiré Kondo lattice--angle-aligned MoTe ₂ /WSe ₂ bilayers. The state is compressible and is manifested by a nearly quantized Hall resistance and a finite longitudinal resistance arising from a bad metallic bulk. 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 through the interplay between band topology and Kondo interactions.

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.

The realization of Coulomb-coupled electron–hole double layers has enabled the exploration of equilibrium excitons—bound electron–hole pairs—without a magnetic field. Doping an exciton fluid with additional electrons or holes further creates an equilibrium fluid of trions—bound states of two electrons and one hole and vice versa. Therefore, a trion Hall effect under a magnetic field is expected but has not been demonstrated. Here we report the observation of the trion Hall effect in MoSe ₂ /WSe ₂ heterostructures, which support Coulomb-coupled layers of electrons and holes with continuously tunable densities. The effect arises from the Lorentz force on trions subjected to a perpendicular magnetic field. By combining magnetotransport and Coulomb drag measurements, we find that an electron-like Hall effect emerges on the hole layer in the presence of negatively charged trions because of Coulomb drag. Furthermore, the effect disappears when trions dissociate at elevated temperatures or high doping densities. These results suggest that transition metal dichalcogenide heterostructures are a candidate for realizing trion-based quantum Hall phenomena.

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 MoTe 2 / WSe 2 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 electronlike Fermi surface evolves into an itinerant Fermi liquid with a holelike 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.

Topologically nontrivial materials are distinguished by their inherent robustness against symmetry-preserving perturbations. However, the resilience of the topologically protected degeneracies against electronic instabilities remains largely unexplored. Here, by employing transport measurements, we demonstrate a layer-number-dependent metal-insulator transition in the topological semimetal Nb₃SiTe₆, which has a layered structure and exhibits Fermi-liquid-like metallicity with a hole carrier density of ∼3.3 × 10²¹ cm^-3 down to the bilayers. An unexpected insulating ground state only emerges in the monolayer and can be effectively tuned by electrostatic gating. The charge neutrality point coincides with a sign change of the Hall signal. Such ambipolar gating behavior contradicts the degeneracy protection under the glide-mirror symmetry in the single-particle picture. Combined with theoretical calculations, we suggest that electronic instabilities could offer a plausible mechanism for gap opening at the symmetry-protected nodal lines. Our findings provide crucial insights for understanding the rich dimension-related metal-insulator transitions in topological materials.

Consecutive topological phase transitions (TPTs) between strongly correlated electronic phases that differ simultaneously in symmetry breaking and topological order are of fundamental interest in condensed matter physics, yet, rarely realized experimentally. We report two consecutive electric-field-driven TPTs at half filling (ν=1) in angle-aligned MoTe_{2}/WSe_{2} moiré heterobilayers. With increasing out-of-plane displacement field, a geometrically frustrated Mott insulator evolves into a ferromagnetic quantum anomalous Hall (QAH) Mott insulator, i.e., a spin-polarized topological Mott insulator without an observable charge-gap closure, and subsequently into an antiferromagnetic, valley-coherent Mott insulator (VC-AFM) accompanied by a continuous charge-gap collapse and the emergence of a critical metallic state. Layer-resolved magnetic circular dichroism (MCD), magneto-transport, and compressibility measurements jointly determine the phase diagram. The high-field evolution of the antiferromagnetic state reveals a metamagnetic-like transition at a critical field B^{*}, above which a Chern insulating transport response reappears. Our results establish the MoTe_{2}/WSe_{2} moiré platform as a tunable realization of an extended Kane-Mele-Hubbard model hosting sequential correlation-topology intertwined transitions.

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.

Topological Kondo insulators are a topologically protected insulating state induced by Kondo exchange interactions between itinerant electrons and local magnetic moments, as opposed to single-particle band inversion. They are characterized by an insulating bulk with Dirac surface states in three dimensions and helical edge states in two dimensions. Although experiments have supported the emergence of these insulators in the rare-earth compound SmB ₆ , their observation in two-dimensional systems has not been demonstrated. Here we report the experimental evidence of a two-dimensional topological Kondo insulator in MoTe ₂ /WSe ₂ moiré bilayers. Using dual-gated devices, we prepare a triangular lattice of local moments in the MoTe ₂ layer and a half-filled dispersive band in the WSe ₂ layer with a chiral Kondo coupling. Using transport and compressibility measurements, we show that the state supports metallic transport at high temperature and an insulating bulk with helical edge conduction protected by spin- S _z conservation at low temperature. Under high magnetic fields, the insulating state at low temperature becomes metallic. Our results demonstrate a highly tunable platform based on moiré materials for studying the interplay of strong interactions and topological order.