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.

💡 Research Summary

In this work the authors realize an artificial Kondo lattice using angle‑aligned MoTe₂/WSe₂ heterobilayers, where the MoTe₂ layer is tuned into a Mott insulating state that hosts a triangular array of localized magnetic moments, while the WSe₂ layer remains metallic and supplies itinerant carriers. By fabricating dual‑gate devices they can independently control the total moiré hole filling ν, the perpendicular electric displacement field D, and an out‑of‑plane magnetic field B. The moiré period (~4.8 nm) and density (~4.5 × 10¹² cm⁻²) are set by the ~7 % lattice mismatch, and the sub‑lattice potential between the two layers can be tuned with D.

The phase diagram contains five regions previously identified in similar systems. Region IV is the Kondo‑lattice regime (ν = 1 + ν_W) where the Mo layer is a Mott insulator (ν_Mo = 1) and the W layer hosts a tunable hole density ν_W. In this regime the low‑energy physics maps onto a chiral Kondo lattice model with an exchange J_K, an intralayer hopping t for the W‑band, and a p‑wave chiral hybridization that can open a topological Kondo gap.

The central discovery is a symmetry‑broken Chern insulator that appears at the fractional moiré filling ν = 4/3. Transport measurements at 20 mK reveal a deep minimum in the longitudinal resistance (R_xx ≈ 200 Ω) accompanied by a quantized Hall conductance σ_xy = e²/h. This quantization persists only within a narrow magnetic‑field window (≈ 2–5 T) and disappears abruptly when the field exceeds a critical value B_c. The same field B_c marks the Kondo breakdown: Zeeman energy overcomes the effective Kondo exchange, the heavy‑fermion Fermi surface collapses, and Shubnikov‑de Haas oscillations from the decoupled W‑layer holes emerge. The Hall resistance changes sign at B_c, indicating a reconstruction from a large, Kondo‑hybridized electron‑like Fermi surface to a small, hole‑like one.

The symmetry‑broken Chern insulator is therefore stabilized precisely at the brink of Kondo breakdown, where local‑moment fluctuations are strongest. It is observed only in the mixed‑valence region V (where the Mo upper Hubbard band and the W band are both partially filled) and not in the pure Kondo‑lattice region IV, underscoring the importance of charge‑fluctuation physics in this regime.

A second device with a finite twist angle of 1.5° reproduces the phenomenon but shifts the required magnetic field to 5–7 T, consistent with the higher moiré density enhancing the effective Kondo coupling and raising B_c. The twist‑angle dependence is systematic across four devices: larger angles increase the overall field scale but slightly narrow the field window where the ν = 4/3 state survives. In the 1.5° device the Hall quantization is weaker (maximum σ_xy ≈ 10 kΩ) and the longitudinal dip is absent, indicating that the Chern insulator is more fragile away from perfect alignment.

The authors discuss that the chiral Kondo interaction introduces a p‑wave hybridization gap that carries a non‑uniform Berry curvature across the moiré Brillouin zone. This non‑uniformity can allow an integer Chern number (≈ 1) to be realized even at a fractional filling, effectively forming a topological charge‑density‑wave state. However, a detailed microscopic theory of why the ν = 4/3 state appears only in region V and how the fluctuating local moments cooperate with the topological gap remains an open problem.

In summary, the paper provides (i) the first experimental observation of a symmetry‑broken integer Chern insulator at a fractional moiré filling, (ii) clear evidence that its stability is tied to the proximity of a magnetic‑field‑induced Kondo breakdown, and (iii) a demonstration that both twist angle and magnetic field are powerful knobs to tune the interplay of strong correlations, topology, and Kondo physics in moiré semiconductors. The work opens new avenues for exploring exotic topological phases emerging from Kondo hybridization and for engineering correlated topological states in two‑dimensional heterostructures.