Topological Chiral Superconductivity Mediated by Intervalley Antiferromagnetic Fluctuations in Twisted Bilayer WSe$_2$

Motivated by the recent observations of superconductivity in twisted bilayer WSe$_2$ (tWSe$_2$), we theoretically investigate the superconductivity driven by electronic mechanism. We first demonstrate that the multi-band screened Coulomb interaction within the random phase approximation is insufficient to induce observable pairing instability. Nevertheless, by further including the intervalley antiferromagnetic fluctuations, the pairing interaction is substantially enhanced, yielding superconductivity with critical temperature $T_c$ of hundreds of millikelvin at van Hove singularities. The predicted $T_c$ increases with increasing the displacement field and corresponds to a doubly-degenerate $d$-wave-like pairing, which evolves into topological chiral $d \pm id$ superconductor below $T_c$. The interplay between superconductivity and intervalley antiferromagnetism results in a phase diagram consistent with experimental observations.These findings establish intervalley fluctuations as the primary pairing glue in tWSe$_2$.

💡 Research Summary

The paper addresses the origin of superconductivity observed in twisted bilayer WSe₂ (tWSe₂) at a twist angle of 5°, focusing on an electronic pairing mechanism. The authors first construct a realistic continuum model for the moiré bands, incorporating spin‑valley locking, effective mass (m*≈0.43 m₀), and a displacement‑field‑controlled interlayer potential Vz. The model reproduces a narrow bandwidth (~80 meV) and shows that saddle points in the band structure move with Vz, giving rise to van Hove singularities (VHS) whose density of states (DOS) grows as Vz increases.

Next, they evaluate the screened Coulomb interaction within a multi‑band random phase approximation (RPA). The bare 2D Coulomb potential V₀(q)=2πe²/(εq) tanh(q ds) is strongly screened by the static charge polarization Π(q), which includes contributions from all moiré bands. The resulting RPA‑screened interaction V_s is too weak to generate a measurable pairing instability: a Kohn‑Luttinger analysis yields critical temperatures of only a few tens of millikelvin, far below the experimentally reported 0.2–0.4 K.

To resolve this discrepancy, the authors introduce intervalley antiferromagnetic (IV‑AF) fluctuations, which are enhanced by the displacement field. In tWSe₂, the K and K′ valleys are related by time‑reversal symmetry, and spin‑valley locking forces intervalley particle‑hole excitations to carry an antiferromagnetic character. The intervalley susceptibility χ_KK′(q_ex) diverges at low temperature for nesting vectors q_ex that connect opposite‑valley Fermi surfaces, especially near the VHS. By dressing the RPA‑screened Coulomb interaction with this susceptibility, they obtain an effective pairing interaction

V_eff(q_ex)=V_KK′(q_ex) /