Spin-triplet superconductivity from excitonic effect in doped insulators

Abstract

Significance We present a mechanism for unconventional superconductivity in doped band insulators, where short-ranged pairing interaction arises from Coulomb repulsion due to virtual interband or excitonic processes. Remarkably, electron pairing is found upon infinitesimal doping, giving rise to Bose–Einstein condensate (BEC)–Bardeen–Cooper–Schrieffer (BCS) crossover at low density. Our theory explains puzzling behaviors of superconductivity and predicts spin-triplet pairing in electron-doped ZrNCl and WTe 2 .

Non-Abelian topological superconductivity in maximally twisted double-layer spin-triplet valley-singlet superconductors

“…Similar results have also been reported in Bernal bilayer graphene (BBG) [35], and both experimental observations were interpreted theoretically as a signature of spintriplet f -wave pairing [36,37]. More recently, Crépel and Fu proposed that a novel 'three-particle' mechanism involving virtual excitons generically gives rise to spintriplet pairing in a two-valley electron liquid formed in doped insulators such as ZrNCl [38].…”

“…The proposed mechanism applies in general to any material with hexagonal symmetry that exhibits gapped f -wave superconductivity in its monolayer form. Here we discuss how the exotic T -broken phase and the non-Abelian excitations can be realized in twisted double layers formed by the two most promising candidate materials, rhombohedral graphene [34,35] and ZrNCl [38], which are thought to be STVS superconductors.…”

“…2a). Such Hamiltonians are widely used to model systems with hexagonal symmetry whose FS consist of disconnected segments around K-points [38,42] shown in Fig. 2b.…”

“…Motivated by the phenomenology observed in RTG and BBG systems [34,35] and by theoretical ideas introduced in Ref. [38], we focus here on equal-spin pairing between electrons belonging to opposite valleys and drop the spin index in our discussions. Moreover, we consider hole doping near K-points by setting t > 0 with band maxima located at ±K given by E max = 3t in each isolated layer (the case of electron doping can be covered by setting t < 0 with E min = 3t).…”

“…While microscopic mechanisms leading to STVS superconductivity may vary across materials such as RTG/BBG [36,37] and ZrNCl [38], on general grounds the interaction responsible for STVS pairing boils down to an effective attraction between electrons in the spintriplet f -wave channel. In the momentum-space representation, the interaction within each isolated layer of STVS superconductor has the form…”