Unconventional superconducting states of interlayer pairing in bilayer and trilayer graphene

TL;DR

This paper develops a theory for unconventional interlayer superconductivity in bilayer and trilayer graphene, revealing unique temperature behaviors, doping effects, and phase transitions influenced by stacking order and doping differences.

Contribution

It introduces a novel theory for interlayer pairing in graphene, describing unconventional superconducting states with unique temperature and doping dependencies, and analyzes phase transitions in trilayer configurations.

Findings

Gapless superconductivity with temperature-induced condensation in bilayer graphene.

Doping opens a gap and alters the temperature dependence of pairing.

Distinct phase transition types in ABA and ABC trilayer graphene.

Abstract

We develop a theory for interlayer pairing of chiral electrons in graphene materials which results in an unconventional superconducting (S) state with s-wave spin-triplet order parameter. In a pure bilayer graphene, this superconductivity exhibits a gapless property with an exotic effect of temperature-induced condensation causing an increase of the pairing amplitude (PA) with increasing temperature. We find that a finite doping opens a gap in the excitation spectrum and weakens this anomalous temperature-dependence. We further explore the possibility of realizing variety of pairing patterns with different topologies of the Fermi surface, by tuning the difference in the doping of the two layers. In trillayer graphene, the interlayer superconductivity is characterized by a two components order parameter which can be used to define two distinct phases in which only one of the components is non vanishing. For ABA stacking the stable state is determined by a competition between these two phases. By varying the relative amplitude of the corresponding coupling strenghes, a first order phase transition can occur between these two phases. For ABC stacking, we find that the two phases coexist with a possibility of a similar phase transition which turns out to be second order.