Chirality crossover of Andreev reflection in a twisted graphene bilayer

Abstract

We investigate the Andreev reflection in a normal conductor/superconductor junction based on a twisted graphene bilayer, which gives rise to a feasible crossover between linear and quadratic dispersion relations characterized by Berry phases. It is found that in the low-energy linear region, the specular Andreev reflection shows an obvious enhancement with the subgap differential conductance being twice that in a graphene monolayer, demonstrating the chirality of a single layer; however, by tuning the external potential, the special spinor wave functions with 2π Berry phase lead to a remarkable suppression of conductance. The transition of different chiralities on the saddle point corresponding to the van Hove singularity in the electronic density of states can be confirmed by measurable signal reversal in conductance. These results provide a facile way to control the Andreev reflection in a twisted graphene bilayer.