Pairing symmetry of twisted bilayer graphene: a phenomenological synthesis

TL;DR

This paper synthesizes experimental data on twisted bilayer graphene, arguing that its superconducting order parameter is likely nodal and a mixture of spin-singlet and triplet pairs, with the orbital parity remaining undetermined.

Contribution

It provides a phenomenological analysis suggesting the order parameter's nodal nature and mixed spin pairing, proposing experimental methods to distinguish orbital parity.

Findings

Superconducting order parameter is likely nodal.

Order parameter is an admixture of spin-singlet and triplet.

Orbital parity remains undetermined, with p-wave or d-wave possibilities.

Abstract

One of the outstanding questions in the study of twisted bilayer graphene -- from both experimental and theoretical points of view -- is the nature of its superconducting phase. In this work we perform a comprehensive synthesis of existing experiments, and argue that experimental constraints are strong enough to allow the structure of the superconducting order parameter to be nearly uniquely determined. In particular, we argue that the order parameter is nodal, and is formed from an admixture of spin-singlet and spin-triplet Cooper pairs. This argument is made on phenomenological grounds, without committing to any particular microscopic model of the superconductor. Existing data is insufficient to determine the orbital parity of the order parameter, which could be either p-wave or d-wave. We propose a way in which the measurement of Andreev edge states can be used to distinguish between the two.