Superconducting Orbital Magnetoelectric Effect and its Evolution across the Superconductivity Normal Metal Phase Transition

TL;DR

This paper investigates the orbital magnetoelectric effect in superconductors, revealing how orbital magnetic moments influence magnetization across phase transitions and proposing experimental detection methods in materials like twisted bilayer graphene.

Contribution

It introduces a unified theory for current-induced spin and orbital magnetization in superconductors, highlighting the role of orbital moments and predicting effects in novel materials.

Findings

Orbital magnetic moments can induce significant orbital magnetization in superconductors.

The current-induced magnetization remains unchanged in uniform superconductors across the phase transition.

Nonuniform pairing causes abrupt changes in magnetization near the transition.

Abstract

Superconducting magnetoelectric effect, which is the current-induced magnetization in a superconductor, mainly focused on the spin magnetization in previous studies, but ignore the effect of the orbital magnetic moments carried by the paired Bloch electrons. In this work, we show that orbital magnetic moments in superconductors can induce large orbital magnetization in the presence of a current. We constructed a unified description for the current-induced spin and orbital magnetization across the superconductivity normal metal phase transition. We find that in a superconductor with uniform pairing, the current-induced magnetization at a given current density is the same as that in its normal metal state, while with the nonuniform superconducting pairing, the current-induced magnetization exhibits an abrupt change in magnitude near the superconductivity normal metal phase transition. Importantly, our theory predicts the orbital magnetoelectric effect in superconducting twisted bilayer graphene which has paired Bloch electrons with large orbital magnetic moments and negligible spin-orbit coupling. We propose that the measurement of the current-induced orbital magnetoelectric effect can be used to detect the possible nonuniform pairings in twisted bilayer graphene and other newly discovered superconductors with non-trivial Berry curvatures.