The compliance of the upper critical field in magic-angle multilayer graphene with the Pauli limit

TL;DR

This paper re-evaluates the upper critical field in magic-angle multilayer graphene, demonstrating that it aligns with the Pauli limit when considering strong coupling effects, contrary to previous claims of violation.

Contribution

It provides a revised calculation of the Pauli limiting field for strong-coupled superconductors, resolving apparent violations in magic-angle multilayer graphene.

Findings

Observed upper critical fields are consistent with the Pauli limit when strong coupling is considered.

Previous reports of violation were due to weak-coupling assumptions in calculations.

The results clarify the fundamental limits of superconductivity in Moire multilayer graphene.

Abstract

The Pauli limiting field represents fundamental magnetic field at which superconducting state collapses due to the spin-paramagnetic Cooper pair-breaking effect. Cao et al [1] reported that the upper critical field (i.e., the magnetic field at which the superconducting state collapses in experiment) in magic-angle twisted trilayer graphene (MATNG, N=3) in 2-3 times exceeds the Pauli limiting field in this material. This observation was interpreted as a violation of the Pauli-limiting field in MAT3G. Similar conclusions were recently reported by the same research group in MATNG (N=4,5) superlattices [2]. Here we point out that Cao et al [1] calculated the Pauli limiting field by the use of reduce (to the weak-coupling limit) full equation of the electron-phonon mediated superconductivity. Considering, that in the same paper, Cao et al [1] reported that MATNGs are strong coupled superconductors, we calculate the Pauli limiting field for strong coupled case and show that the observed upper critical fields in MATNGs comply with the Pauli limit. This implies that there is no violation of the Pauli limiting field in the Moire multilayer graphene superlattices.