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

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.