Vestigial singlet pairing in a fluctuating magnetic triplet superconductor and its implications for graphene superlattices

Abstract

Stacking and twisting graphene layers allows to create and control a two-dimensional electron liquid with strong correlations. Experiments indicate that these systems exhibit strong tendencies towards both magnetism and triplet superconductivity. Motivated by this phenomenology, we study a 2D model of fluctuating triplet pairing and spin magnetism. Individually, their respective order parameters, d and N , cannot order at finite temperature. Nonetheless, the model exhibits a variety of vestigial phases, including charge-4 e superconductivity and broken time-reversal symmetry. Our main focus is on a phase characterized by finite d ⋅ N , which has the same symmetries as the BCS state, a Meissner effect, and metastable supercurrents, yet rather different spectral properties: most notably, the suppression of the electronic density of states at the Fermi level can resemble that of either a fully gapped or nodal superconductor, depending on parameters. This provides a possible explanation for recent tunneling experiments in the superconducting phase of graphene moiré systems.