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

TL;DR

Researchers explore a new type of superconducting state in graphene systems that could explain recent experimental observations.

Contribution

They identify a unique superconducting state with three electrons and one hole, which may explain properties seen in graphene moiré systems.

Findings

The model exhibits vestigial phases like charge-4e superconductivity and broken time-reversal symmetry.

A phase with finite d ⋅ N shows BCS-like symmetries and metastable supercurrents but different spectral properties.

The electronic density of states suppression resembles both fully gapped and nodal superconductors.

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-4e 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.

P. Poduval et al. theoretically study the nonzero-temperature vestigial phases of a 2D model exhibiting both triplet superconductivity and magnetism. They show that this model allows for a unique superconducting state in which the condensate consists of entities with three electrons and one hole, with properties similar to those seen in experiments on moiré systems.

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