Bose-Einstein condensation with a twist
Abstract: Superconductivity is a collective state in which many fermions pair up to give rise to a zero-resistance electron transport regime. The majority of superconductors are well described by a Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity in which a weak momentum space attraction between fermions forms bound Cooper pairs separated in real space. When attraction becomes strong the Cooper pair size shrinks until it effectively forms a diatomic Bose molecule that can, in turn, undergo Bose-Einstein condensation (BEC). A solid-state realization of such a weak BCS to a strong BEC pairing crossover has, so far, eluded experimental observations. Recent electron transport experiments in twisted trilayer graphene [1] raise the possibility of such a transition revealing two distinct superconducting regimes. In this talk, I will discuss how these two regimes can be understood as a gate-tuned phase transition separating gapped BEC-like and gapless BCS-like superconducting phases. In particular, I will propose precise tunneling conductance signatures and demonstrate their consistency with the recent scanning tunneling microscopy measurements on twisted trilayer graphene [2].

[1] Park, J.M., Cao, Y., et al. Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene. Nature 590, 249–255 (2021)
[2] Kim, H., Choi, Y., et al. Spectroscopic Signatures of Strong Correlations and Unconventional Superconductivity in Twisted Trilayer Graphene. arXiv: 2109.12127