Theory for nodal BCS-BEC transition in twisted trilayer graphene

Recent transport [1] and scanning tunneling microscopy [2] experiments on hole-doped twisted trilayer graphene reveal two superconducting regimes, differentiated by both the measured coherence length and tunneling conductance. In this talk, we will discuss how these regimes can be understood as a gate-tuned phase transition separating gapped BEC-like and gapless BCS-like superconducting phases with a common nodal order parameter [3]. We specifically analyze the nature of the transition (including interaction effects), the behavior of the coherence length, and tunneling signatures---demonstrating consistency with experiment.

[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

[3] S. S. Botelho and C. A. R. Sá de Melo. Lifshitz transition in d-wave superconductors. Phys. Rev. B 71, 134507 (2005).