Shaffique Adam

When the relative rotation between two sheets of graphene is set to be close to special angles (called “magic angles”), the low-energy effective theory features Dirac fermions with very flat bands. Interest in understanding the competing mechanisms at play was reignited recently with the experimental observations of superconductivity and strongly correlated insulating phases in such twisted moiré heterostructures. This talk will address three important aspects: first, we provide a rigorous theory for the phonon-dominated transport in twisted bilayer graphene. An accurate treatment of the electron-phonon scattering requires us to go well beyond the usual treatment, including both intraband and interband processes, considering the finite-temperature dynamical screening of the electron-phonon matrix element, and going beyond the linear Dirac dispersion [1]. This allows us to explain the unusual signatures in the resistivity seen experimentally (including the variation with electron density, temperature, and twist angle) showing good quantitative agreement with recent experiments in the metallic regime. Second, despite the moire magnification of phonon couplings, we show a phonon-mechanism is unable to yield the range of critical temperatures observed in experiment. Using the Migdal-Eliashberg framework on a one-parameter effective lattice model for tBG we show that a superconducting state can be achieved by purely electronic mechanism [2]. Our theory for unconventional superconductivity gives robust features including an asymmetrical superconducting dome and the magnitude of the critical temperature comparable with experiments. Finally, following our success in understanding the effect of electron-electron interactions on Dirac fermions [3], we discuss how the Coulomb interaction can be added to models for twisted bilayer graphene.

[1] Girish Sharma, Indra Yudhistira, Nilotpal Chakraborty, Derek Ho, Michael Fuhrer, Giovanni Vignale, and Shaffique Adam, “Carrier transport theory for twisted bilayer graphene in the metallic regime”, Nature Communications 12, 5737 (2021).
[2] Girish Sharma, Maxim Trushin, Oleg Sushkov, Giovanni Vignale, and Shaffique Adam “Superconductivity from collective excitations in magic-angle twisted bilayer graphene”, Phys. Rev. Research, Rapid Comm. 2, 022040 (2020).
[3] Hokin Tang, Jia Ning Leaw, Joao Rodrigues, Igor Herbut, Pinaki Sengupta, Fakher Assaad, and Shaffique Adam, " The role of electron-electron interactions in two-dimensional Dirac fermions", Science 361 570 (2018).