Prominent Cooper pairing away from the Fermi level and its spectroscopic signature in twisted bilayer graphene

Abstract

We investigate phonon-mediated Cooper pairing in flat electronic band systems by solving the full-bandwidth multiband Eliashberg equations for superconductivity in magic angle twisted bilayer graphene using a realistic tight-binding model. We find that Cooper pairing away from the Fermi level contributes decisively to superconductivity by enhancing the critical temperature and ensures a robust finite superfluid density. We show that this pairing yields particle-hole asymmetric superconducting domes in the temperature-gating phase diagram and gives rise to distinct spectroscopic signatures in the superconducting state. We predict several such features in tunneling and angle resolved photoemission spectra for future experiments.

Cascade of replica bands in flat band systems: predictions for twisted bilayer graphene

“…This system exhibits two flat bands close to the Fermi energy, as reported by both theory [17][18][19] and experiment [20][21][22][23][24]. These bands are gaped out from the remaining electron energies, and, together with a reasonably large phonon frequency [25,26] that has been shown to be relevant for explaining superconductivity [27], constitute an ideal testing ground for the phenomenon discovered here. Although the predicted intensity of the replica bands is relatively weak, we propose here their detection in TBG in the foreseeable future.…”

“…For further details on the theory see also Refs. [12,27] and Appendix A. For sake of clarity, we note that the hereinvestigated phenomenon cannot be described within the Bardeen-Cooper-Schrieffer (BCS) model, which assumes a frequency-independent order parameter, absence of mass renomalization, and an electronic bandwidth W that is much larger than the phonon frequency, W ≫ Ω.…”

“…This model has two flat bands near the Fermi level with a narrow bandwidth W ≈ 7 meV which are energetically separated from the rest of the bands by energy gaps over 20 meV. Further, we consider the normal state, T = 1.6 K > T c [15,16] and choose an electron-phonon scattering strength g 0 = 1.6 meV [27]. When perform-ing our analysis for the two flat bands only, the resulting spectra are very similar to our model calculations above and we present the outcomes in Appendix D. Next, we take into account a total of four energy bands with bandwidth W ≃ 127 meV > Ω.…”

Coulomb interaction, phonons, and superconductivity in twisted bilayer graphene

“…The pairing interaction responsible for superconductivity in TBG has been intensively studied. Among other possible pairing mechanisms, the effect of phonons (12)(13)(14)(15)(16)(17)(18)(19) (see also ref. 20), the proximity of the chemical potential to a van Hove singularity in the density of states (DOS) (21)(22)(23)(24)(25) and excitations of insulating phases (26)(27)(28) (see also refs.…”

Going beyond the Chandrasekhar-Clogston limit in a flat-band superconductor

“…Early studies identified that the presence of a flat-band could in fact give rise to a linear dependence of the transition temperature on the strength of the attractive interactions [46,47], generating hope of achieving high critical temperatures. With the discovery of superconductivity in magic-angle twisted-bilayer graphene [43], interest in flat-band superconductivity rocketed [48][49][50][51][52]. Recently, it has also been discovered that superconductivity in twisted trilayer graphene can survive in-plane magnetic fields beyond the Chandrasekhar-Clogston limit [53], which has been interpreted as an indication of spin-triplet pairing [53,54].…”