Thermal Decoupling, Quantum Geometry, and Superconductivity from Domain Walls of Magic-Angle Twisted Bilayer Graphene - I

Magic-angle twisted bilayer graphene (MATBG) presents a promising platform for exploring unconventional high-temperature superconductivity through the emergence of flat bands near the Fermi level. These flat bands significantly influence electron mobility and thermal properties. We report on ab initio molecular dynamics (AIMD) simulations and temperature-dependent phonon dispersion analysis to investigate the distinct thermal behaviors across different stacking configurations of MATBG, particularly focusing on domain wall (DW) stacking. Our findings reveal pronounced thermal decoupling in DW stacking, associated with softened in-plane transverse optical phonons—similar to phonon anomalies observed in high-Tc cuprate superconductors. We also found that the relation between the flat band and the superconducting critical temperature (T_c) in MATB perfectly explained by a very small Fermi temperature (T_F) such that the first term, which was neglected in the T_c formula, T_c = (A/α)(T₀²/T_F)+(α/4A)T_F obtained from the thermal decoupling of cuprate [1], can no longer be neglected. The first term facilitates a transition from Uemura regimes to a novel quantum geometrical phase, where T_c inversely correlates withT_F. Our simulation results explain the experimental results of unconventional superconductivity in magic-angle twisted bilayer graphene in the case of extremely small T_F. These results underscore the potential of flat band systems like MATBG to exhibit unique superconducting properties influenced by quantum geometric effects, offering new insights into the interplay between electronic interactions and thermal phenomena in flat band materials. Our study opens pathways for further exploration of many-body localization and quantum geometry’s role in enhancing superconducting phenomena within flat bands, presenting implications for developing new materials with engineered superconductivity.

[1] https://arxiv.org/abs/2303.11600