Mathematical and Theoretical Physics Seminar (MTPS)

Gautam Rai (Hamburg University)

When two layers of graphene are stacked on top of each other with small rotational misalignment--the honeycomb in the upper layer is twisted relative to the honeycomb in the lower layer---the long-wavelength interference pattern between the two layers gives rise to surprising emergent long-wavelength electronic physics. At a twist-angle of around 1.1 degrees (the magic angle), the low temperature phase diagram of TBLG shows a variety of diverse tunable strong correlation phase as a function of temperature and doping, exhibiting correlated insulating, orbital ferromagnetic, strange metallic, and superconducting behaviour. We use dynamical mean-field theory on the topological heavy Fermion model of magic-angle twisted bilayer graphene (TBLG) to investigate the interplay of electronic correlations and long-range order in the non-superconducting phase of this system. Our work [1, 2] identifies three central events in the filling-temperature plane: (i) the formation of local spin and valley isospin moments at around 100K, (ii) the ordering of these local isospin moments at around 10K, and (iii) a cascadic redistribution of charge between localized and delocalized electronic states upon doping. These three phenomena underpin some of the most puzzling aspects of data from scanning tunneling spectroscopy, transport, and compressibility experiments. In my talk, I will show you what we have learned about the nature of the emergent insulating and metallic states, the doping- and temperature-induced transitions between them, the so-called cascade transitions in local density and compressibility, and the microscopic mechanism behind the isospin Pomeranchuk effect in TBLG. [1] arXiv:2309.08529 [2] Phys. Rev. Lett. 131, 166501 (2023).