Signatures of local moments and heavy fermions in the normal state of twisted bilayer graphene

A plethora of correlated phenomena has been found in twisted bilayer graphene (TBG) as a function of doping and temperature. Some of these states, including the unconventional resets around integer fillings of the spectral, thermodynamic and transport properties which characterize the so-called cascades, remain up to tens of kelvin. However, evidence of broken symmetry states has been detected only below a few Kelvin. In many strongly correlated systems, the formation of local moments produces a strong redistribution of the spectrum. These effects remain at temperatures above symmetry breaking ordering, when the system is in its normal state. To address the consequences of the formation of local moments in the normal state of TBG and its role in the correlated phenomena observed we have performed advanced numerical calculations using a heavy fermion model for TBG with fragile topological flat bands in the absence of symmetry breaking.  The very good agreement between our numerical data and Scanning Tunneling Microscopy, Quantum Twisting Microscopy, resistivity and inverse compressibility experimental data shows that the main signatures of the cascades in twisted bilayer graphene are already present in the normal state, at temperatures above those at which broken symmetry states appear. Beyond comparing with available experimental results, we predict characteristic fingerprints in the optical and real space dependent properties that can be probed in future experiments. Our analysis of the low frequency optical conductivity allows us to clarify the important role played by the light electrons in the transport properties, in spite of their tiny contribution to the density of states at the Fermi level.

Publications:

M.J. Calderón, A. Camjayi, A. Datta, and E. Bascones, Phys. Rev. B 112, L041126 (2025)

A. Datta, M.J. Calderón, A. Camjayi, and E. Bascones, Nat. Com 14, 5036 (2023)

M.J. Calderón and E. Bascones, Phys. Rev. B 102, 155149 (2020)