Electronic compressibility of magic-angle graphene superlattices

Recent calculations and measurements have revealed that the superlattice formed by twisting and stacking two monolayers of graphene at particular small twist angles (dubbed 'magic angles') relative to their crystallographic axes can form an electronic system with substantially reduced Fermi velocity compared to monolayer graphene. Moreover, exotic insulating [1] and superconducting phases [2] near fractional filling of the moiré miniband have been observed in initial transport and capacitance measurements, drawing comparisons to the high-Tc cuprates and other superconducting platforms with analogous phase diagrams. Here we report the first thermodynamic compressibility measurements of magic angle twisted bilayer graphene in the low frequency limit. We find strongly incompressible features at quarter- and half-filling of the electron-doped regime, whereas we find substantially weaker features at the equivalent hole-doped densities. We extract the thermodynamic gaps at fractional filling and compare our results to recent transport studies.

[1] Y. Cao et al. Nature 556, 80-84 (2018)
[2] Y. Cao et al. Nature 556, 43-50 (2018)