Heavy fermions and local moments in magic-angle twisted bilayer graphene observed via planar tunneling spectroscopy

Magic-angle twisted bilayer graphene (MATBG) provides a rich playground for competing correlated electron phases that can be controlled in-situ through the application of gate voltages or magnetic fields. Topological heavy fermion models describe the flat bands in MATBG as coexisting localized flat-band orbitals (f-electrons) and nearly free conduction bands (c-electrons). The interaction between the f-electrons and c-electrons gives rise to emergent phenomena, including unconventional superconductivity, non-Fermi liquid behavior, and topologically nontrivial phases. However, the fundamental properties of the f- and c-electrons, such as their respective heavy and light effective mass, remain elusive. We addressed these questions by using planar tunneling spectroscopy to measure the electronic inverse compressibility in an MATBG device. We observed electron mass renormalization, consistent with the topological heavy fermion model prediction of a heavy Fermi liquid as the ground state for most non-integer fillings. Furthermore, these measurements provided direct entropic evidence for 4-fold and 8-fold degenerate isospin local moment states emerging at temperatures of 10K and 20K, respectively.