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

Topological heavy fermion models[1-5] describe the flat bands in magic-angle twisted bilayer graphene (MATBG) as arising from the hybridization between localized flat-band orbitals (f-electrons) and nearly-free conduction bands (c-electrons). The interplay between these f-electrons and c-electrons is theorized to give rise to emergent phenomena, including unconventional superconductivity[6-8], non-Fermi liquid behavior[9-13], and topologically nontrivial phases[12, 14, 15]. However, the fundamental properties of f- and c-electrons, such as their respective heavy and light effective mass and their properties under strain, need experimental verification. Here we report on the electronic inverse compressibility, effective mass, and entropy of MATBG, obtained from planar tunneling spectroscopy. Our results include the observation of electron mass renormalization, found to be consistent with the topological heavy fermion model prediction of heavy charge-one excitations away from integer fillings. Importantly, we present entropic evidence for 4-fold and 8-fold degenerate isospin local moment states emerging at temperatures of 10K and 20K, respectively, consistent with the entropy of 8 heavy-fermions flavors energetically split by the sample strain.