Evidence of heavy fermion physics in the thermoelectric transport of magic angle twisted bilayer graphene

TL;DR

This study provides experimental evidence of heavy fermion physics in magic angle twisted bilayer graphene by analyzing thermoelectric responses, revealing coexistence of light and heavy carriers and strong electron correlations.

Contribution

It offers the first direct experimental proof of heavy fermion behavior in MATBG through thermoelectric measurements, supporting the topological heavy fermion model.

Findings

Sign-preserving, filling-dependent Seebeck oscillations at low temperatures.

Distinct thermoelectric signatures of strong electron correlations.

Agreement with the topological heavy fermion model (THF).

Abstract

It has been recently postulated, that the strongly correlated flat bands of magicangle twisted bilayer graphene (MATBG) can host coexisting heavy and light carriers. While transport and spectroscopic measurements have shown hints of this behavior, a more direct experimental proof is still lacking. Here, we explore the thermoelectric response of MATBG through the photo-thermoelectric (PTE) effect in gate-defined MATBG pn-junctions. At low temperatures, we observe sign-preserving, fillingdependent oscillations of the Seebeck coefficient at non-zero integer fillings of the moir\'e lattice, which suggest the preponderance of one carrier type despite tuning the Fermi level from hole to electron doping of the correlated insulator. Furthermore, at higher temperatures, the thermoelectric response provides distinct evidence of the strong electron correlations in the unordered, normal state. We show that our observations are naturally accounted for by the interplay of light and long-lived and heavy and short-lived electron bands near the Fermi level at non-zero integer fillings. Our observations firmly establish the electron and hole asymmetry of the correlated gaps in MATBG, and shows excellent qualitative agreement with the recently developed topological heavy fermion model (THF).