Signatures of heavy electron bands in the thermoelectric response of MATBG pn-junctions

The strong electronic interactions in the MATBG flat bands give raise to a myriad of low-temperature, many-body ground states. These various phases present a dichotomy between properties of itinerant and localized electrons, along with the strong asymmetry of the charge ±1 excitations in the vicinity of the integer fillings of the moiré superlattice. Furthermore, such asymmetry has been shown to persist beyond the ordering temperature of any of the correlated ground states, hinting at its key role in the MATBG phenomenology.

These observations have motivated the development of multi-orbital models, such as the Topological Heavy Fermion (THF) model, to describe the MATBG flat bands and provide an analytical treatment of electron correlations in the flat bands. Still, the consequences of the coexistence of light (itinerant) and heavy (localized) electron bands in the transport of the MATBG flat bands have not been described. In this work, we present measurements of the thermoelectric (TE) response of the MATBG flat bands and model the low-temperature thermoelectric response using the THF model. We illuminate high-quality, gate-defined MATBG pn-junctions with a focused laser source (1550 nm) and read out the thermoelectric response through the Seebeck-driven photo-thermoelectric (PTE) effect in the presence of non-uniform filling factor along the junction. We establish the PTE-mechanism as the dominant photovoltage-generating mechanism in MATBG for above-gap excitation. Then, we reveal a previously unreported regime of the TE response of the MATBG flat bands, where the response remains electron-like through the conduction flat bands. We find that this response is consistent with the presence of low-energy heavy fermion bands which do not contribute to transport, causing an asymmetry of the PTE signal. We illustrate the role of carrier lifetime, set by the interaction strength, in the transport of MATBG.

Lastly, we study the PTE response at high temperatures, beyond the ordering temperatures for correlated ground states and find evidence for strong electronic interactions. This observation can be accounted for using the Kondo lattice model of MATBG, where the conduction electrons scatter off the moire lattice of interacting, localized electrons.