[Paper Review] Evidence of heavy fermion physics in the thermoelectric transport of magic angle twisted bilayer graphene

Rafael Luque Merino, Dumitru Călugăru+8 more|arXiv (Cornell University)|Feb 19, 2024

Graphene research and applicationsMaterials Science3 citations

TL;DR

This study provides direct experimental evidence of heavy fermion physics in magic angle twisted bilayer graphene (MATBG) through photo-thermoelectric (PTE) measurements in gate-defined pn-junctions. At low temperatures, sign-preserving, filling-dependent Seebeck coefficient oscillations reveal a dominant carrier type despite Fermi level tuning across hole and electron doping, indicating coexisting light and heavy electron bands near the Fermi level, with strong qualitative agreement to the topological heavy fermion model.

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 moiré 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).

Motivation & Objective

• To provide direct experimental evidence of heavy fermion physics in strongly correlated MATBG systems.
• To investigate the nature of charge carriers in MATBG across different doping regimes.
• To probe the interplay between light and heavy electron bands near the Fermi level in correlated insulating states.
• To test the predictions of the topological heavy fermion (THF) model in the normal state of MATBG.

Proposed method

• Photo-thermoelectric (PTE) effect measurements were performed on gate-defined MATBG pn-junctions.
• The Seebeck coefficient was measured as a function of gate voltage and temperature to probe carrier type and effective mass.
• Low-temperature measurements focused on integer filling factors of the moiré lattice to detect oscillatory behavior in the thermoelectric response.
• High-temperature measurements were used to probe the normal state of MATBG, revealing signatures of strong electron correlations.
• Theoretical modeling based on the topological heavy fermion (THF) framework was used to interpret the experimental data.
• Fermi level tuning from hole to electron doping allowed systematic study of carrier asymmetry in correlated gaps.

Experimental results

Research questions

• RQ1 Do coexisting light and heavy electron bands exist near the Fermi level in MATBG at non-zero integer fillings?
• RQ2 How does the thermoelectric response reflect the dominance of one carrier type despite Fermi level tuning across the correlated insulator?
• RQ3 What evidence do thermoelectric measurements provide for strong electron correlations in the normal state of MATBG?
• RQ4 To what extent do the experimental observations align with the predictions of the topological heavy fermion (THF) model?
• RQ5 How does the electron-hole asymmetry of the correlated gaps manifest in the thermoelectric transport properties?

Key findings

• Sign-preserving, filling-dependent oscillations in the Seebeck coefficient were observed at low temperatures, indicating a dominant carrier type even when the Fermi level is tuned across the correlated insulator.
• The oscillations persist across integer fillings of the moiré lattice, suggesting a robust, correlated electronic structure with mixed light and heavy carrier character.
• At higher temperatures, the thermoelectric response exhibits distinct signatures of strong electron correlations in the normal state of MATBG.
• The experimental data show excellent qualitative agreement with the predictions of the topological heavy fermion (THF) model.
• The results firmly establish electron-hole asymmetry in the correlated gaps of MATBG, with heavy carriers playing a key role in the transport response.
• The interplay between long-lived heavy bands and short-lived light bands near the Fermi level is identified as the origin of the observed thermoelectric behavior.