Interaction driven giant thermopower in magic-angle twisted bilayer graphene

Arup Kumar Paul, Ayan Ghosh, Souvik Chakraborty, Ujjal Roy, Ranit Dutta, K. Watanabe, T. Taniguchi, Animesh Panda, Adhip Agarwala, Subroto Mukerjee, Sumilan Banerjee, Anindya Das

Published 2021-09-01Version 1

Thermopower, a non-invasive probe, is a sensitive tool to study the electronic structure near the singular points of Fermi surface as its magnitude and sign depends on the asymmetry of the density of states at the Fermi energy. Here, we report thermopower measurements in magic-angle twisted bilayer graphene (MtBLG) as function of carrier density, temperature and magnetic field using sensitive Johnson noise thermometry. At low temperatures (<$10K$) thermopower or Seebeck coefficient ($S$) exhibits unusual peak-like features around the positive integer fillings ($\nu=1,2,3$), including the Dirac point, and reaches a value as high as $\sim 100\mu V/K$ at $1K$ for half filling. The peak in $S$ at integer fillings, where the resistance also has maxima, is highly anomalous as the well-known Mott formula would predict $S$ to vanish at the resistance maxima. The anomalously large peaks of $S$ indicate strongly particle-hole asymmetric emergent electronic structure of MtBLG at integer Moire fillings. We show that the thermopower peaks and its non-monotonic temperature dependence can be qualitatively understood within a simple theoretical model with cascade of Dirac revivals arising due to interaction-driven quantum phase transitions. Furthermore, the $S$ shows anomalous peak around the superconducting transition on the hole side and points toward the possible role of enhanced superconducting fluctuations in MtBLG.