Thermopower probing emergent local moments in magic-angle twisted bilayer graphene

TL;DR

This study uses thermopower measurements to detect emergent local moments in magic-angle twisted bilayer graphene, revealing strong correlation effects and their magnetic field dependence through sign changes and entropy signatures.

Contribution

It introduces thermopower as a global transport probe to identify local moments and correlation effects in MATBLG, providing new insights into flatband-induced magnetism.

Findings

Sign changes in thermopower at specific filling factors indicate local moments.

Magnetic field suppresses thermopower, consistent with spin entropy polarization.

Robust crossing points suggest dominant contribution from localized moments.

Abstract

Recent experiments on magic-angle twisted bilayer graphene (MATBLG) have revealed the formation of flatbands, suggesting that correlation effects are likely to dominate in this system. Yet, a global transport measurement showing distinct signatures of strong correlations like local moments arising from the flatbands is missing. Utilizing thermopower as a sensitive global transport probe for measuring entropy, we unveil the presence of emergent local moments through their impact on entropy. Remarkably, in addition to sign changes at the Dirac point ($\nu = 0$) and full band filling ($\nu = \pm 4$), the thermopower of MATBLG demonstrates additional sign changes at the location, $\nu_{cross} \sim \pm 1$, which do not vary with temperature from $5K$ to $\sim 60K$. This is in contrast to sensitive temperature-dependent crossing points seen in our study on twisted bilayer graphene devices with weaker correlations. Further, we have investigated the effect of magnetic field ($B$) on the thermopower, both $B_{\parallel}$ and $B_{\perp}$. Our results show a $30\%$ and $50\%$ reduction, respectively, that is consistent with suppression seen in the layered oxide due to the partial polarization of the spin entropy. The observed robust crossing points, together with suppression in a magnetic field, cannot be explained solely from the contributions of band fermions; instead, our data is consistent with the dominant contribution arising from the entropy of the emergent localized moments of a strongly correlated flatband.