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

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.

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Cited by in corpus (5)

• Nonlocal Moments in the Chern Bands of Twisted Bilayer Graphene
• Intrinsic nonlinear Nernst and Seebeck effect
• Cascades in transport and optical conductivity of Twisted Bilayer Graphene
• Heavy fermion phase diagram in magic-angle twisted trilayer graphene
• Flat-Band Driven Kondo Breakdown and Reentrant Effects in Heavy-Fermion Moiré Superlattices