Excess entropy and breakdown of semiclassical description of thermoelectricity in twisted bilayer graphene close to half filling

TL;DR

This study reveals non-Fermi liquid behavior in twisted bilayer graphene near half-filling through violations of the Mott relation and unusual thermoelectric and resistive properties, indicating breakdown of semiclassical descriptions.

Contribution

It provides experimental evidence of non-Fermi liquid physics in tBLG at small twist angles and half-filling, highlighting excess thermopower and Planckian scattering rates.

Findings

Violation of Mott relation near half-filling

Presence of excess thermopower (~2 μV/K) at low T

Metallic T-linear resistivity with Planckian scattering rate

Abstract

In moir\'{e} systems with twisted bilayer graphene (tBLG), the amplification of Coulomb correlation effects at low twist angles ($\theta$) is a result of nearly flat low-energy electronic bands and divergent density of states (DOS) at van Hove singularities (vHS). This not only causes superconductivity, Mott insulating states, and quantum anomalous Hall effect close to the critical (or magic) angle $\theta = \theta_{c} \approx 1.1^\circ$, but also unconventional metallic states that are claimed to exhibit non-Fermi liquid (NFL) excitations. However, unlike superconductivity and the correlation-induced gap in the DOS, unambiguous signatures of NFL effects in the metallic state remain experimentally elusive. Here we report simultaneous measurement of electrical resistivity ($\rho$) and thermoelectric power ($S$) in tBLG at $\theta \approx 1.6^\circ$. We observe an emergent violation of the semiclassical Mott relation in the form of excess $S$ close to half-filling. The excess $S$ ($\approx 2$ $\mu$V/K at low temperature $T \sim 10$ K) persists up to $\approx 40$ K, and is accompanied by metallic $T$-linear $\rho$ with transport scattering rate ($\tau^{-1}$) of near-Planckian magnitude $\tau^{-1} \sim k_{B}T/\hbar$. The combination of non-trivial electrical transport and violation of Mott relation provides compelling evidence of NFL physics intrinsic to tBLG, at small twist angle and half-filling.