Breakdown of semiclassical description of thermoelectricity in near-magic angle twisted bilayer graphene

The planar assembly of twisted bilayer graphene (tBLG) hosts multitude of interaction-driven phases when the relative rotation is close to the magic angle (theta(m) = 1.1 degrees). This includes correlation-induced ground states that reveal spontaneous symmetry breaking at low temperature, as well as possibility of non-Fermi liquid (NFL) excitations. However, experimentally, manifestation of NFL effects in transport properties of twisted bilayer graphene remains ambiguous. Here we report simultaneous measurements of electrical resistivity (rho) and thermoelectric power (S) in tBLG for several twist angles between theta similar to 1.0 - 1.7 degrees. We observe an emergent violation of the semiclassical Mott relation in the form of excess S close to half-filling for theta similar to 1.6 degrees that vanishes for theta greater than or similar to 2 degrees. The excess S (approximate to 2 mu V/K at low temperatures T similar to 10 K at theta approximate to 1.6 degrees) persists upto approximate to 40 K, and is accompanied by metallic T-linear rho with transport scattering rate (tau(-1)) of near-Planckian magnitude tau(-1) similar to k(B)T/PLANCK CONSTANT OVER TWO PI. Closer to theta(m), the excess S was also observed for fractional band filling (nu approximate to 0.5). The combination of non-trivial electrical transport and violation of Mott relation provides compelling evidence of NFL physics intrinsic to tBLG.

Automated summary

When the relative rotation of twisted bilayer graphene is close to the magic angle, it exhibits interaction-driven phases with correlation-induced ground states and a possibility of non-Fermi liquid excitations. However, the manifestation of non-Fermi liquid effects in the transport properties of twisted bilayer graphene remains ambiguous.