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 (θ_m = 1.1^∘). 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 (ρ) and thermoelectric power (S) in tBLG for several twist angles between θ ~ 1.0 - 1.7^∘. We observe an emergent violation of the semiclassical Mott relation in the form of excess S close to half-filling for θ ~ 1.6^∘ that vanishes for θ ≳ 2^∘. The excess S (≈2 μV/K at low temperatures T ~ 10 K at θ ≈ 1.6^∘) persists upto ≈40 K, and is accompanied by metallic T-linear ρ with transport scattering rate (τ^-1) of near-Planckian magnitude τ^-1 ~ k_BT/ℏ. Closer to θ_m, the excess S was also observed for fractional band filling (ν ≈ 0.5). The combination of non-trivial electrical transport and violation of Mott relation provides compelling evidence of NFL physics intrinsic to tBLG.