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

Abstract

The planar assembly of twisted bilayer graphene (tBLG) hosts multitude of interaction-driven phases when the relative rotation is close to the magic angle (θm = 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 ~ kBT/ℏ. Closer to θm, 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.

Interaction-driven giant thermopower in magic-angle twisted bilayer graphene

“…Motivated by these observations, we have extensively explored the thermopower response of MATBLG and non magic-angle twisted bilayer graphene (TBLG) devices. Unlike previous work involving graphene and TBLG [27][28][29][30][31][32][33][34] , we have utilized Johnson noise thermometry [35][36][37][38] to directly measure the temperature gradient across the MATBLG device and accurately determine S across a temperature ranging from 100 mK to 10 K. Our measurements reveal an intricate dependence of S on carrier density (ν), temperature (T) and magnetic field (B). Our key observations are as follows.…”

Topological Mixed Valence Model for Twisted Bilayer Graphene

“…The discovery of magic angle twisted bilayer graphene (MATBG), developing flat bands at "magic angles" [1][2][3][4][5][6], has opened a new avenue for the exploration of quantum materials. At integral filling, novel spin and valley polarized [7][8][9][10][11] Mott insulators develop, which on doping transform into strange metals [12][13][14][15][16][17][18][19][20] and novel superconductors [4,21], all of which have been a subject of intense theoretical study [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. It is as if, by tuning the voltage, one can explore a family of compounds along an entire row of the periodic table . Various experiments suggest that electrons localized in the moiré hexagons of MATBG resemble quantum dots [42,43], forming localized moments with valley and spin degeneracy near integer filling.…”

Quantum Hall Interferometry in Triangular Domains of Marginally Twisted Bilayer Graphene

“…The sign reversals in V xx 2ω are usually attributed to changes in the quasi-particle excitations or Fermi surface topology near Lifshitz transitions . The near-symmetric sign change of V xx 2ω on the both side of the CNP (blue arrows in the right inset of Figure d) are attributed to the positions of the van Hove singularities (vHS) in the lowest conduction/valence band of the moiré lattice . On the hole doping side, the additional sign reversal of V xx 2ω (black arrow in the right inset of Figure d) indicates the full-filling of the lowest energy valence band.…”

“…36 The near-symmetric sign change of V xx 2ω on the both side of the CNP (blue arrows in the right inset of Figure 1d) are attributed to the positions of the van Hove singularities (vHS) in the lowest conduction/valence band of the moirélattice. 35 On the hole doping side, the additional sign reversal of V xx 2ω (black arrow in the right inset of Figure 1d) indicates the full-filling of the lowest energy valence band. Assuming filling of four electrons per moiréunit cell, we estimate θ ≈ 0.16°from the magnitude of n s which matches with the twist-angle estimation from the Hofstadter butterfly in pattern in R xx .…”

“…To study the thermoelectric transport in mtBLG, we employ local Joule heating in the extended monolayer region by passing a sinusoidal current ( I ω ). , This creates a temperature gradient (Δ T ) along the length of the channel (left inset of Figure d). The thermoelectric voltage ( V 2ω ) is generated in the second-harmonic (2ω) that is recorded in the tBLG region of the Hall bar ( x -direction) with varying doping and heating currents.…”