Evidence of Lifshitz Transition in the Thermoelectric Power of Ultrahigh-Mobility Bilayer Graphene

Abstract

Resolving low-energy features in the density of states (DOS) holds the key to understanding a wide variety of rich novel phenomena in graphene-based 2D heterostructures. The Lifshitz transition in bilayer graphene (BLG) arising from trigonal warping has been established theoretically and experimentally. Nevertheless, the experimental realization of its effects on transport properties has been challenging because of its relatively low energy scale (∼1 meV). In this work, we demonstrate that the thermoelectric power (TEP) can be used as an effective probe to investigate fine changes in the DOS of BLG. We observed additional entropy features in the vicinity of the charge neutrality point (CNP) in gapped BLG. This apparent violation of the Mott formula can be explained quantitatively by considering the effects of trigonal warping, thereby serving as possible evidence of a Lifshitz transition.

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

“…3d for different ν (see SI, section V). Evidently, S exhibits a linear dependence on T at all doping including the higher-energy dispersive band, except in the vicinity of ν = ±2, thus validating the estimation of ∆T from Mott fitting [25]. The S ∝ T behavior is expected in a degenerate weakly or non-interacting metal within the semiclassical framework, and has been verified for monolayer graphene [17] as well as tBLG at slightly larger θ (2 • θ 5 • ) [28].…”

“…Although the Mott formula has been verified in a range of graphene-based devices [17,25], it can be violated in the hydrodynamic regime [29] and due to phonon drag in cross-plane thermoelectric transport in tBLG at θ > 6 • [20]. While the hydrodynamic regime is expected to appear at higher temperatures (> 100 K), we eliminate the possibility of phonon drag from the observation of S ∝ T (away from ν = ±2, Fig.…”

“…While the sign reversals near CNP and the super-lattice gaps at ν = ±4 are due to changes in the quasiparticle excitations, those near integer values of 0 < ν < 4 can be attributed to the correlated states. The sign-reversal of V 2ω near each correlated states is fascinating since it indicates a change in the topology of the Fermi surface, which is naturally associated with the Lifshitz transition [25,26]. Although the correlated states are metallic in nature (Fig.…”

Electronic band crossing in sliding bilayer graphene: Tight-binding calculations and symmetry group representation analysis

“…This dramatic change in the symmetry properties leads to a significant change in the interlayer coupling, and thus the physical properties. [35][36][37][38][39][40][41] Electronic structure calculations for the SBG configurations suggested the presence of Dirac points, but their existence has not yet been rigorously proven. In this paper, by using a tight-binding model for the p z electrons, we show the emergence of Dirac points in the vicinity of the corner points of the hexagonal Brillouin zone.…”

Role of the Lifshitz topological transitions in the thermodynamic properties of graphene

“…In ref. 31, experimental evidence was obtained of the Lifshitz transition in the thermoelectric power of ultrahigh mobility bilayer graphene. Resolving lowenergy features in the density of states (DOS) holds the key to understanding a wide variety of rich and novel phenomena in graphene based on 2D heterostructures.…”