Quantum Hall Interferometry in Triangular Domains of Marginally Twisted Bilayer Graphene.

Quantum Hall (QH) interferometry provides an archetypal platform for the experimental realization of braiding statistics of fractional QH states. However, the complexity of observing fractional statistics requires phase coherence over the length of the interferometer, as well as suppression of Coulomb charging energy. Here, we demonstrate a new type of QH interferometer based on marginally twisted bilayer graphene (mtBLG), with a twist angle θ ≈ 0.16°. With the device operating in the QH regime, we observe distinct signatures of electronic Fabry-Pérot and Aharonov-Bohm oscillations of the magneto-thermopower in the density-magnetic field phase space, at Landau level filling factors ν = 4, 8. We find that QH interference effects are intrinsic to the triangular AB/BA domains in mtBLG that show diminished Coulomb charging effects. Our results demonstrate phase-coherent interference of QH edge modes without any additional gate-defined complex architecture, which may be beneficial in experimental realizations of non-Abelian braiding statistics.

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

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.

Misorientation-Controlled Cross-Plane Thermoelectricity in Twisted Bilayer Graphene.

The introduction of "twist" or relative rotation between two atomically thin van der Waals membranes gives rise to periodic moiré potential, leading to a substantial alteration of the band structure of the planar assembly. While most of the recent experiments primarily focus on the electronic-band hybridization by probing in-plane transport properties, here we report out-of-plane thermoelectric measurements across the van der Waals gap in twisted bilayer graphene, which exhibits an interplay of twist-dependent interlayer electronic and phononic hybridization. We show that at large twist angles, the thermopower is entirely driven by a novel phonon-drag effect at subnanometer scale, while the electronic component of the thermopower is recovered only when the misorientation between the layers is reduced to <6°. Our experiment shows that cross-plane thermoelectricity at low angles is exceptionally sensitive to the nature of band dispersion and may provide fundamental insights into the coherence of electronic states in twisted bilayer graphene.

Seebeck Coefficient of a Single van der Waals Junction in Twisted Bilayer Graphene.

When two planar atomic membranes are placed within the van der Waals distance, the charge and heat transport across the interface are coupled by the rules of momentum conservation and structural commensurability, leading to outstanding thermoelectric properties. Here we show that an effective "interlayer phonon drag" determines the Seebeck coecient (S) across the van der Waals gap formed in twisted bilayer graphene (tBLG). The cross-plane thermovoltage, which is nonmonotonic in both temperature and density, is generated through scattering of electrons by the out-of-plane layer breathing (ZO'/ZA2) phonon modes and differs dramatically from the expected Landauer-Buttiker formalism in conventional tunnel junctions. The tunability of the cross-plane Seebeck effect in van der Waals junctions may be valuable in creating a new genre of versatile thermoelectric systems with layered solids.

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 (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.