ABSTRACT
The ability to tune the twist angle between different layers of two-dimensional (2D) materials has enabled the creation of electronic flat bands artificially, leading to exotic quantum phases. When a twisted blilayer of graphene (tBLG) is placed at the van der Waals proximity to a semiconducting layer of transition metal dichalcogenide (TMDC), such as WSe2, the emergent phases in the tBLG can fundamentally modify the functionality of such heterostructures. Here we have performed photoresponse measurements in few-layer-WSe2/tBLG heterostructure, where the mis-orientation angle of the tBLG layer was chosen to lie close to the magic angle of 1.1°. Our experiments show that the photoresponse is extremely sensitive to the band structure of tBLG and gets strongly suppressed when the Fermi energy was placed within the low-energy moiré bands. Photoresponse could however be recovered when Fermi energy exceeded the moiré band edge where it was dominated by the photogating effect due to transfer of charge between the tBLG and the WSe2 layers. Our observations suggest the possibility of the screening effects from moiré flat bands that strongly affect the charge transfer process at the WSe2/tBLG interface, which is further supported by time-resolved photo-resistance measurements.
ABSTRACT
Recent experiments in magic-angle twisted bilayer graphene have revealed a wealth of novel electronic phases as a result of interaction-driven spin-valley flavour polarisation. In this work, we investigate correlated phases due to the combined effect of spin-orbit coupling-enhanced valley polarisation and the large density of states below half filling of the moiré band in twisted bilayer graphene coupled to tungsten diselenide. We observe an anomalous Hall effect, accompanied by a series of Lifshitz transitions that are highly tunable with carrier density and magnetic field. The magnetisation shows an abrupt change of sign near half-filling, confirming its orbital nature. While the Hall resistance is not quantised at zero magnetic fields-indicating a ground state with partial valley polarisation-perfect quantisation and complete valley polarisation are observed at finite fields. Our results illustrate that singularities in the flat bands in the presence of spin-orbit coupling can stabilise ordered phases even at non-integer moiré band fillings.
Subject(s)
Graphite , Electronics , Environment , Excipients , Magnetic FieldsABSTRACT
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 â³ 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/â. 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.
