Thermoelectric Probing of Flat Bands in Twisted Bilayer Tungsten Diselenide

Due to the relatively strong electron coulombic interaction compared to the small kinetic energy of the flat bands, twisted bilayer graphene moiré system exhibits strongly correlated behaviors including superconducting, insulating, and topological states at different band fillings. Compared to twisted bilayer graphene, transition metal dichalcogenides (TMDs) moiré system with a large range of twisted angles can produce very flat bands and strong correlations based on tight-binding calculation. Transport measurements of twisted bilayer tungsten diselenide (WSe2) reveal insulating and superconducting state at half-filling of the first moiré valence band. With a twisted angle less than 3 degrees in this system, the topmost valence band appears at the Gamma-valley, with narrow bandwidths and strong correlation. Here, we employ thermoelectric measurement as a unique probe of the flat band structures and interactions in twisted bilayer WSe2 of different twisting angles. As a measure of the average entropy per charge, the Seebeck coefficient is highly sensitive to the asymmetry of the electron density of states near the chemical potential and is expected to switch sign when the chemical potential is moved across a flat band. For a sample with a twisted angle near 3 degrees, the measured Seebeck coefficient shows pronounced modulation as the hole concentration is varied by a gate voltage. The observed dependences of the measured Seebeck coefficient on the band filling and temperature provide detailed insight into both the flat band features such as the bandwidth, as well as electron-electron and electron-phonon interactions that cause a deviation of the Seebeck coefficient from the semiclassical Mott relation.