Optical probe of electronically correlated states in atomically thin semiconductors

Abstract

Emergent strongly correlated electronic phenomena in atomically thin transition metal dichalcogenides (TMDs) provide a versatile platform for exploring many-body physics, including electron solids, Mott insulators, and putative exciton condensates. In this thesis, we use excitons as sensitive optical probes of such correlated electron states in gated TMD heterostructures at cryogenic temperatures. In the first part, we investigate indirect excitons in naturally grown homobilayer MoS$_2$ integrated into a dual-gate device that allows independent control of carrier density and out-of-plane electric field. In a regime where single-particle electron tunneling between layers is negligible, we observe that two interlayer excitons with opposite out-of-plane dipole moments undergo an unusual stochastic hybridization upon electron doping, distinct from conventional level crossing or anti-crossing. A phenomenological model with static random coupling, whose strength increases with electron density and decreases with temperature, quantitatively captures the reflectance spectra. We interpret these observations as signatures of a spatially fluctuating interlayer electron coherence order parameter, providing optical evidence for a theoretically proposed many-body state outside the quantum Hall regime. In the second part, we study correlated insulating phases in angle-aligned WSe$_2$/WS$_2$ moiré heterobilayers. Using reflectance-contrast spectroscopy, we employ the WSe$_2$ intralayer exciton as a nanoscale dielectric sensor of the moiré lattice, revealing discrete shifts and intensity modulations at integer and fractional fillings associated with generalized Wigner crystals and Mott insulators. Polarization-resolved measurements in a perpendicular magnetic field uncover valley-resolved exciton responses. GHz microwave transmission is also used to search for pinning-mode and magnetic resonances of the correlated states. Together, the optical and microwave approaches provide complementary, non-transport route to access and characterize emergent many-body physics in two-dimensional TMD heterostructures.