Rydberg Excitons and Doubly Resonant Raman Scattering in Transition-Metal Dichalcogenides

Abstract

Monolayer transition-metal dichalcogenides (TMDs) are emergent semiconductor materials with a wide range of potential applications. Rydberg excitons are similar to the Rydberg atomic states having a large principal quantum number n. The huge binding energies found in TMD semiconductors, up to 900 meV, facilitate studies of the Rydberg excitons, opening a new perspective of research by employing their long lifetimes, strong dipolar interactions, and the potential for coherent effects and quantum chaos. In the framework of a microscopic theory, we provide a complete description of the first-order resonant Raman scattering (RRS) valid for two-dimensional (2D) TMD semiconductors. Assuming as electronic intermediate states the high-n exciton states, we present explicit expressions for the RRS intensity, which are valid for incident laser energies close to the excitonic resonances. The intravalley Pekar−Froḧlich polar longitudinal optical mode and A 1 -homopolar mode deformation potential coupling mechanisms are considered. We report a large enhancement of the Raman efficiency due to the simultaneous incoming and outgoing resonances with bound exciton states, occurring for an appropriate choice of the excitation photon energy. We show that the 2D semiconductors guarantee the necessary conditions for the doubly RRS (DRRS) when intravalley transitions occur between two different Rydberg states separated by an optical phonon energy. The observation of the intravalley DRRS process can open new perspectives for deeper studies of the role of Rydberg excitons in solidstate physics.

Raman scattering owing to magneto-polaron states in monolayer transition metal dichalcogenides

“…It is clear that, for a realistic treatment of Raman scattering, it is necessary to know the renormalized spectrum of the EHP in an applied field B due to the electron interaction with the phonon field. The Raman efficiency in a 2D TMD is defined as 11 where S is the normalization area, dI/d� is a dimensionless quantity, ∂ 2 σ/(∂�∂ω s ) is the scattering cross-section in a volume V = Sb (b is the sample width) per unit solid angle and per unit scattered light frequency ω s , η l (η s ) is the refractive index for the incoming (scattered) light, c is the speed of light in the vacuum, W FI is the scattering amplitude from the initial state I of the system in the presence of the incident photon ω l to the final state F with a scattered photon of frequency ω s and a phonon ω o . At the resonance, the main contribution to the scattering amplitude W FI , considering the MP effects (see Fig.…”

“…the long-range interaction mechanism in a strictly 2D system is not active. Invoking a deviation from the normal incidence of light 11 , in the presence of impurities or defects 16 , the q = 0 selection rule is lifted, which leads to the light scattering via an LO-phonon mode. In the present treatment, we are focusing on strict backscattering configurations on the surface S. The dependence of the scattering efficiency in Eq.…”