Breakdown of the Static Dielectric Screening Approximation of Coulomb Interactions in Atomically Thin Semiconductors

Coulomb interactions in atomically thin materials are remarkably sensitive to variations in the dielectric screening of the environment, which can be used to control exotic quantum many-body phases and engineer exciton potential landscapes. For decades, static or frequency-independent approximations of the dielectric response, where increased dielectric screening is predicted to cause an energy redshift of the exciton resonance, have been sufficient. These approximations were first applied to quantum wells and were more recently extended with initial success to layered transition metal dichalcogenides (TMDs). Here, we use charge-tunable exciton resonances to investigate screening effects in TMD monolayers embedded in materials with low-frequency dielectric constants ranging from 4 to more than 1000, a range of 2 orders of magnitude larger than in previous studies. In contrast to the redshift predicted by static models, we observe a blueshift of the exciton resonance exceeding 30 meV in higher dielectric constant environments. We explain our observations by introducing a dynamical screening model based on a solution to the Bethe-Salpeter equation (BSE). When dynamical effects are strong, we find that the exciton binding energy remains mostly controlled by the low-frequency dielectric response, while the exciton self-energy is dominated by the high-frequency one. Our results supplant the understanding of screening in layered materials and their heterostructures, introduce a knob to tune selected many-body effects, and reshape the framework for detecting and controlling correlated quantum many-body states and designing optoelectronic and quantum devices.. Authors: Mhenni, Amine Ben [Technical University of Munich, Garching (Germany); Munich Center for Quantum Science and Technology (MCQST) (Germany)]; Van Tuan, Dinh [University of Rochester, NY (United States)]; Geilen, Leonard [Technical University of Munich, Garching (Germany); Munich Center for Quantum Science and Technology (MCQST) (Germany)]; Petrić, Marko M. [Technical University of Munich, Garching (Germany); Munich Center for Quantum Science and Technology (MCQST) (Germany)] (ORCID:000000026688326X); Erdi, Melike [Arizona State University, Tempe, AZ (United States)]. DOE Contract: SC0014349; SC0020653. Subjects: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Coulomb interaction engineering; bandgap modulation; dielectric screening in 2D semiconductors; dynamical dielectric screening effects; excitonic properties in van der Waals heterostructures; high-dielectric-constant (high-K) materials; transition metal dichalcogenides (TMDs)