Dynamical Coulomb Screening in Atomically Thin Semiconducting Transition Metal Dichalcogenides

Reduced screening contributes to the particularly strong Coulomb interaction characteristic of 2D materials¹, which is behind the emergence of exotic quantum many-body phases found therein. While the 2D nature of these material systems complicates the screening description, its understanding is, to date, considered well-established, mainly based on static approaches². Here, we use exciton resonances as probes to study screening in monolayer WSe₂, which we embed in dielectric environments with dielectric constants ranging from 5 to more than 1000. At odds with previous reports³, we find evidence for an optical bandgap <i>blueshift </i>for larger dielectric constants. We understand the experimental findings by developing a fully dynamical approach to the environment dielectric response. Starting from the 3χ model⁴, we take the frequency dependence into account via the dynamical dielectric function, both in the bandgap renormalization and in the exciton binding energy calculations. While binding energy remains mainly controlled by low-frequency dielectric screening, we find that high-frequency dielectric response is predominant in bandgap renormalization. Our results show that a fundamental understanding of Coulomb interaction in atomically thin materials cannot ignore dynamical effects. The achieved tunability of the optical bandgap by more than 30 meV, together with the qualitatively different theoretical framework developed, offer new opportunities for tuning the optoelectronic properties of 2D semiconductors and advance the study of many-body interactions in layered materials and their heterostructures.<br/><br/>References:<br/>¹Chernikov, A., Berkelbach, T. C., Hill, H. M., Rigosi, A., Li, Y., Aslan, B., Reichman, D. R., Hybertsen, M. S., & Heinz, T. F. (2014a). Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS₂. Physical Review Letters, 113(7).<br/>²Cho, Y., & Berkelbach, T. C. (2018). Environmentally sensitive theory of electronic and optical transitions in atomically thin semiconductors. Physical Review B, 97(4).<br/>³Raja, A., Chaves, A., Yu, J., Arefe, G., Hill, H. M., Rigosi, A. F., Berkelbach, T. C., Nagler, P., Schüller, C., Korn, T., Nuckolls, C., Hone, J., Brus, L. E., Heinz, T. F., Reichman, D. R., & Chernikov, A. (2017). Coulomb engineering of the bandgap and excitons in two-dimensional materials. Nature Communications, 8(1).<br/>⁴Van Tuan, D., Yang, M., & Dery, H. (2018). Coulomb interaction in monolayer transition-metal dichalcogenides. Physical Review B, 98(12).