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Phys Rev Lett. 2014 Aug 15;113(7):076802. Epub 2014 Aug 13.

Exciton binding energy and nonhydrogenic Rydberg series in monolayer WS(2).

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Departments of Physics and Electrical Engineering, Columbia University, 538 West 120th Street, New York, New York 10027, USA.
Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, USA.
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973-5000, USA.


We have experimentally determined the energies of the ground and first four excited excitonic states of the fundamental optical transition in monolayer WS_{2}, a model system for the growing class of atomically thin two-dimensional semiconductor crystals. From the spectra, we establish a large exciton binding energy of 0.32 eV and a pronounced deviation from the usual hydrogenic Rydberg series of energy levels of the excitonic states. We explain both of these results using a microscopic theory in which the nonlocal nature of the effective dielectric screening modifies the functional form of the Coulomb interaction. These strong but unconventional electron-hole interactions are expected to be ubiquitous in atomically thin materials.

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