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Interlayer excitons in TMDCs

Date : Wednesday, February 6th, 2019 4:00 pm - 5:00 pm Place : Seminar Room 5 (A615), 6th Floor, ISSP Lecturer : Dilna Azhikodan Affiliation : Division of Condensed Matter Theory, The Institute for Solid State Physics, The University of Tokyo Committee Chair : Osamu Sugino (ext. 63290)
e-mail: sugino@issp.u-tokyo.ac.jp

The few layer transition metal dichalcogenides (TMDCs) are two dimensional materials that have an intrinsic gap of the order of ≈ 2eV. The reduced screening in two dimensions implies a rich excitonic physics and, as a consequence, many potential applications in the field of opto-electronics. Here I show that a layer perpendicular electric field, by which the gap size in these materials can be efficiently controlled, generates an anomalous inter-layer exciton whose binding energy is independent of the gap size. I show this originates from the rich gap control and screening physics of TMDCs in a bilayer geometry: gating the bilayer acts on one hand to increase intra-layer screening by reducing the gap and, on the other hand, to decrease the inter-layer screening by field induced charge depletion. This constancy of binding energy is both a striking exception to the universal reduction in binding energy with gap size that all materials are believed to follow, as well as evidence of a degree of control over inter-layer excitons not found in their well studied intra-layer counterparts. The ground-state density functional theory (DFT) calculations are performed using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional using the PAW method as implemented in the VASP code. I have employed the non-self-consistent GW method to determine the many-body gaps, with excitonic properties calculated by solving the Bethe Salpeter equation (BSE) in the q→0 limit.


(Published on: Friday January 25th, 2019)