Nonanalyticity, Valley Quantum Phases, and Massless Excitons in Monolayer Transition Metal Dichalcogenides

ORAL

Abstract

Exciton dispersion as a function of center-of-mass momentum \textbf{Q} is essential to the understanding of exciton dynamics, relaxation, and condensation. We use the ab initio GW-Bethe-Salpeter equation(GW-BSE) method to calculate the dispersion of excitons in monolayer MoS$_{2}$ and find a nonanalytic lightlike dispersion. This behavior arises from the interplay of an unusual $|Q|$-term in both the intra- and intervalley exchange of the electron-hole interaction, which concurrently gives rise to a valley quantum phase of winding number two. We have derived a simple, effective Hamiltonian and analytic solutions, which quantitatively describe this physics, and we predict that signatures of this unusual dispersion can be measured with a linearly polarized optical beam tilted away from normal incidence. The existence of a nonanalytic exciton dispersion can be generalized to other 2D semiconductors with excitons whose amplitudes are localized in a small region of the Brillioun zone.

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Authors

  • Diana Y. Qiu

    Physics Department, UC Berkeley and Lawrence Berkeley National Lab, University of California at Berkeley and Lawrence Berkeley National Laboratory

  • Ting Cao

    Physics Department, UC Berkeley and Lawrence Berkeley National Lab, University of California at Berkeley and Lawrence Berkeley National Laboratory, University of California, Berkeley

  • Steven G. Louie

    University of California at Berkeley and Lawrence Berkeley National Lab, Physics Department, UC Berkeley and Lawrence Berkeley National Lab, University of California at Berkeley, University of California, Berkeley, University of California at Berkeley and Lawrence Berkeley National Laboratory, UC Berkeley and LBNL, UCB Physics and LBNL MSD