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.

*This work was supported by NSF grant No. DMR15-1508412 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC and XSEDE.

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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