Intervalley plasmons in crystals.

Collective charge excitations in solids have been the subject of intense research ever since the pioneering works of Bohm and Pines in the 1950s. Most of these studies focused on long-wavelength plasmons that involve charge excitations with a small crystal-momentum transfer, $q \ll G$, where $G$ is the wavenumber of a reciprocal lattice vector. Less emphasis was given to collective charge excitations that lead to shortwave plasmons in multivalley electronic systems (i.e., when $q \sim G$). We present a theory of intervalley plasmons, taking into account local-field effects in the dynamical dielectric function. Focusing on monolayer transition-metal dichalcogenides where each of the valleys is further spin-split, we derive the energy dispersion of these plasmons and their interaction with external charges. Emphasis in this work is given to sum rules from which we derive the interaction between intervalley plasmons and a test charge, as well as a compact single-plasmon pole expression for the dynamical Coulomb potential.