  We find an intervalley wave collective mode in two- and three-dimensional
Dirac semimetals in the presence of a valley population imbalance. The
dispersion relation of this mode is gapless, proportional to the square of the
wave vector at small frequencies, and inversely proportional to the
electron-electron exchange interaction energy. The valley wave serves as an
energy gain source for the external field, that generates the intervalley
transitions. The spin wave analog is discussed for the case of a semimetal with
nonequilibrium spin orientation.
