Coherent states of excitons

The concept of a coherent exciton state is formulated. It is shown that for this state, a macroscopic wave function can be introduced such that it satisfies a nonlinear equation of the type familiar in the phenomenological theory of a superfluid liquid. The corresponding nondissipative flux is the flux of energy. For excitons interacting with an electromagnetic field, a coupled system of Maxwell equations and Ginzburg–Pitaev„skii-type equations (phenomenological theory of Bose liquid) is obtained.