Collective Excitations of a Granular Superconductor in the Presence of a Steady Uniform Current Flow
By a suitable linearization of the isospin equations of motion, a calculation is made of the collective excitations of a granular superconductor in the presence of a steady, uniform current flow. Aside from the case of vanishing frequency or wave vector, only collective excitations with wave vectors perpendicular to the direction of current flow can occur physically (all others violating the condition of electric charge neutrality in the superconductor). This insures that the uniform supercurrent is stable against decay by excitation of collective oscillations (excitations bounded in space and time). Those excitations which can occur in the presence of the steady, uniform current are appreciably modified by the latter.