  Nonequilibrium effects created by stationary current injection in layered
d-wave superconductors forming a stack of intrinsic Josephson junctions are
studied. Starting from a nonequilibrium Green function theory we derive
microscopic expressions for the charge-imbalance (difference between electron-
and hole-like quasi-particles) on the superconducting layers and investigate
its influence on the quasi-particle current between the layers. This
nonequilibrium effect leads to shifts in the current-voltage curves of the
stack. The theory is applied to the interpretation of recent current injection
experiments in double-mesa structures.
