Spontaneous appearance of rocking localized current filaments in a nonequilibrium distributive system

In a ${\mathit{p}}^{+\mathrm{-}}$ ${\mathit{n}}^{+\mathrm{-}}$p-${\mathit{n}}^{\mathrm{-}}$ device driven by a dc-voltage source via a load resistor we experimentally observe a sequence of transitions when the total current is increased. A first transition from a stationary spatially homogeneous low-current state to a stationary localized current-density filament is followed by a second transition into a spontaneously rocking current filament oscillating with a definite frequency around a fixed position in space. For large total currents a third transition occurs. The rocking current filament transforms into a traveling current filament oscillating between the edges of the sample. A two-layer model for the device is proposed. The physical mechanism and the conditions for the appearance of rocking and traveling current filaments are considered and generalized for a wide class of nonequilibrium systems.