Collective modes of soliton-lattice states in double-quantum-well systems

In strong perpendicular magnetic fields double-quantum-well systems can sometimes occur in unusual broken symmetry states which have interwell phase coherence in the absence of interwell hopping. When hopping is present in such systems and the magnetic field is tilted away from the normal to the quantum-well planes, a related soliton-lattice state can occur which has kinks in the dependence of the relative phase between electrons in opposite layers on the coordinate perpendicular to the in-plane component of the magnetic field. In this paper we evaluate the collective modes of this soliton-lattice state in the generalized random-phase approximation. We find that, in addition to the Goldstone modes associated with the broken translational symmetry of the soliton-lattice state, higher-energy collective modes occur that are closely related to the Goldstone modes present in the spontaneously phase-coherent state. We study the evolution of these collective modes as a function of the strength of the in-plane magnetic field and comment on the possibility of using the in-plane field to generate a finite wave probe of the spontaneously phase-coherent state.