Current self-oscillations, spikes, and crossover between charge monopole and dipole waves in semiconductor superlattices

We thank Rosa Lo´pez and Professor Holger Grahn for helpful discussions. This work has been supported by the DGES ~Spain!, Grant Nos. PB97-0088 and PB96-0875, by the European Union TMR Contract Nos. ERB FMBX-CT97- 0157 and FMRX-CT98-0180, and by the Community of Madrid, Project No. 07N/0026/1998.