Wavefronts may move upstream in doped semiconductor superlattices
  In weakly coupled, current biased, doped semiconductor superlattices, domain walls may move upstream against the flow of electrons. For appropriate doping values, a domain wall separating two electric field domains moves downstream below a first critical current, it remains stationary between this value and a second critical current, and it moves upstream above. These conclusions are reached by using a comparison principle to analyze a discrete drift-diffusion model, and validated by numerical simulations. Possible experimental realizations are suggested. 
A. Carpio (U. Complutense de Madrid), L. L. Bonilla (U. Carlos III de   Madrid), A. Wacker (Technical U. Berlin), and E. Sch...