Hole crystallization in semiconductors
  When electrons in a solid are excited to a higher energy band they leave behind a vacancy (hole) in the original band which behaves like a positively charged particle. Here we predict that holes can spontaneously order into a regular lattice in semiconductors with sufficiently flat valence bands. The critical hole to electron effective mass ratio required for this phase transition is found to be of the order of 80. 
M. Bonitz, V.S. Filinov, V.E. Fortov, P.R. Levashov, and H. Fehske