Roton-maxon spectrum and instability for weakly interacting dipolar   excitons in a semiconductor layer
  The formation of the roton-maxon excitation spectrum and the roton instability effect for a weakly correlated Bose gas of dipolar excitons in a semiconductor layer are predicted. The stability diagram is calculated. According to our numerical estimations, the threshold of the roton instability for Bose-Einstein condensed exciton gas with roton-maxon spectrum is achievable experimentally, e.g., in GaAs semiconductor layers. 
A.K. Fedorov, I.L. Kurbakov, Yu.E. Lozovik
cond-mat.quant-gas