Coherent interactions between phonons and exciton or exciton-polariton   condensates
  We analyse the interaction of exciton and exciton-polariton condensates in semiconductor microcavity with a coherent acoustic wave. An analytical solution for the dispersion of excitations of coupled condensate-phonon system is found in the approximation of k-independent interactions. Accounting for k-dependence results in a stronger modification of the dispersion, and even in the appearance of the "roton instability" region. 
D.V. Vishnevsky, N.A. Gippius, I.A. Shelykh, D.D. Solnyshkov, G.   Malpuech
cond-mat.mtrl-sci cond-mat.mes-hall