Algebraic order and the Berezinskii-Kosterlitz-Thouless transition in an   exciton-polariton gas
  We observe quasi-long range coherence in a two-dimensional condensate of exciton-polaritons. Our measurements are the first to confirm that the spatial correlation algebraically decays with a slow power-law, whose exponent quantitatively behaves as predicted by the Berezinskii-Kosterlitz-Thouless theory. The exciton-polaritons are created by non-resonant optical pumping of a micro-cavity sample with embedded GaAs quantum-wells at liquid helium temperature. Michelson interference is used to measure the coherence of the photons emitted by decaying exciton-polaritons. 
Wolfgang H. Nitsche, Na Young Kim, Georgios Roumpos, Christian   Schneider, Martin Kamp, Sven Hoefling, Alfred Forchel, Yosh...
cond-mat.quant-gas