Collective properties of indirect excitons in coupled quantum wells in a random field

The influence of a random field induced by impurities, boundary irregularities etc. on the superfluidity of a quasi-two-dimensional (2D) system of spatially indirect excitons in coupled quantum wells is studied. The interaction between excitons is taken into account in the ladder approximation. The random field is allowed to be large compared to the dipole-dipole repulsion between excitons. The coherent potential approximation (CPA) allows us to derive the exciton Green's function for a wide range of the random field, and the CPA results are used in the weak-scattering limit, which results in the second-order Born approximation. The Green's function of the collective excitations for the cases of (1) equal electron and hole masses and (2) the "heavy hole" limit are derived analytically. For quasi-two-dimensional excitonic systems, the density of the superfluid component and the Kosterlitz-Thouless temperature of the superfluid phase transition are obtained, and are found to decrease as the random field increases. This puts constraints on experimental efforts to observe excitonic superfluidity.