Perturbation expansion for the diluted two-dimensional XY model
  We study the quasi-long-range ordered phase of a 2D XY model with quenched site-dilution using the spin-wave approximation and expansion in the parameter which characterizes the deviation from completely homogeneous dilution. The results, obtained by keeping the terms up to the third order in the expansion, show good accordance with Monte Carlo data in a wide range of dilution concentrations far enough from the percolation threshold. We discuss different types of expansion. 
Oleksandr Kapikranian (ICMP, LPM), Bertrand Berche (LPM), Yurij   Holovatch (ICMP)