Spin-spin correlation function of the 2D XY model with weak site or bond dilution

Abstract: The spin-spin correlation function of the 2D XY model decays as a power law at all temperatures below the Berezinskii-Kosterlitz-Thouless transition point with a temperature dependent exponent $\eta=\eta(T/J)$ ($J$ is the ferromagnetic coupling strength). It is known from computer experiments that in the 2D XY model with site or bond dilution this exponent depends on concentration $p$ of removed sites/bonds as well. Knowing the slope $\partial\eta/\partial p$ at point $p=0$, one can predict the value of the exponent for small dilution concentrations: $\eta(p)\simeq\eta(0)+p(\partial\eta/\partial p)|{p=0}$. As it is shown in this paper, the spin-wave Hamiltonian allows to obtain exact results for this slope: $(\partial\eta/\partial p)|{p=0} = T/(2J) + O((T/J)^2)$ and $T/(\pi J) + O((T/J)^2)$ for site and for bond dilution, respectively.