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

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 $\eta =\eta (T\mathrm{/}J)$ ($J$ 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 $\mathrm{\partial }\eta \mathrm{/}\mathrm{\partial }p$ at point $p=0$, one can predict the value of the exponent for small dilution concentrations: $\eta (p)\simeq \eta (0)+p(\mathrm{\partial }\eta \mathrm{/}\mathrm{\partial }p){\mathrm{\mid }}_{p=0}$. As it is shown in this paper, the spin-wave Hamiltonian allows to obtain exact results for this slope: $(\mathrm{\partial }\eta \mathrm{/}\mathrm{\partial }p){\mathrm{\mid }}_{p=0}=T\mathrm{/}(2J)+O((T\mathrm{/}J{)}^2)$ and $T/(\pi J)

• O((T/J)^2)$ for site and for bond dilution, respectively.