Impurity effects at finite temperature in the two-dimensional S = 1 ∕ 2 Heisenberg antiferromagnet

We discuss effects of various impurities on the magnetic susceptibility and the specific heat of the quantum $S=1∕2$ Heisenberg antiferromagnet on a two-dimensional square lattice. For impurities with spin ${S}_{i}g0$ (here ${S}_{i}=1∕2$ in the case of a vacancy or an added spin, and ${S}_{i}=1$ for a spin coupled ferromagnetically to its neighbors), our quantum Monte Carlo simulations confirm a classical-like Curie susceptibility contribution ${S}_{i}^{2}∕3T$, which originates from an alignment of the impurity spin with the local N\'eel order. In addition, we find a logarithmically divergent contribution, which we attribute to fluctuations transverse to the local N\'eel vector. We also study frustrated and nonfrustrated bond impurities with ${S}_{i}=0$. For a simple intuitive picture of the impurity problem, we discuss an effective few-spin model that can distinguish between the different impurities and reproduces the leading-order simulation data over a wide temperature range.