Disorder effects in the quantum kagome antiferromagnetZnCu3(OH)6Cl2

Motivated by the recent NMR experiments on ${\text{ZnCu}}_3{\left(\text{OH}\right)}_6{\text{Cl}}_2$, we study the effect of nonmagnetic defects on the antiferromagnetic spin-$\frac{1}{2}$ kagome lattice. We use exact diagonalization methods to study the effect of two such defects on finite-size systems. Our results, obtained without adjustable parameters, are in good quantitative agreement with recent ${}^{17}\text{O}$ NMR data. They provide support for the experimental interpretation of the presence of defects within the kagome layers due to Zn/Cu substitutions. Our results also show that disorder effects become relevant at lower temperatures, raising questions about the experimental evidence for the absence of an intrinsic spin gap in the kagome two-dimensional layers.