Nonmagnetic impurities in a S=1/2 frustrated triangular antiferromagnet: Broadening of 13C NMR lines in κ-(ET)2Cu2(CN)3

We study effects of nonmagnetic impurities in a spin-1/2 frustrated triangular antiferromagnet with the aim of understanding the observed broadening of C-13 NMR lines in the organic spin liquid material kappa-(ET)(2)Cu-2(CN)(3). For high temperatures down to J/3, we calculate local susceptibility near a nonmagnetic impurity and near a grain boundary for the nearest-neighbor Heisenberg model in high-temperature series expansion. We find that the local susceptibility decays to the uniform one in few lattice spacings, and for a low density of impurities we would not be able to explain the line broadening present in the experiments already at elevated temperatures. At low temperatures, we assume a gapless spin liquid with a Fermi surface of spinons. We calculate the local susceptibility in the mean field and also go beyond the mean field by Gutzwiller projection. The zero-temperature local susceptibility decays as a power law and oscillates at 2k(F). As in the high-temperature analysis we find that a low density of impurities is not able to explain the observed broadening of the lines. We are thus led to conclude that there is more disorder in the system. We find that a large density of pointlike disorder gives broadening that is consistent with the experiment down to about 5 K, but that below this temperature additional mechanism is likely needed.