We investigate diamagnetic fluctuations in percolating granular superconductors. Granular superconductors are known to have a rich phase diagram including normal, superconducting and spin glass phases. Focusing on the normal-superconducting and the normal-spin glass transition at low temperatures, we study he diamagnetic susceptibility $\chi^{ (1)}$ and the mean square fluctuations of the total magnetic moment $\chi^{ (2)}$ of large clusters. Our work is based on a random Josephson network model that we analyze with the powerful methods of renormalized field theory. We investigate the structural properties of the Feynman diagrams contributing to the renormalization of $\chi^{ (1)}$ and $\chi^{ (2)}$. This allows us to determine the critical behavior of $\chi^{ (1)}$ and $\chi^{ (2)}$ to arbitrary order in perturbation theory.
Categories: cond-mat.stat-mech