Anomalous percolation and quantum criticality in diluted rare-earth nickelates

A microscopic model for the diluted spin-mixed compounds (R_xY_(1-x))_2BaNiO_5 (R=magnetic rare-earth) is studied using Quantum Monte Carlo (QMC). The ordering temperature is shown to be a universal function of the impurity concentration x and the intrinsic Ni-chain correlation length. An effective model for the critical modes is derived. The possibility of a quantum critical point driven by the rare earth-concentration and the existence of a Griffiths phase in the high dilution limit is investigated. Several possible experimental approaches to verify the results are put forward.