Novel magnetism and superconductivity in a Ru-based double perovskite

We report the observations, using magnetic susceptibility and specific heat measurements, of the coexistence of superconductivity and magnetic ordering at low temperatures in the Cu-doping double perovskites Sr(2)ARu(1-x)Cu(x)O(6), with A = Y or Ho, and x < 0.2. The observed striking features can be understood in terms of a plausible theoretical model based on the double exchange mechanism, which occurs indirectly via Ru5+ spin coupling to itinerant electrons hopping from one Ru site to another.