Pairing state in the ruthenocuprate superconductor RuSr2GdCu2O8: A point-contact Andreev reflection spectroscopy study

The results of point contact Andreev reflection spectroscopy on polycrystalline RuSr2GdCu2O8 pellets are presented. The wide variety of the measured spectra are all explained in terms of a modified Blonder–Tinkham–Klapwijk model considering a d-wave symmetry of the superconducting order parameter. Remarkably low values of the energy gap Δ=(2.8±0.2) meV and of the 2Δ∕kBTc≃2 ratio are inferred. From the temperature evolution of the dI∕dV vs V characteristics we extract a sublinear temperature dependence of the superconducting energy gap. The magnetic field dependence of the conductance spectra at low temperatures is also reported. From the Δ vs H evolution, a critical magnetic field Hc2≃30 T is inferred. To properly explain the curves showing gap-like features at higher voltages, we consider the formation of a Josephson junction in series with the point contact junction, as a consequence of the granularity of the sample