Ja n 20 06 Pairing state in the rutheno-cuprate superconductor RuSr 2 GdCu 2 O 8 : A point contact Andreev reflection spectroscopy study

The results of Point Contact Andreev Reflection Spectroscop y on polycrystalline RuSr 2GdCu2O8 pellets are presented. The wide variety of the measured spectra are a ll explained in terms of a modified BTK model considering ad-wave symmetry of the superconducting order parameter. Remarkab ly low values of the energy gap = (2:8 0:2)m eV and of the2 =kB Tc ’ 2 ratio are inferred. From the temperature evolution of the dI=dV vsV characteristics we extract a sublinear temperature depend nce of the superconducting energy gap. The magnetic field dependence of the conductance spectra at l ow temperatures is also reported. From the vs H evolution, a critical magnetic field H c2 ’ 30T is inferred. To properly explain the curves showing gap-lik e features at higher voltages, we consider the formation of a J sephson junction in series with the Point Contact junction, as a consequence of the granularity of the sample.