Point-contact study of gap amplitude and symmetry in RuSr2GdCu2O8

We present the results of point-contact spectroscopy measurements in polycrystalline RuSr2GdCu2O8 samples previously characterized by AC susceptibility and resistivity measurements, as well as SEM and optical microscopy. Possibly because of the granularity and of the partial sinterization, point-contact measurements turned out to be rather difficult. Best reproducibility of the conductance curves was achieved in point contacts obtained by pressing Au or Pt-Ir tips on the surface of the samples. The low-temperature conductance curves reproducibly show a clear zero-bias cusp that might be interpreted as due to Andreev reflection at the normal metal-superconductor interface in the case where the superconducting order parameter has a pure d-wave symmetry - even though alternative explanations cannot be ruled out. In this paper we show that these curves can indeed be fitted, in the framework of the generalised BTK model, by using pure d-wave symmetry and a gap of the order of 6 meV. Finally, we present the temperature dependence of the conductance curves from 4.2 K up to the critical temperature of the junction. Edited by Antonello Andreone, Giovanni Piero Pepe, Roberto Cristiano, Giovanni Masullo. It is present also in arXiv:cond-mat/0508002v