Pairing state in the ruthenocuprate superconductorRuSr2GdCu2O8: A point-contact Andreev reflection spectroscopy study

Abstract

The results of point contact Andreev reflection spectroscopy on polycrystalline $\mathrm{Ru}{\mathrm{Sr}}_{2}\mathrm{Gd}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$ 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 $\ensuremath{\Delta}=(2.8\ifmmode\pm\else\textpm\fi{}0.2)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and of the $2\ensuremath{\Delta}∕{k}_{B}{T}_{c}\ensuremath{\simeq}2$ 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 $\ensuremath{\Delta}$ vs $H$ evolution, a critical magnetic field ${H}_{{c}_{2}}\ensuremath{\simeq}30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ 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.