Pairing state in the rutheno-cuprate superconductor RuSr2GdCu2O8: A point contact Andreev Reflection Spectroscopy study

TL;DR

This study uses point contact Andreev reflection spectroscopy to investigate the pairing state in RuSr2GdCu2O8, revealing a d-wave symmetry, low energy gap, and high critical magnetic field, with implications for understanding its superconducting properties.

Contribution

It introduces a modified BTK model considering d-wave symmetry to analyze spectra and accounts for granularity effects with a Josephson junction model.

Findings

Energy gap Δ ≈ 2.8 meV and 2Δ/kBTc ≈ 2.

Critical magnetic field Hc2 ≈ 30 T.

Sublinear temperature dependence of the energy gap.

Abstract

The results of Point Contact Andreev Reflection Spectroscopy on polycrystalline RuSr$_2$GdCu$_2$O$_8$ pellets are presented. The wide variety of the measured spectra are all explained in terms of a modified BTK model considering a \emph{d-wave} symmetry of the superconducting order parameter. Remarkably low values of the energy gap $\Delta=(2.8\pm 0.2)meV$ and of the $2\Delta/k_BT_c\simeq 2$ 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 $\Delta$ vs $H$ evolution, a critical magnetic field $H_{c_2}\simeq 30 T$ 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.