Point-contact study of gap amplitude and symmetry in RuSr_2GdCu_2O_8

TL;DR

This study uses point-contact spectroscopy to investigate the superconducting gap and its symmetry in polycrystalline RuSr_2GdCu_2O_8, revealing a d-wave symmetry with a 6 meV gap and temperature-dependent conductance behavior.

Contribution

First direct point-contact spectroscopy analysis of RuSr_2GdCu_2O_8 revealing d-wave symmetry and gap magnitude in polycrystalline samples.

Findings

Reproducible conductance curves show zero-bias cusp consistent with d-wave symmetry.

Estimated superconducting gap of approximately 6 meV.

Temperature dependence of conductance curves analyzed from 4.2 K to T_c.

Abstract

We present the results of point-contact spectroscopy measurements in polycrystalline RuSr_2GdCu_2O_8 samples previously characterized from both the structural and the electrical point of view. AC susceptibility and resistivity measurements, as well as SEM and optical microscopy, indicate that the polycrystalline material is made up of small, weakly connected grains. Possibly because of this morphology, 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 junctions.