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We successfully synthesized a superconducting GdSr2RuCu2O8−δ (RuGd-1212) single crystal via the partial melting technique. Structural analysis of this crystal indicated the substitution of Cu and Gd at the Ru and Sr sites, respectively. The disappearance of superconductivity in non-superconducting samples could be attributed to the substitution of Ru in the CuO2 planes and/or Gd at the Sr sites. The distance between the CuO2 planes in the superconducting RuGd-1212 single crystal was shorter, indicating that the valence of Cu in the CuO2 plane had decreased. A superconducting transition with Tc-onset of 23.0 K and Tc-zero of 6.2 K was observed.

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The occurrence of bulk superconductivity in the magnetosuperconductor GdSr2RuCu2O8−δ (RuGd-1212) remains controversial. Recently, we have successfully grown single crystals of RuGd-1212 with a size of 100 μm by partial melting. Although most single crystals prepared by conventional methods do not exhibit superconductivity, the superconductivity could be improved optimizing the synthetic conditions, and subtle changes in the sintering temperature were found to influence the emergence of superconductivity. Although most of the superconducting samples contained a small amount of the impurity phase Sr2GdRuO6, superconducting RuGd-1212 single crystals were the predominant phase. Furthermore, we successfully observed an onset transition temperature (Tc-onset) of 30 K and an in-plane critical current (Ic) of 12 mA. These experimental results indicate that the obtained RuGd-1212 single crystals exhibited bulk superconductivity.

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