Superconductivity in the Nb-Ru-Ge σ phase

We show that the previously unreported ternary σ-phase material Nb20.4Ru5.7Ge3.9 (Nb0.68Ru0.19Ge0.13) is a superconductor with a critical temperature of 2.2 K. Temperature-dependent magnetic susceptibility, resistance, and specific-heat measurements were used to characterize the superconducting transition. The Sommerfeld constant γ for Nb20.4Ru5.7Ge3.9 is 91mJmolf.u.-1K-2 (∼3mJmolatom-1K-2) and the specific-heat anomaly at the superconducting transition, ΔC/γTc, is approximately 1.38. The zeroerature upper critical field [μ0Hc2(0)] was estimated to be 2 T by resistance data. Field-dependent magnetization data analysis estimated μ0Hc1(0) to be 5.5 mT. Thus, the characterization shows Nb20.4Ru5.7Ge3.9 to be a type-II BCS superconductor. This material appears to be the first reported ternary phase in the Nb-Ru-Ge system, and the fact that there are no previously reported binary Nb-Ru, Nb-Ge, or Ru-Ge σ phases shows that all three elements are necessary to stabilize the material. An analogous σ phase in the Ta-Ru-Ge system did not display superconductivity above 1.7 K, which suggests that electron count cannot govern the superconductivity observed. Preliminary characterization of a possible superconducting σ phase in the Nb-Ru-Ga system is also reported.