Superconductivity of Zr-Nb-Si amorphous alloys

Abstract

Superconducting amorphous alloys with high strength and good ductility have been found in rapidly quenched alloys of the Zr-Nb-Si system. These alloys were produced in a continuous ribbon form of 1 to 2 mm width and 0.02 to 0.03 mm thickness using a modified single roller quenching apparatus. The amorphous alloys were formed over the whole composition range between zirconium and niobium, but the silicon content was limited to the relatively narrow range between about 12 and 24 at%. All the amorphous alloys showed a superconducting transition whose temperature, Tc, increased from 2.31 to 4.20 K with increasing niobium content or with decreasing silicon content. The upper critical magnetic field, Hc2, and the critical current density, Jc, for Zr15Nb70Si15 alloy were of the order of 4.5 Tesla(T) and 5.5×106 A m−2 at 1.5 K in the absence of applied field. The upper critical field gradient at Tc, \((dH_{c2} /dT)_{T_c } \), and the electrical resistivity at 4.2 K, ρn, decreased from 2.89 to 2.10 T K−1 and from 2.70 to 1.80μΩm, respectively, with the amount of niobium. The Debye temperature, θD, the electron-phonon coupling constant, λ, and the bare density of electronic states at the Fermi level, N(Ef) were calculated from the experimentally measured values of ρn, \((dH_{c2} /dT)_{T_c } \), Young's modulus and density by using the strong-coupling theories. From the comparison of Tc with their calculated parameters, it was found that λ is the most dominant parameter for Tc. The GL parameter, κ, and the GL coherence length, ξGL(0), were estimated to be 70 to 100 and about 7.6 nm, respectively, from the experimental values of \((dH_{c2} /dT)_{T_c } \) and ρn by using the GLAG theory and hence it is concluded that the present amorphous alloys are an extremely “dirty” type-II superconductor having a very weak flux pinning force.