Correlation between structural relaxation enthalpy and superconducting properties of amorphous Zr70Cu30 and Zr70Ni30 alloys

Abstract

The anneal-induced change in the superconducting properties together with the irrecoverable relaxation enthalpy (ΔHi,exo) and recoverable relaxation enthalpy (ΔHr,endo) of amorphous Zr70Cu30 and Zr70Ni30 alloys was examined. The increase in Δi,exo and the degradation ofTc progress logarithmically with annealing timeta in a temperature range of 373 to 523 K. The activation energy and the attempted frequency were respectively estimated to be 1.5eV and 6.6 × 1013 sec−1 for the increase in ΔHi,exo and 1.5eV and 1.9×1014 sec−1 for the degradation ofTc. The recoverable structure relaxation exerts little effect onTc. Based on the agreement between the kinetic parameters for the changes of ΔHi,exo andTc, it appears that the degradation ofTc on annealing is associated with the irrecoverable structural relaxation as a result of the annihilation of frozen-in defects and the topological and compositional atomic rearrangement. The values of the attempted frequency being of the order of Debye frequency suggest that the irrecoverable structural relaxation processes occur more or less independently from each other. The dressed density of electronic states at the Fermi level,N(Ef)(1+λ), determined from the measured values ofρn and -(dHc2/dT)Tc using GLAG (theory), was found to have a similar annealing dependence to that ofTc. The degradation ofTc by the irrecoverable relaxation was thus inferred as resulting from the decrease inλ due to the decrease inN(Ef) and the increases inM and ω. Furthermore, the irrecoverable structural relaxation resulted in a significant depression of fluxoid pinning force and was interpreted as due to an enhanced structural homogeneity on the scale of coherence length.