Effect of annealing on the superconducting properties of two amorphous alloys: Nb ₇₀ Zr ₁₅ Si ₁₅ and Zr ₈₅ Si ₁₅

The changes in the superconducting and electronic properties of amorphous Nb ₇₀ Zr ₁₅ Si ₁₅ and Zr ₈₅ Si ₁₅ alloys with annealing were examined with an aim to evaluate the effect of structural relaxation on the superconductivity of metal-metalloid type amorphous alloys. T _c rises once from 3.99 to 4.42 K on annealing at temperatures below about 473 K for the Nb-Zr-Si alloy and from 2.71 to 2.75 K at temperatures below about 373 K for the Zr-Si alloy, and with further rising annealing temperature, t _d , lowers monotonically to a final relaxed value (≃3.15 K for Nb ₇₀ Zr ₁₅ Si ₁₅ and ≃2.49 K for Zr ₈₅ Si ₁₅ ), which is independent of the previous thermal cycling. These results indicate that the thermal relaxation of an amorphous phase occurs through at least two stages. The lowering of T _c occurs exponentially with t _d , and an activation energy for the relaxation process and the frequency of jump over the barrier were estimated to be about 2.03 eV and 2.4×10 ¹⁴ sec ^-1 for Nb ₇₀ Zr ₁₅ Si ₁₅ and about 1.28 eV and 1.2×10 ¹¹ sec ^-1 for Zr ₈₅ Si ₁₅ , respectively. The high frequencies indicate that the relaxations occur more or less independently of each other in a non-co-operative manner. The dressed density of electronic states at the Fermi level, N(E _f ) (1+λ), which was calculated from the measured values of ρ _n and (d H _c2 dT)T _c , exhibited a similar annealing temperature dependence to that of T _c . From this the change in T _c on thermal relaxation was interpreted as due to the changes in λ and/or N(E _f ). From the depressions of J _c (H) and fluxoid pinning force on annealing in a temperature range of 473 to 873 K, it was concluded that the structural relaxation from a less homogeneous quenched-in state to a homogeneous stable state occurred on the scale of coherence length (≃7.5 nm) during the annealing.