Superconducting Properties of Metastable bcc Solid Solution in Melt-quenched Zr-Ge Binary Alloys

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Superconducting metastable bcc phase has been found in rapidly quenched Zr-Ge binary alloys in a composition range 9 to 12 at% Ge. With decreasing germanium content, the T_c value of the bcc alloys increased from 3.30 to 3.88 K, while the H_<c2> gradient at T_c, -(dH_<c2>/dT)_<T_c>, and the electrical resistivity at 4.2 K, ρ_n, decreased from 1.86×10^6 to 1.79×10^6 Am^<-1> K^<-1> and 2.00 to 1.70 μΩm, respectively. There existed a strong correlation between ρ_n, T_c and -(dH_<c2>/dT)_<T_c>; with increasing ρ_n, T_c decreased and - (dH_<c2>/dT)_<T_c> increased. A correlation between the temperature coefficient of resistivity (TCR), 1/ρ_<R.T.> (dρ/dT) and T_c was also found ; the larger the TCR the higher is the T_c. The existence of such correlations was inferred to originate from the close relation between the electron-phonon coupling constant λ and ρ_n or TCR. The electronic dressed density of states at the Fermi level N(E_f) (1+λ) was calculated from the experimentally measured values of -(dH_<c2>/dT)_<T_c> and ρ_n by using theories for strong-coupling superconductors. It was found that the values of N(E_f)(1+λ) significantly reflect T_c. The GL parameter k and the GL coherence length ζ_<GL>(0) were estimated to be 69-79 and about 7.5 nm, respectively, from the experimental values of -(dH_<c2>/dT)_<T_c> and σ_n. The metastable bcc Zr-Ge alloys are concluded to be extremely "dirty" type-II superconductors.
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