Superconductivity in amorphousT5T9transition-metal alloys (T5=Nb,Ta; T9=Rh,Ir)

Abstract

The superconducting transition temperature $T_c$ has been measured for amorphous $T_5{T}_9$ alloys (where $T_5=\mathrm{Nb}\mathrm{or}\mathrm{Ta}$, $T_9=\mathrm{Rb}\mathrm{or}\mathrm{Ir}$) as a function of composition. Nb and Ta dominated the $T_c$ behavior with $T_c$ for Nb alloys ≈ 4.8-5.1 K and $T_c$ for Ta alloys ≈ 3.1-3.3 K. Changing the average group number by varying the $\frac{T_5}{T_9}$ ratio had little effect on $T_c$ within the ∼ 10 at.% range of composition in which the amorphous alloys form. $T_c$ decreased approximately linearly with $x$ for ${\left({\mathrm{Nb}}_{1-x}{\mathrm{Ta}}_x\right)}_{55}{\mathrm{Rh}}_{45}$ and ${\left({\mathrm{Nb}}_{1-x}{\mathrm{Ta}}_x\right)}_{55}{\mathrm{Ir}}_{45}$. The electrical resistivity ${\rho }_n$ and the upper critical field $B_{c2\mathrm{}}$ were measured as functions of temperature. The Sommerfeld constant $\gamma$ was calculated from ${\rho }_n$ and ${\frac{d{B}_{c2\mathrm{}}}{\mathrm{dT}}|}_{T_c}$ using the Ginzburg-Landau-Abrikosov-Gor'kov theory. The Debye temperature ${\Theta }_D$ was calculated from measurements of Young's modulus. The above parameters allowed calculations to be made of $N\left(E_F\right)$, the density of electronic states at the Fermi level, and $\lambda$, the electron-phonon coupling constant. It was found that $\lambda$ more accurately reflected the $T_c$ behavior than did $N\left(E_F\right)$ and that both electronic and phonon contributions had to be considered in the determination of $\lambda$.