Effects of interstitial oxygen on the superconductivity of niobium

Abstract

Superconductivity in niobium-oxygen body-centered-cubic solid-solution alloys (oxygen content 0.024-3.50 at.%) was studied by calorimetric, magnetic, and resistive measurement techniques. These measurements included low-temperature-specific-heat capacity, superconducting-normal transition temperature $T_c$, direct-current magnetization, and electrical resistivity, as well as x-ray lattice parameter, microhardness, and optical metallography to characterize the samples. Oxygen in solid solution lowers the $T_c$ of niobium. In contrast to the prediction of DeSorbo, we found that $\gamma$, the electronic coefficient of low-temperature-specific-heat capacity, also decreases with oxygen concentration. Our data indicated that the "band-structure" electronic density of states at the Fermi level $N_{\mathrm{bs}}\mathrm{}\left(0\right)\mathrm{}$ and the electron-phonon coupling constant $\lambda$ both decrease with oxygen content. Therefore, both the density of electronic states and the phonon spectrum may be controlling the magnitude of $T_c$ in the niobium-oxygen system. Additional superconducting parameters were calculated for the niobium-oxygen alloys from our calorimetric, magnetic, and resistive data. The Ginzburg-Landau parameter ${\kappa }_{\mathrm{GL}}$ was found to increase from less than 1.0 for essentially pure Nb to about 10 for the Nb-3.5-at.%-O alloy. Calculated values of $H_{c2\mathrm{}}\left(4.2\mathrm{} \mathrm{K}\right)$ versus atomic-percent oxygen exhibit a maximum at 2-at.% oxygen which was observed experimentally.