Superconductivity and crystal and electronic structures in hydrogenated and disorderedNb3Ge andNb3Sn layers withA15 structure

Superconducting and transport properties of ${\mathrm{Nb}}_3$Ge and ${\mathrm{Nb}}_3$Sn layers have been varied over a wide range by hydrogenation, ion irradiation, and annealing. After hydrogenation, both compounds remain in the A15 structure and no effects of hydride precipitations at low temperatures could be observed. At high ion-irradiation doses ${\mathrm{Nb}}_3$Ge becomes amorphous, but ${\mathrm{Nb}}_3$Sn remains in the A15 structure, although $T_c$ behaves similarly. The long-range order parameter $S_A$ and the mean displacement amplitude 〈$u^2$ ${〉}^{1/2}$ were determined for ${\mathrm{Nb}}_3$Sn with x-ray diffraction. Distinct differences between the irradiation- and annealing-induced correlations of $T_c$ versus $S_a$, $T_c$ versus 〈$u^2$〉, and $T_c$ versus lattice parameter were observed. This indicates the influence of topological short-range order. The correlations of $T_c$ versus residual resistivity and $T_c$ versus the temperature derivative of the upper critical field at $T_c$ are distinctly different for hydrogenated and irradiated ${\mathrm{Nb}}_3$Sn and ${\mathrm{Nb}}_3$Ge, but the derived correlations of $T_c$ versus the coefficient of the electronic specific heat are very similar. The results are interpreted by a dominant influence of the ${\Gamma }_{12}$ band on high $T_c$. The measurements of the Hall constant $R_H$ indicate a filling of steep electronic bands as a result of hydrogenation. Irradiation has a similar influence as thermal-induced disorder on $R_H$. A maximum in the temperature dependence of $R_H$ indicates a martensitic transformation of ${\mathrm{Nb}}_3$Sn at 50–55 K, which is unchanged in slightly hydrogenated samples with higher $T_c$ but vanishes in irradiated samples.