Enhanced electron-phonon coupling of Ti-doped superconducting Nb3Sn films investigated at microwave frequencies

Abstract

The intermetallic superconductor Nb3Sn is an interesting reference system, due to its high transition temperature and strong coupling, to compare with superconductors like MgB2 or the cuprates. We have prepared high-quality stoichiometric Nb3Sn films on sapphire by tin vapor diffusion. This technique enabled controlled doping of Nb3Sn films with Ti, which substitutes for Nb along the quasi-one-dimensional chain sites. The microwave response of the films was studied at 87GHz using a cavity-endplate technique. The temperature-dependent surface resistance of undoped films was in perfect agreement with the predictions of strong-coupling theory. However, the behavior of the penetration depth contradicted them, showing an unexpected weakly curved temperature dependence. Ti doping affected this behavior. Increasing the Ti concentration introduced a monotonically increasing curvature of Δλ(T). Furthermore, key superconducting properties like the reduced energy gap Δ∕kTc and the quasiparticle conductivity underwent nonmonotonic changes for increasing Ti concentration. Our observations are analyzed in terms of impurity scattering and changes of the electron-phonon coupling strength, most likely resulting from an altered quasiparticle density of states at the Fermi level.

Source-reported materials — not catalogue approval

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