Resistivity anomalies and intrinsic spin-orbit coupling in superconducting thin film solid solutions of Nb$_{1-x}$U$_x$ for $0.15 < x < 0.40$

TL;DR

This work investigates Nb$_{1-x}$U$_x$ thin films in the low U-content regime ($0.15 \le x \le 0.40$) to probe how intrinsic spin-orbit coupling from U affects superconductivity and spin relaxation. Structural analysis shows substitutional U incorporation in the Nb bcc lattice, accompanied by lattice expansion and increased electron density. Transport and magnetotransport data reveal a resistivity feature linked to EEI and localization phenomena and superconductivity below $2$ K; WHH analysis indicates Elliott-Yafet-type spin relaxation with a tight linkage between $\tau_{\mathrm{tr}}$ and $\tau_{\mathrm{so}}$ as U content grows. These findings establish a solid-state platform where strong SOC coexists with conventional superconductivity, with potential implications for superconducting spintronics and topological superconductivity in homogeneous Nb–U alloys.

Abstract

Polycrystalline thin films of $\mathrm{Nb}_{1-x}\mathrm{U}_{x}$ solid solutions with $0.15\leq x \leq 0.40$ were prepared via d.c. magnetron sputtering at ambient conditions. X-ray characterisation of the samples revealed a systematic shift of the (110) Nb Bragg reflection with U concentration, consistent with substitutional replacement of the Nb by U. Superconductivity was observed in all samples below $2$ K. Analysis of the superconducting critical fields revealed a direct scaling of the spin-orbit scattering and transport scattering times, indicating Elliott-Yafet-type spin relaxation in the system. Magnetoresistivity measurements showed a feature in the range $4$ K $\leq T \leq30$ K suggesting a possible a complex interplay between electron-electron interaction and localisation physics.

Figures (11)

• Figure 1: Intensity versus $2\theta$ around the (110) peak of Nb for the four samples and a Nb reference. The solid lines shown are Gaussian fits made to the respective data sets. The peak positions clearly shift to lower angles with increasing U concentration. A reduction of the FWHM of the peak with increasing U concentration is also observed.
• Figure 2: Lattice constant, $a_{\mathrm{xrd}}$ versus at$.$ % U. $a_{\mathrm{xrd}}$ was calculated from the peak of the Gaussian fit made from the $2\theta - \omega$ scan of the (110) Nb Bragg peak. The dashed line represents the expected variation in the lattice constant, $a_{\mathrm{V}}$, calculated from Vegard's law using values of $a_{\mathrm{Nb}} = 3.306$ Å roberge_lattice_1975 and $a_{\mathrm{U}}=3.53$ Å chakraborty_micro-structural_2015. Other lines are guides to the eye. Error bars on the $a_{\mathrm{xrd}}$ data are small compared to the point size.
• Figure 3: Fit to the reflectivity data for the sample with U composition = 19.3 at$.$ % U. Data are open symbols and line is the best fit. Inset: Root mean square roughness, $\sigma_{\mathrm{rms}}$, (triangle symbols) versus composition of the Nb:U alloy layer extracted from the reflectivity fit. Variation with composition of the electron density, extracted from XRD data, $\rho_{\mathrm{e}}^{\mathrm{XRD}}$, and from the XRR data, $\rho_{\mathrm{e}}^{\mathrm{XRR}}$ are also shown as closed and open circles, respectively.
• Figure 4: $\rho_{\mathrm{xx}}(T)$ for $4.2$ K $< T < 300$ K measured in a Van der Pauw configuration. Composition at$.$ % U content determined from the EDXS data.
• Figure 5: Magnetic field dependence of $T^{\star}$. Line is a guide to the eye. Inset: Points are the measurements of resistance, $R$, for the sample with at$.$ % U = 15.8 with the lowest and highest applied in-plane field, as shown. Here an offset of $R_{\mathrm{0}} = 14.6$$\Omega$ has been applied. Lines are the parabolic fits to the determine the position of $T^{\star}$, the minimum of the parabolas, indicated by the vertical dashed lines.