Paper

Superconducting properties of thin film \\(\\mathrm{Nb_{1-x}Ti_xN}\\) studied via the NMR of implanted \\(^8\\)Li

Asaduzzaman, Md, MacFarlane, W Andrew, Ticknor, John O, Laxdal, Robert E, Dunsiger, Sarah R, Morris, Gerald D, Kalboussi, Yasmine, Proslier, Thomas, Thoeng, Edward, Li, Ruohong, McFadden, Ryan M L, Junginger, Tobias, Curci, Ivana

2025

Overview

We report measurements of the normal-state and superconducting properties of thin-film \\(\\mathrm{Nb_{1-x}Ti_xN}\\) using \\(^{8}\\)Li \\(\\beta\\)-detected nuclear magnetic resonance (\\(\\beta\\)-NMR). In these experiments, radioactive \\(^{8}\\)Li\\(^{+}\\) probes were implanted \\(\\sim21\\) nm below the surface of a \\(\\mathrm{Nb_{1-x}Ti_xN}\\)(91 nm) film in \\(\\mathrm{Nb_{0.75}Ti_{0.25}N}\\)(91 nm)/AlN(4 nm)/Nb and its NMR response recorded (via \\(^{8}\\)Li's \\(\\beta\\)-emissions) between 4.6 K and 270 K in a 4.1 T field applied normal to its surface. Resonance measurements reveal wide, symmetric lineshapes at all temperatures, with significant additional broadening below the film's superconducting transition temperature \\(T_\\mathrm{c}(0 \\; \\mathrm{T}) = 15.4 \\pm 0.7\\) K due to vortex lattice formation. Fits to a broadening model find a magnetic penetration depth \\(\\lambda(0 \\; \\mathrm{K})= 180.57 \\pm 0.30\\) nm and upper critical field \\(B_\\mathrm{c2}(0 \\; \\mathrm{K})= 18 \\pm 4\\) T, consistent with literature estimates. Spin-lattice relaxation (SLR) measurements find a Korringa response at low temperatures, with dynamic (i.e., thermally activated) contributions dominating above \\(\\sim100\\) K. Below \\(T_\\mathrm{c}\\), we observe a small Hebel-Slichter coherence peak characterized by a superconducting energy gap \\(\\Delta(0 \\; \\mathrm{K}) = 2.60 \\pm 0.12\\) meV and modest Dynes-like broadening. Our measurements suggest a gap ratio \\(2\\Delta(0 \\; \\mathrm{K})/k_\\mathrm{B}T_\\mathrm{c}(0 \\; \\mathrm{T}) = 3.92 \\pm 0.25\\), consistent with strong-coupling behavior. Sources for the dynamic high-\\(T\\) relaxation are suggested.

• Publisher: Cornell University Library, arXiv.org
• Subject: Critical field (superconductivity) / Energy gap / Low temperature / NMR / Nuclear magnetic resonance / Penetration depth / Spin-lattice relaxation / Superconductivity / Thin films / Transition temperature