Superconducting properties of thin film Nb1-xTixN studied via the NMR of implanted ⁸Li

We report measurements of the normal-state and superconducting properties of thin-film Nb 1-x Ti x N using 8 Li β -detected nuclear magnetic resonance ( β -NMR). In these experiments, radioactive 8 Li + probes were implanted 21 nm below the surface of a Nb 0.75 Ti 0.25 N(91 nm) film in Nb 0.75 Ti 0.25 N/(91 nm)/AlN(4 nm)/Nb and its NMR response recorded (via 8 Li's β -emissions) between 4.6 K and 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 c (0 T) = 15.4(7) K due to vortex lattice formation. Fits to a broadening model find a magnetic penetration depth λ (0 K) = 180.57(30) nm and upper critical field B c2 (0 K) = 18(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 ~100 K. Below T c , we observe a small Hebel-Slichter coherence peak characterized by a superconducting energy gap Δ(0 K) = 2.60(12) meV and modest Dynes-like broadening. Our measurements suggest a gap ratio 2Δ(0 K)/k B T c (0 T) = 3.92(25), consistent with strong-coupling behavior. Sources for the dynamic high-T relaxation are suggested.