Ivana Curci | Searcharxiv

arXiv2025

Microwave characterization of superconducting coplanar resonators made out of granular aluminium

We have designed, fabricated and characterized microwave resonators made out of thin films of granular aluminium (grAl) with different oxygen content. We extract the contribution of the large kinetic-inductance of this disordered superconductor from the frequency shift of the resonators as a function of temperature, and discuss the correlation with the resistivity of the films. At low temperatures, measurements of the internal quality factor as a function of microwave power indicate the presence of two-level systems, which are inherent to the growth process of the granular aluminium. The characterization methods presented here may be useful for the design of MKIDs, high-impedance resonators and superinductors.

cond-mat.supr-con↗

arXiv2025

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

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.

cond-mat.supr-con↗