Dark Characterization of Ti/Al LEKIDs for the Search of Axions in the $W$-Band

We report the electrical (dark) characterization of lumped-element kinetic inductance detectors (LEKIDs) fabricated from a Titanium/Aluminum bilayer and designed for broadband absorption in the W–band (75-110 GHz). These detectors are prototypes for future Quantum Chromodynamics (QCD) axion search experiments within the Canfranc Axion Detection Experiment (CADEx), which demand sub-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-19}~\text{W}/\sqrt{\text{Hz}}$</tex-math></inline-formula> sensitivities under low optical backgrounds. We combine a Mattis-Bardeen analysis to the temperature dependence of the detector parameters with noise spectroscopy to determine the electrical noise equivalent power (NEP). The minimum measured value for the electrical NEP is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim 3 \times 10^{-19}~\text{W}/\sqrt{\text{Hz}}$</tex-math></inline-formula>. Across the measured temperature range, we find that quasiparticle lifetime deviates from the expected BCS recombination law. Our analysis suggests that non-equilibrium relaxation is governed by spatial inhomogeneities in the superconducting gap and phonon diffusion effects. This work sets the road-map to achieve suitable and ultra-sensitive detectors in the W–band for dark matter axion search experiments.