Dark characterization of Ti/Al LEKIDs for the search of axions in the W-band

TL;DR

This work characterizes Ti/Al LEKIDs designed for broadband W-band absorption to support axion dark-matter searches (CADEx) by performing dark electrical measurements and phase-noise spectroscopy. Using Mattis–Bardeen analysis of resonance frequency and quality factor, the authors extract a superconducting gap $\Delta_0$ around $124\ \mu$eV and an electromagnetic fractional kinetic inductance $\alpha\approx0.37$, validating operation across $75$–$110$ GHz. Phase-noise analysis reveals quasiparticle recombination times that deviate from the standard Kaplan model, best described by phenomenological forms implying subgap states and phonon-diffusion bottlenecks; this indicates non-equilibrium processes govern detector dynamics. The study reports a best dark electrical NEP of $NEP_{dark}\approx3\times10^{-19}$ W/√Hz at $f\approx200$ Hz and $T_{bath}=100$ mK, outlining concrete engineering paths (phonon-engineered substrates, reduced absorber volume, improved thermalization) to reach optical NEP targets for W-band axion searches.

Abstract

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 QCD axion search experiments within the Canfranc Axion Detection Experiment (CADEx), which demand sub 1e-19 W/Hz^0.5 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 3e-19 W/Hz0.5. 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.

Figures (4)

• Figure 1: (a) Resistance as a function of temperature for a meander of 10 $\mu$m width and 10.785 mm long measured with a DC current. (b) Transmission amplitude as a function of readout frequency for various bath temperatures between 40 mK and 240 mK. The microwave readout power at the chip level is fixed at –90 dBm. (b) Extracted resonance frequency (top) and internal quality factor (bottom) as functions of bath temperature. The colormap is identical to that used in panel (c). Gray solid lines represent fits of the experimental data using the Mattis–Bardeen model. The Dashed line a is guideline for the eye.
• Figure 2: Measured phase noise Power Spectral Density as a function of audio frequency for a single detector measured at different bath temperatures between 40 mK and 240 mK, increasing from bottom to top. Microwave readout power is -90 dBm, while the frequency of the readout signal is fixed at the resonance frequency obtained from fitting each trace.
• Figure 3: (a) Fits of measured noise power spectral densities together with their corresponding roll-off components, shown for four representative bath temperatures (100 mK, 130 mK, 150 mK, and 170 mK). The fitting is performed using the model described in Eq. \ref{['eq5']}. Dashed lines indicate the full model fit, while dash–dot lines highlight the quasiparticle Lorentzian roll-off contribution. PSD curves are shown after subtraction of the off-resonance spectra, thereby removing the amplifier noise floor. (b) Quasiparticle recombination time as a function of bath temperature, extracted from the Lorentzian fits in the PSDs. Black solid, dashed and dash-dotted curves represent the Kaplan, modified Arrhenius and phonon-electron coupling models.
• Figure 4: (a) Phase response (top) and phase responsivity (bottom) as functions of bath temperature. Each phase response is referenced to the center of the corresponding resonance trace measured at the same temperature. (b) Measured electrical noise-equivalent power (NEP) in the resonator phase as a function of audio frequency for bath temperatures between 100 mK and 240 mK.