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Cryogenic calorimetric experiments to search for neutrinoless double-beta decay ($$0\\nu \\beta \\beta $$0 ν β β ) are highly competitive, scalable and versatile in isotope. The largest planned detector array, CUPID, is comprised of about 1500 individual Li$$_{2}$$2$$^{100}$$100 MoO$$_4$$4 detector modules with a further scale up envisioned for a follow up experiment (CUPID-1T). In this article, we present a novel detector concept targeting this second stage with a low impedance TES based readout for the Li$$_2$$2 MoO$$_4$$4 absorber that is easily mass-produced and lends itself to a multiplexed readout. We present the detector design and results from a first prototype detector operated at the NEXUS shallow underground facility at Fermilab. The detector is a 2-cm-side cube with 21 g mass that is strongly thermally coupled to its readout chip to allow rise-times of$$\\sim $$∼ 0.5 ms. This design is more than one order of magnitude faster than present NTD based detectors and is hence expected to effectively mitigate backgrounds generated through the pile-up of two independent two neutrino decay events coinciding close in time. Together with a baseline resolution of 1.95 keV (FWHM) these performance parameters extrapolate to a background index from pile-up as low as$$5\\cdot 10^{-6}$$5 · 10 - 6  counts/keV/kg/yr in CUPID size crystals. The detector was calibrated up to the MeV region showing sufficient dynamic range for$$0\\nu \\beta \\beta $$0 ν β β searches. In combination with a SuperCDMS HVeV detector this setup also allowed us to perform a precision measurement of the scintillation time constants of Li$$_2$$2 MoO$$_4$$4 , which showed a primary component with a fast O(20 $$\\upmu $$μ s) time scale.

A superconducting film with a tunable low transition temperature (Tc) is required in high-resolution Transition-Edge Sensor (TES) detectors, which have applications including dark matter detection, low threshold coherent elastic neutrino nucleus scattering measurement, and X-ray spectroscopy. We have been investigating a new approach to tune the Tc of superconducting thin films fabricated by co-sputtering Iridium and Platinum. The effects of Pt concentration and deposition parameters on the films' structural, electrical, and superconducting properties have been studied. AFM and XRD techniques and low temperature resistance measurements have been utilized for film characterization. By varying the Pt concentration and deposition parameters when co-sputtering, we have successfully achieved controllable tuning of Tc in the range of 30–200 mK. The experimental results demonstrate co-sputtering as a viable method for controlling the Tc of Ir-based thin films that can be applied to fabricating high-resolution TESs.

One way of making a transition-edge sensor (TES) is by utilizing the proximity effect, in which the T C of a superconducting film is reduced with a normal metal film in metallic contact. The T C of a bilayer TES can be estimated by solving the Usadel equations with given boundary conditions. The classical boundary conditions of a bilayer include a specific interface resistance being temperature-independent. In this paper, we will introduce a temperature-dependent specific interface resistance. By fitting the measured T C data of Ir/Au bilayers from the literature to a T C calculation model, we will compare the fit parameters and fit errors with the temperature-dependent specific interface resistance described in this work and with the classical temperature-independent specific interface resistance.

Superconducting resonators are now found in a broad range of applications that require high-fidelity measurement of low-energy signals. A common feature across almost all of these applications is the need for an increased number of resonators to further improve sensitivity, combined with the desire to limit cryogenic readout channels and complexity. One of the major limitations of current resonator arrays is the observed scatter in the resonator frequencies when compared to the initial design. Here we present recent progress toward identifying one of the dominant underlying causes of resonator scatter - inductor line width fluctuation. We designed and fabricated an array of lumped-element resonators in which the inductor line width changes from 1.8  μ m to 2.2 μ m in steps of 0.1   μ m . The inductor is defined using electron-beam lithography to probe and quantify the systematic variation of resonance frequencies. Paired with two different capacitor geometries the resonators showed a linear frequency spacing of ≈ 20 MHz and 30 MHz, respectively, or 1.48 % and 1.96 % in fractional frequency shift ( Δ f / f o ). This linear relationship matches our theoretical prediction. Our result demonstrates significant improvement in resonator array frequency scatter is readily achievable if inductor line width variation is sufficiently controlled.

A microwave kinetic inductance detector (MKID) is a superconducting pair breaking detector that offers a number of unique advantages for realizing large-format arrays of ultra-sensitive detectors, such as inherent multiplexibility and relative ease of fabrication. With the detection threshold being set by the Cooper pair binding energy, and correspondingly, the superconducting critical temperature ( T c ), typically well-understood MKID materials such as aluminum (Al) present a lower limit on the operating frequency. Aluminum manganese (Al-Mn) is a promising candidate material for MKIDs because it can be fabricated with nearly identical processing as pure Al, but allows for control of the T c with varying levels of Mn doping or post-deposition heat treatment. We present initial results from an early characterization of AlMn using a series of lumped-element superconducting microwave resonators, including measurements of T c , internal quality factor, and noise performance over a range of Mn doping.

Direct detection of nuclear scatterings of sub-GeV dark matter (DM) particles favors low-Z nuclei. Hydrogen nucleus, which has a single proton, provides the best kinematic match. The characteristic nuclear recoil energy is boosted by a factor of a few tens from those for larger nuclei used in traditional Weakly Interacting Massive Particles searches. Furthermore, hydrogen is optimal for detecting spin-dependent nuclear scatterings of sub-GeV DM, where large parameter space still remains unconstrained yet. In this paper, we first introduce several hydrogen-rich targets, which emit two classes of signals under kinetic excitations. One class of the signals is infrared photons, which are from fundamental vibrational and rotational modes of molecules and at several characteristic wavelengths. Another is acoustic phonons and optical phonons that decay into acoustic phonons. We then discuss the technical status and future researches of low- T c transition-edge sensor (TES) detectors, which measure the infrared photons and acoustic phonons with desirable sensitivities. Utilization of hydrogen-rich targets and ultra-sensitive low- T c TES detectors for light DM detection requires both theoretical modeling and experimental prototyping.

We propose a multiplexible kinetic inductance ammeter, which uses a high-quality-factor, superconducting, lumped-element, kinetic inductance resonator as a current sensor, a short, superconducting coplanar waveguide (CPW) for current input, and a CPW transmission line for the sensor readout. The resonator consists of an interdigitated capacitor and a superconducting loop that inductively couples to the input CPW. Current running through the central line of the input CPW generates magnetic fields which are focused into the gaps of the input CPW. These magnetic fields can be measured collectively as the magnetic flux through the superconducting loop. The kinetic inductance of the superconducting loop depends on the screening current for the magnetic flux, so the input current is converted to a change in the frequency of the resonator. We analyze the response and noise of a kinetic inductance ammeter with a high-resistivity NbN loop.

The reference design for the next-generation cosmic microwave background (CMB) experiment, CMB-S4, relies on large arrays of transition-edge sensor (TES) bolometers coupled to Superconducting Quantum Interference Device (SQUID)-based readout systems. Mapping the CMB to near cosmic variance limits will enable the search for signatures of inflation and constrain dark energy and neutrino physics. AlMn TESes provide simple film manufacturing and highly uniform arrays over large areas to meet the requirements of the CMB-S4 experiment. TES parameters such as critical temperature and normal resistance must be tuned to experiment specifications and can be varied based on geometry and steps in the fabrication process such as deposition layering, geometry, and baking time and temperature. Using four-terminal sensing, we measured T C and R N of AlMn 2000 ppm films and devices of varying thicknesses fabricated at Argonne National Laboratory to motivate device geometries and fabrication processes to tune T C to 150–200 mK and R N to ∼ 10 m Ω . Measurements of IV curves and time constants for the resulting devices of varying leg length were made using time-division SQUID multiplexing and determined T C , G , k , f 3 db , and R N . We present the results of these tests along with the geometries and fabrication steps used to tune the device parameters to the desired limits.

We are developing a low- T c TES-based large-area and low-threshold detector targeting a variety of potential applications. The detector consists of a 50.8-mm-diameter Si wafer as the substrate and radiation absorber, a single Ir/Pt bilayer TES sensor in the center, and normal metal Au pads added to the TES to strengthen the TES–absorber thermal coupling. Tight TES–absorber thermal coupling improves detector sensitivity and response uniformity. Here, we report on the electron–phonon (e–ph) coupling strengths for the Ir/Pt bilayer and Au that are measured with our prototype detectors and TES devices. We found that a second weak thermal link besides the one due to e–ph coupling in Ir/Pt or Au was required to explain our data. With the effects of the second weak link accounted for, the extracted e–ph coupling constant Σ for Ir/Pt bilayer in the T c range between 32 and 70 mK is 1.9 × 10 8 WK - 5 m - 3 , and Σ ’s for Au at 40 mK and 55 mK are 2.2 × 10 9 WK - 5 m - 3 and 3.2 × 10 9 WK - 5 m - 3 , respectively.

Frequency-domain multiplexing (fMux) is an established technique for the readout of large arrays of transition-edge sensor (TES) bolometers. Each TES in a multiplexing module has a unique AC voltage bias that is selected by a resonant filter. This scheme enables the operation and readout of multiple bolometers on a single pair of wires, reducing thermal loading onto sub-Kelvin stages. The current receiver on the South Pole Telescope, SPT-3G, uses a 68x fMux system to operate its large-format camera of ∼ 16,000 TES bolometers. We present here the successful implementation and performance of the SPT-3G readout as measured on-sky. Characterization of the noise reveals a median pair-differenced 1/f knee frequency of 33 mHz, indicating that low-frequency noise in the readout will not limit SPT-3G’s measurements of sky power on large angular scales. Measurements also show that the median readout white noise level in each of the SPT-3G observing bands is below the expectation for photon noise, demonstrating that SPT-3G is operating in the photon-noise-dominated regime.