[en] The critical current density of a granular superconductor, modeled as an array of Josephson-coupled grains, is calculated using a Ginzburg-Landau approach that accounts for suppression of the superconducting gap parameter in the grains by supercurrent. For a wide range of experimental parameters, the critical current density versus temperature is found to have an Ambegaokar-Baratoff dependence at low temperatures but to exhibit a crossover to a Ginzburg-Landau (1-T/T/sub c/)/sup 3/2/ dependence near T/sub c/, the crossover occurring at the temperature for which the Josephson coupling energy of a junction is approximately equal to the superconducting condensation energy of a grain. Experimental results displaying this behavior are reported for a NbN film

[en] The most sensitive heterodyne receivers used for millimeter wave and submillimeter wave radioastronomy employ superconductor-insulator-superconductor (SIS) tunnel junctions as the nonlinear mixing element. Good performance has recently been reported for SIS junctions used in planar mixer circuits and waveguide mixers from about 300 GHz to 500 GHz. The authors have developed a submillimeter wave SIS heterodyne receiver for observing important rotational transitions of molecules in the interstellar medium near 550 GHz and 630 GHz. This receiver is based on a waveguide mixer with an adjustable backshort and E-plane tuner. The mixer uses a high current density, 0.25 μm² Nb-AlO_x-Nb tunnel junction defined by electron beam lithography. The capacitance of the junction is compensated with an integrated RF superconductive microstrip tuning circuit. The receiver performance has been measured over the frequency range 460 GHz to 640 GHz. DSB receiver noise temperatures as low as 200 ± 17 K at 540 GHz and 362 ± 33 K at 626 GHz have been obtained. In addition, negative differential resistance has been observed in the DC I-V curve at frequencies around 491 GHz. These results indicate that the superconductive Nb microstrip transmission lines used in the tuning circuits are low-loss and perform well up to at least 90% of the superconductor energy gap frequency

[en] A method to decrease radiation damage to VLSI devices when using x-ray lithography is described. To decrease the x-ray radiation reaching the device SiO₂ gates, an x-ray absorbing thin film is interposed between the resist and the device structure. Tungsten has been shown to meet all the requirements for the absorbing layer material. In particular, a RIE technique to obtain straight profiles in tungsten using a conventional CF₄/O₂ mixture is described in the paper. In addition, the dosimeter film technique utilized to calibrate the x-ray source is described in the appendix

[en] Integrated, miniature, thin-film dc SQUID susceptometers have now been applied in the investigation of a number of interesting physical systems. The leverage of such susceptometers derives from their ability to measure with extreme sensitivity the magnetic properties of micron size samples at low temperatures. In this paper the authors propose a figure of merit for miniature susceptometers and then discuss how this varies with scaling of dimensions. The range of applications for miniature susceptometers can be considerably broadened through improved magnetic, optical, and thermal isolation between the pickup loops and active SQUID elements. Techniques for integrating such features into single chip structures are reviewed as well as modular designs where SQUID and pickup loops are on physical separate substrates and connected by low inductance wire bonds. Extension of this latter technique should make possible the incorporation of commercial SQUIDs as well as high Tc superconductors. Experimental results on some of the systems are also presented

[en] Measurements of TlBaCaCuO-based dc superconducting quantum interference devices (SQUIDs) performed using a practical feedback circuit gave a flat frequency response and an energy sensitivity of 6 x 10^-30 J/Hz at 5 kHz and 77 K with an 80 pH SQUID. Flux-locked loop operation was demonstrated to be strongly effective in eliminating the magnetic field hysteresis of the SQUID. The SQUIDs were operated in the flux-locked loop at temperatures over 90 K and the 1/f noise was found to decrease with increasing temperature near 77 K

[en] Hafnium is an elemental superconductor which crystallizes in a hexagonal close packed structure, has a transition temperature T${}^{\mathrm{C}}\simeq <!--\; ???\; -->$ 400 mK, and has a high normal state resistivity around 90 µ ω cm. In Microwave Kinetic Inductance Detectors (MKIDs), these properties are advantageous since they allow for creating detectors sensitive to optical and near infra-red radiation. In this work, we study how sputter conditions and especially the power applied to the target during the deposition, affect the hafnium T${}^{\mathrm{C}}$, resistivity, stress, texture and preferred crystal orientation. We find that the position of the target with respect to the substrate strongly affects the orientation of the crystallites in the films and the internal quality factor, Q${}^{\mathrm{i}}$, of MKIDs fabricated from the films. In particular, we demonstrate that a DC magnetron sputter deposition at a normal angle of incidence, low pressure, and low plasma power promotes the growth of compressive (002)-oriented films and that such films can be used to make high quality factor MKIDs with Q${}^{\mathrm{i}}$ up to 600,000. (letter)

[en] We are developing of arrays of membrane isolated resonator-bolometers, each with a kinetic inductance device (KID) to measure the temperature of the membrane. The KIDs are fabricated out of the high temperature superconductor YBCO to allow operation at relatively high temperatures. The bolometers are designed to offer higher sensitivity than sensors operating at 300 K, but they require less expensive and lighter weight cooling than even more sensitive conventional superconducting detectors operating at lower temperatures. The bolometer arrays are applicable as focal planes in infrared imaging spectrometers, such as for planetary science missions or earth observing satellites. We describe the devices and present measurements of their sensitivity.

[en] Here, we present a status update on the development of a phonon-mediated particle detector using kinetic inductance detector (KID). The design is intended for $\mathcal{O}(1)$ kg substrate, using $\mathcal{O}({10}^2)$ KIDs on a single readout line, to image the athermal phonon distribution at < 1 mm position resolution and $\mathcal{O}(10)$ eV energy resolution. The design specification is set by the need to improve position reconstruction fidelity while maintaining low energy threshold for future rare-event searches such as for low-mass dark matter. Furthermore, we report on the design, which shows negligible crosstalk and > 95% inductor current uniformity, using the coplanar waveguide feedline, ground shield, and a new class of KIDs with symmetric coplanar stripline (sCPS) inductor. The multiplexing is designed upon the frequency-geometry relation we develop for the sCPS KIDs. We introduce the fabrications of the Nb RF assessment prototypes and the high phonon collection efficiency Al–Nb devices. We achieve $\lesssim <!--\; ???\; -->$ 0.07% frequency displacement on a 80-KID RF assessment prototype, and the result indicates that we may place more than 180 resonances in our 0.4 GHz readout band with minimal frequency misordering. The coupling quality factors are $\sim <!--\; ???\; -->$ 10${}^5$ as designed. Finally, we update our work in progress in fabricating the $\mathcal{O}({10}^2)$ KID, bi-material, $\mathcal{O}(1)$ kg detectors, and the expected position and energy resolutions.

[en] We demonstrate position and energy-resolved phonon-mediated detection of particle interactions in a silicon substrate instrumented with an array of microwave kinetic inductance detectors (MKIDs). The relative magnitude and delay of the signal received in each sensor allow the location of the interaction to be determined with < or approx. 1mm resolution at 30 keV. Using this position information, variations in the detector response with position can be removed, and an energy resolution of σ_E = 0.55 keV at 30 keV was measured. Since MKIDs can be fabricated from a single deposited film and are naturally multiplexed in the frequency domain, this technology can be extended to provide highly pixelized athermal phonon sensors for ∼1 kg scale detector elements. Such high-resolution, massive particle detectors would be applicable to rare-event searches such as the direct detection of dark matter, neutrinoless double-beta decay, or coherent neutrino-nucleus scattering.

[en] A practical strategy for synchronizing the properties of compound Josephson junction (CJJ) radio frequency monitored superconducting quantum interference device (rf-SQUID) qubits on a multi-qubit chip has been demonstrated. The impact of small (∼1%) fabrication variations in qubit inductance and critical current can be minimized by the application of a custom-tuned flux offset to the CJJ structure of each qubit. This strategy allows for a simultaneous synchronization of the qubit persistent current and tunnel splitting over a range of external bias parameters that is relevant for the implementation of an adiabatic quantum processor.