Norton-Corrected Measurement of Complex Impedances of a Large Format Bolometer Array

We report on our efforts to measure simultaneously a well-calibrated complex impedance of a large number of detectors in a long wavelength bolometer array. The array is described in other presentations. A method for correcting the complex impedance measurements of bolometers and calorimeters has been presented by Lindeman et al. (Rev. Sci. Instrum. 78:043105, [2007]) using a Thévenin equivalent circuit to represent the bias network. We have built on this method for superconducting bolometers with a Norton equivalent circuit and have used it to improve our impedance data. We further describe our method for extracting a Norton-corrected complex impedance as a function of frequency from a stream of multiplexed time-ordered data. This method is well-suited to producing simultaneous complex impedance measurements for a large number of detectors.      

Elastic deformation of a hypoid gearbox

High load carrying capacity in the gear mesh can be achieved when the contact load is well distributed. Designing an adequate tooth flank geometry requires a thorough calculation approach. This is particularly important for automotive hypoid gearboxes with aluminum housing. In this paper, the deformation analysis of housing, bearings and shafts for a hypoid gearbox is validated by measurements at a static test rig. The challenge of compensating friction effects to get high-quality measurement results is successfully met by the presented approach.There are several computer programs to calculate the elastic deformation behavior of the system shaft-bearing-housing. Most of the calculation programs use analytical approaches. The advantage of these programs, in comparison to a full FEM approach, is an efficient parameterization of gears and a very short calculation time, while delivering high precision results. The deformation behavior of housings can be considered in different ways. The deformation behavior of a bearing seat can be modelled by its stiffnesses. For housings with lower stiffnesses (for example aluminum housings), crossover influences become more and more important. The deformation behavior of a housing can be taken into account by a stiffness matrix. The stiffness matrix of a housing can be determined by using the finite elements method. Therefore, the complex elastic deformation behavior of housings can be considered in analytical calculation approaches. Within the scope of a research project, investigations have been made to validate the calculated elastic deformations of the shaft-bearing-housing system. For this purpose, deformation measurements of a test gearbox were taken by using a 3D coordinate measuring machine. The test gearbox is static and consists of a hypoid gearbox for automotive applications. The input torque is provided by a lever mechanism. The elastic deformation of the test gear box has been measured and compared to calculation results. Therefore, different approaches of modelling the elastic deformation behavior of the housing can be validated and compared to each other.     

Determination of the Photon Impact Position within a DROID

Distributed read-out imaging detectors (DROIDs) combine a good energy resolution with a high position resolving power. Therefore they are promising candidates to build cameras with a huge detection area. A new method is proposed to determine the photon energy and impact position from the two signals of a one-dimensional DROID. For this purpose the diffusion equation was solved for asymmetric devices with arbitrarily trapping. The new method is then applied to fit the experimental data obtained from the illumination with soft X-ray of a 400 μm long Ta absorber strip with one Ta/Al read-out STJ at each end.      

Performance of TES X-ray Microcalorimeters with a Novel Absorber Design

Superconducting transition-edge sensor (TES) microcalorimeters have demonstrated the Constellation-X requirements for spectral resolution, speed, and pixel size in a close-packed geometry. We will present our recent breakthrough energy resolution with sensors that have all gold and bismuth-gold absorbers. This has been enabled by cantilevered absorbers that make contact to the TES only in regions that are not part of the active thermometer. With this approach, rapid thermalization of the x-ray energy is achieved and interaction between the absorber and TES sensor films is avoided. This design allows us to obtain uniform high performance and is compatible with large-format, high fill-factor arrays. We will discuss this design, the results we have achieved in 8×8 arrays of these pixels, and the dependence of the performance on the geometry of the absorber contact area and on stress within the sensor.      

Modeling of TES X-Ray Microcalorimeters with a Novel Absorber Design

We have successfully modeled our new TES (transition-edge sensor) X-ray microcalorimeters with a novel X-ray absorber design that is suitable for close-packed array with high quantum efficiency. We have determined device parameters that reproduce complex impedance curves and noise spectra throughout the transition. Observed pulse height, decay time and baseline energy resolution were in good agreement with simulated results using the same parameters.      

Observation of Zero-Sound at Atomic Wave-Vectors in a Monolayer of Liquid 3He

The elementary excitations of a strongly interacting two-dimensional Fermi liquid have been investigated by inelastic neutron scattering in an experimental model system: a monolayer of liquid ³He adsorbed on graphite preplated by a monolayer of solid ⁴He. We observed for the first time the particle-hole excitations characterizing the Fermi liquid state of two-dimensional liquid ³He, and we were also able to identify the highly interesting zero-sound collective mode above a particle-hole band. Contrarily to bulk ³He, at low wave-vectors this mode lies very close to the particle-hole band. At intermediate wave-vectors, the collective mode enters the particle-hole band, where it is strongly broadened by Landau damping. At high wave-vectors, where the Landau theory is not applicable, the zero-sound collective mode reappears beyond the particle-hole band as a well defined excitation, with a dispersion relation quite similar to that of superfluid ⁴He. This spectacular effect is observed for the first time in a Fermi liquid (including plasmon excitations in electronic systems).     

The Decay of a Quantized Vortex Ring and the Influence of Tracer Particles

We capture the decay of a quantized vortex ring in superfluid helium-4 by imaging particles trapped on the vortex core. The ring shrinks in time, providing direct evidence for the dissipation of energy in the superfluid. The ring with trapped particles collapses more slowly than predicted by an available theory, but the collapse rate can be predicted correctly if the trapping of the particles on the core is taken into account. We theoretically explore the conditions under which particles may be considered passive tracers of quantized vortices and estimate, in particular, that their dynamics on the large-scale is largely unaffected by the burden of trapped particles if the latter are spaced by more than ten particle diameters along the vortex core, at temperatures between 1.5 K and 2.1 K.     

GISMO, a 2 mm Bolometer Camera Optimized for the Study of High Redshift Galaxies

The 2 mm spectral range provides a unique terrestrial window enabling ground based observations of the earliest active dusty galaxies in the universe and thereby allowing a better constraint on the star formation rate in these objects. We have built GISMO (the Goddard-IRAM Superconducting 2-Millimeter Observer), a 2 mm, 128 element superconducting Transition Edge Sensor (TES) based bolometer camera for the IRAM 30 m telescope in Spain. The camera uses an 8×16 planar array of multiplexed TES bolometers, which incorporates our recently designed Backshort Under Grid (BUG) architecture, described elsewhere. The optical design incorporates a 100 mm (4 inches) diameter silicon lens cooled to 4 K, which provides the required fast beam of 0.9 λ/D. With this spatial sampling, GISMO will be very efficient at detecting sources serendipitously in large sky surveys, while the capability for diffraction-limited observations is preserved. With the background limited performance of the detectors, the camera provides significantly greater imaging sensitivity and mapping speed at this wavelength than has previously been possible. The major scientific driver for the instrument is to provide the IRAM 30 m telescope with the capability to rapidly observe galactic and extragalactic dust emission, in particular from high-z Ultra Luminous Infrared Galaxies (ULIRGs) and quasars, even in the summer season. The instrument will fill in the SEDs of high redshift galaxies at the Rayleigh-Jeans part of the dust emission spectrum, even at the highest redshifts. Our source count models predict that GISMO will serendipitously detect one galaxy every four hours on the blank sky, and that one quarter of these galaxies will be at a redshift of z 6.5. We expect to install GISMO at the 30 m telescope in the second half of 2007.      

Simultaneous Measurements of an Ultrasound and a Torsional Oscillator for Pressurized 4He in a Nanoporous Glass

Recently, torsional oscillator (Yamamoto et al. in Phys. Rev. Lett. 93:075302, 2004) and ultrasound (Kobayashi et al. in AIP Conf. Proc. 128:797, 2007) measurements were carried out for pressurized ⁴He filled in a nanoporous glass (Gelsil), and the superfluid transition temperature T _C shows a different pressure dependence. Thus motivated, we have performed simultaneous measurements with a torsional oscillator and with ultrasound for pressurized ⁴He in Gelsil. T _C is in agreement between the two techniques at all pressures, and the superfluid component above 0.5 K shows most of the same temperature dependence. Furthermore, it was found that the χ-factor (the fraction of superfluid which remains locked to the substrate ) is independent of measuring frequency between the torsional oscillator (kHz-order) and the ultrasound (MHz-order) ranges.     

Development and Characterization of Microcalorimeters for a Next Generation 187Re Beta-Decay Experiment

It has been demonstrated in the past that observing the β-decay spectrum of ¹⁸⁷Re with microbolometers provides a suitable method to determine the mass of the electron anti-neutrino from β-endpoint measurements. In a first step, with the experiment MIBETA a sensitivity of m _νe≤15 eV/c² was achieved. To compete with the sensitivity of m _νe≤2.2 eV/c² established by the Mainz/Troitsk tritium β-decay experiment and the limit of m _νe≤0.2 eV/c² aimed at with KATRIN, a new experiment MARE has been initiated. As a first stage (MARE-1), 300 detectors consisting of silicon implanted thermistors, produced by NASA/GSFC, and absorbers of AgReO₄ crystals will be mounted. To optimize the experimental setup, a test array was equipped with 10 AgReO₄ crystals of various size and shape. The influence of the crystal quality as well as of different types of resin on rise time and energy resolution was investigated.      

Static and dynamic scaling of semiflexible polymer chains—a molecular dynamics simulation study of single chains and melts

A molecular dynamics study of the effect of increasing molecular chain stiffness on the dynamic properties of single linear chains and polymer melts is presented. The chain stiffness is controlled by a bending potential. By means of a normal mode analysis the systematic crossover behavior of coarse-grained linear polymer systems from Rouse modes to bending modes with increasing mode number p is presented systematically for the first time via simulation data. The magnitude and the onset of the region where crossover behavior occurs is investigated in dependence of a systematic variation of the chains’ persistence lengths. For long wavelength modes the well-known p ⁻² behavior is observed which represents the Rouse scaling, whereas for the bending modes one observes a distinct p ⁻⁴ scaling of the mean square mode amplitudes and the relaxation times. Additionally, the effect of this crossover behavior on the monomer dynamics given by their mean square displacements is investigated. The findings are contrasted with previous simulation studies of semiflexible chain behavior and it is shown that those studies—due to too small persistence lengths L _p —were actually limited to the crossover regime where both, Rouse and bending modes contribute to the chain dynamics, and no distinct p ⁻⁴ scaling can be observed. Using an expansion of the monomer position vector, a simple theory of the observed scaling behavior is proposed which allows for deriving analytic expressions that describe the presented simulation data very well.     

Empirical likelihood confidence intervals for the endpoint of a distribution function

Estimating the endpoint of a distribution function is of interest in product analysis and predicting the maximum lifetime of an item. In this paper, we propose an empirical likelihood method to construct a confidence interval for the endpoint. A simulation study shows the proposed confidence interval has better coverage accuracy than the normal approximation method, and bootstrap calibration improves the accuracy.     

Development of a Digital Signal Readout System for Large TES Arrays

We are developing a digital signal readout system for arrays of high-resolution gamma and fast-neutron detectors based on superconducting transition edge sensors (TESs). The readout system allows for real time data acquisition and analysis at count rates exceeding 100 Hz for pulses with several ∼ms decay times with minimal loss of energy resolution compared to optimum filtering. This digital signal processing system had originally been developed for gamma-ray analysis with HPGe detectors, and we have modified the hardware and firmware to accommodate the slower TES signals. Parameters of the filtering algorithm have been optimized to maximize either resolution or throughput. Here we present a summary of the digital signal processing hardware and discuss its initial performance.      

Heat Capacity of Liquid 4He Confined in a Nanoporous Glass

We report a preliminary study of heat capacities of ⁴He confined in a nanoporous Gelsil glass that has nanopores of 2.5 nm in diameter. The heat capacity has a broad peak at a temperature far above the superfluid transition temperature obtained by torsional oscillator technique. The heat-capacity peak is attributed to formation of localized Bose-Einstein Condensates in the nanopores, in which the long-range superfluid coherence is destroyed by pore size distribution or random potential inherent to the porous glass.      

Direct Measurement of the Critical Velocity Above Which a Tuning Fork Generates Turbulence in Superfluid Helium

The dynamics of an electrically-driven 8 kHz quartz tuning fork has been studied experimentally in liquid helium-4 in the temperature range 1.3<T<4.2 K under the saturated vapour pressure. The fork has relatively large dimensions compared to standard 32 kHz fork used in recent investigations. The velocity of the tip of the fork prong is measured by the indirect electromechanical equivalent method and is compared with the velocity of another 8 kHz fork (from the same batch) determined by direct optical measurement of the oscillation amplitude through Michelson interferometry. A comparison of these results has provided absolute values for the critical velocity of the transition to the turbulent state.     

Thermal Conductivity of a Molecular Crystal with Rotational Degrees of Freedom: Orientational Defect Scattering

Measurements of thermal conductivity of solid methane-deuteromethane solutions at equilibrium vapor pressure in the temperature range 1.2÷20 K are reported. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH₄-CD₄ system. The contribution of the orientational defect scattering to the thermal conductivity is discussed in frame of the model of local changes in the moments of inertia of molecules.      

The Vortex State and Josephson Critical Current of a Diffusive SNS Junction

We study theoretically the electronic and transport properties of a diffusive superconductor-normal metal-superconductor (SNS) junction in the presence of a perpendicular magnetic field. We show that the field dependence of the critical current crosses over from the well-known Fraunhofer pattern in wide junctions to a monotonic decay when the width of the normal wire is smaller than the magnetic length $\xi_{\mathrm{H}}=\sqrt{\varPhi_{0}/H}$ , where H is the magnetic field and Φ ₀ the flux quantum. We demonstrate that this behavior is intimately related to the appearance of a linear array of vortices in the middle of the normal wire, the properties of which are very similar to those in the mixed state of a type II superconductor. This novel vortex structure is also manifested in a strong modulation of the local density of states along the transversal dimension, which can be measured with existing experimental techniques.     

Piezoelectric characterization of a single zinc oxide nanowire using a nanoelectromechanical oscillator

In this paper, we present a nanoelectromechanical oscillator with a single semiconducting zinc oxide nanowire (ZnO) doubly clamped and suspended on two metal electrodes by which the piezoelectric property on the growth of the ZnO nanowire along the c-axis, [0001], is characterized by the resonant frequency shift of the oscillator. We report that the resonance of the nanowire oscillator can be detected in ambient air and the effective piezoelectric coefficient on the growth of a ZnO nanowire along the c-axis, [0001], is significantly larger than that of bulk (0001) ZnO.     

Spectral Properties of a Doped Antiferromagnet with Pairing Correlations

In this paper, we study the spectral properties of a phenomenological model for a weakly doped antiferromagnet. In this model, it is assumed that each carrier moves in one of the two sublattices where it was introduced. Such a situation corresponds to a case of underdoped high-temperature superconductors with the free carrier spectra maximum at k=(±π/2,±π/2) and with a four-pocket Fermi surface. We study the spectral properties of the model by taking into account both the fluctuations of the phases of the superconducting order parameter and spins of the antiferromagnetic background. It is shown that the hole spectral function and the density of states are strongly affected by these fluctuations. In particular, we argue that these fluctuations can be responsible for the temperature evolution of the Fermi pockets in cuprate superconductors.     

Free damped vibrations of a viscoelastic oscillator based on Rabotnov’s model

Free damped vibrations of a linear viscoelastic oscillator based on Rabotnov’s model involving one fractional parameter and several relaxation (retardation) times are investigated. The analytical solution is obtained in the form of two terms, one of which governs the drift of the system’s equilibrium position and is defined by the quasi-static processes of creep occurring in the system, and the other term describes damped vibrations around the equilibrium position and is determined by the systems’s inertia and energy dissipation. The drift is governed by an improper integral taken along two sides of the cut of the complex plane. Damped vibrations are determined by two complex conjugate roots of the characteristic equation, which are located in the left half-plane of the complex plane. The behaviour of the characteristic equation roots as function of the system’s parameters is shown in the complex plane. Dedicated to the bright memory of Academician Yury N. Rabotnov.