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.      

Development of Micro-SQUID Magnetometers for Investigation of Quantum Tunneling of Magnetization in Nanometer-Size Magnetic Materials

We report development of micro superconducting quantum interference device (μ-SQUID) magnetometers for investigation of quantum tunneling of magnetization in μm- and nm-size magnetic materials. Both high- and low-temperature superconductor (HTS and LTS) based μ-SQUID magnetometers were fabricated and a three dimensional magnetic coil system was constructed for this purpose. The HTS-μ-SQUIDs with a hole of 4×9 μm² work at temperatures between 4.2 and 70 K and in magnetic fields up to 120 mT. A magnetization measurement of a ferrimagnetic micro-crystal was carried out at 35 K with an accuracy of 10^?9 emu. The development of LTS-μ-SQUIDs has been started in order to study much smaller magnetic materials in a mK temperature range. We present a preliminary result on the LTS-μ-SQUID with a hole of 1×1 μm². The critical current as a function of applied magnetic field shows the SQUID modulation at 4.2 K and up to 30 mT. The heat release associated with the present measurement method is estimated to be on the order of several microwatts.     

Role of Coexistence of Superconductivity and Paramagnetism in Magnetostriction of High-T c Superconductors

Total magnetostriction in the superconducting state for high T _c superconductors has been separated into critical state and paramagnetic components in terms of a H(x) dependent magnetic flux density. We show that the paramagnetic part is χ(2+χ)〈H(x)²〉, where χ is paramagnetic susceptibility. We have reproduced successfully ΔL/L−H _a curves measured by de la Fuente et al. (Phys. C 244:214, [1995]), in which they clearly observed coexistence of superconductivity and paramagnetism, employing the concepts presented in this work.      

Quantum Size Effects in the Growth, Coarsening, and Properties of Ultra-thin Metal Films and Related Nanostructures

This review addresses the quantum mechanical nature of the formation and stability of ultrathin metal films. The competition between quantum confinement, charge spilling effects, and Friedel oscillations determines whether an atomically smooth metal film will be marginally, critically, or magically stable or totally unstable against roughening. Pb(111) films represent a special case, not only because of strong quantum oscillations in the stability of two-dimensional thin films but also because of the exceptionally fast coarsening of Pb nanoclusters. The latter appears to be due to the combined effects of size quantization and the existence of a unique mass exchange medium in the form of an unusually dense and highly dynamic wetting layer. The consequences of size quantization on the physical and chemical properties of the films are profound, some of which will be highlighted in this review.     

Electrical and Thermal Characterization of Membrane-Isolated, Antenna-Coupled, TES Bolometers for the SPIDER Experiment

We have fabricated and measured the thermal and DC electrical properties of transition-edge sensing (TES) bolometers to be employed on the SPIDER experiment, a balloon-based observatory for studying the polarization of the cosmic microwave background (CMB). The bolometers consist of Al and Ti thermistors in series and a termination resistor which couples to an on-chip, polarization sensitive, 150 GHz slot-array antenna through a superconducting microstrip transmission line. Several important parameters were measured. Transition temperature measurements were performed by measuring the Johnson noise in the Ti thermistor. Current-voltage characteristic measurements were performed at various temperatures allowing for the deduction of the thermal conductance and the temperature coefficient of resistance. Electrical noise equivalent power was measured to sub-Hertz frequencies. Finally, the time constant of the bolometers was measured within the Al and Ti transitions where electrothermal feedback speeds up the bolometer response compared with the natural time constant measured just above the Ti transition temperature. The results of these measurements are within the design specifications for SPIDER.      

Characterization of the Response of CaWO4 on Recoiling Nuclei from Surface Alpha Decays

A potentially harmful background for experiments attempting direct dark matter detection like the CRESST (= Cryogenic Rare Event Search with Superconducting Thermometers) experiment is caused by recoiling nuclei from ²¹⁰Po alpha decays on surfaces close to the detector. In order to characterize this kind of background in CRESST, calibration measurements have been performed at the TU München. A for this purpose an optimized version of the CRESST detector has been developed consisting of a 38 g CaWO₄ crystal and a separate cryogenic light detector, both equipped with Ir/Au transition edge sensors (TESs). The simultaneous measurement of the phonon signal and the scintillation light from the CaWO₄ crystal allows to discriminate between electron and nuclear recoils using their different light outputs. The unexpected results of a first measurement with a ²¹⁰Po source can be understood with the help of a Monte Carlo simulation performed for a similar system.      

A Kilopixel Array of TES Bolometers for ACT: Development, Testing, and First Light

The Millimeter Bolometer Array Camera (MBAC) will be installed on the 6-meter Atacama Cosmology Telescope (ACT) in late 2007. For the first season of observations, MBAC will comprise a 145 GHz diffraction-limited, 1024-pixel, focal plane array of Transition Edge Sensor (TES) Bolometers. This will be the largest array of pop-up-detector bolometers ever fielded as well as one of the largest arrays of TES bolometers. We discuss the design specifications for the array and pre-assembly testing procedures for the cryogenic components. We present dark measurements of the TES bolometer properties of numerous 32-pixel columns that have been assembled into the first kilopixel array for ACT, as well as optical measurements made with our 256-pixel prototype array, including first light measurements on ACT.      

Enhancement of Sudden Death of Entanglement for Driven Qubits

We study the recently discovered phenomena of sudden death of entanglement for a system of two qubits, each of them independently longitudinally damped by a reservoir and subjected to a continuous driving. We show that driving produces, in the interaction picture, an effective bath that has elements amounting to various extra sources of noise (transverse, thermal squeezed, thermal longitudinal). As a result, the time of sudden death decreases due to driving, which we verify as well by direct numerical calculation. We suggest that this phenomenon can be studied systematically using superconducting qubits driven by microwave fields.     

Synthesis, Crystal Growth and Epitaxial Layers Growth of FeSe0,88 Superconductor and Other Poison Materials by Use of High Gas Pressure Trap System

The FeSe material was prepared from pure components under inert gas atmosphere. Typically, synthesized material was HIP-ed under pressure of 0.45 GPa of 5N purity argon with use of the High Gas Pressure Trap System (HGPTS). The thin films were obtained by epitaxial process performed on substrates. Thin layers were manufactured by mixed procedures with the use of DC sputtering on the substrate from various types of targets. The FeSe_0.88 material has T _c from 8 to 12 K. It was synthesized at high Se vapor pressure at equilibrium conditions of pressure and temperature. The used HGPTS assure the full separation of the active inner volume for synthesis or crystal growth of material and the outside gas medium. The material has been investigated by SEM, EDX, XRD, magnetic susceptibility and resistivity measurements.     

Dispersion-Induced Splitting of the Collective Mode Spectrum in Axial and Planar Phases of Superfluid 3He

The whole collective mode spectrum in axial and planar phases of superfluid ³He with dispersion corrections is calculated for the first time. In axial A-phase the degeneracy of clapping modes depends on the direction of the collective mode momentum k with respect to the vector l (mutual orbital moment of Cooper pairs), namely: the mode degeneracy remains the same as in case of zero momentum k for k‖l only. For any other directions there is a threefold splitting of these modes, which reaches maximum for k ⊥ l. In planar 2D-phase, which exists in the magnetic field (at H>H _C ) we find that for clapping modes the degeneracy depends on the direction of the collective mode momentum k with respect to the external magnetic field H, namely: the mode degeneracy remains the same as in case of zero momentum k for k‖H only. For any other directions different from this one (for example, for k ⊥ H) there is twofold splitting of these modes. The obtained results imply that new interesting features can be observed in ultrasound experiments in axial and planar phases: the change of the number of peaks in ultrasound absorption into clapping mode. One peak, observed for these modes by Ling et al. (J. Low Temp. Phys. 78:187, 1990), will split into two peaks in a planar phase and into three peaks in an axial phase under the change of ultrasound direction with respect to the external magnetic field H in a planar phase and with respect to the vector l in an axial phase. In planar phase, some Goldstone modes in the magnetic field become massive (quasi-Goldstone) and have a similar twofold splitting under the change of ultrasound direction with respect to the external magnetic field H. The obtained results as well will be useful under interpretation of the ultrasound experiments in axial and planar phases of superfluid ³He.     

Theory of Decay of Superfluid Turbulence in the Low-Temperature Limit

We review the theory of relaxational kinetics of superfluid turbulence—a tangle of quantized vortex lines—in the limit of very low temperatures when the motion of vortices is conservative. While certain important aspects of the decay kinetics depend on whether the tangle is non-structured, like the one corresponding to the Kibble-Zurek picture, or essentially polarized, like the one that emulates the Richardson-Kolmogorov regime of classical turbulence, there are common fundamental features. In both cases, there exists an asymptotic range in the wavenumber space where the energy flux is supported by the cascade of Kelvin waves (kelvons)—precessing distortions propagating along the vortex filaments. At large enough wavenumbers, the Kelvin-wave cascade is supported by three-kelvon elastic scattering. At zero temperature, the dissipative cutoff of the Kelvin-wave cascade is due to the emission of phonons, in which an elementary process converts two kelvons with almost opposite momenta into one bulk phonon. Along with the standard set of conservation laws, a crucial role in the theory of low-temperature vortex dynamics is played by the fact of integrability of the local induction approximation (LIA) controlled by the parameter Λ=ln?(λ/a ₀), with λ the characteristic kelvon wavelength and a ₀ the vortex core radius. While excluding a straightforward onset of the pure three-kelvon cascade, the integrability of LIA does not plug the cascade because of the natural availability of the kinetic channels associated with vortex line reconnections. We argue that the crossover from Richardson-Kolmogorov to the Kelvin-wave cascade is due to eventual dominance of local induction of a single line over the collective induction of polarized eddies, which causes the breakdown of classical-fluid regime and gives rise to a reconnection-driven inertial range.     

Cytotoxicity of single-walled carbon nanotubes suspended in various surfactants

The cytotoxicity of single-walled carbon nanotubes (SWCNTs) suspended in various surfactants was investigated by phase contrast light microscopy characterization in combination with an absorbance spectroscopy cytotoxicity analysis. Our data indicate that individual SWCNTs suspended in the surfactants, sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS), were toxic to 1321N1 human astrocytoma cells due to the toxicity of SDS and SDBS on the nanotube surfaces. This toxicity was observed when cells were exposed to an SDS or SDBS solution having a concentration as low as 0.05?mg?ml(-1) for 30?min. The proliferation and viability of the cells were not affected by SWCNTs alone or by conjugates of SWCNTs with various concentrations of sodium cholate (SC) or single-stranded DNA. The cells proliferated similarly to untreated cells when surrounded by SWCNTs as they grow, which indicated that the nanotubes did not affect cells adversely. The cytotoxicity of the nanotube-surfactant conjugates was controlled in these experiments by the toxicity of the surfactants. Consequently, when evaluating a surfactant to be used for the dispersion of nanoscale materials in applications such as nanoscale electronics or non-viral biomolecular transporters, the cytotoxicity needs to be evaluated. The methodology proposed in this study can be used to investigate the cytotoxicity of other nanoscale materials suspended in a variety?of?surfactants.     

The Viscosity of Aqueous Alkali-Chloride Solutions up to 623 K, 1,000 bar,and High Ionic Strength

An accurate viscosity (dynamic viscosity) model is developed for aqueous alkali-chloride solutions of the binary systems, LiCl–H₂O, NaCl–H₂O, and KCl–H₂O, from 273 K to 623 K, and from 1 bar to 1,000 bar and up to high ionic strength. The valid ionic strengths for the LiCl–H₂O, NaCl–H₂O, and KCl–H₂O systems are 0 to 16.7 mol · kg⁻¹, 0 to 6 mol · kg⁻¹, and 0 to 4.5 mol · kg⁻¹, respectively. Comparison of the model with about 4,150 experimental data points concludes that the average absolute viscosity deviation from experimental data in the above range is within or about 1 % for the LiCl–H₂O, NaCl–H₂O, and KCl–H₂O mixtures, indicating the model is of experimental accuracy. With a simple mixing rule, this model can be extrapolated to predict the viscosity of ternary aqueous alkali-chloride solutions, making it useful in reservoir fluid flow simulation. A computer code is developed for this model and can be obtained from the author: (maoshide@cugb.edu.cn).     

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.      

Properties of Tungsten Thin Films Produced with the RF-Sputtering Technique

For the purpose of building very sensitive light and phonon detectors, as e.g. applied in the Dark Matter (DM) experiment CRESST (Cryogenic Rare Event Search with Superconducting Thermometers), transition edge sensors (TESs) in combination with a massive absorber crystal are used. To ensure high sensitivity of the detectors, low heat capacities, i.e. low working temperatures of about 10 mK are aimed at. Therefore, TESs made of tungsten thin films exhibiting the alpha-tungsten (α-W) phase with transition temperatures of T _c =10–15 mK are required. We have produced tungsten thin films with T _c in the range of 25–55 mK by rf-sputtering. To decouple the thermometer production from the choice of the target material and to avoid heating cycles of the absorber crystal, a composite design for detector production is applied. The composite design includes fabrication of the TES on a separate substrate and then attaching of this separate TES to a massive absorber crystal by gluing. For this purpose small sapphire substrates are used for the deposition of the TES. Properties of tungsten thin films grown with the rf-sputtering technique as well as first results of composite detectors built with these films acting as TESs will be presented.      

Dynamic J-R curves and tension-impact properties of AISI 308 stainless steel weld

In this paper, instrumented tension-impact (dynamic tensile) and instrumented Charpy impact test results for AISI 308 stainless steel welds at room temperature are reported. A few Charpy specimens precracked to a/W (crack length to width ratio) ratios of 0.42 to 0.59 were also tested. Dynamic yield strength obtained from tension-impact test agrees well with that from Charpy V-notch specimens. The strain rates obtained during the tension-impact test are compared with the various estimates of strain rates for V-notch and precracked Charpy specimens. A variation of the compliance changing rate method was necessary for determining the crack initiation point while crack growth was determined by power law key-curve procedure. J-R curves thus obtained from Charpy (precracked and V-notch) specimens are compared with those computed using handbook procedures using dynamic tensile results. Key words: Tension-impact testing, 308 stainless steel weld, Charpy V-notch, dynamic fracture toughness, dynamic yield strength, J-R curve, strain rate, key-curve.     

Controlled synthesis of InAs wires, dot and twin-dot array configurations by cleaved edge overgrowth

We present experimental results on the controlled synthesis of InAs ordered nanostructures with three different grades of complexity: nanowires, quantum dot arrays, and double quantum dot arrays. A model for the diffusion of In adatoms on (110) surfaces explains the observed ordering and establishes general criteria for the optimized fabrication of the three different InAs nanostructure configurations, as a function of the growth conditions. These results are important for the use of ordered InAs nanostructures in future optoelectronic applications.     

Equal-Channel Multi-Angle Pressing Effect on the Properties of NbTi Alloy

An improvement of NbTi alloy functional properties by equal-channel multi-angle pressing (ECMAP) combined with hydrostatic extrusion, drawing and thermal treatment is revealed. The ECMAP method allows to increase the billet accumulated deformation with preserving its initial dimensions. The formed highly dispersed and homogeneous nanocrystalline structure with a more uniform distribution of α-phase precipitations as a result of ECMAP treatment improves the functional properties of the alloy. In the field of 5 T, the critical current density in wire samples produced with application of the ECMAP method is enhanced approximately by a factor of 2 in comparison with the values obtained for the samples produced without the ECMAP treatment.     

Emergence of Pairing Interaction in the Hubbard Model in the Strong Coupling Limit

We implement the rotationally-invariant formulation of the two-dimensional Hubbard model, with nearest-neighbors hopping t, which allows for the analytic study of the system in the low-energy limit. Both U(1) and SU(2) gauge transformations are used to factorize the charge and spin contribution to the original electron operator in terms of the corresponding gauge fields. The Hubbard Coulomb energy U-term is then expressed in terms of quantum phase variables conjugate to the local charge and variable spin quantization axis, providing a useful representation of strongly correlated systems. It is shown that these gauge fields play a similar role as phonons in the BCS theory: they act as the “glue” for fermion pairing. By tracing out gauge degrees of freedom the form of paired states is established and the strength of the pairing potential is determined. It is found that the attractive pairing potential in the effective low-energy fermionic action is non-zero in a rather narrow range of U/t.     

Numerical Simulations of Superfluid Turbulence under Periodic Conditions

This paper is devoted to numerical simulation of vortex tangle dynamics in superfluid helium. The problem is solved on the base of the so called reconnection ansatz consisting of the equation of motion for vortex lines plus reconnection of a loop. A new algorithm, which is based on consideration of crossing lines, is used for the reconnection processes. Calculations are performed for a cubic box. Periodic boundary conditions are applied in all directions. We use the 4th order Runge-Kutta method for the integrations in time. The dynamics of quantized vortices with various counterflow velocities is studied. The density of vortex lines and number of reconnections as functions of vortex line density are calculated.