Simulations of Noise in Phase-Separated Transition-Edge Sensors for SuperCDMS

We briefly review a simple model of superconducting-normal phase-separation in transition-edge sensors (TESs) in the SuperCDMS experiment. After discussing some design considerations relevant to the TESs in the experiment, we study noise sources in both the phase-separated and phase-uniform cases. Such simulations will be valuable for optimizing the critical temperature and TES length of future SuperCDMS detectors.     

A Two-Fluid Model for the Transition Shape in Transition-Edge Sensors

Superconducting microcalorimeters based on transition-edge sensors (TESs) are being successfully used in applications ranging from optical photon counting to gamma-ray and alpha particle spectroscopy. Practical instruments often require a complex optimization among speed, linearity and energy resolution. However, a?lack of understanding of the superconducting transition limits our ability to predict the behavior of a new TES design. Specifically, there is an unmet need for a model that predicts the current and temperature dependent resistance surface that describes the transition: R(I,T). This paper describes the predictions of a two-fluid model for the resistance of a TES based on a Ginzburg-Landau form of the critical current. We compare the predictions of the model for the logarithmic derivatives of resistance with temperature and current (α and β) to measurements of TESs used in x-ray and gamma spectrometers. The model shows excellent qualitative agreement that provides useful insight into the dependence of α and β on the current density and bias point of the TES.     

Reducing Excess Noise in Au/Ti Transition-Edge Sensors

We have studied the noise spectra of Au/Ti bilayer transition-edge sensors with transition temperatures around ∼100 mK. The sensors did not include an absorber element, and were patterned into either a circular Corbino geometry, or a Corbino disk divided into four parallel sections. Both sensor types contained noise in excess of the thermodynamic phonon noise and the equilibrium Johnson noise of the sensor. However, the divided Corbino detector had a significantly lower noise level than the undivided Corbino disk, especially at bias points below the mid-point of the transition.      

Anomalously strong small-q electron-phonon coupling in cuprates due to dielectricity

We consider the role of the enormous ionic dielectric constant ε₀≈ 30–50 in the cuprates. The ionically-screened Thomas-Fermi screening parameter ~q_TF = [4θe²N(E_F)/εin0 ^1/2 is extremely small, namely 0.2-0.3 A^-1. The electron-phonon coupling constant I(q,w) is found to be anomalously large forq ≈ ~q_TF and smallco, accounting for the CDW observed in overdoped BSCCO with wavevector 0.24 A^-1. Using the Eliashberg theory we derive from this I(q,w) a maximumT _c of about 200 K. The small electron-phonon scattering angle △θ = ~q _{F ≈} 0.3 rad acounts for the observedd-wave pairing, as originating from a phonon-mediated mechananism.     

Suppression of excess noise in Transition-Edge Sensors using magnetic field and geometry

We report recent progress at NIST on Mo/Cu Transition-Edge Sensors (TESs). While the signal-band noise of our sensors agrees with theory, we observe excess high-frequency noise. We describe this noise and demonstrate that it can be strongly suppressed by a magnetic field perpendicular to the plane of the sensor. Both the excess noise and =(T/R)(dR/dT) depend strongly on field so our results show that accurate comparisons between devices are only possible when the field is well known or constant. We also present results showing the noise performance of TES designs incorporating parallel and perpendicular normal metal bars, an array of normal metal islands, and in wedge-shaped devices. We demonstrate significant reduction of high-frequency noise with the perpendicular bar devices at the cost of reduced . Both the bars and the magnetic field are useful noise reduction techniques for bolometers.     

Site and sample dependent electron-phonon coupling of Eu ions in epitaxial-grown GaN layers

The incorporation of Eu ions into GaN has been studied using combined excitation-emission spectroscopy for samples that were in situ doped during organometallic vapor-phase epitaxy (OMVPE) growth. The obtained fingerprints of characteristic emission spectra are subsequently used to determine the coupling of the Eu ions to the crystal lattice. We find a majority site, which exhibits coupling to bulk-like phonon modes as well as a localized phonon mode. For this site, we also find that the zero-phonon line of the ⁷F₀ to ⁵D₀ transition, which is forbidden, is much weaker than the phonon-coupled excitation transitions. The ratio of zero-phonon to phonon-coupled transition strength depends on the crystalline quality of the layer. These observations are consistent with the assignment of the majority site to an unperturbed Eu ion on Ga position. We find that the relative abundance of the majority site is strongly underestimated whenever the zero-phonon ⁷F₀ to ⁵D₀ excitation transition is used as a measure.     

Development of a Position Sensitive Neutron Detector with High Efficiency and Energy Resolution for Use at High-Flux Beam Sources

We are developing a high-efficiency neutron detector with 1 cm position resolution and coarse energy resolution for use at high-flux neutron source facilities currently proposed or under construction. The detector concept integrates a segmented ³He ionization chamber with the position sensitive, charged particle collection methods of a MicroMegas detector. Neutron absorption on the helium produces protons and tritons that ionize the fill gas. The charge is amplified in the field region around a wire mesh and subsequently detected in current mode by wire strips mounted on a substrate. One module consisting of a high-voltage plate, a field-shaping high-voltage plate, a grid and wire strips defines a detection region. For 100 % efficiency, detector modules are consecutively placed along the beam axis. Analysis over several regions with alternating wire strip orientation provides a two-dimensional beam profile. By using ³He, a 1/v absorption gas, each axial region captures neutrons of a different energy range, providing an energy-sensitive detection scheme especially useful at continuous beam sources.     

Energy Resolution of a Superconducting Nanowire Single-Photon Detector

An energy resolution of 0.6 eV was achieved with an NbN superconducting nanowire single-photon detector for near infrared photons in the energy range from 1.2 eV to 0.8 eV. The detector operates at 6 K and is read out with a room-temperature amplifier. The photon-energy dependent response of the detector is associated with the change of the detection scenario from the hot-spot formation to unbinding of paired magnetic vortices.      

Analysis of the Photoacoustic Detector Signal for Thermal Diffusivities of Gases

A resonant and a nonresonant photoacoustic detector were used to determine thermal diffusivities of gases. With a nonresonant detector thermal diffusivities can be determined in a wide range between 10^–3 and 10^–7 m²·s^–1, whereas experiments with the resonant detector deliver thermal diffusivities in a range that is about a factor of 100 smaller. As refrigerants—HFCs, HCFCs, and hydrocarbons—are absorbents in the infrared at a wavelength of 3.39 m, their thermal diffusivity can be determined without the addition of a trace gas, particularly at pressures below 0.01 MPa. At pressures above 0.1 MPa, the addition of ammonia as a trace gas is recommended. The absorption wavelength is then 1.531 m. A simulation model for the nonresonant photoacoustic detector is presented for the design of a detector and for an extended error analysis.     

On the Critical Point of Tungsten

Experimental results of exploding tungsten wire experiments with heating rates of 10¹⁰ to 10¹¹ Ks^–1 are interpreted using a one-dimensional hydrodynamic model. The vaporization dynamics under these conditions are discussed. It is shown that for the wires used the superheating of the liquid phase is small and vaporization starts close to the binodal line of the phase diagram. Due to inertia, a volume vaporization takes place in a thin surface layer, while in the bulk of the column formed by the exploding wire a pressure of the order of 10 kbar is maintained. Sufficiently uniform density and temperature distributions are formed in the liquid core surrounded by the two-phase layer. This behavior of vaporizing wires was used to obtain the thermal expansion coefficient of liquid tungsten along with its critical point parameters.     

澳大利亚能效符合性管理体系和产品能效符合性状况

本文重点介绍澳大利亚能效符合性管理体系和产品符合性状况,为我国乃至其他国家完善本国能效符合性管理体系、了解典型发达国家产品符合性状况提供经验参考.     

Anisotropy of the superconducting gap in the presence of electron-phonon and Coulomb interactions

d-wave ands-wave-type solutions of the BCS gap equations are studied numerically with account of long-range and short-range interaction with optical phonon and Coulomb repulsion. It is found that for realistic values of parameters the maximal value of the gap is determined mainly by unscreened long-range interactions of charge carriers with long-waveA _lg , -andB _lg -phonons. The type of the stable solution (d-wave or s-wave) essentially depends on the strength of the Coulomb repulsion as well as on the strength of other short-range interaction potentials. The conclusion is drawn that in some members of cuprate superconductors Cooper pairs may haved- wave symmetry while in others the symmetry may be of anisotropics-wave type.     

Energy and length scales in scintillator nonproportionality

The causes of energy nonlinearity (nonproportionality) in gamma-ray spectroscopy of scintillators have not been fully explained quantitatively, but are believed to involve the spatial density of thermalized electrons and holes, their transport and interactions, and competition between radiative and nonradiative recombination. Here, we relate electron–hole density to the initial ray's energy through expressions for the stopping power and distance, and then attempt to fit some “plateau-type” (e.g. BaF₂, Lu₂SiO₅, etc.) light curves via competing radiative and nonradiative processes obeying different energy power laws. We find that reasonable power laws and the assumption of a uniform energy density in the excited region lead to a fairly good agreement with many experimental curves. We then show that a simple approximation for averaging over regions of different excitation density leads to improved agreement with experiment, at the expense of making it more difficult to uncover the underlying kinetics from the experimental data. These considerations should carry over to more sophisticated models or simulations of scintillation processes.     

Effects of infrared detector nonlinearity on thermal diffusivity measurements using the flash method

When using an infrared detector to measure temperature changes as in the case of the flash technique, the effects of detector nonlinearity can have drastic effects on the experimental data. In the flash technique, the detector nonlinearity tends to shift the calculated half-time to larger values, resulting in underpredicted values of thermal diffusivity especially in experiments performed at room temperature. In order to predict the error in the diffusivity calculation, the nonlinear relationship between the detector signal and the temperature change was developed into a Taylor series expansion used in the flash technique's mathematical model. The nonlinear detector model proves to yield accurate correction factors for the presently calculated values of diffusivity. In order to utilize the model, it is necessary to estimate the maximum temperature rise of the back surface and the degree of detector nonlinearity.     

空气净化器能效限定值及能效等级国家标准研究

本文论述了制定空气净化器能效限定值及能效等级国家标准的背景和意义,介绍了标准研制的总体思路和原则,空气净化器能效限定值的确定,能效等级的划分,以及试验方法等.通过国标的研制与实施,将淘汰高耗能空气净化产品的生产,有效提升空气净化器产品的能效水平.