Negative-resistance models for parametrically flux-pumped superconducting quantum interference devices

A Superconducting QUantum Interference Device (SQUID) modulated by a fast oscillating magnetic flux can be used as a parametric amplifier, providing gain with very little added noise. Here, we develop linearized models to describe the parametrically flux-pumped SQUID in terms of an impedance. An unpumped SQUID acts as an inductance, the Josephson inductance, whereas a flux-pumped SQUID develops an additional, parallel element which we have coined the ``pumpistor.'' Parametric gain can be understood as a result of a negative resistance of the pumpistor. In the degenerate case, the gain is sensitive to the relative phase between the pump and signal. In the nondegenerate case, gain is independent of this phase. We develop our models first for degenerate parametric pumping in the three-wave and four-wave cases, where the pump frequency is either twice or equal to the signal frequency, respectively. We then derive expressions for the nondegenerate case where the pump frequency is not a multiple of the signal frequency, where it becomes necessary to consider idler tones which develop. For the nondegenerate three-wave case, we present an intuitive picture for a parametric amplifier containing a flux-pumped SQUID where current at the signal frequency depends upon the load impedance at an idler frequency. This understanding provides insight and readily testable predictions of circuits containing flux-pumped SQUIDs.Comment: 27 pages, 6 figures, 1 tabl

Thermoelectric figure of merit of tau-type conductors of several donors

Dimensionless thermoelectric figure of merit $ZT$ is investigated for two-dimensional organic conductors $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, $\tau$-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$and$\tau$-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ ($y \le 0.875$), respectively. The $ZT$ values were estimated by measuring electrical resistivity, thermopower and thermal conductivity simultaneously. The largest $ZT$ is 2.7 $\times$ 10$^{-2}$ at 155 K for $\tau-(EDT-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, 1.5 $\times$ 10$^{-2}$ at 180 K for $\tau-(EDO-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$ and 5.4 $\times$ 10$^{-3}$ at 78 K for $\tau-(P-S,S-DMEDT-TTF)_2(AuI_2)_{1+y}$, respectively. Substitution of the donor molecules fixing the counter anion revealed EDT-S,S-DMEDT-TTF is the best of the three donors to obtain larger $ZT$.Comment: proceedings of ISCOM 2009 (to be published in Physica B

The pumpistor: a linearized model of a flux-pumped SQUID for use as a negative-resistance parametric amplifier

We describe a circuit model for a flux-driven SQUID. This is useful for developing insight into how these devices perform as active elements in parametric amplifiers. The key concept is that frequency mixing in a flux-pumped SQUID allows for the appearance of an effective negative resistance. In the three-wave, degenerate case treated here, a negative resistance appears only over a certain range of allowed input signal phase. This model readily lends itself to testable predictions of more complicated circuits.Comment: 4 pages, 3 figure

Public R&D Innovation: The Case of Wind Energy in Denmark, Germany and the United Kingdom

This paper examines the impact of public research and development (R&D) support on cost reducing innovation for wind turbine farms in Denmark, Germany and the United Kingdom (UK). First we survey the literature in this field. The literature indicates that in Denmark R&D policy has been more successful than in Germany or the UK in promoting innovation of wind turbines. Furthermore, such studies point out that (subsidy-induced) capacity expansions were more effective in the UK and Denmark in promoting cost-reducing innovation than in Germany. The second part of the paper describes the quantitative analysis of the impact of R&D and the capacity expansion on innovation. This is calculated using the two-factor learning curve (2FLC) model, in which investment cost reductions are explained by cumulative capacity and the R&D based knowledge stock. Time-series data were collected for the three countries and organized as a panel data set. The parameters of the 2FLC model were estimated, focusing on the heterogeneity of the parameters across countries. We arrive at robust estimations of a learning-by-doing rate of 5.4% and a learning-by-searching rate of 12.6%. The analysis underlies the homogeneity of the learning parameters, enhancing the validity of the 2FLC formulation

Co-Teaching During Teacher Training Periods : Experiences of Finnish Special Education and General Education Teacher Candidates

publishedVersio

Signature of wide-spread clumping in B supergiant winds

We seek to establish additional observational signatures of the effects of clumping in OB star winds. The action of clumping on strategic wind-formed spectral lines is tested to steer the development of models for clumped winds and thus improve the reliability of mass-loss determinations for massive stars.The SiIV 1400 resonance line doublets of B0 to B5 supergiants are analysed using empirical line-synthesis models. The focus is on decoding information on wind clumping from measurements of ratios of the radial optical depths (tau_(rad)(w)) of the red and blue components of the SiIV doublet. We exploit in particular the fact that the two doublet components are decoupled and formed independently for targets with relatively low wind terminal velocities. Line-synthesis analyses reveal that the mean ratio of tau_(rad)(w) of the blue to red SiIV components are rarely close to the canonical value of ~ 2 (expected from atomic constants), and spread instead over a range of values between ~1 and 2. These results are interpreted in terms of a photosphere that is partially obscured by optically thick structures in the outflowing gas.The spectroscopic signatures established in this study demonstrate the wide-spread existence of wind clumping in B supergiants. The additional information in unsaturated doublet profiles provides a means to quantify the porosity of the winds.Comment: Accepted for publication in A&A Letter