Temperature Relaxation Time Research Articles

Conversion gain and noise of niobium superconducting hot-electron-mixers

A study has been done of microwave mixing at 20 GHz using the nonlinear (power dependent) resistance of thin niobium strips in the resistive state. Our experiments give evidence that electron-heating is the main cause of the nonlinear phenomenon. Also a detailed phenomenological theory for the determination of conversion properties is presented. This theory is capable of predicting the frequency-conversion loss rather accurately for arbitrary bias by examining the I-V characteristic, Knowing the electron temperature relaxation time, and using parameters derived from the I-V-characteristic also allows us to predict the -3-dB IF bandwidth. Experimental results are in excellent agreement with the theoretical predictions. The requirements on the mode of operation and on the film parameters for minimizing the conversion loss (and even achieving conversion gain) are discussed in some detail. Our measurements demonstrate an intrinsic conversion loss as low as 1 dB. The maximum IF frequency defined for 3-dB drop in conversion gain, is about 80 MHz. Noise measurements indicate a device output noise temperature of about 50 K and SSB mixer noise temperature below 250 K. This type of mixer is considered very promising for use in low-noise heterodyne receivers at THz frequencies. >

Sensitivity of eddy‐induced heat transport to diabatic forcing

Compensation of the poleward eddy heat transport by the heat transport of an eddy‐induced mean meridional overturning cell is a common feature in many eddy‐resolving ocean models. It has been argued that this is the result of the weak thermal driving of the ocean. As the actual air/sea coupling is scale dependent, it might be questioned whether the approximation of weak thermal driving is relevant for the oceanic eddy field. In this paper the role of diabatic forcing in modifying eddy‐mean flow interaction is investigated. Emphasis has been placed on the sensitivity of the eddy‐induced change in heat transport to the sea surface thermal boundary condition. Experiments have been performed with a multilayer isopycnic primitive equation model of an idealized North Atlantic subtropical and subpolar gyre. For different values of the air/sea coupling, solutions with and without transient eddies have been compared. The air/sea coupling mostly affects the upper ocean thermal and velocity fields. A decrease of the coupling timescale pushes the separation point of the midlatitude jet further northward and induces a tight recirculation southwest of the separated jet. These effects are enhanced by the eddies. In the present model there is compensation of the eddy heat transport for sea surface temperature (SST) relaxation times longer than 150 days; a breakdown of the compensation occurs for SST relaxation times shorter than 50 days (the average upper layer depth is 200 m). In between is a transition regime. For strong thermal driving the eddy‐induced change in total heat transport is of the same order as the eddy heat transport.

Hydrodynamic fluctuations of a hot-electron gas.

Hydrodynamical fluctuations of the electron gas comprise low-frequency and long-range stochastic excitations over a steady state of the system. In this work we develop the theory of hydrodynamic fluctuations in the nonequilibrium electron gas which can be described by its electron temperature. For this case the characteristic spatial-time parameters are the electric charge decay time ${\mathrm{\ensuremath{\tau}}}_{\mathit{M}}$, the electron temperature relaxation time ${\mathrm{\ensuremath{\tau}}}_{\mathit{T}}$, and two corresponding diffusion lengths ${\mathit{L}}_{\mathit{M}}$ and ${\mathit{L}}_{\mathit{T}}$, respectively. The spectral densities of the fluctuations are found and investigated for arbitrary relationships between the fluctuation frequency \ensuremath{\omega} and times ${\mathrm{\ensuremath{\tau}}}_{\mathit{M}}$ and ${\mathrm{\ensuremath{\tau}}}_{\mathit{T}}$, for wave vectors q and lengths ${\mathit{L}}_{\mathit{M}}$, ${\mathit{L}}_{\mathit{T}}$. In the first case, we established the effect of the crossover correlation of the electron density \ensuremath{\delta}n(r,t) and temperature \ensuremath{\delta}T(r,t) fluctuations. The cross correlation depends on \ensuremath{\omega} and changes its sign, which indicates the existence of the frequency range of correlation and anticorrelation of \ensuremath{\delta}n(r,t) and \ensuremath{\delta}T(r,t). In the general case, spectral densities of the fluctuations have non-Lorentz form, which also holds with thermal equilibrium. At the equilibrium the cross-correlation effect leads to only redistribution of the fluctuation intensity over the frequency region. Under nonequilibrium conditions the cross correlation also changes the integral intensities of the fluctuations and is directly related to additional kinetic correlation of the hot electrons caused by electron-electron interaction. The results are applied to the calculation of light scattering by electron plasma fluctuations. The cross-correlation effect gives the essential contribution to the cross section of the light scattering. It is shown that the additional kinetic correlation of hot electrons can be directly measured by means of the light-scattering experiment.

Triple electron scattering rates in a single mode quantum wire

In quantum limit when only lower subband in a quasi-one dimensional system such as the electron gas confined in the quantum wire is occupied by electrons, pair electron collisions do not cause momentum and energy exchange between electrons. In this case more complex scattering events will determine electron momentum, energy, and phase relaxation. We calculated electron-electron scattering rates associated with triple scattering events with participation of two electrons and one charged impurity. The electron temperature relaxation time in triple collisions is estimated. We show that this time can be comparable with momentum relaxation time.

Calculated Spin-lattice Relaxation of 103Ru Compared with Low Temperature Quadrupole Orientation Measurements

Abstract The spin-lattice relaxation time T1 in 103Ru has been determined on the basis of the Dirac theory and strict relativistic band structure calculations. The low temperature relaxation time T(of 103Ru in an axially symmetric quadrupole field and the quadrupole moment Q have been measured by Green and Stone using the technique of low temperature quadrupole orientation. For the usual reference value T1 T, which corresponds to relaxation in a Zeeman spectrum, they obtain 39(6) sK, which exceeds our value by 134%. This large discrepancy is attributed to the fact that the spin relaxation by direct quadrupole scattering of conduction electrons, the so-called Mitchell contribution, is dominant. According to our calculations it amounts to 81% of the total relaxation rate. This contribution could not be included in the evaluation of the experimental data.

Characterization of hyperfine‐shifted 1H resonances in oxidized and reduced putidaredoxin, an Fe2S2 ferredoxin from Pseudomonas putida

AbstractHyperfine‐shifted 1H resonances in the two accessible oxidation states of putidaredoxin (Pdx), an Fe2S2 ferredoxin from Pseudomonas putida, were observed and their temperature dependence and relaxation times measured using rapid‐recycle NMR methods. As with other Fe2S2 ferredoxins, the hyperfine‐shifted resonances observed in oxidized (Fe3+−Fe3+) Pdx exhibit anti‐Curie behavior, their signals moving away from the diamagnetic region with increasing temperature. The hyperfine‐shifted resonances of reduced Pdx (Fe3+−Fe2+) exhibit Curie‐type behavior, and closely resemble the appearance and behavior of the hyperfine shifted resonances of human ferredoxin (HFdx), with which Pdx shares significant sequence and functional homology. Electron self‐exchange is slow on the 1H chemical shift time‐scale, as half‐reduced samples of Pdx exhibit hyperfine‐shifted resonances characteristic of both forms.

Hot carriers and the frequency response of quantum well lasers

A simple analytical model is obtained to describe the effect of carrier heating on the frequency response of a quantum well laser. The principal factors are taken to be injection heating, recombination heating, and hot phonons. The model is applied to the GaAs/GaInAs strained layer system and is shown to qualitatively account for many of the nonideal features observed. The nonlinear effects cannot be described satisfactorily by a single phenomenological ‘‘gain suppression’’ factor. However, at low drives the conventional gain suppression factor can be expressed in terms of the phonon lifetime and the temperature-relaxation time. The response is mediated by several time constants which, in our example, combine to give an effective time constant of about 10 ps. The modulation frequency response becomes seriously impaired when the differential gain is lowered by a factor of 2 and the time constants describing scattering and phonon lifetime are increased by a factor of 2.

Molecular Orientation in Injection-Molded Polypropylene Copolymers with Ethylene

Abstract The molecular orientation process in injection molding of polypropylene copolymers with ethylene was theoretically analyzed from a view point of growth of melt orientation (recoverable shear strain) at the gate and its relaxation in the mold cavity. Regardless of the characteristics of raw resin (copolymerization type and ethylene content) and molding conditions (cylinder temperature), there exist unitary correlations between calculated degree of molecular orientation and experimentally measured degrees of molecular orientation such as the thickness of skin layer and crystalline c-axis orientation function. According to this analysis, the fact that molecular orientation is decreased by copolymerization with ethylene is due to decrease in melt orientation (recoverable shear strain), decrease in the relaxation time and reduction of crystallization temperature.

Conduction and thermally stimulated discharge currents in aramid paper

Conduction currents and thermally stimulated discharge (TSD) currents were studied in aramid paper at various field strengths and temperatures. At low field strengths the conductivity is ohmic, and at higher field strengths (up to 30 MV/m), ionic conduction is a possible mechanism. The activation energy of low field conduction is determined and at high electric fields the hopping distance for ions is in the range of 4.4 to 8.0 nm. TSD currents are also measured at various poling fields, formation and storage times, and rates of heating. The TSD current shows a peak at 135 degrees C for a rate of heating of 1 K/min and shifts to a higher value when the rate of heating is increased. The peak current and the charge released to the external circuit is proportional to the poling field. The observed peak is attributed to the depolarization of permanent dipoles with a room temperature relaxation time of 5.3*10/sup 5/ s. >

"Zonally averaged dynamical and compositional response of the thermosphere to auroral activity during September 18–24, 1984""

A time‐dependent calculation has been performed to study the dynamical and compositional response of the thermosphere to auroral disturbances. The heat and momentum inputs associated with auroral activity were estimated by using an empirical model constructed from two data bases: the precipitating particles monitored by the TIROS/NOAA satellites and the ionospheric convection electric fields measured by the Millstone Hill incoherent scatter radar. The level of auroral activity was given in terms of hemispherical power input due to particle precipitation. The model of high‐latitude energy input was incorporated into a zonally averaged thermospheric model. This paper presents the results of a simulation done for the period September 18–24, 1984. Time histories of the composition are given as well as those of dynamical parameters such as temperature and winds. The spatial structures in these parameters are also discussed. The main new result of the simulation is that during auroral disturbances the radiative cooling by nitric oxide (NO) is greatly enhanced by the increase in NO number density as well as by the temperature increase. As a result the temperature increase in response to auroral activity is damped, and the temperature relaxation time is shortened following a quieting of activity. A comparison between the simulated temperatures and the temperatures observed by the Millstone Hill incoherent scatter radar during the 7‐day interval shows reasonable agreement in terms of the absolute magnitude of the variations as well as the time constants of those variations.

Effect of carrier injection into MESFET substrates : comparison of MESFET on a semi-insulating buffer, MESFET on a P substrate, and substrate-less MESFET

The carrier injection from the active-layer of the submicron-gate-lengthMesfet into the buffer-layer, or substrate in general, is studied by means of a two-dimensional computer simulation in which the energy conservation equation is simultaneously solved with the carrier conservation equation and Poisson’s equation. The mobility, electron temperature and energy relaxation time are treated as energy dependent parameters. This model is capable of simulating the non-stationary conditions associated with the submicron-gate-length devices. The effect of the carrier injection on the I-V characteristics as well as on the small-signal parameters is investigated by simulating twoMesfet structures; the first is aMesfet on a perfect buffer-layer while the second is a symmetricalMesfet which has no substrate. It is found out that the drain current is increased by the carrier injection, whereas the transconductance is reduced due to the increase of the device dynamic range. TheMesfet with an interfacial potential barrier is also simulated. It exhibits characteristics intermediate between those of the other two devices.

The preparation and properties of zinc coordination complexes of poly(4‐vinylpyridine‐co‐n‐butyl methacrylate) copolymers

AbstractTwo poly(4‐vinylpyridine‐co‐n‐butyl methacrylate) copolymers containing 14.2 and 30.0 mol% of 4‐vinylpyridine residues were prepared by free radical copolymerization reactions. Insoluble complexes were obtained for each copolymer by reaction with zinc chloride. The coordination complexes obtained had a molar ratio of 4‐vinylpyridine residues to zinc atoms close to 2 in both cases. The rheological behavior of each of these complexes has been compared to that of the corresponding metal‐free copolymers, which were recovered by decomplexation. The metal complexed polymers exhibited much higher glass transition temperatures and much longer high temperature relaxation times than the corresponding uncomplexed copolymers.

Oh − reorientability in hydroxyapatites: Effect of F − and Cl −

The effects of partially substituted F − and Cl − in hydroxyapatite on the co-operative reorientation motions in the OH-dipole chains associated with the monoclinic-hexagonal phase transition have been examined with the fractional-polarization thermally stimulated current (TSC) method. The motions can be sensed with the TSC method even though various chain segments may be oppositely oriented and thus mandate the hexagonal phase. Substitution of one in four OH − ions by F − causes the OH − dipole chains to be too short, and too bound at the ends, to display this co-operative reorientation. With 16% F substitution, there are enough OH − chains of sufficient length to show the co-operative motion, identifiable by the occurrence of a compensation phenomenon. The occurrence of three compensations with essentially the same temperature (209–216°C) but different characteristic relaxation times is ascribed to the existence of different OH − chain lengths between interrupting F − ions. Chlorhydroxyapatite (Cl 0.5OH 0.5) shows no compensation below 800°C even though both end-members do, at the monoclinic-hexagonal phase transition temperature. This is ascribed to short chain lengths and to direct OH −−Cl − interactions. Fluorapatite shows no compensation below 1200°C, and no phase transition in that temperature range is known.

An emerging technology: Nuclear magnetic resonance microscopy

Much progress in nuclear magnetic resonance microscope has been made in the last few years as a result of improved instrumentation and techniques being made available through basic research in magnetic resonance imaging (MRI) technologies for medicine. Nuclear magnetic resonance (NMR) was first observed in the hydrogen nucleus in water by Bloch, Purcell and Pound over 40 years ago. Today, in medicine, virtually all commercial MRI scans are made of water bound in tissue. This is also true for NMR microscopy, which has focussed mainly on biological applications. The reason water is the favored molecule for NMR is because water is,the most abundant molecule in biology. It is also the most NMR sensitive having the largest nuclear magnetic moment and having reasonable room temperature relaxation times (from 10 ms to 3 sec). The contrast seen in magnetic resonance images is due mostly to distribution of water relaxation times in sample which are extremely sensitive to the local environment.

Spin fluctuations in the magnetic superconductor Y9Co7

The experimental data of magnetisation, Curie temperature and spin-lattice relaxation time for the magnetic superconductor Y9Co7 were analysed in the frame of the recently developed magnetic spin fluctuation models. The basic parameters of the models and the relaxation frequency function of a spontaneous spin fluctuation were calculated.

Modeling of a dc glow plasma loudspeaker

A plasma loudspeaker is a transducer that produces sound by means of an ionized gas channel rather than the voice coil of a conventional loudspeaker. Plasma loudspeakers characteristically have a flat frequency response at the higher acoustic frequencies because of their ‘‘massless’’ membrane; however, their low-frequency response is limited. In this article, a comprehensive model for the electrical and thermodynamic operation of the electrical discharge is presented that explains the frequency-dependent behavior of the discharge’s electrical impedance and acoustic response. The characteristic acoustic frequency response is shown to result from a spatial variation of the temperature relaxation time throughout the plasma channel due to the temperature dependence of that time constant. To support the model, electrical and sound-pressure level measurements in helium, argon, and air are presented.

Temperature dependence of proton relaxation times in vitro

Accurate measurement of tissue relaxation characteristics is dependent on many factors, including field strength and temperature. The purpose of this study was to evaluate the relationship between sample temperature, viscosity and proton spin-lattice relaxation time ( T1) and spin-spin relaxation time ( T2). A review of two basic models of relaxation (a) the simple molecular motion model and (b) the fast exchange two state model is given with reference to their thermal dependencies. The temperature dependence for both T1 and T2 was studied on a 0.15 Tesla whole body magnetic resonance imager. Thirteen samples comprising both simple and complex materials were investigated by using a standard spin-echo (SE) technique and a modified Carr-Purcell-Meiboom-Gill (CPMG) multi-echo sequence. A simple linear relationship between T1 and temperature was observed for all samples over the range of 20°C to 50°C. There is an inverse relationship between viscosity and T1 and T2. A quantity called the temperature dependence coefficient (TDC) is introduced and defined as the percent rate of change of the proton relaxation time referenced to a specific temperature. The large TDC found for T1 values, e.g. 2.37%/°C for CuSO 4 solutions and 3.59%/°C for light vegetable oils at 22°C, indicates that a temperature correction should be made when comparing in-vivo and in-vitro T1 times. The T2 temperature dependence is relatively small.

ESR AND CONDUCTIVITY STUDIES OF DOPED POLYACETYLENE

The temperature, doping and microwave power dependence of ESR spectra of polyacetylene doped with AsF5 or SbF5 is studied. Coexistence of two types of spins is found in all but the most heavily doped samples. The superimposed ESR lines differ in shape and width, temperature dependence and relaxation time. They are attributed to localized and itinerant spins, respectively. These results are indicative of sample inhomogeneities (at least on an microscopic level).

Influence of electron heating during recombination of copper atoms in copper halide vapor lasers on their output parameters

A theoretical analysis is made of the influence of electron heating during recombination of copper atoms in copper halide vapor lasers on the characteristic relaxation time of the electron temperature. The results of the analysis are compared with experimental data.

Picosecond transient photocurrents in amorphous silicon

Three different types of $a\ensuremath{-}\mathrm{S}\mathrm{i}$, prepared by rf glow discharge, chemical vapor deposition, and evaporation, were found to have the same initial mobility of 1 ${\mathrm{cm}}^{2}$/V s at room temperature and different carrier relaxation times ranging from 200 to 4 ps. The initial mobility of $a\ensuremath{-}\mathrm{S}\mathrm{i}$:$\mathrm{H}$ is thermally activated with a prefactor of 8 ${\mathrm{cm}}^{2}$/V s. These results give strong support to the existence of a mode of conduction in extended states which is an intrinsic property of $a\ensuremath{-}\mathrm{S}\mathrm{i}$.