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At least 307 records · Page 17Linked to original sources

PubMed2000

Dynamic scaling of ultrasonic damping near the nematic-smectic- A transition of TBBA

We report a detailed study of the ultrasonic damping near the nematic-smectic- A phase transition of terephtal-bis- p-p(')-butylaniline (TBBA). Two mechanisms contribute to the damping. One is isotropic; it is associated with the critical fluctuations, and leads to a scaling behavior of the 3D- XY type which is observed up to 15 degrees C from the transition, even though the transition is first order. The other is anisotropic; it appears only in the smectic- A phase, and is associated with the relaxation of the order-parameter modulus.

Journal Article↗

PubMed2000

Phase transition in the collisionless damping regime for wave-particle interaction

Gibbs statistical mechanics is derived for the Hamiltonian system coupling a wave to N particles self-consistently. This identifies Landau damping with a regime where a second order phase transition occurs. For nonequilibrium initial data with warm particles, a critical initial wave intensity is found: above it, thermodynamics predicts a finite wave amplitude in the limit N-->infinity; below it, the equilibrium amplitude vanishes. Simulations support these predictions providing new insight into the long-time nonlinear fate of the wave due to Landau damping in plasmas.

Journal Article↗

PubMed2000

Mechanical damping by intercrystalline diffusion of hydrogen in metallic polycrystals.

Anelastic relaxation by intercrystalline atomic diffusion-analogous to thermal diffusion (i.e., thermoelastic) effects but never observed hitherto-was recently suggested as a new mechanism to explain part of the hydrogen damping spectra in intermetallic compounds. A critical experimental test of this model is now presented using a Zr65Cu17.5Ni10Al7.5 alloy, which allows for a quantitative kinetic analysis in comparison to a closely related reorientation process. The results are in full agreement with the predictions of the model and clearly corroborate the proposed "intercrystalline Gorsky effect" as a new type of mechanical damping in sufficiently fine-grained polycrystals.

Journal Article↗

PubMed2001

Temperature dependence of damping and frequency shifts of the scissors mode of a trapped Bose-Einstein condensate.

We have studied the properties of the scissors mode of a trapped Bose-Einstein condensate of 87Rb atoms at finite temperature. We measured a significant shift in the frequency of the mode below the hydrodynamic limit and a strong dependence of the damping rate as the temperature increased. We compared our damping rate results to recent theoretical calculations for other observed collective modes, finding a fair agreement. From the frequency measurements we deduce the moment of inertia of the gas and show that it is quenched below the transition point, because of the superfluid nature of the condensed gas.

Journal Article↗

PubMed2002

Enhanced gilbert damping in thin ferromagnetic films.

The precession of the magnetization of a ferromagnet is shown to transfer spins into adjacent normal metal layers. This "pumping" of spins slows down the precession corresponding to an enhanced Gilbert damping constant in the Landau-Lifshitz equation. The damping is expressed in terms of the scattering matrix of the ferromagnetic layer, which is accessible to model and first-principles calculations. Our estimates for permalloy thin films explain the trends observed in recent experiments.

Journal Article↗

PubMed2002

Rotational damping in ytterbium nuclei.

We have made the first clear measurements of rotational damping widths in nuclei. In a mixture of three Yb nuclei, these widths are 300 +/- 60 keV between 1.2 and 1.5 MeV gamma-ray energy [approximately (37-57)Planck's constant]. Compound damping and motional narrowing are discussed in connection with these results.

Journal Article↗

PubMed2002

Radiation damping effects on the interaction of ultraintense laser pulses with an overdense plasma.

A strong effect of radiation damping on the interaction of an ultraintense laser pulse with an overdense plasma slab is found and studied via a relativistic particle-in-cell simulation including ionization. Hot electrons generated by the irradiation of a laser pulse with a radiance of I lambda(2)>10(22) W microm(2)/cm(2) and duration of 20 fs can convert more than 35% of the laser energy to radiation. This incoherent x-ray emission lasts for only the pulse duration and can be intense. The radiation efficiency is shown to increase nonlinearly with laser intensity. Similar to cyclotron radiation, the radiation damping may restrain the maximal energy of relativistic electrons in ultraintense-laser-produced plasmas.

Journal Article↗

PubMed2002

Accidental suppression of landau damping of the transverse breathing mode in elongated bose-einstein condensates.

We study transverse radial oscillations of an elongated Bose-Einstein condensate using finite-temperature simulations, in the context of a recent experiment at ENS [F. Chevy, Phys. Rev. Lett. 88, 250402 (2002)]]. The mode observed experimentally corresponds to an in-phase collective oscillation of both the condensate and the thermal cloud. Our simulations are in excellent agreement with experiment, accounting for the very small damping rates observed. In contrast to other condensate modes, interatomic collisions are found to be the dominant damping mechanism in this case.

Journal Article↗

PubMed2002

Beliaev damping of quasiparticles in a Bose-Einstein condensate.

We report a measurement of the suppression of collisions of quasiparticles with ground state atoms within a Bose-Einstein condensate at low momentum. These collisions correspond to Beliaev damping of the excitations, in the previously unexplored regime of the continuous quasiparticle energy spectrum. We use a hydrodynamic simulation of the expansion dynamics, with the Beliaev damping cross section, in order to confirm the assumptions of our analysis.

Journal Article↗

PubMed2005

Collapse of thermal activation in moderately damped Josephson junctions.

We study switching current statistics in moderately damped Nb-InAs-Nb and intrinsic Bi2Sr2CaCu2O8+delta) Josephson junctions. A paradoxical collapse of thermal activation with increasing temperature is reported and explained by the interplay of two conflicting consequences of thermal fluctuations, which can both assist in premature escape and help in retrapping back into the stationary state. We analyze the influence of dissipation on the thermal escape by tuning damping with a gate voltage, magnetic field, temperature, and an in situ capacitor.

Journal Article↗

PubMed2006

Time-resolved spin-torque switching and enhanced damping in permalloy/Cu/permalloy spin-valve nanopillars.

We report time-resolved measurements of current-induced reversal of a free magnetic layer in Permalloy/Cu/Permalloy elliptical nanopillars at temperatures T=4.2 K to 160 K. Comparison of the data to Landau-Lifshitz-Gilbert macrospin simulations of the free layer switching yields numerical values for the spin torque and the Gilbert damping parameters as functions of T. The damping is strongly T dependent, which we attribute to the presence of an antiferromagnetic oxide layer around the perimeter of the Permalloy free layer. This adventitious antiferromagnetic oxide can have a major impact on spin-torque phenomena.

Journal Article↗

PubMed2006

Origin of increase of damping in transition metals with rare-earth-metal impurities.

The damping due to rare-earth-metal impurities in transition metals is discussed in the low concentration limit. It is shown that all established damping mechanisms based on spin-orbit and/or spin-spin interactions cannot explain experimental observations even qualitatively. We introduce a different relaxation channel due to the coupling of the orbital moments of the rare-earth-metal impurities and the conduction p electrons that leads to good agreement with experiment. Using an itinerant picture for the host ions, i.e., write their magnetization in terms of the electronic degrees of freedom, is key to the success of our model.

Journal Article↗

PubMed2005

Relay feedback tuning of robust PID controllers with iso-damping property.

A new tuning method for proportional-integral-derivative (PID) controller design is proposed for a class of unknown, stable, and minimum phase plants. We are able to design a PID controller to ensure that the phase Bode plot is flat, i.e., the phase derivative w.r.t. the frequency is zero, at a given frequency called the "tangent frequency" so that the closed-loop system is robust to gain variations and the step responses exhibit an iso-damping property. At the "tangent frequency," the Nyquist curve tangentially touches the sensitivity circle. Several relay feedback tests are used to identify the plant gain and phase at the tangent frequency in an iterative way. The identified plant gain and phase at the desired tangent frequency are used to estimate the derivatives of amplitude and phase of the plant with respect to frequency at the same frequency point by Bode's integral relationship. Then, these derivatives are used to design a PID controller for slope adjustment of the Nyquist plot to achieve the robustness of the system to gain variations. No plant model is assumed during the PID controller design. Only several relay tests are needed. Simulation examples illustrate the effectiveness and the simplicity of the proposed method for robust PID controller design with an iso-damping property.

Algorithms↗

PubMed1990

Allergy to storage mites in asthmatic patients and its relation to damp housing conditions.

The relation between allergy to house dust mites and allergy to storage mites was studied in 326 patients with bronchial asthma. A positive RAST to house dust mites was found in 24.5% and to storage mites in 7.4%. There was a close association between a positive RAST to house dust mites and a positive RAST to storage mites, and 91.6% had a positive RAST to both. Only 0.9% with a negative RAST to house dust mites had a positive RAST to storage mites, whereas the frequency was 27.5% in those with a positive RAST to house dust mites. Present occupation with exposure to storage mites was found in 20.8% with a positive RAST to storage mites. A positive RAST to both house dust mites and storage mites was significantly associated with anamnestic indicators of damp housing conditions. The close association between allergy to house dust mites and allergy to storage mites in this study is probably caused by a common environmental factor, i.e. damp housing conditions.

Agricultural Workers' Diseases↗

PubMed1992

Presence of viable mould propagules in indoor air in relation to house damp and outdoor air.

The presence of viable mould propagules in indoor air was investigated using the N6-Andersen sampler in combination with DG18-agar, in relation to house damp (characterized with a checklist) and in relation to the presence of moulds in outdoor air. The first part of the study was conducted in 46 houses in the autumn of 1987, the second part in 84 houses in May 1989. Further, in the second part, the results obtained with settlement plates (OPD) were compared with those obtained with the N6-Andersen sampler. The number of CFU/m3 in the indoor and outdoor air varied widely. A large variety of mould genera and species was isolated. Species of Cladosporium, Penicillium and Wallemia predominated. The variability in time was high and the reproducibility of the measurements in terms of CFU/m3 and of species isolated was only moderate. The low predictive value of these measurements limits their use in epidemiological studies of the relationship between exposure to moulds and respiratory symptoms. Overall, the geometric mean concentration was somewhat higher outdoors than indoors. However, the clear differences found between the number of CFU/m3 belonging to different mould species in in- and outdoor air show that the presence of viable mould propagules in indoor air is not simply a reflection of the presence of moulds in outdoor air. The presence of moulds in indoor air was only weakly related to house damp as characterized by the checklist. High, statistically significant correlations were found between the CFU yield obtained with the OPD and the CFU/m3 yield obtained with the N6-Andersen sampler.(ABSTRACT TRUNCATED AT 250 WORDS)

Air Pollutants↗

PubMed1983

Analysis of competition for substrate sites in an allosteric enzyme with co-operative kinetics. Effects of dAMP and dUMP on donkey spleen deoxycytidylate aminohydrolase.

Hypotheses about the interactions of effectors with conformations of allosteric enzymes having co-operative kinetics can be tested simply and exactly without knowledge of the nature of the intersubunit co-operativity by using a linkage approach to the analysis of steady-state kinetics. Applying this approach to competition for substrate sites in the allosteric enzyme donkey spleen dCMP aminohydrolase, we show that the kinetics are consistent with the hypothesis that the substrate dCMP and the competitor dAMP, as well as the allosteric activator dCTP, bind exclusively to the same conformation of the enzyme subunits. The linkage test can be applied in the presence of other effectors without knowledge of how these interact with the enzyme. Our tests showed that dCMP and DAMP are still bound exclusively to this same conformation in the presence of the product dUMP or of the allosteric inhibitor dTTP. We give evidence that dUMP binds to the same conformation as dCMP, but that it is also bound to other conformation(s). The advantages of the linkage approach, and some general problems in steady-state kinetics of allosteric enzymes, are discussed.

Allosteric Site↗

PubMed1990

On the mechanistic origin of damped oscillations in biochemical reaction systems.

A generalized reaction scheme for the kinetic interaction of two reactants in a metabolic pathway has been examined in order to establish what minimal mechanistic patterns are required to support a damped oscillatory transient-state kinetic behaviour of such a two-component system when operating near a steady state. All potentially oscillating sub-systems inherent in this scheme are listed and briefly characterized. The list includes several mechanistic patterns that may be frequently encountered in biological system (e.g. involving feedback inhibition, feed-forward activation, substrate inhibition or product activation), but also draw attention to some hitherto unforeseen mechanisms by which the kinetic interaction of two metabolites may trigger damped oscillations. The results can be used to identify possible sources of oscillations in metabolic pathways without detailed knowledge about the explicit rate equations that apply.

Chemical Phenomena↗

PubMed1997

A sighted man with non-24-hour sleep-wake syndrome shows damped plasma melatonin rhythm.

Twenty-four-hour profiles of plasma melatonin, cortisol and rectal temperature were measured longitudinally in a sighted man who has been suffering from sleep disorders for more than 10 years. The sleep-wake rhythm of this subject free-ran, despite his routine life, and occasionally showed a sign of internal desynchronization, where sleep was lengthened up to 30 h. These states were classified into the non-24-hour sleep-wake syndrome. Plasma melatonin concentrations in the subjective night remained at a low level and showed a damped circadian rhythm. At the same time, robust circadian rhythms were detected in plasma cortisol and rectal temperature, indicating that the circadian pacemaker was intact. The causal relationship between the damping of nocturnal melatonin rise and a failure of entrainment of the sleep-wake cycle is discussed.

Adult↗