Phase Relaxation Rate Research Articles

Weak localization in a GaAs heterostructure close to population of the second subband.

Weak-localization magnetoresistance has been measured as a function of carrier density using a backgate technique. The phase relaxation rate and the spin-orbit relaxation rate have been determined. The dominating contribution to the phase relaxation rate comes from electron-electron interaction, as demonstrated for samples covering densities from 2.5 to 7.5×10 15 m -2 . Interband scattering is found to enhance phase relaxation. A strictly two-dimensional electron gas (2DEG) quenches the spin-orbit relaxation rate in the plane of the 2DEG, 1/τ so xy , whereas the orthogonal part, 1/τ so z appears added to the phase relaxation rate in the interpretation of the weak-localization magnetoresistance, and can be determined as the saturation value of the phase relaxation at low temperatures

Fluctuation conductivity above the superconducting transition: Regularization of the Maki-Thompson term

The Maki-Thompson (MT) fluctuation correction to the conductivity in impure electron systems close to the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$ is reconsidered. It is shown that the results of earlier works concerning the case when the pair breaking is due to the electron-electron interaction in the Cooper channel are incorrect. The essential point of our calculations is that the electron phase-relaxation rate due to the electron-electron interaction in the Cooper channel is a singular function of the electron frequency. This singularity, which was missed in earlier works, indicates strongly interacting fluctuations near the transition temperature. According to our results the MT correction close to the transition temperature is less singular than the Aslamazov-Larkin (AL) correction. A little farther from the transition both corrections are the same order, e.g., close to transition in the two-dimensional case, the MT correction is proportional to ln\ensuremath{\alpha}, where \ensuremath{\alpha}=(T-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$, while the AL correction is proportional to ${\mathrm{\ensuremath{\alpha}}}^{\mathrm{\ensuremath{-}}1}$. We also consider the magnetoconductivity close to the transition temperature in the two-dimensional case.

Exciton-hole collision in staggered type-II Al0.34Ga0.66As/AlAs multiple quantum wells.

We report the phase relaxation of \ensuremath{\Gamma}-\ensuremath{\Gamma} direct excitons in staggered type-II ${\mathrm{Al}}_{0.34}$${\mathrm{Ga}}_{0.66}$As/AlAs multiple quantum wells by means of the time-resolved degenerate four-wave mixing. The dephasing rate increases with the increase in the excitation density. Its excitation density dependence sharply changes at a critical density of 0.2 \ensuremath{\mu}J/${\mathrm{cm}}^{2}$. The dependence agrees with the excitation density dependence of the long-lived \ensuremath{\Gamma}-hole density studied by means of the pump-and-probe experiment. The long-lived hole density saturates at the same excitation density of 0.2 \ensuremath{\mu}J/${\mathrm{cm}}^{2}$. These facts definitely indicate that the phase relaxation rate of \ensuremath{\Gamma}-\ensuremath{\Gamma} excitons is dominated by the collision between the excitons and long-lived holes.

Femtosecond time-resolved coherent anti-Stokes Raman scattering from carotenoids in vivo and in vitro: comparison of vibrational relaxation times ( T2) of the in-phase CC stretching bands

Femtosecond time-resolved coherent anti-Stokes Raman scattering has been observed from two carotenoids, rhodopin and spirilloxanthin, in vivo and in vitro. Intracytoplasmic membranes of a species of photosynthetic bacteria have been used as the in vivo sample where the carotenoids exist in the pigment—protein complexes. The vibrational phase relaxation rate in vitro is almost the same as that of β-carotene previously reported, but that in vivo is definitely faster.

Localization and interaction effects in GaAs/AlGaAs heterostructures modified by 4He-ion implantation

Magnetoresistance is used to study localization and interaction in the 2-dimensional electron layer of 4He-ion implanted GaAs/Al-GaAs modulation doped heterostructures. At very low magnetic fields weak localization magnetoresistance can be fitted to theory, thereby determining the diffusion constant and the phase relaxation rate. An unexpected saturation of the phase relaxation rate at low temperature was found to have an interesting relation to mobility and sample size. In the magnetic field range, where the cyclotron radius becomes of the order of the mean free path, the magnetoresistance was related to the effect of electron-electron interaction. By comparison of the magnetoresistance at different implantation doses, we extracted a remnant quantum correction to the conductivity, which has not earlier been noticed. In samples with two Subbands populated interband scattering is observed to cause spin-orbit effects in the weak localization magnetoresistance.

Relaxation rates of ethylene obtained from their effect on coherent transients

In a three-level cascade type double resonance experiment on a free molecular jet, under conditions of reduced, but not eliminated collisional relaxation, the upper level population is shown to exhibit an oscillatory behaviour as a function of overlap between pump and probe laser foci. A qualitative interpretation of the results in the framework of the dressed molecule picture is provided. By numerical integration of the optical Bloch equations, allowing for collisional relaxation processes involving all levels, the observed behaviour has been simulated. Numerical results on the rates of depletion of upper and middle level populations as well as on the dephasing rates were obtained in the temperature range between 55 and 30 K, yielding population rates of about 30 MHz/Torr* for the upper level and 5–13 MHz/Torr* for the middle level (1 Torr* is used as a unit of number density corresponding to 3.27 × 10 22 molecules/m 3). The phase relaxation rate was a factor of 1–3.5 larger in the same temperature range, implying the breakdown of the strong collision approximation at low temperatures.

Weak localization experiments on magnesium films: effects of substrate, ion implantation, microwaves, temperature, and resistivity/thickness on the phase and spin-orbit relaxation

Weak localization magnetoresistance is used to investigate spin-orbit and the phase relaxation rates. The spin-orbit rate is independent of temperature. By plotting the spin-orbit rate as a function of the inverse thickness of the films, we separate the spin-orbit relaxation into two parts: one from the scattering off the bulk imperfections, and one from the scattering against the two surfaces. The ratio between the spin-orbit and the impurity relaxation rates in the bulk was found to be close to 2 × 10−5 for all the different samples. The influence of implanted heavy ions on the spin-orbit relaxation was also investigated. The dependence of the phase relaxation rate on resistivity, film thickness, and temperature has been studied. Theoretical results for electron-electron and electron-phonon scattering are compared to our data. We consider two novel temperature-independent phase relaxation mechanisms which may explain the residual rate we observe. The influence of a high-frequency electromagnetic field on the phase relaxation rate was investigated. For small microwave power levels the phase relaxation rate was found to increase linearly with microwave power. In the absence of a magnetic field and for samples having a dominating spin-orbit interaction (antilocalization) the resistance increases with microwave power, but turns into a decrease at high microwave power levels. For those samples having very weak spin-orbit interaction the resistance decreases continuously as we apply microwaves. This is roughly the expected behavior, but the observed change in resistance was larger than that calculated at small microwave power levels.

The vibrational relaxation of DCl(v=1) in liquid xenon and HCl(v=1) in liquid krypton

The relaxation rate constants for DCl(v=1) in liquid xenon at 211 K and HCl(v=1) in liquid krypton at 190 K are reported. Both the DCl–DCl and HCl–HCl rate constants are similar to the corresponding gas phase rate constants at the same temperature, and the relaxation appears amenable to an isolated binary collision (IBC) description. As in the gas phase, the rate constant for DCl–DCl deactivation in liquid xenon is found to be smaller than that for HCl–HCl relaxation in liquid xenon, despite the smaller energy defect for the DCl relaxation process. This suggests that rotational degrees of freedom are also involved in the liquid phase vibrational relaxation of these molecules. The observed relaxation rates for DCl(v=1) by liquid xenon and HCl(v=1) by liquid krypton are found to be within an order of magnitude of values estimated from various forms of the IBC model. More careful comparisons await values of the appropriate gas phase relaxation rate constants near 200 K. Finally, certain features of the mechanism for production of vibrationally excited HCl and DCl are considered.

New Light on the Scattering Mechanisms in Si Inversion Layers by Weak Localization Experiments

The quasi two-dimensional resistivity and magnetoresistance has been measured on Si(100) MOSFET electron inversion layer as a function of carrier density, temperature and magnetic field. The magnetoresistance is interpreted by the weak localization theory, and we determine the phase relaxation time. The phase relaxation rate has three components, of which two can be identified as the electron-electron scattering rate and the so-called Nyquist (or impurity mediated electron-electron) scattering rate. A third contribution becomes important at low carrier densities and suggests itself as being related to the screened impurity relaxation rate, which however is normally considered to be elastic and therefore should not contribute to the phase relaxation. The screened impurity relaxation rate and the Nyquist scattering rate are the dominating contributions to the temperature dependent mobility of our samples. The screened impurity relaxation rate is compared to recent calculations by Gold and Dolgopolov. At magnetic fields intermediate between the weak-localization-regime and the Landau-level-dominated-regime the weak localization magneto-resistance has been measured and compared to current theories.

Measurement of the electron phase relaxation rate in In0.53Ga0.47As: a possible diagnostic test of compositional disorder

The authors present low-field magnetoresistance measurements made at low temperatures (<or=4.2K) on n-type samples of the semiconductor alloy In0.53Ga0.47As doped in excess of the metal-insulator transition. The samples were strongly degenerate at the temperatures of the measurements. The low-field magnetoresistance is found to be governed by quantum interference (weak-localisation) effects. The authors find that good fits to the data are obtained with a phase relaxation time as the sole variable. This time is much shorter than the calculated magnetic or spin-orbit scattering times, supporting their assertion that these are only minor perturbations.

Dynamic conductivity of a two-dimensional Josephson junction array

The author calculates the response of an N*N Josephson junction array to an oscillatory transverse vector potential. Duality transformations, and a dynamic version of Kosterlitz real space scaling, are used with the resistivity shunted junction model as a starting point. The long-wavelength low-frequency conductivity sigma (q to 0, omega , T) identical to sigma 1+i sigma 2 is calculated, for T near TKT, the Kosterlitz-Thouless transition temperature. For fixed frequency, sigma 1( omega ,T) peaks at, and sigma 2( omega ,T) falls off at, a frequency-dependent T=Tw>TKT. In the limit of mod T-TKT mod /TKT<<1,(1n( Gamma 0/ omega ))-1<<1, dispersion relations and a sum rule are approximately satisfied. The sigma 1,2( omega ,T) behaviour is similar to the (phenomenological) response function of helium films to an oscillating substrate, with the difference that the quantum phase relaxation rate Gamma 0 is not a fitted parameter, but is related to the isolated-junction resistance.

On statistics of an ensemble of oscillators under excitation. III. Quantum non-linear oscillators with fast phase relaxation in a non-monochromatic external field

The statistics of an ensemble of oscillators excited by an external field in a heat bath is considered with the help of the double averaging method and the quasi-probability distribution function method. The parametric resonance and the force-induced resonance cases are considered. The present models show that there are conditions when non-linearity of oscillators, or fast phase relaxation, or non-monochromaticity of the external field promotes the excitation of oscillators. Experiments in which the increase of excitation of polyatomic molecules in an IR laser field was caused by a buffer gas have been discussed in the literature. This effect might accrue due to the increase of the phase relaxation rate. A possible control experiment to verify this statement is given.

Absorption of powerful resonance radiation accompanying collisional line broadening

This review gives a systematic exposition of the qualitative problems of nonlinear laser resonance spectroscopy, taking into account the effect of the light field on the dynamics of the broadening collision and, correspondingly, on the rate of phase relaxation. The results of experiments, in which these nonlinear optocollisional effects, predicted earlier theoretically, were observed, are presented briefly.

Electron spin-echo studies of phase relaxation kinetics in systems containing two types of spins

The electron spin-echo signal decay of hydrogen atoms stabilized in glassy solutions of sulfuric acid containing Cu 2+ and Fe 2+ ions has been investigated at different ion spin-lattice relaxation times T 1 The times T 1 were found by separating the temperature-dependent contribution from the rate of the ion phase relaxation. It has been shown that the uncorrelated sudden-jump model quantitatively describes the observed kinetics of H atom phase relaxation in the case of spin S = 1 2 . At short T 1 ⪡ 10 −6 sec(ferrous ions) the phase relaxation kinetics also fits the conventional theory. The temperature dependences of spin-lattice relaxation times for copper and ferrous ions measured by the electron spin-echo technique are reported.

High-resolution time resolved spectroscopy of collisionless molecular beams. II. Energy randomization and optical phase relaxation of molecules in crossed laser and molecular beams

This paper outlines new ways for separating the different channels of optical dephasing of molecules in beams. It is shown that both the population loss and the optical phase relaxation rates can be obtained under collisionless conditions (molecular beams). These dephasing rates, which measure directly the homogeneous width of the prepared resonance, can be obtained with an energy resolution of better than one part in 108. Under these conditions the solution of the density matrix equations of motion is given for effusive and nozzle beams which represent a statistically open ensemble. The results, which we applied to our recent measurements of optical T1 and T2, are discussed in different limits of power broadening, beam geometry, detector characteristics, temperature of the oven (or material container), and the transit time the molecules of different velocities spend in the laser beam. Furthermore, we indicate that the treatment of coherent transients in beams utilizing conventional Bloch equations is not valid since there is a loss of optically excited molecules. Finally, we discuss possible differences between small and large molecules when they undergo radiationless (or reactive) processes following the selective laser excitation (∼10 kHz–10 MHz).

Theoretical and experimental behaviour of a thermal conductivity detecter for the determination of the ortho/para hydrogen composition of gas mixtures in the temperature-jump region

The theoretical characteristics of a thermistor type thermal conductivity detector were determined for gases in the range 1 to 30 Torr, and were tested experimentally for mixtures of ortho- and para-hydrogen in pure hydrogen and in dilute mixtures of hydrogen in helium. It was found that the detector could be treated as being spherically symmetric. For temperature-jumps of up to 100 K, the total heat transfer could be adequately explained by a simplified form of the Kennard temperature-jump theory, only two parameters being required, the radius of the thermistor and its accommodation coefficient. The differential behaviour, the sensitivity to changes in ortho/para composition, required an additional parameter, the apparent gas phase relaxation rate for rotational energy.