Formation of defects in gallium phosphide grown in the presence of oxygen

The influence of oxygen introduced in the gaseous phase on the formation of defects in GaP epitaxial layers is investigated by deep-level transient spectroscopy. The extremal dependences of the concentrations of charge carriers and electron traps with energy E _c−0.24 eV on the oxygen flux are discussed. Zh. Tekh. Fiz. 67, 52–55 (September 1997)     

Critical conditions for spontaneous relaxation in self-organizing dissipative film systems

It is established that the necessary conditions for spontaneous relaxation of elastic strain energy in a copper-As₆₀Se₄₀ self-organizing dissipative heterostructure is that the elastic deformation energy and the temperature must reach their threshold values. It is shown that in the temperature range 270–340 K the spontaneous relaxation of elastic deformation energy is accompanied by structural-chemical ordering and anomalous diffusion of copper into the glassy chalcogenide semiconductor layer. The maximum concentration of copper dissolved in the films is 40 at. %. Conductivity inversion from p to n type is observed in doped layers obtained by this method. Zh. Tekh. Fiz. 69, 128–129 (August 1999)     

Magnetic Resonance Fingerprinting with short relaxation intervals

PurposeThe aim of this study was to investigate a technique for improving the performance of Magnetic Resonance Fingerprinting (MRF) in repetitive sampling schemes, in particular for 3D MRF acquisition, by shortening relaxation intervals between MRF pulse train repetitions.Material and methodsA calculation method for MRF dictionaries adapted to short relaxation intervals and non-relaxed initial spin states is presented, based on the concept of stationary fingerprints. The method is applicable to many different k-space sampling schemes in 2D and 3D. For accuracy analysis, T₁ and T₂ values of a phantom are determined by single-slice Cartesian MRF for different relaxation intervals and are compared with quantitative reference measurements. The relevance of slice profile effects is also investigated in this case. To further illustrate the capabilities of the method, an application to in-vivo spiral 3D MRF measurements is demonstrated.ResultsThe proposed computation method enables accurate parameter estimation even for the shortest relaxation intervals, as investigated for different sampling patterns in 2D and 3D. In 2D Cartesian measurements, we achieved a scan acceleration of more than a factor of two, while maintaining acceptable accuracy: The largest T₁ values of a sample set deviated from their reference values by 0.3% (longest relaxation interval) and 2.4% (shortest relaxation interval). The largest T₂ values showed systematic deviations of up to 10% for all relaxation intervals, which is discussed. The influence of slice profile effects for multislice acquisition is shown to become increasingly relevant for short relaxation intervals. In 3D spiral measurements, a scan time reduction of 36% was achieved, maintaining the quality of in-vivo T1 and T2 maps.ConclusionsReducing the relaxation interval between MRF sequence repetitions using stationary fingerprint dictionaries is a feasible method to improve the scan efficiency of MRF sequences. The method enables fast implementations of 3D spatially resolved MRF.     

Two forms of polarization relaxation in polydomain ferroelectrics in an electric field

The characteristic features of the polarization and depolarization kinetics of polydomain ferroelectrics with square and narrow “extended” dielectric hysteresis loops are investigated for the model of TGS and Rb₂ZnCl₄ crystals. It is shown that for the second crystal, in contrast to the first crystal, the local free energy is asymmetric relative to the direction of polarization, the coercive field does not have a definite value, and only part of the crystal volume participates in the slow thermoactivational relaxation. The slow relaxation follows a universal empirical power law in all cases. The distribution functions of the relaxation times in crystals are constructed on the basis of experimental data, and comparative estimates are made of the relaxation parameters and the parameters of the energy barriers for domain walls. Fiz. Tverd. Tela (St. Petersburg) 41, 499–504 (March 1999)     

First-principles investigations of ordering in binary α-Ti solid solutions

The ordering tendency in binary f-Ti solid solution containing 3sp or 4sp simple-metal (SM) or 3d transition-metal (TM) solute is investigated systematically by the linear muffin-tin orbital (LMTO) method within the atomic sphere approximation (ASA). We demonstrated that the effective pairwise interaction (EPI) energy in a solid solution is equal to half the solute-solute interaction energies and can be evaluated by a supercell total energy approach. The calculations of EPI energy both with and without volume relaxation of the supercells and local density of states (LDOS) show that the EPI energies of Ti-SM and Ti-TM solutions are dominated by different factors. For Ti-SM solutions, the EPI energies are of large absolute values with a negative sign, indicating strong ordering tendency in these solutions. The volume relaxation does not alter the EPI energy substantially. The calculated LDOS shows that the ordering tendency in Ti-SM solutions may be related to the hybridization between the electrons of the SM atoms when they are close to each other. For most Ti-TM solutions, if calculated without relaxation, the absolute EPI energies are very small; however, if calculated with relaxation, they are of relatively large positive values, indicating a clustering tendency in these solutions. By combining the calculated EPI energy and Flinn's model for short-range order (SRO) strengthening, the increase in critical shear stress sro due to SRO is estimated for Ti-SM alloys, and the results qualitatively agree with experiment.     

Ab initio pseudopotential study of vacancies and self-interstitials in hcp titanium

By means of an ab initio plane-wave pseudopotential method, monovacancy, divacancy and self-interstitials in hcp titanium are investigated. The calculated monovacancy formation energy is 1.97 eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results. The relaxation of the atoms around a single vacancy is observed to be small. Two divacancy configurations, the in-plane and the off-plane, have also been shown to be equally stable. With regards to the interstitials, of the eight configurations studied, two (octahedral and basal octahedral) have relatively lower formation energies and are, thus, the most likely stable configurations. We find small energy differences between them, suggesting their possible co-existence. It is also observed that the tetrahedral configuration decays to a split dumbbell configuration, whereas both the basal tetrahedral and the basal pseudocrowdion interstitials decay to the basal octahedral configuration. Using the nudged elastic band method (NEB), we determine a possible minimum energy path (MEP) for the diffusion of self-interstitial titanium atoms from an octahedral site to the nearest octahedral site. The energy barrier for this migration mechanism is shown to be about 0.20 eV.     

Relaxation of electric fields induced by mechanical loading in natural dielectrics

An investigation is made of electric fields induced in natural dielectrics by mechanical loading and electrical polarization. It is shown that the relaxation of the polarization is identical in nature for both cases and is basically a thermally activated process. The temperature dependence is obtained for the relaxation time of the electric fields. Also estimated is the activation energy for motion of charge carriers leading to the relaxation of these fields. Fiz. Tverd. Tela (St. Petersburg) 39, 1202–1204 (July 1997)     

Long-term evolution of photoinduced light absorption in C60 films

It is found that photoinduced absorption in the energy range 0.6–2.1 eV in C₆₀ films grown by the same method can differ by a factor of 100. This change is attributable a 10⁶-fold increase in the relaxation time τ of electron-hole photoexcitations after working with the films for several months. It is established that the bimolecular recombination mechanism for photoexcitations (triplet excitons and polarons), which is typical of as-prepared and partially aged films, is superseded in fully aged films by thermally activated tunneling of localized photoexcitations. An investigation of the film transmission spectra in the photon energy range 0.2–5.0 eV shows that the long-time variations of the optical properties are associated with a decrease in the concentration of defects forming shallow tails of the density of states. An abrupt decrease of the relaxation time τ and the photoinduced absorption is observed in both types of films at T⩾80 K. Fiz. Tverd. Tela (St. Petersburg) 39, 1303–1309 (July 1997)     

Selected States Magnetic Resonance Spectroscopy (SSMRS): Detection of Paramagnetic Element Dynamics

Abstract

Magnetic resonance (MR) response obtained in a strongly heterogeneous magnetic field with a linear gradient is analysed. It is shown that the employment of strong magnetic field gradients enables the MR spectroscopy to be accomplished in a system with selectively populated energy states (SSMRS). The method can be applied for measuring such physical quantities as the spin diffusion coefficient, D, spin-lattice, T_1m and spin-spin, T_2m, relaxation times and mobility, p_m, of paramagnetic elements in individual Zeeman energy states.

     

Effect of the degree of ordering of structural vacancies on the fine structure of the valence-band top in cubic boron nitride

The electronic energy structure of the defect system of c-BN_x with ZnS-type structure is calculated in the multiple-scattering theory by the local coherent potential method. The cluster version of the MT approximation is used to calculate the crystal potential. The effect of the relaxation of the crystal lattice on the electronic structure of nonstoichiometric boron nitride c-BN_0.75 is studied and a comparison is made with the electronic energy structure of c-BN in the same approximation. Fiz. Tverd. Tela (St. Petersburg) 39, 1064–1065 (June 1997)     

Charge relaxation in polyampholytes of various statistics

We discuss theoretically the relaxation of charge fluctuations in polyampholyte solutions. It has been shown previously by some of us (J. Wittmer et al. Europhys. Lett. 24, 263 (1993)) that the charge distribution along the polyampholyte backbone has a dramatic influence on the polarization energy and hence on the solubility. Here it is demonstrated that a similar effect exists for the charge relaxation. The charge relaxation mechanism qualitatively depends on the statistics: for alternating polyampholytes the relaxation is mainly due to local dipole inversion and is not primarily driven by electrostatic interactions, whereas for random polyampholytes it is driven by electrostatic interactions. Intermediate statistics (with short-ranged (exponential) correlations) appear as a combination of these two limiting cases: short-wavelength modes are insensitive to the loss of correlations along the backbone, whereas long-wavelength modes correspond to a random statistics with renormalized charges. The relaxation of the dielectric constant is also calculated. Received: 20 December 2002 / Accepted: 13 March 2003 / Published online: 24 April 2003 RID="a" ID="a"e-mail: johner@ics.u-strasbg.fr     

Effect of an alternating field on the relaxation of effect of an alternating field on the relaxation of magnetic flux in a Josephson medium

We establish how trapped magnetic flux depends on the frequency and amplitude of an alternating field and how such a field affects the relaxation rate of the flux. We find that the nature of the flux creep changes in the process and that relaxation of the flux stops after the external field is switched off. We examine the dynamics of flux relaxation in a ring in the approximation in which the current density is assumed homogeneous, for various density dependences of the effective vortex activation energy. The critical current density and the vortex activation energy are obtained as functions of the external field strength. Finally, we explain the observed behavior in terms of the different field profiles emerging in the rings. Zh. éksp. Teor. Fiz. 111, 1047–1056 (March 1997)     

Energy relaxation and transport of indirect excitons in AlAs/GaAs coupled quantum wells in magnetic field

The evolution of indirect exciton luminescence in AlAs/GaAs coupled quantum wells after excitation by pulsed laser radiation has been studied in strong magnetic fields (B⩽12 T) at low temperatures (T⩾1.3 K), both in the normal regime and under conditions of anomalously fast exciton transport, which is an indication of the onset of exciton superfluidity. The energy relaxation rate of indirect excitons measured in the range of relaxation times between several and several hundreds of nanoseconds is found to be controlled by the properties of the exciton transport, specifically, this parameter increases with the coefficient of excitonic diffusion. This behavior is qualitatively explained in terms of migration of excitons between local minima of the random potential in the plane of the quantum well. Zh. éksp. Teor. Fiz. 114, 1115–1120 (September 1998)     

Reaction dynamics of Cl O ClO O

Applying the two photon laser induced fluorescence technique for nascent state resolved ClO() detection, the reaction dynamics of Cl+O ClO+O₂ is investigated. The ClO product is formed in its electronic ground state ClO(). A complete product state analysis in terms of vibration, rotation, spin-orbit and -states indicates that nascent ClO radicals are formed in v =0-6 vibrational states peaking at v =3. The ClO fragment shows a moderate rotational excitation, described by a Boltzmann distribution with a temperature parameter of 1300 K 200 K. The spin orbit ratio of :. Most of the excess energy is released as translational energy or as internal energy of the O₂ product. By comparing our results with the trajectory studies of Farantos and Murrell, we favour a reaction mechanism, where the transition complex is planar containing an essentially linear OOCl group. In order to determine the possible influence of vibrationally excited ClO on other trace components of the atmosphere, especially the reaction ClO(v >0)+ O₃, a rough estimate of the vibrational relaxation rate of ClO with the major atmospheric collision partner, N₂, has been performed. A measurement of the vibrational distribution of ClO at different N₂ pressures indicates a mean vibrational relaxation rate of . Received: 27 February 1998 / Revised: 1st April 1998 / Accepted: 15 April 1998     

Creep kinetics of linearly heated metallic glasses

A quantitative theory of creep in linearly heated metallic glasses is developed in terms of new ideas on the kinetics of irreversible structural relaxation under external mechanical stress. The validity of the resulting flow equation has been confirmed by a specially devised experiment. It is shown that the temperature dependence of Newtonian viscosity is determined by the rate of heating and the energy spectrum of irreversible structural relaxation. Fiz. Tverd. Tela (St. Petersburg) 39, 2186–2190 (December 1997)     

Efficiency of stimulation of gaseous chemical reactions by a diffracted light beam whose energy is absorbed by vibrational transitions

Laser-induced reversible chemical reaction kinetics in a gas mixture with the absorption of light energy by molecular vibrational transitions is investigated by mathematical modeling. The influence of beam diffraction, the relaxation time of vibrational energy into heat, and the order of vibrational transition on the structure of the region of high absorption and on the propagation of switching waves is analyzed. The discussion applies to pulses having a duration much longer than the time of energy transfer between vibrational levels. Zh. Tekh. Fiz. 69, 65–73 (April 1999)     

The kinetics of low-temperature electron-phonon relaxation in a metallic film following instantaneous heating of the electrons

The theoretical analysis of experiments on pulsed laser irradiation of metallic films sputtered on insulating supports is usually based on semiphenomenological dynamical equations for the electron and phonon temperatures, an approach that ignores the nonuniformity and the nonthermal nature of the phonon distribution function. In this paper we discuss a microscopic model that describes the dynamics of the electron-phonon system in terms of kinetic equations for the electron and phonon distribution functions. Such a model provides a microscopic picture of the nonlinear energy relaxation of the electron-phonon system of a rapidly heated film. We find that in a relatively thick film the energy relaxation of electrons consists of three stages: the emission of nonequilibrium phonons by “hot” electrons, the thermalization of electrons and phonons due to phonon reabsorption, and finally the cooling of the thermalized electron-phonon system as a result of phonon exchange between film and substrate. In thin films, where there is no reabsorption of nonequilibrium phonons, the energy relaxation consists of only one stage, the first. The relaxation dynamics of an experimentally observable quantity, the phonon contribution to the electrical conductivity of the cooling film, is directly related to the dynamics of the electron temperature, which makes it possible to use the data of experiments on the relaxation of voltage across films to establish the electron-phonon and phonon-electron collision times and the average time of phonon escape from film to substrate. Zh. éksp. Teor. Fiz. 111, 2106–2133 (June 1997)     

Molecular-statics simulation of the cerium impurity in LSO crystals

A model is proposed for the Lu₂SiO₅ crystal with cerium impurity, and the defect formation energy, ion relaxation energy, and the defect-induced changes of the Madelung potentials are calculated. The calculations show substitution of the cerium ion for the lutetium ion in the Lu₁ position to be energetically preferred. Fiz. Tverd. Tela (St. Petersburg) 39, 491–492 (February 1997)     

On the possibility of the direct study of local electron-phonon interaction in semiconductors

Local electron-phonon interaction in deep-level states of defects in semiconductors was studied by induced absorption spectroscopy. Using ZnS:Cu single crystals as an example, it was shown that the laser modulation of two-step impurity absorption is an efficient technique for direct investigations of phonon relaxation effects in deep-level states. It was shown that the localized states in ZnS are prone to extremely strong electron-phonon coupling.