Goldstone mode relaxation in a quantum hall ferromagnet due to hyperfine interaction with nuclei

Spin relaxation in quantum Hall ferromagnet regimes is studied. As the initial non-equilibrium state, a coherent deviation of the spin system from the B direction is considered and the breakdown of this Goldstone-mode state due to hyperfine coupling to nuclei is analyzed. The relaxation occurring non-exponentially with time is studied in terms of annihilation processes in the “Goldstone condensate” formed by “zero spin excitons”. The relaxation rate is calculated analytically even if the initial deviation is not small. This relaxation channel competes with the relaxation mechanisms due to spin-orbit coupling, and at strong magnetic fields it becomes dominating.     

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)     

Anomalous absorption of ultrasound in gadolinium in a magnetic field

The absorption coefficient α _k for longitudinal ultrasonic (15 MHz) waves propagating transverse to the direction of a magnetic field H is measured in single crystal gadolinium. It is found that in fields H⩽600 Oe, the peak in α _k is shifted toward lower temperatures, while the absolute magnitude of the absorption rises with increasing H. It is shown on the basis of dynamic scaling that the anomalous behavior of α _k in fields H⩽600 Oe can be explained by introducing a magnetic field analog of the Landau-Khalatnikov relaxation mechanism. Fiz. Tverd. Tela (St. Petersburg) 39, 339–340 (February 1997)     

Investigation of acceptor centers in semiconductors with the diamond crystal structure by the μ − SR method

The residual polarization of negative muons in crystal silicon samples with phosphorus (P: 1.6×10¹³ cm⁻³) and antimony (Sb: 2×10¹⁸ cm⁻³) impurities is investigated. The measurements are made in a 1000 G magnetic field oriented in a direction transverse to the muon spin in the temperature range 4–300 K. The relaxation rate and shift of the precession frequency in the silicon sample with the phosphorus impurity are measured more accurately than previously. It is found that in antimony-doped silicon the acceptor center _μ A1 at temperatures below 30 K can be in both ionized and neutral states. The experimental data are interpreted on the basis of spin-lattice relaxation of the magnetic moment of an acceptor center, formation of acceptor-donor pairs, and recombination of charge carriers at the acceptor. Preliminary measurements showed a nonzero residual polarization of negative muons in germanium. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 61–66 (10 July 1998)     

An approximate expression for the galvanomagnetic tensor

Using the iterative solution to the Boltzmann equation for electrons in d.c. electric and magnetic fields, an expression for the resistivity tensor can be obtained in the form of an infinite series. This series can be approximated by retaining only the first two terms. In the cases where relaxation times exist — in the sense that the collision term in the Boltzmann equation can be written asg(k)/τ(k), whereτ(k) is the relaxation time, andf (k) = f _E(ɛ k) + [∂f _E(εk)/∂ε]·g(k) the distribution function for electrons with wavevectork — this approximation is exact. For polyvalent metals in the one-OPW approximation, the complete galvanomagnetic tensor can be obtained using this approximation and the result differs from that obtained by using a time of relaxation given by an expression suggested byZiman. A calculation for a simple model Fermi surface, with screened Coulomb scattering, is carried out and the results compared with those of the relaxation time approximation.     

Toward a theory of nuclear relaxation in dielectric glasses at ultralow temperatures

The temperature and frequency dependence of the nuclear relaxation rate in dielectric glasses is investigated. It is shown that at low and ultralow temperatures nuclear relaxation is due to an interaction between the nuclear quadrupole moment and fluctuations of the electric field created by dipole moments of two-level systems. Fluctuations of this field can be associated with the background relaxation or are due only to the dipole-dipole interaction between two-level systems. It is shown that at lower temperatures the second relaxation mechanism begins to dominate. Expressions are obtained for the temperature and frequency of crossover between different nuclear relaxation regimes. The possibility of experimental confirmation of our results is discussed. Zh. éksp. Teor. Fiz. 115, 2254–2262 (June 1999) Russian Scientific Center “Kurchatov Institute”     

Anisotropy inc-axis oriented YBa2Cu3O7−δ

Muon spin relaxation (μSR) data taken at LAMPF on ac-axis oriented fine powder sample of YBa₂Cu₃O_7−δ (YBCO) embedded in epoxy are analyzed for relaxation rate anisotropy. Clear differences beyond simple magnetic field penetration depth anisotropy are observed forB‖c andB c. The low-temperature anisotropy ratio is consistent with oriented ceramic data. Small crystallite size and anisotropic flux pinning characteristics are suggested as the fundamental cause of the additional effects.     

Kinetics of isothermal creep in metallic glasses including the statistical distribution of activation parameters

A generalized theoretical model is proposed for the structural relaxation of metallic glasses under load. Structural relaxation is treated as a set of irreversible, uncorrelated, two-stage atomic displacements in some regions of the structure, the “relaxation centers.” In loaded samples structural relaxation acquires a directional character, leading to the buildup of plastic deformation in accordance with the magnitude and orientation of the applied mechanical stress. General equations are obtained for creep kinetics including a continuous statistical distribution of the principal activation parameters. These equations are compared with the results of a special experiment. The model is found to provide an adequate interpretation of the observed creep kinetics, except for the first 10¹–10² seconds after loading. It is argued that the initial stage of creep is determined by reversible atomic realignments in relaxation centers having symmetric two-well potential. Fiz. Tverd. Tela (St. Petersburg) 39, 2008–2015 (November 1997)     

Nuclear magnetic relaxation in ferrimagnetic Y3Fe5−x SixO12 films

The effect of silicon impurities on the damping of spin-echo signals from the ⁵⁷Fe nuclei of tetrahedral Fe³⁺ ions in epitaxial yttrium-iron-garnet films was investigated. It was found that for silicon concentrations 0.015⩽x⩽0.037 the damping of the spin echo is a two-component process, which made it possible to separate nuclei into two types, differing by both the longitudinal and transverse magnetic relaxation times. For silicon concentrations 0.044⩽x⩽0.073 the decay of the echo can be described by one exponential and all nuclei in the sample have the same transverse relaxation times and the same longitudinal relaxation times. The experimental results are interpreted on the basis of the supposition that impurity “macromolecules” form around the Si⁴⁺ ions. The relaxation times of the iron nuclei in a “macromolecule” are much shorter than the relaxation times of iron nuclei belonging to the matrix ions. The radius of a “macromolecule” is estimated on the basis of percolation theory. Fiz. Tverd. Tela (St. Petersburg) 40, 1494–1497 (August 1998)     

Nuclear relaxations due to paramagnetic impurities in two-level systems

The rates of two types of nuclear spin-lattice relaxation are compared. Transverse relaxation of nuclear spins interacting with paramagnetic centers is also examined under the assumption that the paramagnetic centers form two-level tunneling systems. The transverse relaxation rate is calculated and it is shown that at certain temperatures the transverse relaxation rate is governed by the two-level systems. Fiz. Tverd. Tela (St. Petersburg) 39, 1210–1212 (July 1997) Deceased     

Longitudinal nuclear spin relaxation in solids in a triply rotating coordinate system: Selective observation of the multispin dipole contribution

The longitudinal nuclear spin relaxation in an effective magnetic field H _e3 acting in a triply rotating coordinate system is recorded. Rotating and doubly rotating coordinate systems are employed for strong suppression of the secular nuclear dipole interactions in the first two orders and for separation of higher-order interactions (four-and five-spin). Experiments on protons in polycrystalline benzene showed that the contribution of such multispin dipole interactions to this relaxation can be observed selectively as a pronounced local minimum in the temperature dependence of the relaxation time. This contribution correponds to ultraslow molecular motions with rates ≃ γH _e3≃2π(10¹−10³) s⁻¹ and can be employed to study such motions in detail, including for purposes of identification of the form of the motion. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 5, 335–340 (10 September 1996)     

Relaxation processes in the incommensurate phase of a crystal with defects

The nature of the relaxation of the incommensurate superstructure of a ferroelectric to the equilibrium state is investigated experimentally. It is shown that near a phase transition the temperature dependence of the relaxation time of the incommensurate phase of the defective crystal is exponential. This law agrees qualitatively with the notion of domain wall motion in an inhomogeneous medium containing “random local phase-transition temperature” type defects. Fiz. Tverd. Tela (St. Petersburg) 41, 513–515 (March 1999)     

Kinetics of structural relaxation and regularities of plastic flow of metallic glasses

On the basis of a calculation of the structural relaxation rate and an experimental acoustical-emission determination of the temperature of the transition from localized to uniform flow it is argued that the type of plastic deformation of metallic glasses is uniquely determined by the kinetic structure of the relaxation. In the case of a kinetically hindered structural relaxation, which is characteristic for tests of initial samples at temperatures T<380–420 K, a localized dislocational deformation is realized. At higher temperatures, “memory” of the thermal prehistory of the samples is lost (aging at room temperature), the structural relaxation rate grows abruptly and plastic flow becomes uniform viscoplastic flow. Fiz. Tverd. Tela (St. Petersburg) 41, 2167–2173 (December 1999)     

Spin–Spin Interactions and Nuclear Magnetic Relaxation in Magnetic Solids

The influence of the orientational fluctuations of the electronic magnetization, which modulate nuclear spin–spin interactions (Suhl–Nakamura and dipole–dipole), on the spin-lattice relaxation of magnetic nuclei with spin I = 1/2 in the magnetically ordered solids has been investigated. It has been shown that this mechanism of the spin-lattice relaxation is less effective in comparison with the process of spin-lattice relaxation caused by the direct fluctuations of hyperfine fields, which appear when there are the fluctuations of electronic magnetization direction.     

Dielectric relaxation in deuterated triglycine sulfate crystals

A study has been made of the slow relaxation of the dielectric permittivity of deuterated triglycine sulfate (DTGS) initiated by application of a dc electric field. The field and temperature dependences of the relaxation time associated with domain-wall motion were obtained. The effect of the internal electric field on relaxation processes is seen in different behavior of the field and temperature dependences under different orientations of the external field. The existence in DTGS crystals of two temperature regions of domain-structure rearrangement lying 7–8 and 15–18 °C below the Curie point has been established. Fiz. Tverd. Tela (St. Petersburg) 40, 1542–1545 (August 1998)     

Effect of spin relaxation on the polarization of excitonic luminescence in disordered systems

The kinetics of localized excitons in systems with disorder is studied with allowance for the fine structure of the excitonic state and for spin relaxation processes. The exciton distribution function, formed as a result of the competition between radiative and nonradiative recombination, spin relaxation, and intercenter transitions with an exponentially wide scatter in transition times, is calculated. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 8, 612–617 (25 April 1997)     

Cole-Davidson dielectric relaxation as a self-similar relaxation process

An effort has been undertaken to understand the nature of one type of non-exponential relaxation, namely Cole-Davidson relaxation. Toward this end, a model of relaxation as a self-similar process is proposed. An equation containing operators of fractional integration and differentiation is obtained and solved, which the relaxation function obeys in this case. Fiz. Tverd. Tela (St. Petersburg) 39, 101–105 (January 1997)     

Effect of Mo3+ ions on Nd3+ spin-lattice relaxation in Y3Al5O12

The content of Mo³⁺ ions in YAG:Nd garnet samples prepared by different technologies has been studied, and the spin-lattice relaxation rate of these ions at temperatures of 4–5 K measured. It is concluded is drawn that Mo³⁺ ions can play the part of rapidly relaxing centers mediating the Nd³⁺ spin-lattice relaxation at liquid-helium temperatures. This may account for a number of features in the spin-lattice relaxation of rare-earth ions in garnets, observed earlier at low temperatures. Fiz. Tverd. Tela (St. Petersburg) 40, 2026–2028 (November 1998)     

Slow relaxation experiments in disordered charge and spin density waves: collective dynamics of randomly distributed solitons

We show that the dynamics of disordered charge density waves (CDWs) and spin density waves (SDWs) is a collective phenomenon. The very low temperature specific heat relaxation experiments are characterized by: (i) “interrupted” ageing (meaning that there is a maximal relaxation time); and (ii) a broad power-law spectrum of relaxation times which is the signature of a collective phenomenon. We propose a random energy model that can reproduce these two observations and from which it is possible to obtain an estimate of the glass cross-over temperature (typically T _g≃ 100-200 mK). The broad relaxation time spectrum can also be obtained from the solutions of two microscopic models involving randomly distributed solitons. The collective behavior is similar to domain growth dynamics in the presence of disorder and can be described by the dynamical renormalization group that was proposed recently for the one dimensional random field Ising model [D.S. Fisher, P. Le Doussal, C. Monthus, Phys. Rev. Lett. 80, 3539 (1998)]. The typical relaxation time scales like ∼τexp(T _g/T). The glass cross-over temperature T_g related to correlations among solitons is equal to the average energy barrier and scales like T _g∼ 2xξΔ. x is the concentration of defects, ξ the correlation length of the CDW or SDW and Δ the charge or spin gap. Received 12 December 2001     

Tensor structure of the stationary point of the radiative relaxation operator of an atom

We formulate the problem of the stationary point of the operator of radiative relaxation of an atom: the initial distribution among the sublevels of the excited state, whose nonzero eigenvalues (populations) coincide with the populations of the final distribution (after spontaneous decay) among the sublevels of the ground state. We show that these distributions can be expressed in terms of spherical functions of the complex direction. The results are then used to develop a compact analytical representation of the stationary density matrix of atoms interacting with an elliptically polarized monochromatic field. Zh. éksp. Teor. Fiz. 114, 125–134 (July 1998)