On the calculation of <H 2> molecular integrals

The density matrix equations of motion arising in the triplet mechanism of chemically induced electron spin polarization are solved exactly without the imposition of the Redfield approximation. It is shown that the triplet spin relaxation time occurring in the final expression is not the true relaxation time because the spectral density involved depends both on the rotational correlation time and on the quenching rate. The effective spin relaxation time differs only slightly from the true time. The equations are extended to the case where the initial triplet passes on its polarization to the secondary triplet and exact solutions for the polarizations of the latter's doublets are obtained in the form Π_B = cΠ_A; an explicit expression for c is presented. The consequences of the secondary triplet being able to pass back its polarization to the initial triplet are explored and a ‘coherence effect’ on the polarization on the first triplet's doublets is analysed.     

Relaxation experiments with synchrotron radiation

Relaxation phenomena show up in standard energy domain Mössbauer spectra via line broadening. The evaluation of such spectra is in most cases done by adopting the stochastic theory mainly developed in the 60s and 70s. Due to the time structure and the polarization of the synchrotron radiation nuclear resonance forward scattering in the time domain gives valuable information on relaxation mechanisms. We report here mainly on Nuclear Forward Scattering (NFS) experiments investigating the paramagnetic relaxation of the Fe³⁺ ion in (NH)₄Al_0.95 ⁵⁷Fe_0.05(SO₄)₂·12H₂O and briefly on recent investigations on charge fluctuations in Eu₃S₄.     

Transport properties and polarization phenomena in intercalated Ag x HfSe2 compounds

The electrical properties of intercalated Ag _x HfSe₂ compounds (x = 0.1, 0.2) have been investigated for the first time. Investigations have been performed using various current electrodes, which make it possible to pass either the electron current or the ion current across the sample. Polarization effects, which indicate the self-consistent migration of charge carriers in the samples, have been found for the samples at room temperature. Based on the characteristic features of polarization decay, coefficients of conjugated chemical diffusion have been evaluated.     

Cross-Relaxation Effects in Pulsed-Field-Gradient Stimulated-Echo Measurements on Water in a Macromolecular Matrix

Water diffusion in poly[2-hydroxyethyl-methacrylate] hydrogels has been measured by PFG NMR techniques, using both spin-echo and stimulated-echo methods. It is found that the results differ when evaluated using the standard equations for the echo attenuation. Furthermore, an apparent dependence of the diffusion coefficient on the diffusion time was found in stimulated-echo experiments. This effect is shown to be caused by cross relaxation between the protons of the water and those on the polymer matrix. An equation is derived describing the attenuation of a stimulated echo in the presence of cross relaxation. This equation shows that the additional damping due to cross relaxation depends on the gradient amplitude. If this equation is used with the measured cross-relaxation parameters, both methods are found to agree within experimental error, and no diffusion-time dependence is found for the stimulated-echo experiments. The equation predicts the effect of replacing part of the H₂O by D₂O; this has been experimentally verified. It is concluded that macromolecular systems should be checked for cross relaxation when stimulated-echo methods will be used for self-diffusion measurements.     

Fluorescence quenching in Nd:YAG

We show that quenching of the Nd fluorescence is in principle not associated with the Nd³⁺ ion but with the host. The process is due to near-field electric dipole interaction between Nd pairs, and cross relaxation via the⁴I_{1 5/2} manifold. We present for the first time the complete fluorescence spectrum and level scheme of Nd∶YAG, and find that Nd∶YAG has an exceptional level configuration which boosts cross relaxation. Our results encourage the search for new Nd laser materials which have a slightly different position of the⁴I_{1 5/2} manifold so that higher Nd concentrations can be achieved for integrated-optics applications. Finally, we report on implications of energy migration which we found to be quite effective in Nd∶YAG.     

Study of thermal currents in powder β-spodumene ceramics doped with Cuo, FeO and TiO2

Ternary-phase ceramic system of Li₂O Al₂O₃ 4SiO₂ doped with CuO, FeO and TiO₂ has been prepared and subjected to dc electrical conductivity and thermally stimulated depolarization current (TSDC) measurements as a function of temperature (30-250 °C) and field strength. The electrical conductivity results are explained by assuming both ionic and electronic conduction mechanisms coexist with different contributions over the whole temperature range of experiments. TSDC spectra have been found to be characterized by a broad intense relaxation peak, which can be attributed to an ionic charge polarization. The broad relaxation transitions are apparently a result of the nonuniform nature of this process. Activation energies are calculated for both dc electrical conductivity and TSDC according to Arrhenius equation and initial rise method, respectively.     

Kinetics of polarization switching in relaxor-ferroelectric Sr0.5Ba0.5Nb2O6 crystal doped with chromium

The kinetics of polarization reversal process in relaxor-ferroelectric Sr_0.5Ba_0.5Nb₂O₆ (SBN : 50) crystal is studied by switching current registration. The temporal behavior of the polarization is analyzed in terms of the domain dynamics. It has been found that the observed stretched-exponential behavior of the polarization relaxation can be the result of a wide distribution of relaxation times. It is suggested that the distribution of relaxation times may be attributed to a nonuniform distribution of the local coercive fields within the crystal structure.     

Impurity pairs and excitation relaxation in doped silicon

The kinetics of photoconductivity is studied in silicon doped with B, Al, Ga, In, P, As, and Sb with concentrations of 10₁₆–10₁₈cm_?3 at 4.2 and 10.5 K placed in an 8-mm microwave electric field under pulsed impurity excitation. It is found that infrared absorption by impurity pairs and a slow component of photoresponse relaxation arise at close impurity concentrations. It is shown that this component is due to an increase in the polarization hopping conductivity in the presence of the optical charge exchange of impurity states—isolated impurities and impurity pairs and dipoles (pairs of the major and compensating impurities). The hopping transfer processes of ion charges in the course of relaxation are analyzed. It is shown that the main contribution to polarization photoconductivity comes from hopping transitions in impurity pairs at relatively small concentrations and from hopping with the participation of isolated ions at increased concentrations.     

Measurements and simulation of hysteresis loops of donor-doped strontium bismuth tantalate ceramics

Wolframium ions doped SrBi₂Ta₂O₉ ceramics have been synthesized by the conventional mixed-oxide method. A sawtooth-like hysteresis loop was observed at low measuring-frequency. This phenomenon was thought to be related with the perturbation of the defect-dipole polarization under an external electrical field. A developed physical-model was derived on the basis of simplifying such perturbation to a simple harmonic pendulum oscillator, which combines physical considerations with the well-known Preisach model. On the hypothesis of the simple harmonic pendulum oscillator, the included-angle between the defect-dipole polarization and the elementary-dipole polarization can be expressed as the product of the defect-dipole relaxation velocity and permitted relaxation time. Simulation result was excellently agreed with the observed measurement.     

On the hydrodynamic sound in polarized Fermi liquids

The velocity of sound for polarized Fermi liquids at frequencies intermediate between the inverse spin relaxation time and the inverse typical relaxation time is found via the kinetic equation. This velocity is not expressed by the static compressibility at a fixed magnetic field but at fixed spin polarization. The measurement of static quantities and this velocity allows one to determine the Landau parameters with l = 0 and the effective masses for the polarized liquid.     

On generalizations of the Debye equation for dielectric relaxation

In some previous papers one of us (G.A.K.) discussed dielectric relaxation phenomena with the aid of non-equilibrium thermodynamics. In particular the Debye equation for dielectric relaxation in polar liquids was derived. It was also noted that generalizations of the Debye equation may be derived if one assumes that several microscopic phenomena occur which give rise to dielectric relaxation and that the contributions of these microscopic phenomena to the macroscopic polarization may be introduced as vectorial internal degrees of freedom in the entropy. If it is assumed that there are n vectorial internal degrees of freedom an explicit from for the relaxation equation may be derived, provided the developed formalism may be linearized. This relaxation equation has the form of a linear relation among the electric field E, the first n derivatives with respect to time of this field, the polarization vector P and the first n + 1 derivatives with respect to time of P. It is the purpose of the present paper to give full details of the derivations of the above mentioned results. It is also shown in this paper that if a part of the total polarization P is reversible (i.e. if this part does not contribute to the entropy production) the coefficient of the time derivative of order n + 1 of P in the relaxation equation is zero.     

ZnO压敏陶瓷中缺陷的介电谱研究

从理论上证明了介电松弛过程在介电谱上等效于电子松弛过程,认为室温下10⁵Hz处特征损耗峰起源于耗尽层处本征缺陷所形成的电子陷阱.在-130—20℃范围内测量了三种配方ZnO陶瓷的介电频谱,发现ZnO压敏陶瓷室温下10⁵Hz处的特征损耗峰在低温下分裂为两个特征峰,认为它们起源于耗尽层中的本征缺陷(锌填隙或/和氧空位)的电子松弛过程.发现ZnO-Bi₂O₃二元系陶瓷特征峰仅仅由锌填隙引起,而ZnO-Bi_{2关键词： ZnO压敏陶瓷 本征缺陷 介电谱 热处理}     

Clustering effect in the crystallization process of a CuZr amorphous alloy

The structural relaxation and crystallization processes of the Cu₅₀Zr₅₀ amorphous alloy have been studied by field ion microscopy (FIM) on an atomic scale. An interesting phenomenon which we call the clustering effect was observed for the first time as far as we know. In the temperature range 673–723 K, clusters consisting of 3, 4, or 5 atoms formed and migrated towards certain crystalline centers. They then combined with one another and rearranged to produce an ordered atomic array. This clustering process including the formation, migration, combination and rearrangement of clusters is considered as a structural relaxation process.     

Molecular dynamics simulation of SnF2 nanostructures in the internal channels of single-walled carbon nanotubes

A molecular dynamics simulation of solid tin(II) fluoride nanostructures formed in internal channels of single-walled carbon nanotubes (SWCNTs) has been performed using two types of model potentials—without and with inclusion of the polarization of ions. For the potential taking into account the polarization of ions, an ordered SnF₂@SWCNT structure is reproduced: in SWCNT(10, 10), it has the form of the SnF₂ internal nanotube. At the same time, the SnF₂@SWCNT(11,11) structure is substantially disordered (glass-like). It has been found that heating of the SnF₂@SWCNT model system produces a superionic state characterized by a high mobility of fluorine ions without migration of tin ions. The model potentials disregard the covalent character of Sn-F bonds and the specific interactions of a lone electron pair of the Sn²⁺ ion. This makes it impossible to completely reproduce the properties of SnF₂ at normal pressures. However, some characteristics of the SnF₂ high-pressure modification can be reproduced if the polarization of ions is taken into account.     

Retardation of polarization in mixed K<Subscript>0.88</Subscript>(NH<Subscript>4</Subscript>)<Subscript>0.12</Subscript>H<Subscript>2</Subscript>PO<Subscript>4</Subscript> crystal

The time dependences of polarization of K_0.88(NH₄)_0.12H₂PO₄ mixed crystal have been studied within the temperature range of 74–100 K. Two mechanisms of polarization relaxation were found. The first mechanism is caused by domain walls lateral motion and their interaction with point lattice defects. The second one supposedly is due to polar regions infiltration through the regions of frustrated paraelectric phase.     

Mesopore size dependence of the ionic diffusivity in alumina based composite lithium ionic conductors

Ordered-mesoporous Al₂O₃ was synthesized by a sol–gel method using neutral copolymer surfactants as structure-directing agents. The pore size was controlled over the 3–15 nm range by the use of various surfactants. Composites composed of the synthesized mesoporous Al₂O₃ and a lithium ion conductor (LiI) were prepared. The maximum dc electrical conductivity, 2.6 × 10^− 4 S cm^− 1 at 298 K, was observed for 50 LiI·50 Al₂O₃ composite with 4.2 nm average mesopore size, which was considerably higher than the previously reported LiI-alumina composites. A systematic dependence of conductivity upon pore size was observed, in which conductivity increased with decreasing pore size, except for samples with a pore size of 2.8 nm. The lithium ion diffusion coefficient determined by the ⁷Li pulsed field gradient nuclear magnetic resonance (PFG-NMR) showed excellent agreement with the measured conductivity calculated by the Nernst-Einstein equation. On the other hand, lithium migration activation energies obtained by quasielastic neutron scattering (QENS) and ⁷Li NMR spin-lattice relaxation time (T₁) were considerably smaller than those obtained from electrical conductivity and PFG-NMR. This could be explained by the ion migration mechanism in heterogeneous composites and a possible enhancement of conductivity in mesoscopically confined spaces.     

Temperature-dependent Time-resolved Fluorescence Study of Cinchonine Alkaloid Dication

Photo induced excited state dynamical processes of cinchonine alkaloid dication (C⁺⁺) have been studied over a wide range of temperature using steady state and nanosecond time-resolved fluorescence spectroscopic techniques. The temperature-dependent fluorescence studies of C⁺⁺ clearly indicate the existence of two distinct emitting species having their own characteristic decay rates. The shorter-lived species shows a usual temperature dependence with increasing non-radiative deactivation at higher temperatures, while the longer-lived species show features resembling to the excited state solvent relaxation process with a large solvent relaxation time (τ _r ∼ 6 ns). The species emitting in the lower energy side, having longer decay time is found to be more sensitive towards chloride ion quenching and has a charge transfer character. Further, concentration quenching with decrease in τ _r of long lived species shows the possibility of energy migration along with solvent relaxation in C⁺⁺.     

Anomalously strong relaxation of the polarization of muons in the magnetically ordered and paramagnetic states of the TbMnO<Subscript>3</Subscript> multiferroic

An anomalously strong relaxation of the muon polarization in a magnetically ordered state in the TbMnO₃ multiferroic has been revealed by the method below the μSR Néel temperature (42 K). Such a relaxation is due to the muon channel of relaxation of the polarization and the interaction of the magnetic moment of the muon with inhomogeneities of the internal magnetic field of an ordered state in the form of a cycloid. Above the Néel temperature, beginning with temperatures depending on the applied magnetic field, a two-phase state has been revealed where one phase has an anomalously strong relaxation of the muon polarization for a paramagnetic state. These features of the paramagnetic state are due to short-range magnetic order domains that appear in strongly frustrated TbMnO₃. A true paramagnetic state has been observed only at T ≥ 150 K.     

TSC study of deuteration of polystyrene

A comparative study of natural and deuterated polysterene has been performed by thermally stimulated currents (TSC). Around the glass-rubber transition temperature (T_g a TSC peak is observed whose relaxation times obey an Arrhenius equation. At 50° above T_g another TSC peak is observed whose relaxation time obeys a Vogel equation. Deuteration is found to increase the values of T_g.     

35Cl N.M.R. relaxation in aqueous sodium perchlorate solutions

The relaxation time, T _1ρ, for ³⁵Cl in perchlorate ion in aqueous solutions is found to be 0·25 s. T _1ρ is insensitive to the presence of most first row transition metals; however, a weak complex with manganous ion is demonstrated, and the perchlorate ion-manganous exchange rate is shown to be between 3 × 10⁴ s^-1 and 3·6 × 10⁷ s^-1. In the absence of manganous ion, the relaxation is dominated by the nuclear electric quadrupole interaction; however, a dipolar contribution is observed with manganous ion present.