High-temperature behaviour of spin-lattice relaxation and Knightshift of Te125 in tellurium

The Knightshift and the nuclear spin-lattice relaxation time of Te¹²⁵ in enriched (94% Te¹²⁵) polycrystalline tellurium have been studied from room temperature to the melting point (725 K). Over the whole temperature range, the shift is governed by an Arrhenius law with an activation energy of about 0.3 eV in agreement with the gap energy of tellurium at higher temperatures. Below 420 K, the relaxation time is caused by the conduction electrons, whereas in the high-temperature region the relaxation process is due to translational atomic diffusion. In this region, the relaxation mechanism is found to be determined by the chemical shift anisotropy of Te¹²⁵.     

Some peculiarities of spin-lattice relaxation of impurity rare-earth ions in crystals caused by the structure defects

The spin-lattice relaxation times for Nd³⁺ ions in yttrium-aluminum garnets (YAG) and for Yb³⁺ ions in CaF₂ in the low-temperature range have been measured. For the first system the temperature dependence of the relaxation rate is determined to a great extent by the method of sample preparation. For samples grow by the method of the horizontally oriented crystallization the dependence is described asT ₁ ^?1 =AT ⁿ ,n ? 4.7, which is an evidence of an influence of local structure disordering on the relaxation. The temperature dependence of the relaxation rate in CaF₂:Yb is also “anomalous”:T ₁ ^?1 =AT ^3.3. The results are compared with the previous data on the relaxation in similar systems, and with other cases of observation of “anomalous” temperature dependences. Different manifestations of the local crystal defects in spin-lattice relaxation are discussed.     

Relaxation Behavior of Laser-Polarized Xe Gas: Size Dependency and Wall Effect of the T1 Relaxation Time in Glass and Gelatin Bulbs

Size dependency of the relaxation time T₁ was measured for laser-polarized ¹²⁹Xe gas encapsulated in different sized cavities made by glass bulbs or gelatin capsules. The use of laser-polarized gas enhances the sensitivity a great deal, making it possible to measure the longer ¹²⁹Xe relaxation time in quite a short time. The size dependency is analyzed on the basis of the kinetic theory of gases and a relationship is derived in which the relaxation rate is connected with the square inverse of the diameter of the cavity. Such an analysis provides a novel parameter which denotes the wall effect on the relaxation rate when a gas molecule collides with the surface once in a second. The relaxation time of ¹²⁹Xe gas is also dependent on the material which forms the cavity. This dependency is large and the relaxation study using polarized ¹²⁹Xe gas is expected to offer important information about the state of the matter of the cavity wall.     

Relaxation of vibrationally excited states in solid “nitrate–nitrite” binary systems

The processes of molecular relaxation in the solid NaNO₃–NaNO₂ and KNO₃–KNO₂ “nitrate–nitrite” binary systems have been investigated by Raman spectroscopy. The relaxation time of the vibration ν₁(A) of an NO^- ₃ anion in the binary system is found to be shorter than that in individual nitrate. The increase in the relaxation rate is explained by the existence of an additional mechanism of relaxation of vibrationally excited states of the nitrate ion in the system. This mechanism is related to the excitation of vibration of another anion (NO^- ₂) and generation of a lattice phonon. It has been established that this relaxation mechanism is implemented provided that the difference between the frequencies of the aforementioned vibrations correspond to the range of sufficiently high density of states in the phonon spectrum.     

The T 1 and T 2 Relaxation Times Distribution for the 35Cl and 14N NQR in Micro-composites and in Porous Materials

The paper describes the results of an experimental study of the ³⁵Cl and ¹⁴N nuclear quadrupole resonance in composite and porous materials, the influence of the environment and the crystallite size of powder on the nuclear quadrupole resonance line widths, as well as on the spin–spin and spin–lattice relaxation times. It is established that the spin–lattice relaxation time has a unimodal distribution, and the spin–spin relaxation time—bimodal distributions for all the investigated samples. The idealized distribution function of the relaxation times is obtained on the assumption that the concentration of inhomogeneities and relaxativity decreases with an increasing distance from the surface into the interior of the crystallite exponentially. It is shown that the model with the spin diffusion explains the shortening of the decay signal with decreasing grain size, but this is not confirmed by the experimental distribution of relaxation times obtained by means of the inverse Laplace transformation.     

An electro-optical study of relaxation rates of off-centre ions in SrO

The rate of relaxation between off-centre sites of Ni²⁺ and Co²⁺ substitutional defects in SrO is determined from the response of the optical absorption to an electric field. This is achieved not by direct lifetime measurement but by an alternative and sensitive technique in which the optical signal is monitored exactly out of phase with a modulated electric-field. It is found that the temperature dependence of relaxation lifetimes in the range 70μs to 13ms for Ni²⁺ ions is consistent with a simple classical thermally-activated process, a result which is to be held in contrast with the situation for Co²⁺.     

Time relaxation of the solutions of master equations for large systems

The time relaxation behavior of the solutions of certain classes of discrete master equations is studied in the limit of an infinite number of states. Depending on the range of the transition matrix, a relaxation behavior is found reaching from at ^–1/2 law for short range, over enhanced relaxation to an exponential relaxation for the extreme long-range case. The behavior in the limit of a continuous family of states is also discussed.     

EPR and spin-lattice relaxation of rare-earth ions in LiLuF4 monocrystals

The EPR and spin-lattice relaxation are studied of impurity rare-earth ions in LiLuF₄ crystals at liquid-helium temperatures. It is detected that paramagnetic relaxation of Er³⁺ ions is retarded by the effect of the phonon throat. The effect of resonance attenuation of the phonon throat is clarified in two-phonon resonance relaxation processes of Er³⁺ ions. The Debye temperature of the crystal is determined from an analysis of experimental results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 24–27, February, 1988.     

Mössbauer study of spin-spin relaxation of Fe3+ ions in the presence of other paramagnetic ions

Mössbauer spectroscopy has been used to study the influence of paramagnetic ions, viz. Cu²⁺, Cr³⁺, Co²⁺ Mn²⁺, Gd³⁺ and Dy³⁺ on the spin-spin relaxation time of Fe³⁺ ions in amorphous frozen aqueous solutions. It is found that these ions shorten the relaxation time, but the effect is much smaller than suggested earlier on the basis of measurements of relaxation of Fe³⁺ in an α-Al₂O₃ matrix. It is also found that S-state ions have a greater influence on the relaxation time than other paramagnetic ions. The spectra obtained in presence of S-state impurity ions could only be fitted by allowing the individual transition probabilities to vary independently.     

Excited state dynamics in silver nanoparticles embedded in phosphate glass

Excited state dynamics in silver nanoparticles embedded in aluminophosphate glass was studied by ultrafast optical pump–probe technique. The absorption process of pump radiation and the electron–phonon relaxation on the 10^?13–10^?11 s scale were analyzed in the framework of two-temperature model. The time evolution of the light-induced transient diffraction grating shows an uncommon relaxation on the nanosecond time scale. This relaxation is assigned to phonon–phonon scattering process as well as to the energy transfer from photoexcited electronic states in glass matrix to silver nanoparticles.     

Mössbauer spectra of a paramagnetic ion in an axial crystal field in the presence of spin-lattice relaxation

The influence of spin-lattice relaxation on the Mössbauer hyperfine spectra of the Fe³⁺ ion is investigated in the case of an axial crystal field. All the various influences of the relaxation process on the spectra can be explicitly described using two relaxation parameters. A detailed analysis of the Mössbauer relaxation spectra for the various temperatures, relations between the relaxation parameters and external magnetic field directions is carried out. When the magnetic field direction is nearly collinear with the symmetry axis and one of the relaxation parameters is small, the dynamics of Mössbauer spectra is shown to have anomalous features. The influence of random magnetic fields is shown to give an unconventional development of patterns as a function of the relaxation parameters.     

Temperature dependence of21Ne,83Kr,and131Xe quadrupole relaxation and of xenon diffusion in polyatomic solvents

The main aim of the present work is to contribute, by an extension of the experimental data base, to the understanding of quadrupole relaxation of nuclei of noble gases dissolved in molecular liquids. We have performed temperature dependent spin-lattice relaxation rate measurements of²¹Ne,⁸³Kr, and¹³¹Xe in various non-aqueous solvents (e.g. in methanol, ethanol, n-butanol, acetone, acetonitrile, benzene, carbon tetrachloride, N,N-dimethylformamide, dodecane, tetradecane, p-xylene, and hexafluorobenzene). In nine liquids we also measured translational diffusion coefficients of dissolved xenon as a function of temperature by the NMR spin-echo technique, obtaining partly the very first diffusion data for a number of systems. The comparison of the remarkable low activation energies for the noble gas nuclear quadrupole relaxation with that of the noble gas diffusion reveals that both seem to be closely connected. There are strong hints that the nuclear relaxation process follows an empirical “Dη ^γ =A correlation” found previously by Evans and co-workers for the tracer diffusion of noble gases in polyatomic liquids. For almost all solvents γ decreases from¹³¹Xe to²¹Ne parallel with a decrease of the corresponding activation energy for the quadrupolar relaxation. Possible physical reasons for this behavior are briefly discussed. Essential qualitative results in this paper were found to agree with two MD computer simulations for¹³¹Xe relaxation in benzene and methanol. Further MD simulations are proposed which are obviously required for the deeper understanding of the experimental results found in the present paper.     

Electron paramagnetic resonance and relaxation of amorphous silicon below 1 K

Amorphous silicon, generated within crystalline Si by ²⁸Si⁺ ion implantation, exhibits an electron spin relaxation rate which varies with temperature as T^2.37 between 0.3 and 4.2 K. These results exclude the current model of a phonon-limited, direct relaxation mechanism in a-Si. A relaxation process, consistent with the known temperature variation, is outlined. EPR signal strengths, relative to a known paramagnet at temperatures near 1.2 and 0.4 K, put limits on an antiferromagnetic Curie-Weiss temperature of 0?θ?0.06 K.     

Effect of oxygen on the NMR relaxation properties of crude oils

Nuclear magnetic resonance relaxation measurements of bulk fluids provide a sensitive probe of the dynamics of molecular motion. Dissolved oxygen can interfere with this technique as its paramagnetic nature leads to a reduction of the paramagnetic relaxation times of the fluids. We studied this effect for the relaxation properties of crude oils that are in general characterized by a distribution of relaxation times. The samples were stock tank oils that have been exposed to air. We comparedT ₁ andT ₂ relaxation time distributions and their correlation functions of the initial (oxygenated) samples with those from the deoxygenated samples. Oxygen was removed from the oils with a freeze-thaw technique. As expected, the effect of oxygen is most apparent in oils with long relaxation times. In these oils the effect of oxygen can be described by an additional relaxation rate 1/T _1,2 ^ox to the transverse and longitudinal relaxation rates that is sample dependent but does not vary within the relaxation time distribution of the oil. Values of 1/T _1,2 ^ox for different crude oils were found to be in the range of 2.5 to 8.3 s. For crude oils that have components with relaxation times less than 100 ms, no significant oxygen effect is observed.     

The relaxation rate of the magnetic moment of a shallow acceptor center as a function of impurity concentration in silicon

A study is made into the temperature dependence of residual polarization of negative muons in crystalline silicon with the concentration of impurity of the n-and p-types ranging from 8.7×10¹³ to 4.1× 10¹⁸ cm^?3. The measurements are performed in a magnetic field of 1000 G transverse to the muon spin, in the temperature range from 4.2 to 300 K. The form of the temperature dependence of the relaxation rate v of the magnetic moment of the μAl⁰ acceptor in silicon is determined. For a nondegenerate semiconductor, the relaxation rate depends on temperature as v ∝ T ^q (q ≈ 3). A variation in the behavior of the temperature dependence and a multiple increase in the relaxation rate are observed in the range of impurity concentration in excess of 10¹⁸ cm^?3. The importance of phonon scattering and spin-exchange scattering of free charge carriers by an acceptor from the standpoint of relaxation of the acceptor magnetic moment is discussed. The constant of hyperfine interaction in an acceptor center formed by an atom of aluminum in silicon is estimated for the first time: |A _hf (Al)/2π| ～ 2.5×10⁶s^?1.     

Low-temperature spin-lattice relaxation of8Li in the glass Li2O·2SiO2

Nuclear spin-lattice relaxation in Li₂0·2Si0₂ glass below 200 K has been studied using the asymmetric Β-decay radiation of polarized⁸Li (T_1/2=0.Bs) nuclei produced by capture of polarized neutrons. Transients of the⁸Li polarization follow an exp(?√E/T₁) law. The dependence of the spin-lattice relaxation rate ⊥_¯1 ¹ on temperature T and magnetic field B can roughly be described by T_¯1 ¹～T/B. The interpretation is based on the assumption that for⁸Li, contrary to⁷Li in the same glass, spin-diffusion is absent and that each probe nucleus is coupled by quadrupolar interaction to an individual distribution of nearby centres typical of glasses. The fluctuation of these centres causing relaxation may be induced by either a multi-phonon or a thermally activated motional process.     

Temperature relaxation and energy loss of hot carriers in graphene

The temperature relaxation and energy loss of hot Dirac fermions are investigated theoretically in graphene with carrier–optical phonon scattering. The time evolutions of temperature and energy loss for hot Dirac fermions in graphene are calculated self-consistently. It shows that the carrier–optical phonon coupling results in the energy relaxation of hot carriers excited by an electric field, and the relaxation time for temperature is about 0.5–1 ps and the corresponding energy loss is about 10–25 nW per carrier for typically doped graphene samples with a carrier density range of 1–5×10¹² cm^?2. Moreover, we analyze the dependence of temperature and energy relaxation on initial hot carrier temperature, lattice temperature and carrier density in detail.     

<Superscript>63,65</Superscript>Cu NMR study of low-frequency spin dynamics in the infinite-layer antiferromagnetic compound SrCuO<Subscript>2</Subscript>

The spin-lattice and spin-spin relaxation times have been measured for ^63,65Cu NMR in the infinite-layer anti-ferromagnet SrCuO₂ in the ordered state for temperatures from 4.2 to 361 K. In the region of low temperatures (T≤250 K), both relaxation processes are of the same nature and the main contribution to the relaxation rate is associated with the diffusion of a small number of holes with an activation energy of ～42 meV. In the high-temperature range (T > 250 K), contributions to the transverse relaxation rate exhibit redistribution and this relaxation process is determined predominantly by indirect interactions.     

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)     

Electron spin relaxation of copper(II) ions in diamagnetic crystals

The electron spin-lattice and spin-spin phase relaxation measurements of Cu²⁺ ions in various crystals are reviewed and discussed. Examples of the Debye temperature determination from a wide temperature range measurements of the spin-lattice relaxation time T₁ are shown. An influence of the Jahn-Teller dynamics on T₁ is presented. The phase relaxation described by the phase memory time T_M is affected by temperature due to the spin packet width modulation by molecular motions. The T_M is anisotropic in crystals and can be different for different hyperfine lines of an EPR spectrum.