Proton spin-lattice relaxation of a tunnelling CH2D2 molecule

The proton spin-lattice relaxation time of a CH₂D₂ molecule is calculated. The tunnel splittings are assumed to be much larger than the Zeeman energy. The dependence of the relaxation efficiency on the site symmetry is examined. The results are compared with experimental T₁ values in the solid phases of CH₂D₂. Contrary to CH₄ and CD₄, the symmetry of the crystal field has no influence on the relaxation rate. The calculated relaxation rate is lower than experimentally observed, which indicates that some of the energy levels coincide.     

Phase diagram of YBa2Cu3O7−y at T<T c according to transverse nuclear relaxation data for Cu(2)

Two peaks are observed at T=35 and 47 K in the transverse relaxation rate for Cu(2) nuclei in YBa₂Cu₃O_7?y . A comparison of the relaxation rates for isotopes ⁶³Cu(2) and ⁶⁵Cu(2) at T=47 K indicates the magnetic nature of relaxation. The enhancement of local magnetic field fluctuations perpendicular to CuO₂ planes at T=47 K is associated with critical fluctuations of orbital currents. The peak at T=35 K is attributed to the emergence of an inhomogeneous superconducting phase. The obtained experimental results and the available data from the literature made it possible to propose a qualitatively new phase diagram of the superconducting state.     

空位所引起的晶体弹性常数的改变

认为具有少量空位的晶体是一个均匀的晶体,把完整与不完整晶体的区别,归结为势函数的修改。应用Morse函数推导出弹性常数c₁₁和c₁₂的变化率与空位浓度的关系。因为空位邻近的原子有松弛,考虑了由松弛而改变了的相互作用对弹性常数的影响。首先求得铝中空位的第一和第二最近邻的位移百分率各为1.61和-0.32,这里的正号和负号各表示向空位移进和移出。其次,进行了具体的计算,在铜晶体中,估计1％的空位浓度使弹性常数减小1.3—1.4％。然而,铝晶体中的空位对弹性常数的影响     

表面活性剂CTAB水溶液中的T2弛豫分散研究

用CPMG脉冲序列测定了表面活性剂十六烷基三甲基溴化铵（CTAB）分子中的氮甲基(N-CH₃)质子的横向弛豫时间（T₂表观）,并发现测得的T₂表观\}与序列中的重聚脉冲间隔时间的一半τ _cp有关,说明存在横向弛豫分散现象. 当在τ_cp≤1 ms时,T₂表观与τ²_cp}呈线性关系;而当τ_cp≥4.6 ms时,T₂表观变得与τ_cp无关. 利用Luz-Meiboom两体化学交换模型计算了不同浓度的CTAB溶液中的N-CH₃质子的本征横向弛豫时间（T₂本征）和化学交换速率k_ex,发现k_ex与T₂本征和自扩散系数D一样,在临界胶束浓度（CMC）附近发生突变. 这个突变反映了CTAB分子在从单体到胶束的转变过程中其动力学特性发生了改变.      

NMR Solvent Spin‐Lattice Relaxation Rate in Colostrum

Abstract

In this study, by using a FT‐NMR spectrometer operating at 60 MHz for proton, the solvent spin‐lattice relaxation times (T ₁) in colostrum were measured versus the days of lactation, whereas the T ₁ values in dehydrated colostrum were determined versus concentration of its hydrating solid. Data show that the spin‐lattice relaxation rate (1/T ₁ or R ₁) in colostrum is linearly dependent upon the inverse of time (1/days), and the R ₁ in dehydrated colostrum increases linearly with increasing concentration of its hydrating solid content (C). From data, the total paramagnetic contribution of ions in colostrum to the R ₁ was found to be negligible. The dehydrated colostrum data indicates that the R ₁ in colostrum is linearly dependent upon its hydrating solid content. Therefore, the R ₁ changes in colostrum were analyzed in terms of the relaxivities (increase in relaxation rate per unit concentration of solid) and the concentrations of milk constituents. Such an analysis provides a relation similar to that of the R ₁ in dehydrated colostrum. The current data imply that the relaxation changes in colostrum by days may be explained through changes in the concentrations of milk constituents. Also, the data suggest that the relaxation mechanism in colostrum can be explained in terms of fast chemical exchange of protons between free water and water bound to milk constituents.     

High-precision measurements of the compressibility of chalcogenide glasses at a hydrostatic pressure up to 9 GPa

The volumes of glassy germanium chalcogenides GeSe₂, GeS₂, Ge₁₇Se₈₃, and Ge₈Se₉₂ are precisely measured at a hydrostatic pressure up to 8.5 GPa. The stoichiometric GeSe₂ and GeS₂ glasses exhibit elastic behavior in the pressure range up to 3 GPa, and their bulk modulus decreases at pressures higher than 2–2.5 GPa. At higher pressures, inelastic relaxation processes begin and their intensity is proportional to the logarithm of time. The relaxation rate for the GeSe₂ glasses has a pronounced maximum at 3.5–4.5 GPa, which indicates the existence of several parallel structural transformation mechanisms. The nonstoichiometric glasses exhibit a diffuse transformation and inelastic behavior at pressures above 1–2 GPa. The maximum relaxation rate in these glasses is significantly lower than that in the stoichiometric GeSe₂ glasses. All glasses are characterized by the “loss of memory” of history: after relaxation at a fixed pressure, the further increase in the pressure returns the volume to the compression curve obtained without a stop for relaxation. After pressure release, the residual densification in the stoichiometric glasses is about 7% and that in the Ge₁₇Se₈₃ glasses is 1.5%. The volume of the Ge₈Se₉₂ glass returns to its initial value within the limits of experimental error. As the pressure decreases, the effective bulk moduli of the Ge₁₇Se₈₃ and Ge₈Se₉₂ glasses coincide with the moduli after isobaric relaxation at the stage of increasing pressure, and the bulk modulus of the stoichiometric GeSe₂ glass upon decreasing pressure noticeably exceeds the bulk modulus after isobaric relaxation at the stage of increasing pressure. Along with the reported data, our results can be used to draw conclusions regarding the diffuse transformations in glassy germanium chalcogenides during compression.     

Multiphonon relaxation in fluoride and ternary sulfide laser crystals with neodymium ions

It is shown that, in ternary sulfide crystal matrices, in contrast to fluoride matrices with similar phonon spectra, the multiphonon relaxation rate as a function of the number p _eff of “effective phonons” (the energy gap ΔE _min between the J′ and J levels of the Nd³⁺ ion) has a steep slope, which does not decrease with increasing p _eff. This indicates that, compared to fluoride crystals, the multiphonon relaxation rate of mid-IR transitions (in the range 4–5 μm) in ternary sulfide matrices sharply decreases, by three to four orders of magnitude.     

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     

NMR studies of U0.2Y0.8Pd3 and YPd3 at low temperatures

We report results of ⁸⁹Y-NMR measurements on U_0.2Y_0.8Pd₃ as well as YPd₃, performed at frequencies between 9 and 16MHz and at temperatures between 0.25 and 2 K. In this temperature range the average Knight shift is +0.5%, temperature-independent and the same for both materials. Also the NMR linewidth for both compounds was found to be temperature and field independent but one order of magnitude larger for U_0.2Y_0.8Pd₃ than for YPd₃. For U_0.2Y_0.8Pd₃ our results indicate a distribution of internal static fields at the Y sites and a small temperature-dependent enhancement of the spin-lattice relaxation rate T ₁ ^?1 with respect to YPd₃. The NMR spectra are consistent with the presence of very small frozen U moments, but the temperature dependence of the spin-lattice relaxation rate indicates a more complicated situation. In particular (T₁T)^?1 shows an anomalous temperature dependence.     

Investigation on crystalline structure and dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) film

The dielectric relaxation behaviors of hot pressed poly(vinylidene fluoride) (PVDF) film have been studied using dielectric spectroscopy in the frequency domain from 20 Hz to 5 MHz at temperatures between 20 °C and 200 °C. Crystalline/amorphous interphase is suggested with methods of FTIR, XRD, and DSC. Frequency and temperature dependence of dielectric spectroscopy reveals the relaxation behavior and structural dynamics of the samples, and three types of relaxation processes are suggested, α_Ac relaxation process contributed by the hopping transport process near the periphery of conduction band or valence zones at Fermi energy, α_c relaxation process related to the structure change of crystal lattice trapped dipoles in crystalline regions, and α_a relaxation process arising from segmental dipole rearrangement of interphases in amorphous regions. Cole-Cole and Havriliak-Negami experimental equations were utilized to analyze these relaxation processes, and differences of Arrhenius parameters for α_Ac and α_c relaxation processes obtained from Cole-Cole and Havriliak-Negami equations were discussed in detail. Activity energy of different relaxation processes obtained from Arrhenius equation and VFT equation indicates non-single thermal activation mechanism for hot pressed PVDF film.     

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al_0.25Ga_0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (T_e) of hot electrons was obtained from the lattice temperature (T_L) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range T_e > 100 K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al_0.25Ga_0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.     

Dielectric property of CaCu3Ti4O12 thin film grown on Nb-doped SrTiO3(100) single crystal

Thin film of CaCu₃Ti₄O₁₂ (CCTO) has been deposited on Nb-doped SrTiO₃(100) single crystal using pulsed laser deposition. The dielectric constant and AC conductivity of CCTO film in the metal–insulator–metal capacitor configuration over a wide temperature (80 to 500 K) and frequency (100 Hz to 1 MHz) range have been measured. The small dielectric dispersion with frequency observed in the lower temperature region (<300 K) indicates the presence of small defects in the deposited CCTO thin film. The frequency-dependent AC conductivity at lower temperature indicates the hopping conduction. The dielectric dispersion data has been analyzed in the light of both conductivity relaxation and Debye type relaxation with a distribution of relaxation times. Origin of dielectric dispersion is attributed to the distribution of barrier heights such that some charge carriers are confined between long-range potential wells associated with defects and give rise to dipolar polarization, while those carriers which do not encounter long-range potential well give rise to DC conductivity.     

布居分支比模型——一种定性分析上转换效率的有效方法

通过对在上转换的过程中起关键作用的是发动上转换的关键亚稳态的动力学因素的分析,建立了在连续激光激发下的定性分析上转换的“布居分支比β”物理模型：即上转换可以由关键亚稳态上的布居再次被激发至更高激发态的分支比β来描述;其特别的物理意义在于这种方法易于从总体上对上转换的性能有全面的定性的掌握.还进一步论证了对于Er³⁺离子步进吸收上转换,石英、五磷酸盐等截止声子能量大于1000cm^-1的材料的上转换发光约为氟化物的万分之一的原因是它们的多声子无辐射弛豫速率W_关键词：     

Physical properties of high performance fluoride ion conductor BaSnF4 thin films by pulsed laser deposition

This article presents the results on the growth and characterization of BaSnF₄ thin films on glass substrates prepared by pulsed laser deposition technique. The structural results of BaSnF₄ thin film carried out by glancing angle X-ray diffraction technique indicates the formation of the film with similar structure (tetragonal, P₄/nmm) to the bulk target material. The absorption coefficient and band gap of the film is determined by suitable analysis of the transmittance spectra. The transport properties of the thin films are studied using impedance spectroscopy in the temperature range of 323–573 K. The frequency-dependent imaginary part of impedance plot shows that the conductivity relaxation is non-Debye in nature. The scaling behavior of the imaginary part of impedance at various frequencies indicates temperature-independent relaxation behavior.     

Nuclear-spin-lattice relaxation in the organic superconductor (BEDT-TTF)2Cu[N(CN)2]Br0.8Cl0.2

The behavior of the spin-lattice relaxation rate of ¹H nuclei in the organic superconductor (BEDT-TTF)₂Cu[N(CN)₂]Br_0.8Cl_0.2 in the temperature interval 12–319 K is investigated. A relaxation rate maximum, which depends on the resonance frequency and is associated with thermally activated vibrations of the end ethylene groups in the BEDT-TTF molecules, is observed in the vicinity of 270 K. The relaxation rate is observed to deviate from Korringa behavior at T<100 K. These deviations can be attributed to the existence of strong antiferromagnetic spin correlations. Fiz. Tverd. Tela (St. Petersburg) 39, 39–41 (January 1997)     

Study of dipole dynamics and pre-transitional effects at isotropic to nematic phase transition by low-frequency dielectric relaxation measurements

Thermal microscopy (TM) as well as low-frequency (LF) dielectric relaxation studies are carried out on a nematic liquid crystal (NLC), viz E7 (Merck Ltd, UK). The isotropic to nematic (IN) transition temperatureT _IN determined by TM and LF-dielectric permittivity measurements agrees with the available data. Dielectric loss studies in the frequency region of 5–10?MHz indicate a relaxation (in the kHz region) akin to Debye type off-centered dispersion. The observed nematic relaxation is found to correspond to reorientation (about the short axis) of the nematic dipole to the external field. The temperature variation of the nematic relaxation frequencyf _R is found to follow an Arrhenius shift, with an activation energy of 1.7?eV. Temperature variation of the dielectric strength (Δε = ε _o ? ε_∞) and the distribution parameter α in the nematic phase are discussed. The dynamic response of the nematic dipoles and growth of pre-transitional fluctuations are found to be nonlinear in the vicinity of the IN transition. The value of the exponent α_eff = 0.072 indicates weak growth of transitional fluctuations across the IN transition.     

Infrared absorptions and band widths of dilute solutions of CF3H in liquid Ar,N2, and Xe

The spectra of fluoroform (CF₃H) in the solvents Ar, N₂, and Xe have been obtained in the fundamental region (400–4000cm^?1) using a low temperature cryostat and a Fourier transform infrared spectrophotometer. Ab initio calculations at the HF/6-31G? level have been performed to obtain the calculated vibrational frequencies of the isolated CF₃H molecule and CF₃H in the presence of the solvents (Ar, N₂, and Xe). Comparison of the frequency shifts of CF₂H in solution with respect to the gas phase frequencies is made for the experimental and theoretical results. Lorentzian functions were used to fit the bands and obtain the wavenumber at the peak absorbance and the vibrational band widths. An analysis of the dynamics of relaxation has been made based on the infrared time correlation functions for three of the fundamental modes (ν₁, ν₃, and ν₄). Bandwidths, band moments, and relaxation times were obtained by appropriate fitting of the experimental correlation functions to theoretical models. In liquid argon, the temperature dependence of the second moment (M ₂) indicates that rotational relaxation explains the bandwidth of the ν₃ mode. For the ν₄ mode, the temperature dependence of M ₂ can be attributed to rotational relaxation if it is corrected with a Coriolis coupling term. The bandwidths of the ν₁ mode do not follow the rotational relaxation model, and probably vibrational relaxation is the dominant mechanism.     

Spin dynamics in the impurity-doped one-dimensional heisenberg paramagnet (CH3)4NMn(1-x)CuXCl3: Nuclear magnetic relaxation

Proton spin-lattice relaxation rate has been measured at room temperature in the impurity-doped (CH₃)₄NMn_(1-x)Cu_xCl₃ for X = 0.04, 0.1 and 0.17. The result for Hz. snfc;chain-axis as a function of resonance frequency clearly shows that the spectral density of the spin fluctuations in the impure system remains to have the characteristics 1-D diffusive term (ω^{-$\text{1}\text{2}$}), with a slower rate of the spin diffusion in accordance with the theory by Richards. The result for H⊥ chain-axis indicates, however, the existence of a singularity of the fluctuations near ω = 0.     

Dielectric relaxation in YTTRIA-doped Ceria solid solutions

The nature of the defects present in Y₂O₃-doped CeO₂ is explored by means of dielectric relaxation, employing primarily the thermally-stimulated depolarization current (TSDC) technique. For Y₂O₃ concentrations ? 1 mole %, two relaxation peaks are observed. The lower-temperature peak shows a dielectric relaxation rate, τ^?1 which is $\text{1}\text{2}$ the relaxation rate of the corresponding anelastic relaxation. This proves that the peak is due to nn, 〈111〉 oriented, (YV_O) pairs, where V_O represents the oxygen vacancy. These pairs are positively charged defects which are compensated by an equal number of isolated Ý. The upper peak is a broad peak whose position varies with Y₂O₃ concentration in the same manner as the activation enthalpy for the ionic conductivity. The peak is not due to simple dipoles, but to relaxation of the array of alternately charged (YV_O) and Ý defects by the redistribution of oxygen-ion vacancies. A simple model, in which these defects form a superlattice containing wrong pairs, explains the essential features of the upper peak.     

Neodymium luminescence in chalcogenides of europium-gallium: EuGaS4 and EuGa2Se4

Photoluminescence of chalcogenides of europium-gallium, EuGa₂S₄ and EuGa₂Se₄, doped with neodymium is investigated. The positions of Stark levels are determined from the spectra. The symmetry of luminescence centres is shown to be lower than cubic and the existence of nonequivalent centers is established. At 77 K the decay time of luminescence from the excited levels of Nd³⁺ depends on the spin of the states. That indicates a slow relaxation rate in the crystals under investigation. It is probable that these crystals can be used as effective luminophores.