Effect of Critical State Model on Magnetic Relaxation in Type-II Superconductors

Based on the theoretical model in which magnetic relaxation was considered, exact equations are derived for the relaxation rate, normalized relaxation rate and relaxation time. The relations between the theory on relaxation rate and experimental data are compared, as well as the relaxation time. The valid ranges of several models are specified and the magnitude of relaxation time is estimated. The results indicate that different models are suitable for different temperature regions and the effect of relaxation time on the experimental measurements could not be neglected.     

Stress relaxation in free-standing aluminum beams

The stress-relaxation behavior of 2-μm-thick free-standing thin-film aluminum beams was evaluated by using a piezoelectric-actuated mechanical tester, which subjected them to quasi-static microtensile stress relaxation tests. The microstructure of the Al beams was modified in two ways: by annealing at different temperatures or by alloying with 1.5 at.% Ti. The features of the relaxation curves, such as stress recovery after unloading, suggest an anelastic relaxation behavior. Based on the anelastic relaxation model, the relaxation parameters were estimated from these relaxation curves and samples of various microstructures were compared. The results show that the relaxation strength and relaxation time change markedly with a decreasing grain size. It was also found in Ti-alloyed samples that the presence of the precipitates along the grain boundaries appears to suppress the relaxation significantly. In addition, the measured relaxation time was found to be comparable with the value predicted by the grain boundary sliding model. As such, we conclude that the grain boundary sliding is the dominant relaxation mechanism in free-standing thin films.     

Spin relaxation in InGaN quantum disks in GaN nanowires

The spin relaxation time of photoinduced conduction electrons has been measured in InGaN quantum disks in GaN nanowires as a function of temperature and In composition in the disks. The relaxation times are of the order of ～100 ps at 300 K and are weakly dependent on temperature. Theoretical considerations show that the Elliott-Yafet scattering mechanism is essentially absent in these materials and the results are interpreted in terms of the D'yakonov-Perel' relaxation mechanism in the presence of Rashba spin-orbit coupling of the wurtzite structure. The calculated spin relaxation times are in good agreement with the measured values.     

Giant dielectric constant in titania nanoparticles embedded in conducting polymer matrix

Complex impedance and dielectric permittivity of titania-polypyrrole nanocomposites have been investigated as a function of frequency and temperature at different compositions. A very large dielectric constant of about 13,000 at room temperature has been observed. The colossal dielectric constant is mainly dominated by interfacial polarization due to Maxwell-Wagner relaxation effect. Two completely separate groups of dielectric relaxation have been observed. The low frequency dielectric relaxation arises from surface defect states of titania nanoparticles. The broad peak at high frequency region is attributed to Maxwell-Wagner type polarization originating from the inhomogeneous property of nanocomposite. An abrupt change in grain boundary conductivity and dielectric relaxation associated with titania was observed at around 150 K. Anomalous behavior in conductivity and dielectric relaxation is qualitatively explained by band tail structure of titania nanoparticle.     

聚合物表面分子松弛行为研究进展

聚合物表面的松弛行为是高分子凝聚态物理研究领域中的一个热点问题,位于表面的高分子具有与本体分子截然不同的运动行为特点,从而赋予了高分子表面许多特殊性质,如特殊的环境响应性和表面粘弹性。文中综述了最近二十年关于聚合物表面分子松弛行为的研究进展,包括基于各种表征技术的研究方法、聚合物表面分子松弛行为的特点和影响因素。可以预测,随着新表征技术的开发应用,对聚合物表面分子松弛行为的研究将越来越深入。     

Control of exciton spin relaxation by electron-hole decoupling in type-II nanocrystal heterostructures

The electron spin flip relaxation dynamics in type II CdSe/CdTe nanorod heterostructures are investigated by an ultrafast polarization transient grating technique. Photoexcited charge separation in the heterostructures suppresses the electron-hole exchange interaction and their recombination, which reduces the electron spin relaxation rate in CdSe nanocrystals by 1 order of magnitude compared to exciton relaxation. The electron orientation is preserved during charge transfer from CdTe to CdSe, and its relaxation time constant is found to be approximately 5 ps at 293 K in the CdSe part of these nanorods. This finding suggests that hole spin relaxation determines the exciton fine structure relaxation rate and therefore control of exciton spin relaxation in semiconductor nanostructures is possible by delocalizing or translating the hole density relative to the electron.     

Nuclear Spin Relaxation of Powdered Metallic Antimony in Liquid 3He

Nuclear spin relaxation studies have been carried out to determine the surface relaxation of powdered Sb immersed in liquid ³ He. The surface relaxation mechanism was observed to be significantly more effective than the bulk Korringa relaxation rate for temperatures T < 75 mK. This surface relaxation is attributed to the modulation of the dipole-dipole interaction between the Sb spins at the metal surface and the ³ He spins in the solid-like layer on the surface by the quantum zero-point motion of these ³ He atoms. The total relaxation has been analyzed in terms of the surface relaxation and the spin diffusion through the bulk of the Sb particles. The experimental results are in excellent agreement with the theory developed for this system. Replacement of the ³ He by ⁴ He removes the surface relaxation component and restores the relaxation rates to the bulk value.     

CaCu3Ti4O12中的晶界弛豫

研究了CaCu3Ti4O12在低温低频下的内耗曲线特征,发现金属中来源于晶界的内耗规律也适合CaCu3Ti4O12材料.测量了晶界弛豫激活能的大小和不同温度下该材料的特征弛豫时间.与常温的情况相比,低温下CaCu3Ti4O12的特征弛豫时间明显增加.分析表明:这种弛豫时间的增加来源于在畴区上极化弛豫的动态慢化效应.     

Volumetric relaxation in simple liquid: Molecular dynamics simulation

The example of a mixture of simple liquids with Lennard-Jones potentials and soft spheres is used for considering the general principles of relaxation of energy in dense media. The processes of relaxation to uniform velocity distribution are studied, and the main stages of relaxation from nonisothermal state are identified. The dependences of characteristic relaxation times on parameters of the medium such as the density, initial temperature, and the mass ratio of components of the mixture are obtained. The results are compared with the data on nonideal plasma for determining the importance of the concrete interaction potential.     

Stress relaxation phenomena in vegetable tissue

The stress relaxation in some compressed vegetable fleshes, i.e. potato and kohlrabi tubers, and carrot and parsley roots, has been investigated. The relaxation curves obtained with different prestrain parts (different stress level and different prestrain rate) are approximated in the second part by simple equations, i.e. logarithmic and power equations. The similarity of these equations is demonstrated using similarly defined parameters: the initial slope of the relaxation curve and the time factor. The influence of prestrain on the strain relaxation is shown by the probability density of the decay processes. The activation volume was determined by two different methods: analysis of relaxation curves or/and by push change of the deformation rate. The second method gives systematically higher values than the first one. The activation volume strongly decreases with increasing stress, but only small differences in activation volumes were observed for different vegetable fleshes. The activation volumes between 50 and 110 nm^–3 are in a good agreement with the pore volume in the cell walls. The results obtained agree with the hypothesis of the controlling role of squeezing out of the cellular sap for stress relaxation in vegetable flesh.     

Phase-slip states in the normal-superconducting relaxation of current-biased microstrips

We have solved the generalized time-dependent Ginzburg-Landau (TDGL) equations of Kramer and Watts-Tobin numerically. The solutions are used to predict the relaxation behavior of a microstrip biased below its dc critical current and excited into the resistive state by the application of a supercritical current pulse of a fixed duration. We have studied the relaxation behavior of the gap parameter and voltage along the microstrip as a function of the current pulse amplitude and bias magnitude. The relaxation is found to occur through a succession of phase-slip oscillations. The relaxation behavior is determined by the initial phase-slip state created by the current pulse. The maximum resistance attained increases rapidly, then saturates at a value less than the normal state resistance. The numerical solutions exhibit a region in which a relaxation time anomaly occurs with respect to the current pulse amplitude. In this region, the more highly excited states were found to relax faster. These features of the resistive relaxation behavior are found to be in qualitative agreement with our previous experimental measurements.     

高聚物粘弹性力学模型计算中容易被忽视的一个基本问题

在应用粘弹性力学模型模拟线形高聚物应力松弛现象时,不能忽视一个基本问题:在维持总形变不变的情况下,总应力是在逐渐衰减的,即应力不是恒定值。本文对高聚物力学松弛及粘弹性的本质进行了讨论。     

Thermomechanical relaxation of residual stress in shot peened nickel base superalloy

Abstract

The thermal and thermomechanical behaviour of the relaxation of the residual stresses of a shot peened Astroloy superalloy under tensile cyclic loads has been evaluated by X-ray diffraction and investigated. The stress relaxation under purely thermal conditions (550 and 650°C) and thermomechanical conditions (pulsating tensile loading at 650°C) as afunction of the exposure time is presented. The purely thermal relaxation is interpreted by annihilation and reorganisation of the crystalline defects induced by shot peening, whereas the mechanical relaxation is linked to cyclic plasticity of materials. In consequence, the thermomechanical relaxation is essentially due to the complex mechanism of the concurrent thermal and mechanical effects. A model is used to predict the residual stresses induced by the specified shot peening conditions and their relaxation under the specified thermal/thermomechanical conditions.

MST/1963     

Level recovery curve in the relaxation theory of filtration

A problem on the level recovery curve in the relaxation theory of filtration is considered when there is a continuous spectrum of internal relaxation times. An asymptotics at large times is found as a functional of a relaxation kernel. An explicit expression with two additional parameters characterizing the relaxation kernel is calculated for a power spectrum of internal relaxation processes in a rock-saturating fluid system.     

Structural relaxation kinetics in FeSiB amorphous alloys

Structural relaxation kinetics in the FeSiB amorphous systems are studied on the basis of the activation energy spectrum model, through electrical resistivity variation measurements. The silicon content slightly modifies the features of the spectra, and a quite homogeneous distribution in energies for the processes leading to relaxation is obtained. Thermal stability against structural relaxation and crystallization is discussed.     

Relaxation in dispersion-hardened alloys by volume diffusion

Since elastic inhomogeneities are included in dispersion-hardened alloys, uniform applied stresses are disturbed in the materials. When the applied stresses have non-zero hydrostatic components, relaxation by long-range volume diffusion occurs in the dispersion-hardened alloys at high temperatures. The relaxation by the volume diffusion is discussed by taking into account the change in the Gibbs free energy of the material. The stress state inside a dispersed particle after the complete relaxation and the relaxation time to reach the equilibrium state are derived in a physically clear manner. The relaxation time obtained is compared with previous calculations by other investigators and the origins of the differences are discussed.     

Spin Dynamics in ZnO-Based Materials

In this work, we address the issue of spin relaxation and its relevance to spin detection in ZnO-based materials, by spin-polarized, time-resolved magneto-optical spectroscopy. We have found that spin relaxation is very fast, i.e. about 100 ps for donor bound excitons in wurtzite ZnO, despite of a weak spin–orbit interaction. We also reveal that alloying of ZnO with Cd enhances spin relaxation, prohibiting ZnCdO/ZnO structures for efficient optical spin detection. On the other hand, a variation in strain field induced by lattice mismatch with substrates does not seem to lead to a noticeable change in spin relaxation. The observed fast spin relaxation, together with the limitation imposed by the band structure, are thus identified as the two most important factors that limit the efficiency of optical spin detection in the studied ZnO-based materials.     

Influence of residual stresses in the stress relaxation of cold drawn wires

This study demonstrates the influence of residual stresses in steel wires on the stress relaxation losses. Standard stress relaxation tests were performed on four types of wire, all with the same mechanical properties but with different residual stresses. Surface residual stresses were measured by X-ray diffraction. The experimental results show that stress relaxation losses decrease as the value of surface tensile residual stresses decrease. The role, sometimes controversial, of initial pre-stretching and heat treatments on stress relaxation losses can also be understood in the light of the residual stresses induced during cold-drawing.     

Nuclear spin-lattice relaxation rate in Kondo superconductors in the gapless state

The host nuclear spin-lattice relaxation rates in Kondo superconductors in the gapless state are derived within the framework of the Matsuura, Ichinose, and Nagaoka (MIN) theory of the Kondo effect. The relaxation rates for both low-T _K and high-T _K Kondo superconductors are obtained. The temperature dependence of the relaxation rate in low-T _K Kondo superconductors in the gapless state arising from the pair breaking by the Kondo impurity is obtained. The dependence of the relaxation rate for high-T _K Kondo superconductors in the gapless state on the external magnetic field producing the gapless state is also obtained.