Thermal hardening of Ni3Ge alloy single crystals with an L12 superstructure at low temperatures

The mechanical properties and the dislocation structure of Ni₃Ge alloy single crystals have been experimentally studied at low temperatures. It is found that the flow stresses increase beginning with 4.2 K, and the observed rise in the stresses depends on the orientation of the strain axes of the crystals. The dislocation structure is investigated thoroughly. It is revealed that the mean density of dislocations and the interdislocation interaction parameter α anomalously increase as the temperature increases in the range 4.2–293 K. The mechanisms providing an explanation for the temperature anomaly of flow stresses and the α parameter are considered. The activation energy of thermal hardening is evaluated. It is assumed that the low activation energies of thermal hardening are due to the motion of dislocations at velocities close to the velocity of sound at these temperatures.     

匹配垂直质点振速的浅海地声参数反演

海底声学参数对海洋波导中的声场特性研究和相关应用具有重要意义。针对一次夏季黄海声传播实验,分析了浅海负跃层环境下垂直质点振速的传播特性和简正波结构,说明当声源和接收器均位于负跃层下时,除海底附近外的大部分深度上垂直质点振速能量较高,且与声压相比,号数高的简正波对垂直质点振速的贡献更大,利用垂直质点振速进行匹配场反演能获得更高的海底参数敏感性。分析了海底吸收系数对垂直质点振速匹配场反演的影响,结果表明只有当进行匹配场反演时设置的海底吸收系数接近真实值时,才能获得准确的海底声速、密度和海深反演结果。利用实验中矢量水听器获取的垂直质点振速信号进行匹配场反演,将海底吸收系数在变化范围内取不同值对海底声速、密度和平均海深进行全局搜索,根据代价函数值最大确定了海底声速、密度及平均海深的反演结果,并利用不同距离上的声压传播损失反演出不同频率下的海底吸收系数。根据反演得到的海底声学参数计算声压传播损失,与实验中声压水听器测量结果符合较好。     

高压下固态Kr弹性性质、电子结构和光学性质的第一性原理计算

采用密度泛函理论中平面波基矢,模守恒赝势结合局域密度近似以及广义梯度近似对固态Kr在高压下的结构以及弹性性质进行了研究, 通过计算发现弹性常数,Debye温度以及声速都随压力的增大而增大,所计算的弹性常数与实验和其他的理论符合的很好. 利用Debye模型得到了固态Kr的热力学性质, 熵随压力的增大而减小,随温度升高而升高;而定容热容C_v,定压热容C_p则随温度升高而升高,而且C_v在达到一定温度时趋于定值,所得的热力学性质和实验值是相符的．最后还预测了固态Kr在高压下的电子结构和光学性质, 计算结果表明随压力的增加固态Kr的前沿能带变窄,光吸收系数增大,吸收峰增宽,电子更容易发生跃迁,固态Kr有可能转化为半导体． 关键词： Kr 第一性原理 弹性常数 光学性质     

On the velocity of sound in an itinerant-electron antiferromagnet

The longitudinal sound velocity in an itinerant-electron antiferromagnet is studied, using a jellium model for the ions and a two-sub-band model for the itinerant electrons with an energy gap caused by the antiferromagnetic spin ordering. It is shown that the longitudinal sound velocity decreases with decreasing temperature, due to the electron-photon interaction. The exchange interaction between itinerant electrons is seen to play a particularly important role in the phonon softening phenomena for such a system; and although the model is a great simplification of reality it gives reason to believe that the observed elastic anomalies in the face-centred-cubic 3d transition metal alloys are directly related to their tendency towards antiferromagnetism.     

脉冲激光辐照硅材料引起表面波纹的特性研究

叙述了激光与材料相互作用过程中引起相干受激光散射的机制,以及形成材料表面波纹的特性。在激光波长1．06μm、能量15mJ、光斑直径2mm、脉冲半峰全宽约10ns和入射方向为布儒斯特角的条件下,进行了脉冲激光辐照硅材料形成表面波纹的实验研究。在脉冲激光辐照硅材料表面功率密度略大于材料损伤阈值的条件下,发现了硅材料表面形成的平行等间距直线条纹结构。用光学显微镜和原子力显微镜分别测量了被辐照硅材料表面的波纹形貌特征。在假设硅材料表面波纹的产生与声波在材料中的传播速度有关的条件下,由声波传播速度和激光辐照硅材料的脉冲宽度较好地解释了材料表面形成条纹的宽度．并认为在形成表面波纹的过程中,热应力起主要作用。     

Asymmetric nuclear matter and Skyrme potential

The binding energy, symmetry energy, pressure, incompressibility, and the velocity of sound are calculated for asymmetric nuclear matter using Skyrme interaction SkO’. The behavior of these physical quantities is studied for different values of the asymmetry parameter α _τ , the density ρ, and the temperature T. Good agreement is obtained in comparison with previous theoretical estimates and experimental data. The text was submitted by the authors in English.     

Density of vibrational states of a hyperquenched glass

The vibrational density of states of a hyperquenched and an annealed glass has been measured using nuclear inelastic scattering. The hyperquenched sample shows a higher number of vibrational states in the low-energy region with respect to the annealed glass. It reveals, however, lower density and sound velocity and, therefore, smaller Debye energy. After rescaling the energy axes in Debye energy units and area renormalization, the density of states of both samples becomes identical. Thus, the effect of quenching is described by the transformation of the continuous medium.     

Ab initio calculation of elastic properties of rock-salt and zinc-blend MgS under pressure

The elastic properties of zinc-blend (ZB) and rock-salt (RS) MgS are calculated by ab initio method. The calculation shows that the enthalpy for RS structure and for ZB structure essentially is the same at ambient pressure. The ZB structure becomes unstable above 5 GPa. For these two structures, the pressure dependences of typical elastic properties, i.e. the bulk modulus, the shear modulus, the Young's modulus, the Poisson's ratio, and the anisotropy factor, are presented. The Debye temperature and sound velocity under high pressure have also been calculated. Debye approximation is used to estimate the zero-point vibrational energy.     

“Slow” wave of zero-sound in degenerate Fermi gas

The dynamic Hartree-Fock theory with point-like interaction is used to calculate the speed of sound and damping factor of a zero-sound wave propagating in a degenerate Fermi gas. This wave propagates slower than Fermi velocity. It is shown, that if the interaction is weak and density is small, then the damping of such a wave can be small. A possibility of discovering such waves in ultracold Fermi gases is discussed.     

Spectral analysis of the flow sound interaction at a bias flow liner

The interaction between the flow field and the sound field is responsible for the sound absorption at perforated acoustic liners with bias flow and has to be investigated contactlessly. Based on the optically measured flow velocity spectrum, an energy analysis was performed. As a result, the generation of broadband flow velocity fluctuations in the shear layer surrounding the bias flow caused by the flow sound interaction has been observed. In addition, the magnitude of this acoustically induced flow velocity oscillation exhibits a correlation with the acoustic dissipation coefficient of the bias flow liner. This supports the assumption that an energy transfer between the flow field and the sound field is responsible for the acoustic damping.     

BCS and BEC p-wave pairing in Bose-Fermi gases

The pairing of fermionic atoms in a mixture of atomic fermion and boson gases at zero temperature is investigated. The attractive interaction between fermions, that can be induced by density fluctuations of the bosonic background, can give rise to a superfluid phase in the Fermi component of the mixture. The atoms of both species are assumed to be in only one internal state, so that the pairing of fermions is effective only in odd-l channels. No assumption about the value of the ratio between the Fermi velocity and the sound velocity in the Bose gas is made in the derivation of the energy gap equation. The gap equation is solved without any particular ansatz for the pairing field or the effective interaction. The p-wave superfluidity is studied in detail. By increasing the strength and/or decreasing the range of the effective interaction a transition of the fermion pairing regime, from the Bardeen-Cooper-Schrieffer state to a system of tightly bound couples can be realized. These composite bosons behave as a weakly-interacting Bose-Einstein condensate.     

The steady state growth rate of a recrystallization front in regions of heterogeneous dislocation density

The paper investigates whether a change from a homogeneous to an inhomogeneous dislocation distribution, assumed to be caused by a slight additional deformation, can lead to an increase of the recrystallization temperature of a deformed metal. In this case, the higher temperature would indicate a more stable deformation structure despite the increase of stored energy. The recrystallization temperature is related to the growth rate. Hence, the steady state velocity of a recrystallization front moving either parallel or vertically to the stripes of a simplified two-dimensional heterogeneous dislocation distribution of parallel sections of higher and lower dislocation densities is calculated. The results show that if a front growths through the high and low density sections in series an overall slower rate despite higher mean dislocation density is, indeed, possible. However, growing in the parallel arrangement always leads to a higher growth rate compared with the homogeneous case of slightly less stored energy. Since in a real structure the faster growth is likely to succeed, the recrystallization temperature observed will be lowered with additional deformation in accordance with experimental experience.     

The universal sound velocity formula for the strongly interacting unitary Fermi gas

Due to the scale invariance,the thermodynamic laws of strongly interacting limit unitary Fermi gas can be similar to those of non-interacting ideal gas.For example,the virial theorem between pressure and energy density of the ideal gas P=2E/3V is still satisfied by the unitary Fermi gas.This paper analyses the sound velocity of unitary Fermi gases with the quasi-linear approximation.For comparison,the sound velocities for the ideal Boltzmann,Bose and Fermi gas are also given.Quite interestingly,the sound velocity formula for the ideal non-interacting gas is found to be satisfied by the unitary Fermi gas in different temperature regions.     

纳米通道滑移流动的分子动力学模拟研究

本文采用非平衡分子动力学方法对平板纳米通道滑移流动进行了非平衡分子动力学模拟,获得了不同壁面势能和不同温度时流体的速度分布及密度分布。研究结果表明滑移速度在很大程度上决定于流体温度和壁面吸引力作用强度的大小。由于不同壁面吸引力时流体的密度分布受温度的影响规律不同,使得不同壁面吸引力时流体的滑移速度受温度影响规律也不一致。而且,流体结构受壁面流速的影响要受到壁面势能的制约。     

On the connection between energy velocity, reverberation time and angular momentum

The decay of a steady acoustic field in an enclosure is studied both theoretically and experimentally. Our main result is that the initial part of any local sound decay is driven by an exponential function of time whose rate constant is equal in modulus to the inverse of the mean energy velocity divergence. This is empirically demonstrated by experimental analysis of both 1-D and 3-D case studies, thus showing that the reverberation time is strictly connected with the sound energy velocity field and can be determined from its differential properties. A further property of the mean energy velocity is found: it is related not only with the reverberation time, but also with the angular momentum density and with the non-uniform distribution of energy.     

Sm atomic dynamics in a charge density wave compound SmNiC2

¹⁴⁹Sm nuclear resonant inelastic scattering was carried out in a charge density wave compound SmNiC₂. We have investigated temperature dependences of the Sm partial phonon density of states and recoil-free fraction at the Sm site and the average sound velocity estimated from the Sm partial density of states. The Sm partial density of states exhibits temperature dependence, suggesting that the phonon modes between 20 and 25 meV may correlate with the charge density wave. Temperature dependence of the recoil-free fraction is difficult to prove the correlation with either the charge density wave or ferromagnetic ordering. The average sound velocity obtained by the Sm partial phonon density of states exhibits temperature dependence, agreeing qualitatively with very recent elastic constant measurements.     

The role of the induction zone on the detonation–turbulence linear interaction

Detonation–turbulence linear interaction analysis extends the non-reactive shock–turbulence analog by considering geometrical scaling of the noise with respect to the half-reaction distance. The analysis emphasizes the effect of structure in energizing selective frequencies, and determining acoustic amplification in the farfield. Natural frequencies are determined as eigenvalues of the inviscid non-forced interaction problem. They modify postshock energy spectra by supporting resonant amplification, and cast light on the role of the activation energy on the detonation–turbulence interaction. Detonations with higher activation energies amplify smaller scales by resonant amplification. An analysis of the bifurcation parameters reveals a strong link between detonation overdrive and acoustic attenuation. The damping is correlated with the subcritical nature of the characteristic solutions for high overdrives. For detonation conditions on the stability boundary, a larger overdrive supports a weaker resonant peak in both the temperature and longitudinal velocity spectra. Postshock temperature variances feature a well-defined maximum within the reaction zone, which is found to be sensitive to changes in detonation structure.     

海底沉积物压缩波声速比与物理特性的关系

海底沉积物因组成结构复杂导致声学特性复杂多样,多种理论解释模型存在参数较多且各具适用性现象·通过运用体积平均和等效参数的方法对海底沉积物声波传播机理进行研究,提出一种少参数的海底沉积物与底层海水的压缩波声速比通用模型(General Model of Sound Speed Ratio,GMSSR).GMSSR模型包含弹性结构分布因子、孔隙度、等效密度比、等效弹性模量比4个物理特性参数。分析海底沉积物两相介质结构的串联和并联两种极限情况分析,基于弹性结构分布因子和串并联结构的体积平均分布建立一般情况下海底沉积物的等效弹性模量表达式·应用GMSSR合理地分析各海域测量的海底沉积物声速比经验模型的共性和解释南海实测数据分散的范围,表明:(1)表层海底沉积物主要是以串联结构为主,接近于悬浮液状态或者体积分量很少的并联堆积状态;(2)等效弹性模量比的影响因素大于等效密度比,结构变化往往通过影响弹性结构分布因子而影响等效弹性模量比,从而引起声速比的变化;(3)不同研究海域的声速比-孔隙度经验关系具有相似性,可以通过GMSSR模型分析得出;(4)基于弹性结构分布因子的差异,可以合理解释南海海域测量表层海底沉积物声速比较大的分散性。      

Sound velocity measurements of nuclear-ordered solid 3He along the melting curve

We have measured the temperature dependence of the 10.98 MHz longitudinal sound velocity of solid 3He in the nuclear-ordered U2D2 phase and in the paramagnetic phase along the melting curve. The temperature dependence of the sound was attributed to the contributions from the nuclear spin system and the molar volume change along the melting curve. The sound velocity increased with temperature as T4 in the U2D2 phase and the sound anisotropy due to the exchange interaction was found to be about 10% among single-domain crystals investigated. The average value of the Gruneisen constant of the spin wave velocity in the ordered phase was gamma(c) = 16 and is compared to the calculated value of the multiple-exchange model.     

Sound velocity change in doped displacive ferroelectrics

An expression is obtained for the sound velocity change in doped displacive ferroelectrics, in the presence of an electric field by considering higher order anharmonic terms in the modified Silverman-Joseph Hamiltonian. The mass and force constant changes due to impurity may give cancellation effects. The presence of higher order anharmonicity decreases the sound velocity which in turn varies with the applied electric field. The sound velocity decreases anomalously near the Curie temperature.