Temperature and pressure behaviour of narrow-gap semiconductors including galena

The structural high pressure and temperature investigation of narrow-gap semiconductors (lead chalcogenides) has been performed in the present article. A realistic approach for room temperature and high temperature study of narrow-gap semiconductors has been used. It is examined that the present compounds are more stable in NaCl-phase and they transform to CsCl-phase at high pressure. In the present article, the phase transition pressures and volume collapses of lead chalcogenides have been investigated at room and high temperatures. Phase transition pressures have been reported at high temperature range from 0 to 1200 K. Elastic and anharmonic constants have also been reported at room temperature. A structural study of the narrow-gap semiconductors have been carried out using the realistic model including temperature effect. The temperature and pressure behaviour of elastic constants for the present compounds have also been discussed. Furthermore, various mechanical and thermo dynamical properties like modulus of elasticity, Debye temperatures etc. are also presented.     

On the superconductivity in β-beryllium and related phases

Two high temperature beryllium allotropes have been stabilized at room temperature through the addition of transition elements. Superconductivity has been observed in almost all of them. Beryllium films reported superconducting in the past appear in all probability to require the presence of transition element impurities, in order to show the phenomenon.     

提拉生长铌酸锶钠锂晶体的组分过冷及提高晶体品质的研究

本文采用厚晶体抛光、腐蚀、显微观察的方法研究提拉法生长晶体的组分过冷。在高温偏光下研究了铌酸锶钠锂晶体的结构转变温度。采用最佳的生长和工艺条件克服了铌酸锶钠锂晶体的组分过冷和内部生长条纹,提高了晶体的光学均匀性。 关键词：     

EuMo_6S_8的超导性、电荷密度波和磁性间的相互影响

我们已观察到在120K时Eu_(1.2)Mo_6S_8的热容量、电阻及霍尔系数的反常规象,并确定为一种马氏体结构的相变特征;这种马氏体结构相变产生于一种模糊的电荷密度波的转变,后者在部分费米面上造成能隙。 费米面出现部分能隙说明了在常压下直10mK不出现超导的原因。高压抑制了电荷密度波的转变。从费米面上状态的超导性和这种转变相互竞争出发,计算得到的超导转变温度对压力的依赖性,与实验结果非常一致。在低温时,临界场随温度的变化非常强烈,这种变化已用Jaccarino-Peter补偿效应进行了解释。在高压时与理论的预言比较,EuMo_6S_8的临界场非常高,至少与现有任何的超导体一样高。压力为14Kbar、温度低于2K时在10T到15T范围内已观察到低场下电阻态到超导态的转变,这就是称之为场诱导超导性的一种新效应。     

Competition between thermal fluctuations and disorder in the crystallization of 4He in aerogel

The dynamical transition in the crystallization of 4He in aerogel has been investigated by direct visualization and dynamical phase diagrams have been determined. The crystal-superfluid interface in aerogel advances via creep at high temperatures and avalanches at low temperatures. The transition temperature is higher at a higher interface velocity and lower in higher porosity aerogels. The transition is due to competition between thermal fluctuations and disorder for the crystallization process.     

利用中值检测方法研究近50年中国极端气温变化趋势

利用中国194个站点1957—2001年的逐日温度观测资料和月平均温度资料,分别采用中值和均值两种极端事件检测方法,检测并比较了中国极端高温事件和极端低温事件的变化趋势和空间分布特征.检测表明,年均极端高温的次数在近50年中趋于上升,而年均极端低温的次数有所减少,这与目前观测结果一致,符合全球变暖的特点.在空间分布上,除西南地区部分站点外,近50年中国大部分地区极端低温事件的年均发生次数趋于减少,而极端高温事件发生频率的变化则呈现出东南沿海地区减少、西北内陆地区增加的分布特点. 关键词： 极端事件 全球增暖 变化趋势 中值     

金刚石和石墨之间相转变几率的研究

 以势垒渡越模型为基础，对石墨和金刚石的相转变几率在压强-温度（p-T）图上的分布进行了研究。从理论上得到了金刚石和石墨的亚稳态相区边界，理论计算得到的相转变几率分布与文献报道的相转变速率的实验结果具有相同的规律。高温高压法合成金刚石的p-T特征程序可以用相转变几率的分布来解释。     

Pressure-induced polymorphism in enstatite (MgSiO3) at room temperature: clinoenstatite and orthoenstatite

The phase transition of a synthetic clinoenstatite in a diamond-anvil cell has been studied by using Raman spectroscopy at various pressures and room temperature. The phenomena observed in clinoenstatite have been compared with that observed in orthoenstatite. It is found that the pressure-induced phase transitions in the two enstatites are reversible, but with different transition pressures and transition behavior. An analysis of Raman spectra has revealed that the two enstatites have different high-pressure polymorphs. This result suggests that the space group of the high-pressure polymorph of orthoenstatite is not of C2/c, and that orthoenstatite and orthoferrosilite have different transition routes at room temperature and high pressure. The compressional behavior of the high-PC2/c enstatite is also discussed according to the pressure dependences of Raman frequencies.     

快速冷却条件下银的比热及玻璃化过程的分子动力学模拟

本文采用以嵌入原子模型为基础的分子动力学方法对液态银快速冷却条件下的比热及玻璃化过程进行了模拟，得到了银的比热及其与温度的关系．同时，对快速冷却条件下银的结构特性以及动力学特性进行模拟和分析，结果表明在所模拟的冷却速率下，银在从 １２００ Ｋ冷却到 １０００ Ｋ的过程中发生了玻璃化转变，其玻璃化温度在 １０００ Ｋ左右．     

Selection of oscillation frequency in the 1.315 μm iodine laser

Tuning of an iodine oscillator at 1.315 μm has been carried out with an in-cavity, temperature controlled solid etalon; the hyperfine transition detail and pressure shifts have been observed. This enables the hyperfine transition and the laser frequency of an oscillator-amplifier system to be selected for use in pulse propagation studies or high power laser applications.     

Ultrasonic attenuation by a thin film of high transition temperature Nb3Ge

The electron-phonon contribution to the ultrasonic attenuation coefficient of a 0.5 μm Nb₃Ge film has been measured using 1.2 GHz surface acoustic waves. Analysis of the attenuation data shows that the largest part of the film has a superconducting transition near the bulk transition temperature, 18 K, even though the total film has a high transition temperature, namely 21.5 K. The electron mean free path is calculated from the electron-phonon attenuation data and is found to be an order of magnitude higher than expected for high transition temperature Nb₃Ge films.     

Change in the Crystallization Features of Supercooled Liquid Metal with an Increase in the Supercooling Level

The process of homogeneous crystal nucleation has been considered in a model liquid, where the interparticle interaction is described by a short-range spherical oscillatory potential. Mechanisms of initiating structural ordering in the liquid at various supercooling levels, including those corresponding to an amorphous state, have been determined. The sizes and shapes of formed crystal grains have been estimated statistically. The results indicate that the mechanism of nucleation occurs throughout the entire considered temperature range. The crystallization of the system at low supercooling levels occurs through a mononuclear scenario. A high concentration of crystal nuclei formed at high supercooling levels (i.e., at temperatures comparable to and below the glass transition temperature T_g) creates the semblance of the presence of branched structures, which is sometimes erroneously interpreted as a signature of phase separation. The temperature dependence of the maximum concentration of crystal grains demonstrates two regimes the transition between which occurs at a temperature comparable to the glass transition temperature T_g.     

A comparative Raman study of 0.65(PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript>) -0.35(PbTiO<Subscript>3</Subscript>) single crystal and thin film

We report a comparative Raman study of 0.65(PbMg_1/3Nb_2/3O₃)-0.35(PbTiO₃) (PMN-0.35PT) single crystal and thin film. Raman spectra investigation indicates a change in bulk from the high temperature cubic to the tetragonal phase and then to the low temperature Mc monoclinic phase. The transition temperatures are in good agreement with the ones previously observed by dielectric measurements on the same sample. In contrast, we observe no phase transition to the monoclinic phase in the PMN-0.35PT 4000 Å thick film and only a cubic to tetragonal diffuse transition has been determined at high temperature. The enhanced stability of the tetragonal phase and the absence of low temperature monoclinic phase have been attributed to the in plane strain.     

The spin density waves in chromium detected by hyperfine field measurements

The magnetic hyperfine field of tantalum nuclei in a high purity chromium matrix has been measured using the Time Differential Perturbed Angular Correlation technique. The spectra show that the hyperfine field is proportional to the amplitude of the spin density wave of chromium and that the tantalum probe nuclei do not clamp the phase of the spin density wave. The incommensurate antiferromagnetic first order phase transition as well as the spin flip transition have been observed. The temperature dependence of the hyperfine field is shown to deviate from the temperature dependence of the maximum magnetization of the spin density wave.     

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba₂YCu₃O_7−δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.     

Domain structure in ferroelectric-ferromagnetic films

The conditions for the phase transition to the ferroelectromagnetic phase in ferroelectric-ferromagnetic films through the mechanism of the loss of stability of the initial homogeneous state have been considered. The geometry of the domain structure and the temperature of the transition to the inhomogeneous state have been determined. The condition for the phase transition to the ferroelectromagnetic phase has been established and determined by the relationship between the temperature-dependent coefficients of the expansion of the thermodynamic potential into a series in terms of the polarization and magnetization vector components. The influence of free charge carriers on the geometry of the domain structure and the transition temperature has been studied. The possible existence of a single-domain state has been discussed. The permittivity of a multidomain sample has been determined. The feasibility of exploiting the studied material for nondestructive recording and reading of information has been noted.     

高温高压下碳纳米管的相转变及金刚石的合成

 利用透射电子显微镜（TEM）和扫描电子显微镜（SEM）研究了碳纳米管在高压高温下的相转变。研究发现在高压高温条件下碳纳米管是不稳定的,在5.5 GPa压力下,770 ℃到950 ℃间退火时碳纳米管趋向形成碳纳米葱。在5 GPa、1 000 ℃条件下合成了金刚石。     

Sphere-to-rod transition of triblock copolymer micelles at room temperature

A room temperature sphere-to-rod transition of the polyethylene oxide-polypropylene oxide-polyethylene oxide-based triblock copolymer, (PEO)₂₀(PPO)₇₀ (PEO)₂₀ micelles have been observed in aqueous medium under the influence of ethanol and sodium chloride. Addition of 5–10% ethanol induces a high temperature sphere-to-rod transition of the micelles, which is brought to room temperature upon addition of NaCl. The inference about the change in the shape of the micelles has been drawn from small-angle neutron scattering (SANS) and viscosity studies.     

Electronic transition and the metallization effect in the BiFeO<Subscript>3</Subscript> crystal at high pressures

The optical absorption spectra from bismuth ferrite (BiFeO₃) have been studied at high pressures up to 60 GPa in diamond anvil cells. An electronic transition at which the energy of the optical absorption edge decreases sharply from ～1.5 eV to zero has been observed at room temperature in a pressure range of 45–55 GPa. This indirectly indicates a insulator-metal transition. The observed electronic transition correlates with the recently revealed structural and magnetic transitions induced by high pressures in this crystal. The behavior of the optical absorption edge with decreasing the pressure is completely reversible in correlation with the reversibility of the magnetic transition. The “smearing” of the structural transition in pressure is caused by thermal fluctuations between the high-spin state and low-spin state of the Fe³⁺ ions near the transition.     

Change of thermotropic properties in liquid crystal with multiple phase transitions: Surface “aging” effect

The effect of surface “aging” on thermotropic properties of polymorphic mesogen, 4-butoxyphenylester of 4-decyloxybenzoic acid (BPEDBA) which exhibits the smectic G, smectic C, smectic A, and nematic mesophases has been investigated. Temperatures of direct and reverse phase transitions and temperature widths of the heterophase regions have been determined with high accuracy. The shift of the phase transition temperatures and change of the temperature widths of the heterophase regions under the influence of surface “aging” have been reported.