Oxygenation-induced two peaks in the self-doped La0.9MnO3+δ compounds

A series of self-doped La_0.9MnO_3+δ compounds have been fabricated by the solid-state reaction method with different post-annealing and oxygenation processes. The temperature dependence of resistivity measured at several magnetic fields indicated that all of these samples have an insulator–metal transition with a large negative colossal magnetoresistance (CMR) ratio. The resistivity, maximum CMR ratio and peak position are dominated by the oxygen content. As the oxygen content increases, the resistivity decreases, the peak position shifts to higher temperature and the maximum CMR ratio increases, respectively; meanwhile the peak number in the R–T curve is changed from 1 to 2, then from 2 to 1. The temperature dependencies of magnetization and specific heat show the evidence of magnetic phase transition in these samples. We think that two peaks contributed by the different oxygen-induced ferromagnetic phases resulted in the phase separation of the sample.     

A simplified analysis of the electronic contribution to the elastic constants in ultrathin films of stressed semiconductors under magnetic quantization

An attempt is made to study the electronic contribution to the second- and third-order elastic constants in ultrathin films of strained semiconductors under magnetic quantization in the presence of broadening on the basis of a newly formulated electron dispersion law. It is found, taking stressed n-InSb as an example, that the carrier contribution to the second- and third-order elastic constants oscillates with increasing carrier degeneracy and decreasing film thickness, respectively, in different manners and the stress enhances their numerical values. A relationship between the said contributions and the thermoelectric power has been derived for quantum confined semiconductors having arbitrary dispersion laws and our analysis agrees quite well with the suggested relationship.     

Fe3O4/MgO(100)薄膜外场诱导电阻变化特性

采用激光分子束外延（L-MBE）方法,以MgO(100)为基底生长了Fe3O4单晶薄膜, 研究了Fe3O4/MgO(100)薄膜外场（温度、磁和激光场）诱导电阻变化特性。X射线衍射(XRD)分析表明Fe3O4薄膜是沿MgO(200)晶面外延生长的单晶薄膜;反射高能电子衍射（RHEED）强度振荡曲线分析表明Fe3O4薄膜表面平整,而且生长模式为2维层状生长;原子力显微镜（AFM）分析表明Fe3O4薄膜表面粗糙度为0.201 nm,说明薄膜表面达到原子级平整度。外场作用下Fe3O4薄膜的电阻测试表明:薄膜样品的电阻在120 K(Verwey转变温度)出现一峰值,略微下降后继续增大, 展现出半导体型的导电特性; 在激光作用下,整个测量温度范围内薄膜样品的电阻减小,样品展示出瞬间光电导的特性;从降温曲线可以看出, Verwey转变温度由无激光作用时的120 K上升到有激光作用时的140 K; 光致电阻变化率随着温度的降低而增大,这主要是由于激光作用导致电荷有序态的退局域化。     

Mechanodynamic penetration of helium atoms into nanocrystalline iron

A relation between the characteristics of plastic deformation and the specific features of mechanodynamic penetration of helium into nanocrystalline iron compressively strained at 4.2 K is investigated. Iron samples with a grain size of about 200 nm are prepared by the multiple equal-channel angular pressing technique. The samples deformed in giant (6–7%) sample-averaged serrations, which amounts to several thousand percent strain in a shear band. The amount of helium in samples strained to various degrees is measured, and curves of helium extraction from these samples are obtained in the temperature range 300–1400 K. At a strain of ～50%, the amount of helium built up in a sample is found to be substantially higher (more than hundredfold) than that in samples subjected to lower strains. It is found that an increase in the strain rate gives rise to a strain within a serration (the strain localization is enhanced) and that the amount of accumulated helium decreases, most probably, because of the shorter deformation time. The helium extraction curves obtained with increasing temperature exhibit several peaks. The temperature positions of some of them are about the same for samples strained to different extents, while the other peaks are characteristic of samples subjected to a specific strain only. The results obtained suggest the existence of helium traps of different types, which depend on the original structure and the magnitude of the strain and differ both in the amount of helium they contain and in the temperatures at which helium is released from these traps.     

La0．85Sr0．015MnO3—δ／Fe异质结构的瞬间光电导研究

采用磁控溅射的方法在Si基片上制备了La0.85Sr0.015MnO3-δ/Fe异质结构,对所制备结构中La0.85Sr0.015MnO3-δ薄膜的输运及光诱导特性进行了分析,表明薄膜在整个测量的温度区间内呈现金属一半导体相变,相变温度为130K.在低温金属相,磁场作用导致电阻减小,而高温半导体相则使电阻增大;激光辐照导致样品电阻减小,并产生瞬间光电导效应,即随着激光的关闭电阻瞬间恢复到原值,其相对变化值随着温度的降低而增大,并在温度为80K时达到极大值约1660%,分析认为该瞬态光电导效应可能与薄膜本身的氧缺陷有关.     

Elastic properties,Debye temperature,density of states and electron–phonon coupling of ZrB12 under pressure

The structural parameters, elastic constants and the electronic density of states of ZrB₁₂ under pressure are determined using first-principles calculations with plane-wave pseudopotential density functional theory, within the generalized gradient approximation. From the elastic constants the elastic parameters and Debye temperature were calculated. They increase as the pressure is increased. The density of states at the Fermi level decreases as pressure is increased, changing from 0.576 to 0.515. Using the Debye temperature and the McMillan equation, the electron–phonon coupling constant was obtained as a function of pressure. It is found that the electron–phonon coupling constant is proportional to the logarithm of the ratio between the value of the Debye temperature and the value of the superconducting critical temperature.     

Instability of Epitaxially Strained Thin Films Based on Nonlocal Elasticity

We perform a linear analysis of the elastic fields and stability of epitaxially strained thin films based on nonlocal elasticity. We derive expressions of perturbed stresses to the first order of perturbation amplitude, which show that the stresses are directly proportional to the lattice mismatch and the perturbation amplitude, and decrease with an increase in the perturbation wavelength. The critical perturbation wavelength distinguishes whether the flat film for the perturbation is stable, which is inversely proportional to the square of the mismatch and decreases with the thickness of the film.     

Probing the anisotropy constants of SmCo5 and PrCo5 by Hall resistance measurements in pulsed high magnetic fields up to 47 T

In order to assess the anisotropy constants of highly anisotropic thin film samples with anisotropy fields well above 10 T, Hall resistance measurements were conducted in pulsed magnetic fields. These measurements also deliver the anomalous Hall data, which are proportional to the perpendicular magnetisation. This specific approach combines the high field values obtainable by pulsed fields with a measurement technique sensitive enough to be applied to thin film samples. Two epitaxial Rare Earth-Cobalt thin films with large in-plane uniaxial magnetocrystalline anisotropy at room temperature were studied. The resulting anisotropy fields and constants are discussed with respect to measurements on single crystals and similar films investigated in quasi-static magnetic fields well below the anisotropy field. The present technique proved to be very valuable to highly anisotropic samples, as the approach to saturation is fully monitored and the data thus provides a more extended view on the hard axis magnetisation process.     

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     

Strain induced epitaxial relationships of Cr on Co/Pd(111)

The growth and structure of Co ultra-thin films on Pd(111) and Cr on Co/Pd(111) have been analyzed by grazing incidence X-ray diffraction and low energy electron diffraction. It is shown that the in-plane lattice constant of the epitaxial Co film depends on the growth temperature. Although the strain decreases as a function of the Co film thickness, it persists for 20 monolayer (ML) films or even thicker. When Cr is deposited at room temperature on a strained Co film (10 to 20 ML thick) a Kurdjumov–Sachs epitaxial relationship is observed, whereas when Cr is deposited on a Co(0001) single-crystal or on a very thick Co film on Pd(111), a Nishyama–Wassermann orientation is obtained.     

Wetting layer thickness and early evolution of epitaxially strained thin films

We propose a physical model which explains the existence of finite thickness wetting layers in epitaxially strained films. The finite wetting layer is shown to be stable due to the variation of the nonlinear elastic free energy with film thickness. We show that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We observe the development of faceted islands in unstable films.     

Elastic properties of layered crystals

The characteristic features of the elastic properties of layered crystals and their dependence on temperature and pressure are analyzed. The relations between the elastic constants of hexagonal layered crystals are given. It is shown that the anomalous behavior of the elastic constants in the temperature region of a phase transition affects both the magnitude and sign of the thermal expansion coefficients of layered crystals. From analyzing the pressure and temperature dependences of the elastic constants, it is found that the anharmonicity of the bonding forces between the layers is much greater than the anharmonicity of the intralayer forces. The contribution from thermal expansion to the variations of the elastic constants with temperature is estimated.     

Relative intensities in X-ray photoelectron spectra: Part VII. The effect of elastic scattering in a solid on the angular distribution of photoelectrons escaping from samples covered with thin films of various thicknesses

The angular distributions of X-ray photoelectron peak intensity for (1) a semi-infinite sample, (2) a substrate sample covered with a film, and (3) an overlayer sample are calculated by the Monte-Carlo method. The elastic as well as the inelastic scattering of electrons in a solid is taken into account. In all cases the elastic scattering is shown to have a significant effect on both the absolute value of peak intensity and the angular distribution of photoelectrons. The electron mean free paths without inelastic collisions (λ_n) calculated using formulas derived without taking account of elastic scattering are shown to differ significantly from the real values. Moreover, the λ_n values calculated in this way are not physical constants at all, but depend for example on the film thickness and the intervals of photoelectron take-off angles under consideration. The elastic scattering effect is shown capable of explaining some difficulties which arise in the interpretation of experimental data reported in the literature on the basis of expressions derived taking into account only the inelastic interactions of photoelectrons with a solid.     

The influence of substrate temperature variation on tungsten oxide thin film growth in an HFCVD system

Tungsten oxide (WO₃) thin films were deposited by a modified hot filament chemical vapor deposition (HFCVD) technique using Si (1 0 0) substrates. The substrate temperature was varied from room temperature to 430 °C at an interval of 100 °C. The influence of the substrate temperature on the structural and optical properties of the WO₃ films was studied. X-ray diffraction and Raman spectra show that as substrate temperature increases the film tends to crystallize from the amorphous state and the surface roughness decreases sharply after 230 °C as confirmed from AFM image analysis. Also from the X-ray analysis it is evident that the substrate orientation plays a key role in growth. There is a sharp peak for samples on Si substrate due to texturing. The film thickness also decreases as substrate temperature increases. UV-vis spectra show that as substrate temperature increases the film property changes from metallic to insulating behavior due to changing stoichiometry, which was confirmed by XPS analysis.     

The use of piezoelectric film and ultrasound resonance to determine the complete elastic tensor in one measurement.

The ultrasonic and elastic properties of materials are conventionally measured using quartz, lithium niobate, etc., transducers and a pulse-echo technique with the transducer driven at resonance. Problems with the technique include transducer ringing, transducer-sample coupling, parallelism of sample faces, beam diffraction, and the necessity of remounting transducers in order to measure all of the elastic constants. Usually, these problems can be minimized, but with samples that are only a fraction of a millimeter in size, conventional ultrasound measurements become difficult if not impossible. However, nearly all of these problems may be avoided if a resonance technique is used, and all of the elastic constants may be determined with a single measurement. For the broadband response and minimum transducer loading required for a resonance measurement in a small sample, polyvinylidene fluoride (PVDF) piezoelectric film (as thin as 9 microns) is ideally suitable.     

Temperature dependence of elastic moduli for solid C60

The attenuation and velocity of ultrasonic waves (at a frequency of ～4 MHz) along the 〈111〉 and 〈100〉 directions in solid C₆₀ single-crystal samples are measured in the temperature range 100–300 K. The temperature dependences of the complete set of elastic constants for C₆₀ fullerite are determined from the experimental data. It is shown that the specific features in the behavior of the elastic moduli near the orientational phase transition temperature are associated with different contributions of the relaxation processes to the effective elastic moduli. The activation volume and deformation potentials for the ground and excited states of the C₆₀ low-temperature phase are evaluated from the results obtained in this work and the data available in the literature.     

Anomalous elasticity and anelasticity of GdCu at low temperatures

The temperature dependence of the single crystal and polycrystalline elastic constants of GdCu compound have been determined between the ambient and 77 K, the interval in which the structural instability occurs. From the shear constants, C' and C₄₄ the magnitude of the elastic anisotropy was calculated, and found to be close to unity over the whole temperature range. This is in contrast with the expected behavior of the elastic anisotropy in the vicinity of martensitic transformations, where the anisotropy attains appreciable values. The large changes of the shear elastic constants, relative to the compressional constant suggests that the structural instability in GdCu is induced by a mechanism of isotropic shear. The peak in the ultrasonic attenuation is symmetric, typical of a relaxational process. The behavior of the elastic properties and ultrasonic attenuation in the temperature interval of the structural instability of GdCu is discussed.     

超巨磁阻测辐射热仪

基于超巨磁阻(CMR)材料在金属-绝缘体(M-I)转变点附近的巨大电阻变化,设计并测试了一种新型超巨磁阻测辐射热仪(CMR bolometer).用外延法生长的La_0.67Ca_0.33MnO₃ 薄膜作为测辐射热仪的辐射敏感元件,测量了该器件对黑体和He-Ne激光器的光学响应.所测的信号、噪声都随调制频率的上升而下降,但对于He-Ne激光源,它的信噪比却没有明显的改变.测量了薄膜铁磁金属态和顺磁绝缘态的信号-温度关系,最强的信号出现在靠近M- 关键词： 超巨磁阻     

Fabrication processing effects on the microstructure and morphology of erbium film

The effect of substrate temperature on the microstructure and the morphology of erbium film are systematically investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). All the erbium films are grown by electron-beam vapor deposition (EBVD). A novel preparation method for observing the cross-section morphology of the erbium film is developed. The films deposited at 200°C have (002) preferred orientation, and the films deposited at 450°C have a mixed (100) and (101) texture, due to the different growth mechanisms of surface energy minimization and recrystallization, respectively. The peak positions and the full widths at half maximum (FWHMs) of erbium diffraction lines (100), (002), and (101) shift towards higher angles and decrease with the increasing substrate temperature in a largely uniform manner, respectively. Also, the lattice constants decrease with increasing temperature. The transition in the film stresses can be used to interpret the changes in peak positions, FWHMs, and lattice constants. The stress is compressive for the as-growth films, and is counteracted by the tensile stress formed during the process of temperature cooling to room temperature. The tensile stress mainly originates from the difference in the coefficients of thermal expansion of the substrate-film couple.     

还原温度对氧化石墨烯结构及室温下H_2敏感性能的影响

以氧化石墨凝胶制备的氧化石墨烯(GO)溶胶为前驱体,在100—350℃温度下还原获得不同还原程度的还原氧化石墨烯(rGO)样品,并采用旋涂工艺制备还原氧化石墨烯气敏薄膜元件.采用X射线衍射、拉曼光谱、傅里叶变换红外光谱和气敏测试等手段研究还原温度对样品结构、官能团和气敏性能的影响.结果表明:经热还原处理的氧化石墨烯结构向较为有序的类石墨结构转变,还原温度为200℃时,样品处于GO向rGO转变的过渡阶段,还原温度达到250℃时,则表现出还原氧化石墨烯特性;无序程度随还原温度的升高先由0.85增大至1.59,随后减小至1.41,总体呈现增加趋势;氧化石墨烯表面含氧官能团随还原温度的升高逐渐热解失去,不同含氧官能团的失去温度范围不同;热还原氧化石墨烯具有优异的室温H_2敏感性能,随着还原温度的升高,元件灵敏度逐渐减小,响应-恢复时间逐渐增大,最佳灵敏度为88.56%,响应时间为30 s.