蓝青铜K0.3MoO3单晶Peierls相变研究

测量了蓝青铜K_0.3MoO₃单晶Ｒ-Ｔ曲线，发现曲线在２８０Ｋ左右有异常变化，计算得到１８０Ｋ以下的半导体能隙为１３２０Ｋ（０.１１ｅＶ）．液氮温度下测量了晶体的非线性导电性，得到电场阈值为０.1２９Ｖ／ｃｍ．样品ＤＳＣ研究表明，样品在２４０Ｋ处经历一新的Ｐｅｉｅｒｌｓ相变，且为一级相变，据此对相变的微观性质进行了定量计算．１８０Ｋ附近Δｃ_p-Ｔ曲线表明，１８０Ｋ处的相变为一个二级相变加一个一级相变 关键词：     

量子系统的Ergodlc定理

在Klein的一篇论文中,他指出了当量子系统的ergodic定理成立时,所有的运动积分R必须满足以下的条件:sumfrom r″to (α′γ″|R|β′γ″=常数δ_α′β′, (1) 式中α′,β′,…等代表我们所研究的系统的态,γ″,ρ″…等代表舆我们的系统共同平衡的外界的态。在这篇短文中,我们指出:Klein的讨论在一点是可以怀疑的,而用了另一个方法来讨论ergodic定理。这样,我们证明了(1)式只是在引入另一个假定——外界各态有同一个几率——后才是充分的,而在一般情形下,我们须要更强的条件,例如(α′ρ″|R|β′γ″)=常数δ_α′β′δ_ρ″γ″。(2)以上相当於没有运动积分的情形。有运动积分的情形也在本文中作了讨论。     

Photoinduced phenomena in amorphous As<Subscript>4</Subscript>S<Subscript>3</Subscript>Se<Subscript>3</Subscript>–Sn films

We investigate the kinetics of photodarkening and recording of holographic diffraction gratings in amorphous As₄S₃Se₃ thin-film structures doped with tin (Sn) in concentrations of 0–10 at %. It is established that an increase in the Sn concentration leads to a decrease in the photodarkening rate and degree. The photodarkening kinetics is approximated by a stretched exponential function. It is found that an increase in the Sn concentration leads to a decrease in the transmission (photodarkening) variation in the investigated As₄S₃Se₃–Sn films. It is determined that, in the recording of holographic diffraction gratings at a Sn concentration of 3–8 at %, the As₄S₃Se₃–Sn films exhibit the maximum sensitivity and diffraction efficiency of the recorded gratings. It is shown that the dependence of diffraction efficiency on the As₄S₃Se₃ film thickness has the maximum at a film thickness of 4 µm.     

Silica-coated Y2O3:Eu nanoparticles and their luminescence properties

Crystalline Y₂O₃:Eu is of paramount significance in rare earth materials and research on luminescence spectra. In this work, the nanocrystalline Y₂O₃:Eu was coated with silica by a facile solid state reaction method at room temperature. The transmission electron microscope (TEM) photographs showed that the prepared Y₂O₃:Eu particle is polycrystalline with the size of 20 nm, the size of silica-coated particle is about 25 nm. The XPS spectra indicated that the silica layer is likely to interact with Y₂O₃:Eu by a Si-O-Y chemical bond. The luminescence spectra showed that the intensity of ground samples is lower than that of unground ones, the intensity of silica-coated phosphors is higher than that of the ground samples, while almost the same as that of the unground ones. Therefore, the silica coating decreases the surface defects of nanoparticles of the nanocrystalline Y₂O₃:Eu, thus increasing their luminescent intensity.     

A New Laser Photochemical Process for Isotope Enrichment

Selective laser excitation of a specific isotopic species to its lowest electronic singlet (S₁) and triplet (T₁) states respectively, then followed by a chemical reaction for isotope separation is examined theoretically. It is found that the direct excitation to T₁ state from ground state (S₀) is an unique excitation channel for obtaining a controllable chemical reaction and for achieving a high isotope separation factor. This is due to the following important facts: 1) Resonant energy transfer from a selectively excited triplet isotopic species to an unexcited species of the other isotope present in the mixture is missing (or is very small); 2) The lifetime of the T₁ state is normally longer than that of the S₁ state; 3) There is no intersystem crossing channel that exists for the deactivation of excited isotopic species from T₁ state.

Numerical calculation is performed for the chlorine isotope separation in a mixture of thiophosgene (0.5 torr) and diethoxyethylene (1.5 torr). The result indicates that the isotopic enrichment factor is at least 2.5 × 10³ times larger for selective T₁ excitation than that for S₁ excitation.     

Magnetic properties and temperature stability of a molybdenum-doped finemet-type alloy

The influence of nanocrystallization conditions on the structure, magnetic properties, and temperature stability of nanocrystalline magnetically soft Fe_73.5Cu₁Nb_1.5Mo_1.5Si_13.5B₉ alloy is studied. It is found that preliminary low-temperature annealing exerts an influence on subsequent nanocrystallization of the alloy. In addition, preannealing followed by nanocrystallization considerably improves the magnetic properties of the alloy. It is shown that the magnetic properties of the material can be controlled by varying the frequency of a magnetic field used for thermal treatment causing nanocrystallization. It is established that the magnetic properties of nanocrystalline Fe_73.5Cu₁Nb_1.5Mo_1.5Si_13.5B₉ alloy offer a high temperature stability.     

Effect of CO2 laser irradiation on the transport properties of La0.67Ba0.33MnO3 thin film

The effect of CO₂ laser irradiation on La_0.67Ba_0.33MnO₃ epitaxial thin film was investigated. Epitaxial thin films grown by pulsed laser deposition were irradiated by a CO₂ laser in air for 10-60 s. It is shown that after CO₂ laser irradiation treatment, the crystallinity of the film is strongly enhanced. It is found that a dramatic decrease in the resistivity of the CO₂-laser-irradiated film is accompanied by a remarkable increase in its insulator-metal transition temperature and the temperature coefficient of resistance. This significant improvement of its structure and properties is achieved in several dozens of seconds and surpasses that observed in films annealed in an oxygen atmosphere at 900 ^°C for 12 h, suggesting that CO₂ laser irradiation is a new and effective tool to optimize CMR manganites for bolometric applications.     

Effects of a cap layer on built-in electric fields of AlxGa1-xN/GaN heterostructures non-destructively probed by Franz-Keldysh oscillations

We have investigated electric field strengths in the Al_xGa_1-xN layer, F_AlGaN's, of Al_xGa_1-xN/GaN heterostructures with and without a GaN cap layer using photoreflectance (PR) spectroscopy. Franz-Keldysh oscillations (FKOs) from the Al_xGa_1-xN layer are clearly observed in the PR spectra. It is found from analysis of the FKOs that stacking of the cap layer causes a remarkable enhancement of F_AlGaN. This fact demonstrates that the FKO profile is a non-destructive probe for a change of built-in electric field strength induced by a cap layer. Numerical calculations of F_AlGaN based on a Schr?dinger-Poisson equation clarify that the magnitude of the enhancement of F_AlGaN is dominated by the cap-layer thickness.     

Spin pumping in a composite high overtone bulk acoustic wave resonator

A new way of generating a pure spin current using a magnetoelectric composite high-overtone bulk acoustic-wave resonator (HBAR) based on a layered Al–ZnO–Al–Gd₃Ga₅O₁₂–Y₃Fe₅O₁₂–Pt structure is proposed. It is established that the efficiency of generating the rf magnetic field driving the spin current by this method exceeds that of sources which use surface acoustic waves for the excitation of magnetic dynamics and is not inferior to the efficiency of usual electromagnetic ways of exciting a pure spin current that employ microwave cavities.     

Relaxation structure analysis of Li inserted γ-Fe2O3

γ-Fe₂O₃ has a spinel structure with cation vacancy and is expected to perform as a favorable electrode material for secondary lithium-ion battery. When lithium is inserted electrochemically into γ-Fe₂O₃, prolonged potential change is observed after the insertion. In this study, we inserted various amount of Li into γ-Fe₂O₃ (x = 0.66, 1.1, 1.5 in terms of Li_XFe₂O₃), then made the circuit open, measured X-ray diffraction (XRD) patterns at various elapsed time, and analyzed the crystal structure change of γ-Fe₂O₃ with time by the Rietveld method. The X-ray Rietveld analysis revealed that the iron occupancy of 8a site decreased and that of 16c site increased with lithium insertion process and after lithium insertion, the iron occupancy of 8a site increased and that of 16c site decreased gradually with relaxation time. It is indicated that lithium prefer 8a site to occupy kinetically, on the other hand, prefer 16c site thermodynamically.     

Phase separation of intercalation electrodes

Lithium ion cells are widely used for portable equipment because of their high energy density. These cells employ lithium intercalation materials as their anode and cathode active materials. Lithium intercalation materials sometimes show phase separation as a function of the amount of inserted Li, and the cell voltage becomes a constant at the two-phase region. The thermodynamic criterion of the phase separation of binary mixtures is already known using Gibbs free energy. The criterion of the phase separation was applied to that of lithium intercalation materials. First, the phase separation of spinel LiMn₂O₄ and LiNiO₂ was studied, with the assumption that the cathode materials do not deform during Li insertion and extraction, are completely ionic, and only the Coulomb potential is effective in terms of changing the internal energy. Our calculated phase separation was in agreement with the experiments at 0<x<0.5 for Li_xMn₂O₄ and 0<x<0.25 for Li_xNiO₂. Our calculated voltage of Li_xMn₂O₄ was very high compared with the experimental value, because of a very low value of the Coulomb potential at the lithium tetrahedral site in Li_xMn₂O₄. Therefore, it seems that LiMn₂O₄ is not a perfect ionic crystal. Next, the phase separation of graphite was studied, considering a work (w_a) to expand the layer when Li is inserted. When w_a is small, it is expected that the phase separation is the co-existence of the Li-poor phase and the Li-rich phase. However, if w_a is large, it is expected that the two-phases are a phase without Li and a phase filled with Li completely. From these considerations, graphite seems to have a large w_a. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

(BiFeO3)25/(La0.7Sr0.3MnO3)25 多层膜的光学和电学性质

采用射频磁控溅射方法在(001)SrTiO₃衬底上制备(001)取向的(BiFeO₃)₂₅/(La_0.7Sr_0.3MnO₃)₂₅多层膜.光学测试结果表明,1.3-2.1 eV范围内,相对于衬底而言多层膜光吸收增强; BiFeO₃的带隙为2.7 eV. 另外,结合绝缘介质导电模型分析了所测得的电流-电压数据,在所测试的温度及电压下,所制备的(BiFeO₃)₂₅/(La_0.7Sr_0.3MnO₃)₂₅多层膜的导电机理由空间电荷限制电导主导. 关键词： 多层膜 吸光度 空间电荷限制电导     

Effect of electron correlations on the electronic structure and magnetic properties of the perovskite-like high-pressure phase ErCu3V4O12

The electronic structure and magnetic properties of double perovskite ErCu₃V₄O₁₂ have been investigated in the framework of the projector augmented-wave method. It has been shown that the electron correlations play an important role: the calculations in the one-electron approximation predict that the ground state of ErCu₃V₄O₁₂ is a magnetic metallic state, while with the inclusion of electron-electron correlations, it is a ferrimagnetic semimetallic state. It has been found that the magnetic Cu-Cu and V-V couplings are ferromagnetic and that the Cu-V magnetic coupling is antiferromagnetic.     

NaB6Si: A novel wide band gap semiconductor with great hardness

Using first-principles calculations, a novel B₁₂-based ternary compound, NaB₆Si structure is proposed in this work. This structure is confirmed to be dynamically, thermodynamically, and mechanically stable at ambient pressure. The formation energy of NaB₆Si is lower than that of experimentally synthesized Na₈B_74.5Si_17.5. The Vickers hardness is calculated to be 30.2 GPa, indicating NaB₆Si is a promising hard material. The band structure and density of states reveal that NaB₆Si is a wide band gap semiconductor with a band gap approximately 2.88 eV. This study provides a fundamental understanding of the structural, mechanical, and electronic properties in NaB₆Si.     

Enhancement of the thermoelectric power factor of MnSi1.7 film by modulation doping of Al and Cu

It is well known that aluminum (Al) and copper (Cu) are acceptor impurities with shallow- and deep-energy levels in silicon (Si), respectively. The thermoelectric power factor of Al and Cu codoped Si film is larger than that of only Al-doped Si film. In this report, the Al and Cu codoped Si layer, Si: (Al + Cu), is used as a barrier layer, while a higher manganese silicide (HMS, MnSi_1.7) layer is used as a well layer to enhance the power factor of MnSi_1.7 film. It is found that the Al and Cu modulation doped MnSi_1.7 film, Si: (Al + Cu)/MnSi_1.7, has a power factor almost two times larger than that of only Al modulation doped MnSi_1.7 film, Si: Al/MnSi_1.7. It is also found that an undoped Si spacer layer between the Si: (Al + Cu) barrier layer and the MnSi_1.7 well layer can enhance the power factor further. Finally, it is demonstrated that the MnSi_1.7 film with double Si barrier layers, Si: (Al + Cu)/MnSi_1.7/Si: (Al + Cu), has the highest power factor, 1423×10^?6 W/m?K² at 783 K, which is very close to that of MnSi_1.7 bulk material.     

A metastable phase forming during crystallization of an amorphous Ni70Mo10P20 alloy

The evolution of the structure of an amorphous Ni₇₀Mo₁₀P₂₀ alloy during annealing in the temperature range from 673 to 873 K is studied using x-ray diffraction and transmission electron microscopy. It is found that a previously unknown metastable phase forms during crystallization of the amorphous Ni₇₀Mo₁₀P₂₀ alloy. The morphology of the newly formed crystals is studied, the space group and the lattice constant are determined, and the atom positions for the phase that forms are proposed. It is found that the metastable phase is a phosphide whose composition is close to (Ni(Mo))₃P. We established that, as the temperature or the duration of annealing is increased further, the metastable phase turns into the equilibrium Ni₃P phase. The orientation relations between the metastable and equilibrium phases are determined.     

X-ray absorption spectroscopic study of a mixed valence system,EuPd2Si2

L₃ X-ray absorption spectroscopic study of Eu firmly establishes that the compound, EuPd₂Si₂, is a mixed valence system. It is found that the relative intensities of the absorption peaks corresponding to divalent and trivalent europium ions in EuPd₂Si₂ are a strong function of temperature.     

The Newtonian viscosity of polymer solutions: Scaling relationships

Scaling in terms of temperature, composition, and molecular weight variables has practical and fundamental significance. The viscosity of polymer solutions deviates from that predicted by the Huggins equation when the concentration is higher than a characteristic concentration c_ch. The value of c_ch depends on the molecular weight of the polymer and the thermodynamic conditions of the system. It is also a known fact that the deviations are due to the entanglements and interactions of polymer molecules. Therefore, we believe cch can be used as a concentration-reducing parameter to get the superposition curves. It can be shown that the concentration corresponding to a minimum value of η_sp/ch² (in the case of η_sp/c² vs concentration curves) is the value of c_ch of that system. Moreover, this c_ch is related to the intrinsic viscosity and molecular weight through the Huggins and Mark-Hauwink-Sakurada equations (c_ch = k′M^?a′). Using c_ch values for different systems and plotting log η_r versus C/C_ch, the superposition curves are obtained. In each case these curves are found to be linear, at least when concentrations approach zero. Master curves may be plotted by making use of the initial slopes of the curve (log η_r vs Bc/c_ch) and it is found that the data obtained at different thermodynamic conditions fit these (log η_r vs Bc/c_ch, B being the initial slope of log η vs c/c_ch) curves very well. The slopes are also compared to k′, a′, and the expansion coefficient of the system and the relationships are found to be linear. It is concluded that c_ch is a better parameter for the superposition of viscosity data, as well as being easy to obtain experimentally.     

高温超导薄膜中的涨落超导电性及其维度

对RBa₂Cu₃O_7-δ(R=Y,Gd)超导薄膜电阻温度关系的仔细测量,观察到在T_c附近有相当强的超导涨落现象,研究发现,多晶和外延的YBa₂Cu₃O_7-δ薄膜在T_c以上均呈现出2D涨落超导电性;而当温度趋向于T_c时,有—2D向3D的转变发生,这与Lawrence-Doniach理论很好地相符,实验中还发现,一 关键词：     

A novel metallic silicon hexaboride,Cmca-B6Si

Using first-principles calculations, we propose a novel metallic silicon hexaboride, Cmca-B₆Si. This structure is dynamically and mechanically stable at ambient pressure. Furthermore, the formation energy of Cmca-B₆Si is lower than those of synthesized Pnnm-SiB₆ and SiB₄, indicating that Cmca-B₆Si is thermodynamically stable. The calculations of thermodynamic stability and structural inheritance indicate that Cmca-B₆Si may be obtained by removing Li from the synthesized LiB₆Si, that is, by imitating the preparation of Si₂₄ and T-Graphene C₄. The structure of Cmca-B₆Si is a covalent three dimensional framework linked by B₁₂ icosahedrons and Si atoms. The analysis of band structure and density of states reveals that Cmca-B₆Si is metallic because of the partially occupied 2p and 3p electrons of B and Si atoms, respectively. The electron orbitals reveal that Cmca-B₆Si has a novel two-dimensional metallicity. This study gives a fundamental investigation on the structural, phonon, and electronic properties of Cmca-B₆Si.