Charge and Sodium Ordering in β-Na0.33V2O3

Polarized Raman and optical spectra for the quasi one-dimensional metallic vanadate -Na_0.33V₂O₃ are reported for various temperatures. The spectra are discussed in the light of the sodium and charge ordering transitions occurring in this material, and demonstrate the presence of strong electron–phonon coupling.     

Phonons and Phase Transitions in Finite Nuclei

The nature and evolution of collectivity and coherence in nuclei is one of the most fundamental issues in nuclear structure and its evolution with N and Z. Despite many experiments, the nature of nuclear vibrational modes in deformed nuclei and the nature of nuclear phase/shape transitions are not at all understood. We discuss new experiments on phonon and multi-phonon states in the rare earth nuclei and on new evidence for phase coexistence in Sm that relates to the possible existence of phase transitional behavior in finite nuclei.     

Universal Anisotropic Condensation Transition of Gases in Nanotube Bundles

Gases adsorbed within bundles of carbon nanotubes (inside of the nanotubes or in the interstitial channels between the tubes) exhibit a variety of phase transitions with the help of interactions between molecules in neighboring channels or tubes. Because the channels/tubes are widely separated, these transverse interactions are weaker than the (longitudinal) interactions within the same channel. The transition temperatures that result are therefore lower than those of typical two- or three-dimensional transitions of the same species of molecules. We discuss here the condensation transition of such a gas to form a liquid, expressing the transition behavior in universal form, where the reduced critical temperature T ^* _c is a universal function of the reduced transverse interaction.     

Elaboration and hydriding behavior characterization of a magnesium matrix composite containing intermetallic particles

The AB₂ Laves phase intermetallic compounds, as well as magnesium metal (among a great variety of other metals and alloys) present hydriding properties of interest for practical applications in hydrogen storage devices through the formation of metallic hydrides. This fact has encouraged numerous experimental works concerning the characterization of materials by different available techniques. The application of internal friction measurements to study the hydriding processes of intermetallic compounds is precluded because of the fast decrepitation of the material upon few cycles of formation and decomposition of the hydrid phase. However, a composite where the intermetallic particles are included within a ductile magnesium matrix, is suitable to be studied by internal friction techniques, and possible of obtaining information about the interaction between hydrogen atoms, particles, and matrix. In the present work, it is described the elaboration of a composite material obtained by sintering a pellet prepared from a mixture of equal amounts in weight of Mg and Zr(Fe_0.7Cr_0.3)₂ particles, ranging in sizes up to 250 μm and 50 μm respectively. Details on some experimental procedures and discussion about the material composition are considered and correlated with morphological characteristics determined by SEM, and also assessed from the observed hydriding behavior and internal friction results. Received: 30 March 2000     

In-situ studies on phase transformations under electron irradiation in a high voltage electron microscope

High voltage electron microscopy (HVEM), using electron energies adequate for causing displacements of atoms from lattice sites, is a very effective technique for studying mechanisms of solid state phase transformations and for charting the path of phase evolution in real time. This has been demonstrated in studies on chemical ordering in nickel-molybdenum alloys and on theβ → Ω displacement ordering in zirconium-niobium alloys. The enhanced diffusivity due to electron irradiation makes it possible to explore a sequence of phase evolution at low enough temperatures where even some first-order transformations are driven by free energy (G) instabilities with respect to the relevant order parameter (η). Specific issues addressed in these studies are reviewed in this paper.     

Influence of oxygen content on the magnetic properties of B-site deficient lanthanum manganites

The results of X-ray diffraction and magnetization measurements performed for LaMn_0.94O_x (2.91 ≤ x ≤ 3.00) manganites have been presented. The solid solutions have been found to possess three different structural forms (orbitally ordered orthorhombic, orbitally disordered orthorhombic, and monoclinic ones), depending on the oxygen concentration. The transitions from an antiferromagnetic to an inhomogeneous ferromagnetic, and then to a spin-glass phase have been observed with increasing oxygen content. The experimental results have been discussed in the framework of the ionic distribution model adapted to the case of B-site deficient manganites.     

Local Order and Structure in Mn-Substituted Manganites Studied by EXAFS

We report extended X-ray absorption fine structure measurements at the Mn K edge on La_0.25Ca_0.75Mn_0.92Cr_0.08O₃ samples. Macroscopically, the Mn substitution induces a short-range charge-ordered state in spite of the long-range one present in the parent manganite La_0.25Ca_0.75MnO₃. We show how on a local scale the structural EXAFS parameters still confirm the presence, with only a slight reduction, of the features characteristic of the charge ordered un-substituted compounds.     

Y—Ba—Cu—O超导陶瓷的热压研究

用热压技术制备了高密度的 Y-Ba-Cu-O 材料,在适当条件下热处理,可以形成与烧结材料相同的高 T_c超导体。显微结构和 X-光衍射分析表明:在热压的 Y-Ba-Cu-O 超导陶瓷中,晶粒成片状生长,优先生长的方向与正交晶系中的 c 轴向垂直。片状晶体在一定的近程内平行排列,远程范围内无定向性,本文还对此类显微结构的取向特征与材料的超导特性进行了讨论。     

固相反应合成磷酸四钙的物相演变

采用TGA-DSC、FTIR及XRD等方法研究了固相反应法合成磷酸四钙(TTCP) 的物相演变过程.结果表明,1200℃以上物相变化与CaO-P₂O₅-H₂O)的高温相图基本一致, 1200℃以下的物相变化为绘制CaO-P₂O₅-H₂O低温相图提供重要数据;通过揭示固相合成 TTCP物相演变的全过程,为制备出纯TTCP这一重要的磷酸钙骨水泥组成材料提供重要理 论依据.     

Solid-state wetting transitions at grain boundaries

A previously developed model of the dependence of grain boundary (GB) segregation on GB character has been exercised to investigate solid-state wetting transitions at GBs, and their anisotropy. In the case of binary systems displaying a solid-state miscibility gap, it is shown that the wetting transition temperature for precipitates at a GB is anisotropic, and is inversely related to GB energy. The model also allows calculation of prewetting transitions and associated excursions in adsorption off phase coexistence. These transitions are first order below a prewetting critical point (T_PWC), and higher order at temperatures above T_PWC. Investigation of the prewetting behavior provides the means for construction of the two-dimensional phase diagram of a GB.     

Pseudogap Transition in T-shaped Double Dot

We investigate the transport properties of the T-shaped arrangement of double-quantum-dot (DQD) as a result from a setup which presents zero-field Kondo splitting and quantum-critical transition. In a particular case, this setup appears as a system of a usual quantum dot (QD) with Kondo resonance coupled to another QD containing one or two discreet levels. We show that the setup formed from a dot presenting Kondo resonance connected to the leads only by the second dot maps the system onto an Anderson impurity with an effective energy hybridization function Δ(ω) proportional to |ω| ^r where r=1. A similar behavior but with r=2, has been predicted by Dias da Silva et al. (Phys. Rev. Lett. 97:096603, [2006]) for the parallel double quantum dots setup.     

Structural and electrical properties of the Y-Ba-Cu-O superconducting films prepared by spray pyrolysis

High-T _c , superconducting YBa₂Cu₃O_7– thin films have been grown on (100) MgO substrates by a chemical spray pyrolysis method. The crystal structure and surface morphology have been studied by X-ray diffraction and scanning electron microscopy, respectively. The assprayed films were amorphous and insulating, but upon annealing the films became superconducting and show a textured surface morphology with an average grain size of the order of 5–15m. The films were highly oriented with thec-axis being perpendicular to the substrate surface. Three different microstructures were recorded: long rod-shaped grains, platelet or rounded-shape grains, and a melting-like growth. Electrical measurements were carried out in a low-temperature cryostat using a standard d.c. four-probe technique. The onset transition temperature was around 83–86 K, and the completion of the transition to zero resistance was in the range 73–78 K. The magnitude of the measured critical current density was in the range 750–3750 A/cm² at 30 K. A correlation between the resistance of the tunnelling junctions and the critical current density was found from the theoretical models.     

Microstructure and superconducting properties of Y-Ba-Cu-O films prepared by chemical spray pyrolysis

Y-Ba-Cu oxide superconducting films were prepared by spray pyrolysis on (100) MgO substrates and alumina coated with a silver buffer layer. Acetate, nitrate, and oxide precursors were used as starting materials. The optimum starting compounds have been assessed and the effectiveness of rapid thermal processing as a fabrication tool for annealing of highT _c superconducting films has been established. The superconducting behavior was found to be strongly dependent on the type of the precursor used, on the method of heat treatment, and on the substrate material. The films show preferred orientation and good adherence to the substrate. Transport measurements indicated that the films exhibited a superconducting transition with an onset temperature between 83–93 K and ending between 52–74 K. The critical current density was found to be strongly dependent on film processing.     

Novel Phase Behavior in Quantum Rotors

A system of quantum linear rotors in a crystal field is found to display a novel type of phase diagram. At a certain value of the crystal field a discontinuity point appears at the orientational phase transition line. As the crystal field increases this point splits into two critical points with a single-phase-state discontinuity (hole) between them. This remarkable feature is common both for a system where all angular momentum states are allowed and for one where the angular momentum is restricted to have even values despite the fact that general aspects of the transformations induced by the crystal field differ considerably for the two systems.     

Electric-field-induced phase transitions in co-doped Pb(Zr1−xTix)O3 at the morphotropic phase boundary

The strain- and polarization-electric field behavior was characterized at room temperature for Pb_0.98Ba_0.01(Zr_1−xTi_x)_0.98Nb_0.02O₃, 0.40 ⩽ x ⩽ 0.60. The investigated compositions were located in the vicinity of the morphotropic phase boundary, giving insight into the influence of crystal structure on the hysteretic ferroelectric behavior. The remanent strain of particular compositions is shown to be larger than theoretically allowed by ferroelectric switching alone, indicating the presence of additional remanent strain mechanisms. A phenomenological free energy analysis was used to simulate the effect of an applied electric field on the initial equilibrium phase. It is shown that electric-field-induced phase transitions in polycrystalline ferroelectrics can account for the experimental observations. The experimental and simulation results are contrasted to neutron diffraction measurements performed on representative compositions in the virgin and remanent states.     

Chemical fabrication of superconducting Y-Ba-Cu oxide films

High-T _c superconducting films of Y-Ba-Cu oxide were prepared on (100) MgO substrates by a chemical sol gel method. A procedure is described for preparing a superconducting film using acetate compounds dissolved in salicylic or lactic acids in the presence of ethylene glycol. This solution has superior qualities in terms of homogeneity, viscosity, and stability against atmospheric hydration. The results indicate that the nature of the solvent influences the microstructure and superconducting properties of Y-Ba-Cu-O films. X-ray diffraction analysis reveals that the annealed films are textured and had orthorhombic orientation. Transport measurements indicated that the onset temperatures of the superconductivity transition occurred between 82 and 95 K. The zero-resistance transition temperature was above the boiling point of the liquid nitrogen. The highest critical current density was 2700 A/cm² at 20 K. It is also demonstrated experimentally that the results obtained from the X-ray diffraction analysis alone cannot provide a good criterion for evaluating the quality of a superconductor. A correlation between the crystal structure and the superconducting properties of the Y-Ba-Cu-O films is presented.