Structural phase transitions in ‘hcp’ C70 single-crystals

The thermal expansion of vapor-grownC ₇₀ single crystals ahs been investigated using high-resolution capacitance dilatometry from 5–380 K. Measurements were made both parallel and perpendicular to the hexagonalc-axis. Three first-order phase transitions which we associate with the consecutive disordering of theC ₇₀ molecules are observed upon heating at 280 K (long-axis spinning), 300 K (long-axis precession) and 355 K (quasi-free rotation), respectively. The highest-temperature transition exhibits a very large (50 K) thermal hysteresis. Powder and single-crystal X-ray diffraction show that the crystals are predominantly hexagonal-close-packed (HCP) with an idealc/a1.63 above 360 K andc/a1.84 at 295 K.     

An analytical model to represent crystallization kinetics in materials with metastable phase formation

The aim of this article is to propose a simple analytical model that can describe the isothermal crystallization process in materials when the formation of a stable crystalline phase is preceded by the formation of a metastable phase. This model explains deviations from the well-known Johnson-Mehl-Avrami-Kolmogorov kinetics theory and predicts the three slopes in Avrami’s plot. The model predictions were compared with experimental results obtained from X-ray measurements in the chalcogenide glasses with composition of Ge₂Sb₂Te₅ (thin films) and in aqueous solutions of methylhydrazine monohydrate during isothermal phase transformations. In order to validate the proposed model to represent experimental results, a computer program was developed. This program uses experimental data from measurements of the total volume fraction at different times during isothermal transformations and fits the model parameters that best represent the kinetic behavior of the system.     

Crystal structures and cation ordering of Sr2AlSbO6 and Sr2CoSbO6

The two double perovskite oxides Sr₂AlSbO₆ and Sr₂CoSbO₆ were prepared and their structures studied with the X-ray powder diffraction method. At room temperature the crystal structure of Sr₂AlSbO₆ is cubic , with . It was found that depending on the preparation conditions, the Al³⁺ and Sb⁵⁺ cations can be either entirely or partially ordered. In the case of the partially ordered Sr₂AlSbO₆ sample, the extension of cation ordering was estimated from the -dependent broadening of the diffraction peaks and the results were interpreted as evidence of the formation of anti-phase domains in the material. Low-temperature Raman spectroscopic measurements demonstrated that the cubic phase of Sr₂AlSbO₆ is stable down to 79 K.The room-temperature crystal structure of Sr₂CoSbO₆ is trigonal (space group with and . At 470 K, however, the material undergoes a continuous phase transition and its structure is converted to cubic (space group . The studied Sr₂CoSbO₆ sample was partially ordered, but unlike Sr₂AlSbO₆, no indication of the formation of anti-phase domains was observed.     

Spatial coherence properties of monochromatic electromagnetic beams and of laser modes

It is shown that, contrary to common belief, monochromatic light beams are, in general, not spatially completely coherent, i.e., they will, in general, not produce fringes of unit visibility in a Young's double pinhole interference experiment. We cite experiments with laser modes which confirm this result.     

New Expressions for Dark-Hollow Light Beams

A class of new light beams of dark-hollow beams, named sinusoidal dark-hollow beams, is introduced. The propagation formula for a sinusoidal dark-hollow beam through a paraxial ABCD optical system is derived. The propagation properties of the sinusoidal dark-hollow beam are comparatively studied and illustrated with numerical examples.     

Phase-Locked Fibre Array for Coherent Combination and Atmosphere Aberration Compensation

We report a fibre amplifier array that not only achieves coherent beam combination by compensation of phase noises of fibre amplifier, but also accomplishes correction of atmosphere aberration. It is of master-oscillatormultiple-amplifier （MOPA） configuration, which can be phase-locked by the multidither principle or heterodyne detection principle. First laboratory experiments of atmosphere aberration compensation of a three-element fibre amplifier array are reported. The atmosphere aberration is created by a phase screen in the experiment. The phase changes of the beam, which are introduced by the fibre amplifier and the phase screen, are both detected by the heterodyne detection method. Phase modulators are controlled to compensate for the phase in the three paths. No matter whether there is a phase screen producing atmosphere aberration or not, the dim dynamic interference fringes in the far field turn to a clear and stable pattern, and the peak intensity is maximized. It is indicated that the fibre amplifier array is phase-locked, and coherent combination of the three beams is achieved. It can be used not only to obtain high power fibre laser array but also in laser space communication.     

First-Principles Prediction of High-Pressure Phase of CaC6

The lattice dynamics of rhombohedral GaG6 is studied as a function orpressure to probe Its high pressure phase with low superconducting transition temperature using the density functional liner-response theory. The pressureinduced phase transition in CaC6 is attributable to the softening transverse acoustic （TA） phonon mode at the zone boundary X （0.5, 0.0, 0.5） point. The high pressure phase is then explored by performing fully structural optimization in the supercell which accommodates the atomic displacements corresponding to the eigenvectors of the unstable mode of TA（X）. The high-pressure phase is predicted to be a monoclinic unit cell with space group P21/m.     

Discontinuities in the specific heat of magnesium and associated latent heat at pressure-induced structural phase transitions using a local first principles pseudopotential

After a successful application of a local first principles pseudopotential to the study of the structural pressure-induced phase transition for magnesium, I now use the same local first principles type of pseudopotential, to study the specific heat as function of the pressure, at 300 K, in the region around the predicted phase transitions. I found that the specific presents two discontinuities, one for each transition of phase. These discontinuities are associated to the existence of latent heat at each transition, which has not been yet reported experimentally.     

Coherent thermal emission by microstructured waveguides

Following Greffet et al. [Coherent emission of light by thermal sources. Nature 2002;416:61-4], we study in this article, the possibility to engineer thermal coherent sources with waveguides. The idea is to rule a grating on a waveguide made of a slice of germanium deposited on a participating media such as glass. The guided waves, thermally excited are coupled to the far field by the grating and increase the system emissivity in certain directions and wavelengths. We numerically compute the diffraction of a plane wave by the grating by a rigorous coupled waves algorithm (RCWA). The reflected, transmitted and absorbed energy calculated allows to obtain the system emissivity by means of the Kirchhoff law.     

Realization of an All-Fibre Self-Organization Intra-Cavity Coherent Erbium-Doped Fibre Laser

An intra-cavity coherent coupling Michelson Er-doped fibre （EDF） laser （MCEDFL） is proposed and demonstrated. Characteristics of the MCEDFL are investigated. It is found that the MCEDFL with a polarizer can be coherent combined effectively. By the experiment based on fibre Bragg gratings （FBGs） with different reflectivity, we find that the reflectivity of the FBG play a vital role in improving the performance of the MCEDFL. This outcome adequately shows many favourable features, such as high efficiency, easy operation, and simple all-fibre configuration.