Phase diagram of a hard-sphere system in a quenched random potential: A numerical study

We report numerical results for the phase diagram in the density-disorder plane of a hard-sphere system in the presence of quenched, random, pinning disorder. Local minima of a discretized version of the Ramakrishnan-Yussouff free energy functional are located numerically and their relative stability is studied as a function of the density and the strength of disorder. Regions in the phase diagram corresponding to liquid, glassy, and nearly crystalline states are mapped out, and the nature of the transitions is determined. The liquid to glass transition changes from first to second order as the strength of the disorder is increased. For weak disorder, the system undergoes a first-order crystallization transition as the density is increased. Beyond a critical value of the disorder strength, this transition is replaced by a continuous glass transition. Our numerical results are compared with those of analytical work on the same system. Implications of our results for the field-temperature phase diagram of type-II superconductors are discussed.

A kinetic study of liquid-phase catalytic oxidation of styrene to benzaldehyde with Wilkinson complex

Liquid-phase homogeneous catalytic oxidation of styrene with Wilkinson complex by molecular oxygen in toluene medium gave selectively benzaldehyde and formaldehyde as the primary products. Higher temperatures and styrene conversions eventually led to acid formation due to co-oxidation of aldehyde.A reaction induction period and an initiation period, typical of free-radical reactions, characterized the oxidation process. The effects of temperature and catalyst and styrene concentrations on the conversion of styrene to benzaldehyde and acid formation have been studied. The optimum reaction parameters have been determined as a styrene-to-solvent mole ratio of 0.5, a catalyst-to-styrene mole ratio of 5.0 X lo4, and a reaction temperature of 75 "C. A reaction scheme based upon free-radical mechanism yielded a pseudo-first-order model which agreed well with the observed kinetic data in the absence of co-oxidation of aldehyde. A second-order model was found to fit the experimental data better in the case of aldehyde conversion to acid.

New phase unwrapping strategy for rapid and dense 3D data acquisition in structured light approach

Sinusoidal structured light projection (SSLP) technique, specifically-phase stepping method, is in widespread use to obtain accurate, dense 3-D data. But, if the object under investigation possesses surface discontinuities, phase unwrapping (an intermediate step in SSLP) stage mandatorily require several additional images, of the object with projected fringes (of different spatial frequencies), as input to generate a reliable 3D shape. On the other hand, Color-coded structured light projection (CSLP) technique is known to require a single image as in put, but generates sparse 3D data. Thus we propose the use of CSLP in conjunction with SSLP to obtain dense 3D data with minimum number of images as input. This approach is shown to be significantly faster and reliable than temporal phase unwrapping procedure that uses a complete exponential sequence. For example, if a measurement with the accuracy obtained by interrogating the object with 32 fringes in the projected pattern is carried out with both the methods, new strategy proposed requires only 5 frames as compared to 24 frames required by the later method.

Structural, dielectric and ferroelectric properties of four-layer Aurivillius phase Na0.5La0.5Bi4Ti4O15

Monophasic Na0.5La0.5Bi4Ti4O15 powders were synthesized via the conventional solid-state reaction route. The X-ray powder diffraction (XRD), selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM) studies carried out on the as synthesized powdered samples confirmed the phase to be a four-layer Aurivillius that crystallizes in an orthorhombic A2(1)am space group. The microstructure and the chemical composition of the sintered sample were examined by scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDX). The dielectric properties of the ceramics have been studied in the 27-700 degrees C temperature range at various frequencies (100 Hz to 1 MHz). A sharp dielectric anomaly was observed at 580 degrees C for all the frequencies corresponding to the ferroelectric to paraelectric phase transition. Saturated ferroelectric hysteresis loops were observed at 200 degrees C and the associated remnant polarization (P-r) and coercive field (E-c) were found to be 7.4 mu C/cm(2) and 34.8 kV/cm, respectively. AC conductivity analysis confirmed the existence of two different conduction mechanisms in the ferroelectric region. Activation energies calculated from the Arrhenius plots were similar to 0.24 eV and similar to 0.84 eV in the 300-450 degrees C and 450-580 degrees C temperature ranges, respectively. (C) 2010 Elsevier B.V. All rights reserved.

Infrared Spectra of Dimethylquinolines in the Gas Phase: Experiment and Theory

Infrared spectra of atmospherically important dimethylquinolines (DMQs), namely 2,4-DMQ, 2,6-DMQ, 2,7-DMQ, and 2,8-DMQ in the gas phase at 80 degrees C were recorded using a long variable path-length cell. DFT calculations were carried out to assign the bands in the experimentally observed spectra at the B3LYP/6-31G* level of theory. The spectral assignments particularly for the C-H stretching modes could not be made unambiguously using calculated anharmonic or scaled harmonic frequencies. To resolve this problem, a scaled force field method of assignment was used. Assignment of fundamental modes was confirmed by potential energy distributions (PEDs) of the normal modes derived by the scaled force fields using a modified version of the UMAT program in the QCPE package. We demonstrate that for large molecules such as the DMQs, the scaling of the force field is more effective in arriving at the correct assignment of the fundamentals for a quantitative vibrational analysis. An error analysis of the mean deviation of the calculated harmonic, anharmonic, and force field fitted frequencies from the observed frequency provides strong evidence for the correctness of the assignment.

A note on the Berry phase for systems having one degree of freedom

A one-dimensional arbitrary system with quantum Hamiltonian H(q, p) is shown to acquire the 'geometric' phase gamma (C)=(1/2) contour integral c(Podqo-qodpo) under adiabatic transport q to q+q+qo(t) and p to p+po(t) along a closed circuit C in the parameter space (qo(t), po(t)). The non-vanishing nature of this phase, despite only one degree of freedom (q), is due ultimately to the underlying non-Abelian Weyl group. A physical realisation in which this Berry phase results in a line spread is briefly discussed.

Unconventional mechanism of stabilization of a tetragonal phase in the perovskite ferroelectric (PbTiO3)(1-x)(BiFeO3)(x)

Temperature-dependent x-ray powder-diffraction study of the tetragonal compositions of PbTiO3-BiFeO3 series has revealed that, unlike for all the known ferroelectric perovskites, the compositions exhibiting giant tetragonality is stabilized from the cubic phase via a complex transition pathway which involve (i) formation of minor monoclinic phase with a large pseudotetragonality along with an intermediate tetragonal phase (major) with a small tetragonality, (ii) gradual vanishing of the intermediate tetragonal phase and concomitant increase in the monoclinic regions, and finally (iii) gradual transformation of the monoclinic phase to the tetragonal phase with giant tetragonality.The system seems to adopt such a complex transition pathway to create amicrostructure with very large number of domains and interfaces for stress relief, which would not have been possible in case of a direct cubic-tetragonal transition.

Electron-Paramagnetic-Res study of LiKSO4 in phase IV below -138 degrees C

A detailed single-crystal EPR study of phase IV of lithium potassium sulphate below -138 degrees C has been carried out using NH3+, which substitutes for K+, as the paramagnetic probe. The spin-Hamiltonian parameters have been evaluated at -140 degrees C and yield an isotropic g=2.0034; (AH)XX=(AH)YY=25.3 G and (AH)ZZ=23.8 G; (AN)XX=8.1 G, (AN)YY=21.2 G and (AN)ZZ=25.9 G. In this phase there are 12 magnetically inequivalent K+ sites and their occurrence is ascribed to the loss of a c glide.

Ferroelastic phase transition in CslO4

An optical investigation of the high-temperature structural phase transition in gel-grown single crystals of CslO4 is reported. This crystal undergoes a phase transition from the room-temperature orthorhombic phase of symmetry Pnma to a tetragonal phase at 150°C. The birefringence Δn = |na-nb| falls abruptly at Tc, indicating the first-order nature of the phase transition. Microscopic examination has revealed the existence of ferroelastic domains in the crystal. The domain structure and its dependence on temperature was studied in detail. The experimental results suggest that this crystal can be assigned to the ferroelastic Aizu species 4/mmmFmmm (p).

Retrieval of dust aerosols during night: improved assessment of long wave dust radiative forcing over Afro-Asian regions

Several investigators in the past have used the radiance depression (with respect to clear-sky infrared radiance), resulting from the presence of mineral dust aerosols in the atmosphere, as an index of dust aerosol load in the atmosphere during local noon. Here, we have used a modified approach to retrieve dust index during night since assessment of diurnal average infrared dust forcing essentially requires information on dust aerosols during night. For this purpose, we used infrared radiance (10.5-12.5 mu m), acquired from the METEOSAT-5 satellite (similar to 5 km resolution). We found that the `dust index' algorithm, valid for daytime, will no longer hold during the night because dust is then hotter than the theoretical dust-free reference. Hence we followed a `minimum reference' approach instead of a conventional `maximum reference' approach. A detailed analysis suggests that the maximum dust load occurs during the daytime. Over the desert regions of India and Africa, maximum change in dust load is as much as a factor of four between day and night and factor of two variations are commonly observed. By realizing the consequent impact on long wave dust forcing, sensitivity studies were carried out, which indicate that utilizing day time data for estimating the diurnally averaged long-wave dust radiative forcing results in significant errors (as much as 50 to 70%). Annually and regionally averaged long wave dust radiative forcing (which account for the diurnal variation of dust) at the top of the atmosphere over Afro-Asian region is 2.6 +/- 1.8 W m(-2), which is 30 to 50% lower than those reported earlier. Our studies indicate that neglecting diurnal variation of dust while assessing its radiative impact leads to an overestimation of dust radiative forcing, which in turn result in underestimation of the radiative impact of anthropogenic aerosols.

Thermal expansion and phase transition in NaVO3

The present x-ray study has been undertook in order to correlate the phase transition in sodium metavanadate NaVO3 crystal with its structural aspects. The thermal expansion behaviour of NaVO3 was studied from room temperature up to 500 C, well beyond the transition temperature.

Phase space methods and the Hamilton-Jacobi form of dynamics

A general analysis of the Hamilton-Jacobi form of dynamics motivated by phase space methods and classical transformation theory is presented. The connection between constants of motion, symmetries, and the Hamilton-Jacobi equation is described.

Computer estimation of the Cd---Hg---Te phase diagram

The compounds CdHgTe and its constituent binaries CdTe, HgTe, and CdHg are semiconductors which are used in thermal, infrared, nuclear, thermoelectric and other photo sensitive devices. The compound CdHgTe has a Sphaleritic structure of possible type A1IIB1IIC6VI. The TERCP program of Kaufman is used to estimate the stable regions of the ternary phase diagram using available thermodynamic data. It was found that there was little variation in stochiometry with temperature. The compositions were calculated for temperatures ranging from 325K to 100K and the compositional limits were Cd13−20Hg12−01Te75−79, Hg varying most. By comparison with a similar compound, Cd In2Te4 of forbidden band width. 88 to .90 e.V., similar properties are postulated for Cd1Hg1Te6 with applications in the infra red region of the spectrum at 300K where this composition is given by TERCP at the limit of stability.

Studies on the phase transitions in thiourea

Thiourea (CS(NH2)2) is one of the few examples of molecular crystals exhibiting ferroelectric properties. The dielectric constant along the ferroelectric axis [100] shows maxima at 169, 177 and 202 K. An inflection point occurs at 170.5 KZ Following Goldsmith and White the phases are named as I (F.E. below 169 K), II (A.F.E. 169 Kphases 11, 111 and IV.4,5 The effect of structural modulation is to destroy the local n glide symmetry, though the overall structure is Pnma. Thiourea also exhibits thermoluminescence after being irradiated with x-rays which is accounted for by defects like NH3+ and NH- formed by the breaking of hydrogen bonds.

Phase-shift randomness in one-dimensional disordered conductors in the quasi-metallic domain

An invariant imbedding method yields exact analytical results for the distribution of the phase theta (L) of the reflection amplitude and for low-order resistance moments (pn) for a disordered conductor of length L in the quasi-metallic regime L<phases. A direct proof of the validity of the random-phase assumption for studying scaling of resistance in one dimension is thus obtained.