X-ray studies of the smectic B phase of the 4-bromobenzylidene-4′-alkoxyanilines

The results of X-ray studies of the nine members (4?÷?12) of the 4-bromobenzylidene-4′-alkyloxyaniline homologous series (nBBAA) in smectic B (SmB) and smectic A (SmA) phases are presented. Interestingly, no SmC phase was identified. The thickness of smectic layers in the SmA and SmB phases was estimated showing unexpectedly that in SmA it is bigger than in the SmB. For the SmB phase, orthorhombic unit cell parameters were determined as a function of temperature. The obtained data are discussed in relation to rotational dynamics.     

Calculation of the phase diagram of the TiZr alloy and investigation of the tendency toward separation of the ω phase

The electronic, structural, and thermodynamic properties of the TiZr equiatomic alloy have been calculated in terms of the electron density functional theory and Debye-Grüneisen model. The calculated lattice parameters a and c/a agree well with experimental data for the α, ω, and β phases. It has been shown that the ω phase is stable at atmospheric pressure and low temperatures, and it remains energetically more favorable up to T = 600 K. In the temperature range 600 K < T < 900 K, the α phase becomes stable, and above 900 K, the β phase of the TiZr alloy is stable. The phase diagram constructed in this study agrees qualitatively with the available experimental data. A tendency toward separation of the TiZr equiatomic alloy in the ω phase has been analyzed. It has been demonstrated that, in the ground state, the TiZr equiatomic alloy in the ω phase exhibits a tendency toward ordering rather than toward phase separation.     

Extraction of the CKM phase γ from the charmless two-body B meson decays

Using all experimentally measured charmless B → PP, PV decay modes, where P(V) denotes a light pseudoscalar(vector) meson, we extract the CKM angle γ by a global fit. All hadronic parameters are determined from the experimental data, such that the approach is least model dependent. The contributions of the various decay modes are classified by the topological weak Feynman diagram amplitudes, which are determined by the global fit.To improve the precision of the approach, we consider the flavor SU(3) breaking effects of the topological diagram amplitudes of the decay modes by including the form factors and decay constants. The fit result for the CKM angle γ is( 69.8 ± 2.1 ± 0.9)°. It is consistent with the current world average values but has a smaller uncertainty.     

Structure of the B2 phase in Ti–25Al–25Mo alloy

The structure of the B2 phase has been investigated in Ti–25Al–25Mo alloy using Rietveld refinement of X-ray and neutron diffraction data in as-cast and solution-treated conditions. Different initial structure models have been used for the refinement. The site occupancy of the various chemical constituents in the B2 phase has been calculated and compared with earlier investigations. The relative merits of neutron diffraction over X-ray diffraction for structural refinement of the B2 phase in Ti–25Al–25Mo alloy have been demonstrated.     

Ab initio study of the (2 × 2) phase of barium on graphene

We present a first-principles density functional theory study on the structural, electronic and dynamical properties of a novel barium doped graphene phase. Low energy electron diffraction of barium doped graphene presents clear evidence of (2 × 2) spots induced by barium adatoms with BaC₈ stoichiometry. First principles calculations reveals that the phase is thermodynamically stable but unstable to segregation towards the competitive BaC₆ monolayer phase. The calculation of phonon spectrum confirms the dynamical stability of the BaC₈ phase indicating its metastability, probably stabilized by doping and strain conditions due to the substrate. Barium induces a relevant doping of the graphene π states and new barium-derived hole Fermi surface at the M-point of the (2 × 2) Brillouin zone. In view of possible superconducting phase induced by foreign dopants in graphene, we studied the electron–phonon coupling of this novel (2 × 2) obtaining λ = 0.26, which excludes the stabilization of a superconducting phase.     

The magnetic circular dichroism spectrum of the 1 B 2u ←1 A 1g transition of benzene in the vapour phase

The magnetic circular dichroism (MCD) spectrum of the ¹ B _2u ←¹ A _1g transition of benzene has been measured in the vapour phase. Many of the bands due to transitions between single vibronic levels display A terms. It has been shown that the angular momentum arises by vibronic mixing both of the ¹ E _1u state with the ¹ B _1u state and of the ¹ E _2g states with the ¹ A _1g ground state by e _2g vibrations. The magnitudes and signs of the experimental and calculated ratios, A/D, for the A ₀ ⁰ vibronic origin are in excellent agreement. Two strong MCD progressions of opposite sign with B-term dispersion have been observed in regions of low absorption. These are identified with vibronic origins due to the v ₈ and v ₉ e _2g modes. By contrast the MCD spectrum of hexadeuterobenzene vapour has a much lower magneto-rotational strength and displays none of the striking features of the benzene MCD spectrum.     

Kinetics of A1 ↔ B2 phase transformations in Cu-Pd alloys

The effect of different initial states on the kinetics of formation of an ordered structure in the Cu-Pd alloys with palladium contents of 47, 49, 50, and 55 at % is studied. The activation energy of atomic ordering in an equiatomic CuPd alloy after severe plastic deformation has been estimated. A nearly single-phase ordered B2-type state in the Cu-49 at % Pd and Cu-50 at % Pd alloy has been achieved. For this phase state of the alloys, the lowest reported room temperature resistivity has been achieved. The temperature boundaries of phase transformations in the investigated Cu-Pd alloys are found to be in contradiction with the generally accepted phase diagram.     

Quantum phase transition of the Jaynes-Cummings model

Herein, we propose an experimentally feasible scheme to show the quantum phase transition of the Jaynes-Cummings(JC) model by modulating the transition frequency of a two-level system in a quantum Rabi model with strong coupling. By tuning the modulation frequency and amplitude, the ratio of the effective coupling strength of the rotating terms to the effective cavity(atomic transition) frequency can enter the deep-strong coupling regime, while the counter-rotating terms can be neglected. Thus, ...     

On the ferroelectric phase transition in polytypes of β-TlInS2 crystals

Permittivity measurements and x-ray diffraction study were performed for polytypes c and 2c of β-TlInS₂ crystals in the temperature range T = 160–250 K. Substantial differences are revealed in the temperature, sequence, and character of the structural phase transitions associated with the formation of incommensurate modulated structures and the occurrence of a ferroelectric state in these polytypes.     

Influence of shifted phase value on the output spectra of DFB lasers with phase shifter

DFB lasers with π/2 phase shifter can run stably on single mode and providevery high side mode suppression ratio (MSR). This attractive feature may be degraded ifphase shift deviates from π/2. In this paper, we analyze theoretically how the output spectraand side mode suppression ratio are influenced by the deviation value of phase shift from π/2.     

Anisotropy of the solid–liquid interface properties of the Ni–Zr B33 phase from molecular dynamics simulation

Solid–liquid interface (SLI) properties of the Ni–Zr B33 phase were determined from molecular dynamics simulations. In order to perform these measurements, a new semi-empirical potential for Ni–Zr alloy was developed that well reproduces the material properties required to model SLIs in the Ni_50.0Zr_50.0 alloy. In particular, the developed potential is shown to provide that the solid phase emerging from the liquid Ni_50.0Zr_50.0 alloy is B33 (apart from a small fraction of point defects), in agreement with the experimental phase diagram. The SLI properties obtained using the developed potential exhibit an extraordinary degree of anisotropy. It is observed that anisotropies in both the interfacial free energy and mobility are an order of magnitude larger than those measured to date in any other metallic compound. Moreover, the [0 1 0] interface is shown to play a significant role in the observed anisotropy. Our data suggest that the [0 1 0] interface simultaneously corresponds to the lowest mobility, the lowest free energy and the highest stiffness of all inclinations in B33 Ni–Zr. This finding can be understood by taking into account a rather complicated crystal structure in this crystallographic direction.     

XY theoritical phase diagrams of the thiospinels AB2xB′2−2xS4

Randomly diluted frustrated helimagnetic and ferromagnetic systems with spinel structure, ZnCr_2xAl_2−2xS₄ and CdCr_2xln_2−2xS₄, respectively, were studied in the framework of the XY model. Using the replica method, the variational principle and taking into account exchange integrals up to the fourth nearest neighbour (nn), J₄ in the case of HM and up to the second (nn) J₂ in the case of FM, the T vs. x magnetic phase diagrams were established. The dramatic effect of frustration is evidenced, which leads to the disappearance of long-range order and the appearance of spin-glass state for great dilution. The obtained results are in agreement with those obtained by magnetic measurements.     

Dynamics of the normal–superconductor phase transition and the puzzle of the Meissner effect

The analysis of Pippard (1950) for the growth of the normal phase into the superconducting phase in the presence of a magnetic field H>H_c$H>H_c$ is applied in reverse to the case H<H_c$H<H_c$ (H_c=$H_c=$ critical magnetic field). We carry out the analysis both for a planar and a cylindrical geometry. As the superconducting phase grows into the normal phase, a supercurrent is generated at the superconductor–normal phase boundary that flows in direction opposite to the Faraday electric field resulting from the moving phase boundary. This supercurrent motion is in direction opposite to what is dictated by the Lorentz force on the current carriers, and in addition requires that mechanical momentum of opposite sign be transferred to the system as a whole to ensure momentum conservation. In the cylindrical geometry case, a macroscopic torque of unknown origin acts on the body as a whole as the magnetic field is expelled. We argue that the conventional BCS-London theory of superconductivity cannot explain these facts, and that as a consequence the Meissner effect remains unexplained within the conventional theory of superconductivity. We propose that the Meissner effect can only be understood by assuming that there is motion of charge in direction perpendicular to the normal–superconductor phase boundary and point out that the unconventional theory of hole superconductivity describes this physics.     

Study of the structural phase transformation,and optical behavior of the as synthesized ZnO–SnO2–TiO2 nanocomposite

Bulk nanocomposites ZnO–SnO₂–TiO₂ were synthesized by solid-state reaction method. The X-ray diffraction patterns and Raman spectra of bulk nanocomposite as a function of sintering temperature (700 °C–1300 °C) indicate that the structural phases of SnO₂ and TiO₂ depend on the sintering temperature while the ZnO retains its hexagonal wurtzite phase at all sintering temperatures and SnO₂ started to transform into SnO at 900 °C and completely converted into SnO at 1100 °C, whereas the titanium dioxide (TiO₂) exhibits its most stable phase such as rutile at low sintering temperature (≤900°C) and it transforms partially into brookite phase at high sintering temperature (≥ 900 °C). The optical band gap of nanocomposite ZnO–SnO₂–TiO₂ sintered at 700 °C, 900 °C, 1100 °C and 1300 °C for 16 hours is calculated using the transformed diffuse reflectance ultra violet visible near infra red (UV–VisNIR) spectra and has been found to be 3.28, 3.29, 3.31 and 3.32 eV, respectively.     

Study of point defects for B2 structures with a ω-type martensitic phase

The molecular dynamics method has been used to study the effects of the inheritance from point defects in B2 structures having an -like martensitic transformation with different final reaction products. For order defects, antistructural atoms, and their simplest complexes it has been shown that in several cases the number of atoms in the defects increases sharply for martensite during the transition to austenite. Here the fraction of chemical energy accumulated by the defects grows. A certain type of defect in the B2 structure in martensite corresponds to the appearance of high-energy linear disordered chains in directions of the [111]_B2 type. This can play a substantial role in the determine the path of the reverse martensitic transformation.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 54–59, June, 1995.     

The meaning of the terms “phase” and “phase transition”

Various meanings of the terms phase and phase transition encountered in scientific literature are discussed. These terms supplement each other and cover only together all the macroscopic situations which are now denoted by this term.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 67–71, August, 1988.     

Effect of manganese ions concentration on the anatase–rutile phase transformation of TiO2 films

The phase structures of Ti_1?xMn_xO₂ (0?x<0.08) films synthesized by sol–gel spin coating have been investigated. The effect of Mn dopants on the stability of titanium dioxide (TiO₂) was studied by X-ray diffraction and Raman spectra for isochronally annealed samples. The increased Mn dopant concentration decreased the onset temperature of anatase–rutile (A–R) phase transformation. The calculated activation energy for the phase transformation decreased from 173.6 to 89.4 kJ/mol with Mn dopants concentration increasing from 0% to 7.51%. The Mn ions incorporated into the TiO₂ lattice reduce the rutile nucleation barrier and promote the nucleation rate.     

Evidence of antiferroelectric-like ordering from the study of dielectric permittivity–voltage behavior in bent core liquid crystalline material showing B2 phase

Detailed dielectric relaxation behavior of a bent core liquid crystalline material showing B₂ phase has been investigated. Only one relaxation mode is detected in the dielectric relaxation spectrum which was attributed to the long-axis rotation process. Though the field-dependent dielectric study does not reveal any other relaxation mode, the appearance of two humps in the dielectric permittivity–voltage spectrum, similar to the antiferroelectric (AFLC) case, confirms the signature of weak AFLC ordering in the B₂ phase.     

Contribution to the solution of heisenberg's model for phase transitions in the presence of an external field: The A2 → B2 disorder-order transition in nonstoichiometric binary alloys

Abstract

In this article, the A2 → B2 transition is studied in nonstoichiometric binary alloys: Heisenberg's model for phase transitions in the presence of an external field is reduced to the resolution of a couple of differential equations. Introduction of the Gibbs grand canonical ensemble enables us to show that the interaction Hamiltonian between a pair ij and the rest of the crystal is written simply as ? _int =—μ[Ncirc] (where [Ncirc] is the particle number operator corresponding to the long-range-order parameter and μ is its intensive conjugate variable). Among others, the state vector and the density matrix of the pair ij, and the grand partition function are obtained as functions of the intensive variables of the problem. The long-range-order parameter is not a constant of the motion: it fluctuates with time in the vicinity of the transition.