Phase Transition Parameters Research Articles

Quantum phase transitions in theJ-J' Heisenberg andXYspin-½ antiferromagnets on a square lattice: finite-size scaling analysis

We investigate the critical parameters of an order-disorder quantum phase transitions in the spin-1/2 $J-J'$ Heisenberg and XY antiferromagnets on square lattice. Basing on the excitation gaps calculated by exact diagonalization technique for systems up to 32 spins and finite-size scaling analysis we estimate the critical couplings and exponents of the correlation length for both models. Our analysis confirms the universal critical behavior of these quantum phase transitions: They belong to 3D O(3) and 3D O(2) universality classes, respectively.

Vapor pressure of precursors for CVD on the base of platinum group metals

Metal beta-diketonates are broadly used in the processes involving deposition of different types of coatings by CVD. Acetylacetonates of platinum group metals are the most suitable precursors for use in highly effective CVD processes because of their thermal properties, low price of initial ligand and accessible synthesis methods. Much attention is given to the investigation of such an important property as volatility, which is necessary to optimize the parameters of coating deposition. At present, a lot of experiments studying temperature dependencies of saturated vapor pressure of volatile metal chelates have been carried out by different methods. However, the data on vaporization parameters often do not coincide with each other. In order to consider the reasons of such differences, we measured the temperature dependencies of saturated vapor pressure of acetylacetonates of palladium(II) (1), platinum(II) (2), rhodium(III) (3), iridium (III) (4), ruthenium(III) (5) by flow and Knudsen methods. This allowed extending the temperature range within which vapor pressure was measured. The thermodynamic parameters of phase transition from crystal to gas were calculated. The comparison with the literary data obtained by static method with the membrane zero-gauge was carried out; the differences were enunciated and explained. The results obtained in the study were co-processed. The following meanings of the coefficients A and B in equation In P alm = - (A/T) + B were obtained for the investigated complexes: 13425; 22,05 (1); 12737; 20,19 (2); 14290; 22,97 (3); 15391; 25,88 (4); 15228; 25,56 (5) (flow method).

Interaction of a dental filling material eluate and membrane lipids.

The possibility that 4-N,N-dimethyl amino benzoic acid ethylester (DMABEE), a leachable lipophilic component of dental fillings, could interact with biological membranes was investigated. Interaction of DMABEE with phospholipids was studied by two methods: First, by determining the surface pressure/molecular area isotherms at 37degrees C of glycerophospholipids monolayers, using the Langmuir technique: and second, by phase transition parameters in liposomes of the same lipids, using differential scanning calorimetry (DSC). DMABEE clearly interacted, in a concentration-independent manner, with monolayers of saturated phosphatidylcholines (PC, i.e., markers of the outer membrane leaflet) and phosphatidylserines (PS, i.e., markers of the inner membrane leaflet). This interaction increased with increasing acyl length in the lipids and was greater with PC than with PS. These observations with monolayers were confirmed by the studies of liposomes of PC and PS.

Axial Pressure Influence on Dielectric and Ferroelectric Properties of Na 0.5 Bi 0.5 TiO 3 Ceramic

It was shown that axial pressure strongly changes the dielectric and ferroelectric properties of Na 0.5 Bi 0.5 TiO 3 ceramic. This includes: (i) the shift of rhombohedral-tetragonal phase transition to higher temperature, suppressing the dielectric dispersion, and reduction of thermal hysteresis of the electric permittivity which can be evidence of transition nature changes to second order one, and (ii) the decrease of polarization in the long-range ferroelectric phase and the shift of phase decay temperature to higher temperature. Also the thermodynamic parameters of the rhombohedral-tetragonal phase transition were estimated.

The Influence of External Electric Field on Local Orientations and Phase Transitions in Polymer Liquid Crystals (PLCs)

Full Paper: The starting point is our previous study of influence of the internal molecular mean field of dipoledipole interactions on local orientation and phase transitions in polymer liquid crystal (PLC) systems of longitudinal chains. [1, 2] Electric dipoles are created by LC mesogen moieties. The longitudinal PLC is a macromolecule of consecutively copolymerized LC and flexible polymer sequences. We now amplify the model by inclusion of dipole-external electric field interactions. We find that the external fields can seriously modify the local orientational order of the system and affect phase transition parameters dependent on that order. In particular, the external fields induce the formation of disoriented nematic phases with negative values of the second order orientation parameter pP2P for LC sequences in the longitudinal PLCs while the first order parameter pP1P is positive. However, some rapid decreases in pP1P are observed at points of positiveto-negative transitions of pP2P; thus the LC disorientation manifests itself. The limiting case of the monomer liquid crystal (MLC) systems is included also.

Chiral thermodynamics in a magnetic field

We study thermodynamic properties of the QCD vacuum in a magnetic field below chiral phase transition. The hadronic phase free energy in a constant homogeneous magnetic field is calculated in the framework of the chiral perturbation theory at non-zero pionic mass. It is demonstrated that the order parameter of the chiral phase transition remains constant provided temperature and magnetic field strength are related through obtained equation (the phenomenon of ''quark condensate freezing'').

Stability and Structures of Liquid Crystalline Phases Formed by Branched-Chain Phospholipid Diastereomers

Diastereomers of 1-alkyl-2-acylglycerophosphocholines containing 2-methyl-branched fatty acids at the C2 position of the glycerol are synthesized and separated, varying the acyl-chain systematically (m = 13, 15, 17). The physicochemical properties are studied in monolayers at the air−water interface by pressure area isotherms and grazing incidence X-ray diffraction (GIXD) measurements. The phase behavior is examined by a combination of differential scanning calorimetry and time-resolved X-ray diffraction experiments. In monolayers at the air−water interface, the introduction of a methyl-branch disturbs the lateral packing drastically, resulting in phases with strongly tilted chains and an undefined tilt azimuth. The monolayer structures of the diastereomeric fractions cannot be distinguished. The comparison of the lyotropic mesophases of the aqueous dispersions shows comparable structural parameters. The temperature dependency of the d spacings is nonlinear and is discussed in terms of a critical swelling of the bilayer. The present experiments show that the two racemic pairs of the resolved diastereomers (SS + RR/SR + RS) exhibit pronounced differences in the thermodynamic parameters of phase transitions both in two-dimensional (monolayers at the air/water interface) and three-dimensional systems (aqueous dispersions). However, the influence of the chiral carbon atoms is not sufficient to change the structure of the phases despite being able to affect their stability.

Hybridization of low frequency phonon branches and incommensurate instability in Sn2P2Se6

Dispersion curves of the low frequency phonon modes related to the order parameter of the structural phase transition to the incommensurate phase in Sn2P2Se6 crystal are analyzed in the framework of a simple semi-empirical model. It is shown that the phase transition instability appears out of the Brillouin zone center due to the hybridization (eigenvector mixing) of these branches.

Lattice dynamics and the ferroelectric phase transition inSn2P2S6

This paper presents a quasiharmonic model for the lattice dynamics of the ${\mathrm{Sn}}_{2}{\mathrm{P}}_{2}{\mathrm{S}}_{6}$ crystal. The model, constructed as a combination of the valence force field and the polarizable ion model with a majority of potential parameters fixed by auxiliary Hartree-Fock calculations, enables us to calculate vibrational spectra for both the paraelectric and the ferroelectric structure. The calculated eigenvector of the lowest frequency optic mode corresponds to the order parameter of the ferroelectric phase transition and can be identified with the symmetry-breaking soft mode. Interpretation of the obtained results and their comparison with available experimental data is provided.

Comment on "Photon transmission technique for studying multiple phase transitions in a liquid crystal".

Contrary to the recently published results by Ozbek et al. [Phys. Rev. E 59, 6798 (1999)], we argue that the transmitted light intensity through an unoriented sample is by no means a good measure of the order parameters of the different phase transitions of a liquid crystal.

Chiral symmetry and the spectrum of the QCD Dirac operator

The order parameter of the chiral phase transition is directly related to the infrared part of the spectrum of the QCD Dirac operator. This part of the spectrum follows from the low energy limit of QCD which is given by a partition function of weakly interacting Goldstone modes. We find that the slope of the Dirac spectrum is determined by the pion decay constant whereas for $\lambda \ll 1/L^2 \Lambda_{\rm QCD}$ the correlations of the Dirac eigenvalues are given by a random matrix theory with the global symmetries of the QCD partition function. A possible observation of these continuum results in lattice QCD with staggered fermions is discussed.

Saturated Vapor Pressure of Iridium(III) Acetylacetonate

The temperature dependency of the saturated vapor pressure of Ir(acac) 3 has been measured by the method of calibrated volume (MCV), the Knudsen method, the flow transpiration method, and the membrane method. The thermodynamic parameters of phase transition of a crystal to gas were calculated using each of these methods, and the following values of ΔH o T (kJ mol -1 ) and ΔS o T (J mol -1 K -1 ), respectively, were obtained: ○ MCV: 101.59, 156.70 ○ Knudsen: 130.54, 224.40 ○ Flow transpiration: 129.34, 212.23 ○ Membrane: 95.45, 149.44. Coprocessing of obtaining data (MCV, flow transportation method and Knudsen method) at temperature ranges 110-200°C as also conducted: ΔH o T =127.9±2.1 (kJ mol -1 ); ΔS o T =215.2±5.0 (J mol -1 K -1 ).

The Specific Heat of Tetramethylammonium Salts

New measurements of the (N(CH3 )4 )2 MnBr4 specific heat by adiabatic calorimetry around the ferro- paraelastic phase transition shown by the crystal around 276 K are compared with previous calorimetric studies on similar tetramethylammonium bromide compounds. The thermodynamic behaviour of the tribromides and tetrabromides derivatives together with the influence on the phase transition parameters of the cation and halogen molecular substitutions are examined. The thermal relaxation experiments permit to study the behaviour of the crystals thermal conduction as a function of the temperature. Finally, the Landau theory for second order phase transitions is used to describe the thermodynamic behaviour of some of these crystals.

Coupling between a first-order gas-liquid phase transition and a second-order orientational transition in Langmuir monolayers.

The Ginzburg-Landau theory is developed to investigate coupling between a gas-liquid first-order phase transition (FOPT) and an orientational second-order phase transition (SOPT) in Langmuir monolayers. It is found that the coupled SOPT and FOPT takes place simultaneously if the uncoupled FOPT occurs prior to the SOPT with compression, and that in the opposite case, the coupling makes the FOPT take place at a lower pressure, but still behind the SOPT. Gas-liquid phase separation always leads to a decrease in the averaged order parameter of the orientational phase transition, which is qualitatively consistent with experimental data.

Interaction of glycosylated tetraphenyl porphyrins with model lipid membranes of different compositions

Porphyrin derivatives are one of the major groups of compounds applied in photodynamic therapy. Their effect and interaction with different cell organelles and macromolecules is dependent on their chemical structure.We have examined the effect of the molecular structure of porphyrin derivatives on binding to liposomes without net charge and with negative charge. In this study a series of substituted tetraphenyl porphyrins (TPPs) has been used. Two of them, TP(4-OGluOH) 4P and TP(4-OXylOH) 4P, are symmetrically substituted derivatives having four carbohydrate — glucose or xylose — moieties linked to the macrocycle. The other two derivatives, TP(4-OGluOH) 3P and TPF5(4-OGalOH) 3P, were selected as asymmetrically substituted amphiphilic molecules with slightly different hydrophobic character due to the different ligand moieties. Association constants ( K L) of derivatives to liposomes and rate constants of liposome binding have been determined and also the gel-to-liquid-crystalline phase-transition parameters have been monitored in order to determine the type and strength of interaction. According to our measurements, the K L values follow the order TP(4-OGluOH) 3P>TPF5(4-OGalOH) 3P>TP(4-OXylOH) 4P for both types of liposomes. TP(4-OGluOH) 4P is able to bind only to vesicles holding negative charges on their surface. At 18°C, the measured data are best fitted by two exponentials for three TPP derivatives. The binding of TP(4-OGluOH) 4P follows single-exponential kinetics. At 37°C, TP(4-OGluOH) 3P binds to DMPC liposomes according to biexponential kinetics, but all the other binding processes follow monoexponential kinetics under similar conditions. The strongest effect on phase-transition parameters is caused by TP(4-OGluOH) 3P. From the results it can be concluded, that the binding process of TPPs, especially that of the symmetrically substituted derivatives, is drastically influenced by the surface charges of the model membranes. According to present data, the negative surface charges of DMPC/DMPG liposomes can facilitate the association of the symmetrically substituted hydrophilic porphyrins to the liposomes. However, the asymmetrically substituted derivative TP(4-OGluOH) 3P, which is the most lipophilic compound in the present investigation, still has the largest association constant to both neutral and negatively charged liposomes. In general, in both types of liposomes the order of the association constant follows the order of the lipophilicity of the porphyrin derivatives.

Perturbation theory for discotic nematic liquid crystals of axially symmetric molecules: effect of dispersion interaction

We investigate the influence of dispersion interaction on a variety of thermodynamic properties of discotic nematic liquid crystals at the discotic nematic-isotropic transition. We report calculations for a hard oblate ellipsoidal system, superposed with an attractive interaction represented by dispersion interaction subjected to different external pressures ranging from 1 to 300 bar. We consider a model system (which simulates a discotic nematic liquid crystal) in which molecules are assumed to interact via a pair potential having both repulsive and attractive parts. The repulsion part is represented by a repulsion between hard oblate ellipsoids of revolution and is a short range, rapidly varying potential. The attractive potential, a function of centre of mass distance and relative orientation between two molecules, is represented by dispersion interaction. The properties of the reference system and first order perturbation term are evaluated using a decoupling approximation which decouples orientational from translational degrees of freedom. The inclusion of fourth and sixth rank orientational order parameters in the calculation slightly improves the result. The role of pressure on phase transition parameters has also been studied.

Perspectives of phase-transition based sensors and that yttrium-doped BaTiO 3 semiconductor with posistor effect

The synthesis of Y-doped Ba titanate semiconductor Ba 0.998Y 0.002TiO 3 with posistor effect was carried out on the basis of ceramic technology. The influence on conductivity of the bulk ceramic samples in the region of posistor transition temperature of the reducing gases of CO and H 2S was investigated. The unique sensitivity of the material to CO in the concentration range of 0.1–1.5 vol.% at low temperature (108°C) was found. The phenomena is based on the extreme reactivity of ternary grain oxygen atoms at the temperature of phase transition. At the same time the selectivity of the material to CO against H 2S and very low sensitivity to H 2S was found out. It is suggested posistor ceramics of Ba 0.998Y 0.002TiO 3 to be perspective material for low temperature chemical sensors creation. Also posistor effect as well as phase transition parameters influence of the chemically active gases is a new way in chemical sensors development.

Quantizing field-induced magnetic phase in a three-dimensional electron gas

Theory of magnetic-induction bifurcation in the Condon domain phase of a three-dimensional (3D) electron gas under high-magnetic field is presented. The results\char22{}the diamagnetic phase transition temperature and the induction bifurcation values\char22{}are in good agreement with nuclear magnetic resonance data in silver. The temperature dependence of the order parameter of the diamagnetic phase transition\char22{}the magnetization in each Condon domain\char22{}is obtained by a comparison of the theory with the experiment. The mean-field approach has been shown to lead to the universal behavior of an electron gas of 2D and 3D dimensionality.

Calorimetric investigations of phase transitions in the cryolites (NH4)3Ga1−x ScxF6(x=1.0,0.1,0)

The specific heat of the cryolites (NH4)3Ga1−xScxF6 with x=1.0, 0.1, and 0 was measured. The thermodynamic parameters of the phase transitions were determined. A previously unknown phase transition was found in the scandium compound at T=243 K.

Aharonov-Bohm effect, center monopoles and center vortices in SU(2) lattice gluodynamics

SU(2) gluodynamics is investigated numerically and analytically in the (Indirect) Maximal Center gauge at finite temperature. The center vortices are shown to be condensed in the confinement phase and dilute in the deconfinement phase. A new physical object, center monopole, is constructed. We show that the center monopole condensate is the order parameter of deconfinement phase transition. The linking of the vortex worldsheets and quark trajectories is identified with the Aharonov-Bohm interaction in an effective Abelian Higgs theory. We conclude that the confinement in the Maximal Center gauge can be explained by a new mechanism called “ the real superconductor mechanism” .