The effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system

We investigate the effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system by applying the concept of negativity. It is found that the nonlinear couplings favor the thermal entanglement creating. Only when the nonlinear couplings vertical bar K vertical bar are larger than a certain critical value does the entanglement exist. The dependence of the thermal entanglement in this system on the magnetic field and temperature is also presented. The critical magnetic field increases with the increasing nonlinear couplings constant vertical bar K vertical bar. And for a fixed nonlinear couplings constant, the critical temperature is independent of the magnetic field B. (c) 2005 Elsevier B.V. All rights reserved.

Analytical study of the antiferromagnetic Heisenberg films

With the variational cumulant expansion (VCE) method, the thermodynamic behaviors of S = 1/2 antiferromagnetic Heisenberg films in simple cubic lattices are studied analytically. From the analytic properties of the free energy, in principle we are able to calculate analytically the critical temperatures T-c(L) and the thermodynamic functions, to any order cumulant as the functions of the number of L (the hyperlayers in the hyperfilm). Explicit expressions for T-c(L) up to the fourth order are given. A comparison with the existing results for 3-dimensional system is given. The effective range of the interaction is obtained from numerical results.

GROWTH OF WIDE-BAND GAP IMMISCIBLE II-VI ALLOYS BY METALORGANIC VAPOR-PHASE EPITAXY

Although metalorganic vapor phase epitaxy (MOVPE) is generally regarded as a non-equillibrium process, it can be assumed that a chemical equilibrium is established at the vapor-solid interface in the diffusion limited region of growth rate. In this paper, an equilibrium model was proposed to calculate the relation between vapor and solid compositions for II-VI ternary alloys. Metastable alloys in the miscibility gap may not be obtained when the growth temperature is lower than the critical temperature of the system. The influence of growth temperature, reactor pressure, input VI/II ratio, and input composition of group VI reactants has been calculated for ZnSSe, ZnSeTe and ZnSTe. The results are compared with experimental data for the ZnSSe and ZnSTe systems.

Molecular dynamics study on the phase diagrams of linear and branched chain molecules

The particle transfer molecular dynamics is used to study the phase equilibria of linear and branched chain molecules. The scaling of the critical temperature versus chain length is obtained and the critical densities are found to decrease with increasing chain length, which are in agreement with the results of experiment and theory. The phase diagrams of the linear and the branched chain molecules nearly overlap with each other. Moreover, the radial distribution functions of linear and branched chain molecules in gas phase are very similar, but in the liquid phase, they are different for different kinds of chains.

Gas transport properties of a series of cardo polyarylethers

Gas transport of H-2, CO2, O-2, N-2, and CH4 in a series of cardo polyarylethers were examined over a temperature range of 30 similar to 100 degreesC. These polymers include three poly(aryletherketone)s, two poly(arylethersulfone)s, and one poly(aryletherketoneketone). It was found that the large length/diameter ratio of the polymer repeat unit for cardo polyaryletherketoneketone (PEKK-C) and strong intermolecular interaction in hydrogen-bonded polyarylethersulfone (PES-H) and hydrogen-bonded polyaryletherketone (PEK-H) resulted in a considerable increase in gas permselectivity. Alkyl-substituted polyaryletherketone (PEK-A), bearing a pendant bulky propyl group on the cardo ring, simultaneously exhibited 62.5% higher H-2 permeability and 59.8% higher H-2/N-2 permselectivity than unmodified poly(aryletherketone) (PEK-C). The causes of the trend were interpreted in terms of chain packing density, segmental motion ability, steric factor, and intermolecular interaction of polymers, together with gas kinetic diameter and critical temperature data.

Gas transport properties of novel cardo poly(aryl ether ketone)s with pendant alkyl groups

Gas transport of hydrogen, oxygen, nitrogen, carbon dioxide, and methane in four cardo poly(aryl ether ketone)s containing different alkyl substituents on the phenyl ring has been examined from 30 to 100 degrees C. The permeability, diffusivity, solubility, and their temperature dependency were studied by correlations with gas shape, size, and critical temperature as well as polymeric structural factors including glass transition, secondary transition, cohesive energy density, and free volume. The bulky, stiff cardo and alkyl groups in tetramethyl-substituted TMPEK-C resulted in increased H-2 permeability (by 55%) and H-2/N-2 permselectivity (by 106%) relative to bisphenol A polysulfone (PSF). Moreover, the weak dependence of gas transport on temperature in TMPEK-C made it maintain high permselectivities (alpha(H2/N2) in 68.3 and alpha(O2/N2) in 5.71) up to 100 degrees C, exhibiting potential for high-temperature gas separation applications.

Transport property of polyaniline and its molecular weight dependence

The electrical conductivity of polyaniline doped with camphor sulfonic acid (PAn-CSA) was studied. The results indicate that there is a critical temperature (T-c) and the temperature dependence of PAn-CSA conductivity shows metallic and semiconductor characteristics above and below T-c, respectively. The higher the molecular weight of PAn, the lower the T-c. The conductivity was enhanced remarkably when PAn-CSA film was stretched, its room temperature conductivity is up to 750 S/cm when elogonation is 60%; however, T-c was independent of elongation.

Structural and kinetic studies of spin crossover in an Iron(II) complex with a novel tripodal ligand

Configurational and ligand conformational influences on the kinetics of (1)A(1) reversible arrow T-5(2) spin crossover in the Fe(II) complex with the novel tripodal ligand, 1,1,1-tris((N-(2-pyridylmethyl)-N-methylamino)methyl)ethane (tptMetame), have been explored. Despite having six chelate rings and three chiral nitrogen atoms, only one enantiomeric pair of isomers, Delta, SSS, and Lambda, RRR, of the complex ion is observed. The conformation of the three rings forming the upper ''cap'' of the complex structure can be assigned delta or lambda with respect to the 3-fold molecular axis. X-ray data at 300 and 153 K, above and below the critical temperature for the spin transition, show that the conformation of the ligand ''cap'' is the same as the absolute configuration of the complex, with the same Lambda lambda(CAP)(or Delta delta(CAP)) combination prevailing for both the LS ((1)A(1)) and HS (T-5(2)) isomers. Molecular mechanics calculations further show that the ligand energy remains lowest for this Lambda lambda(CAP) (or Delta delta(CAP)) combination at all Fe-N distances over the range spanning the LS and HS isomers. Measurements of the spin crossover relaxation time have been carried out in solution over the temperature range 293-170 K. The observed monophasic relaxation traces are also consistent with the absolute configuration of the complex remaining unaltered during the spin crossover.

Monte Carlo study of the XY-model on quasi-periodic lattices

Monte Carlo Simulations were carried out using a nearest neighbour ferromagnetic XYmodel, on both 2-D and 3-D quasi-periodic lattices. In the case of 2-D, both the unfrustrated and frustrated XV-model were studied. For the unfrustrated 2-D XV-model, we have examined the magnetization, specific heat, linear susceptibility, helicity modulus and the derivative of the helicity modulus with respect to inverse temperature. The behaviour of all these quatities point to a Kosterlitz-Thouless transition occuring in temperature range Te == (1.0 -1.05) JlkB and with critical exponents that are consistent with previous results (obtained for crystalline lattices) . However, in the frustrated case, analysis of the spin glass susceptibility and EdwardsAnderson order parameter, in addition to the magnetization, specific heat and linear susceptibility, support a spin glass transition. In the case where the 'thin' rhombus is fully frustrated, a freezing transition occurs at Tf == 0.137 JlkB , which contradicts previous work suggesting the critical dimension of spin glasses to be de > 2 . In the 3-D systems, examination of the magnetization, specific heat and linear susceptibility reveal a conventional second order phase transition. Through a cumulant analysis and finite size scaling, a critical temperature of Te == (2.292 ± 0.003) JI kB and critical exponents of 0:' == 0.03 ± 0.03, f3 == 0.30 ± 0.01 and I == 1.31 ± 0.02 have been obtained.

Analyse des propriétés électroniques de supraconducteurs à l’aide de la théorie de la fonctionnelle de la densité

Cette thèse traite de la structure électronique de supraconducteurs telle que déterminée par la théorie de la fonctionnelle de la densité. Une brève explication de cette théorie est faite dans l’introduction. Le modèle de Hubbard est présenté pour pallier à des problèmes de cette théorie face à certains matériaux, dont les cuprates. L’union de deux théories donne la DFT+U, une méthode permettant de bien représenter certains systèmes ayant des électrons fortement corrélés. Par la suite, un article traitant du couplage électron- phonon dans le supraconducteur NbC1−xNx est présenté. Les résultats illustrent bien le rôle de la surface de Fermi dans le mécanisme d’appariement électronique menant à la supraconductivité. Grâce à ces résultats, un modèle est développé qui permet d’expliquer comment la température de transition critique est influencée par le changement des fré- quences de vibration du cristal. Ensuite, des résultats de calcul d’oscillations quantiques obtenus par une analyse approfondie de surfaces de Fermi, permettant une comparaison directe avec des données expérimentales, sont présentés dans deux articles. Le premier traite d’un matériau dans la famille des pnictures de fer, le LaFe2P2. L’absence de su- praconductivité dans ce matériau s’explique par la différence entre sa surface de Fermi obtenue et celle du supraconducteur BaFe2As2. Le second article traite du matériau à fermions lourds, le YbCoIn5. Pour ce faire, une nouvelle méthode efficace de calcul des fréquences de Haas-van Alphen est développée. Finalement, un dernier article traitant du cuprate supraconducteur à haute température critique YBa2Cu3O6.5 est présenté. À l’aide de la DFT+U, le rôle de plusieurs ordres magnétiques sur la surface de Fermi est étudié. Ces résultats permettent de mieux comprendre les mesures d’oscillations quan- tiques mesurées dans ce matériau.

Étude du champ magnétique interne de deux matériaux magnétiques et d'un supraconducteur sans symétrie d'inversion.

Cette thèse est divisée en trois parties. Une première section présente les résultats de l'étude de la formation de polarons magnétiques liés (BMP) dans le ferroaimant EuB6 par diffusion de neutrons à petits angles (SANS). La nature magnétique du système ferromagnétique est observée sous une température critique de 15K. La signature des BMP n'apparaît pas dans la diffusion de neutrons, mais ces mesures permettent de confirmer une limite inférieure de 100\AA à la longueur de cohérence des BMP (xi_{Lower}). Dans un second temps, l'étude du LaRhSi3, un supraconducteur sans symétrie d'inversion, par muSR et ZF-muSR nous permet de sonder le comportement magnétique du système dans la phase supraconductrice. Aucun champ magnétique interne n'a été détecté en ZF-muSR sous la température critique (T_c = 2.2K). Cela indique que la phase supraconductrice ne porte pas de moment cinétique intrinsèque. L'analyse du spectre d'asymétrie sous l'application d'un champ magnétique externe nous apprend que le système est faiblement type II par l'apparition de la signature de domaines magnétiques typique d'un réseau de vortex entre H_{c1}(0) et H_{c2}(0), respectivement de 80+/- 5 et 169.0 +/- 0.5 G. Finalement, la troisième section porte sur l'étude du champ magnétique interne dans l'antiferroaimant organique NIT-2Py. L'observation d'une dépendance en température des champs magnétiques internes aux sites d'implantation muonique par ZF-muSR confirme la présence d'une interaction à longue portée entre les moments cinétiques moléculaires. Ces valeurs de champs internes, comparées aux calculs basés sur la densité de spins obtenue par calculs de la théorie de la fonctionnelle de la densité, indiquent que la moitié des molécules se dimérisent et ne contribuent pas à l'ordre antiferromagnétique. La fraction des molécules contribuant à l'ordre antiferromagnétique sous la température critique (T_c = 1.33 +/- 0.01K) forme des chaines uniformément polarisées selon l'axe (1 0 -2). Ces chaines interagissent antiferromagnétiquement entre elles le long de l'axe (0 1 0) et ferromagnétiquement entre les plan [-1 0 2].

Induced random fields in the LiHoxY1-xF4 quantum ising magnet in a transverse magnetic field

The LiHoxY1-xF4 magnetic material in a transverse magnetic field Bxx̂ perpendicular to the Ising spin direction has long been used to study tunable quantum phase transitions in a random disordered system. We show that the Bx-induced magnetization along the x̂ direction, combined with the local random dilution-induced destruction of crystalline symmetries, generates, via the predominant dipolar interactions between Ho3+ ions, random fields along the Ising ẑ direction. This identifies LiHoxY1-xF4 in Bx as a new random field Ising system. The random fields explain the rapid decrease of the critical temperature in the diluted ferromagnetic regime and the smearing of the nonlinear susceptibility at the spin-glass transition with increasing Bx and render the Bx-induced quantum criticality in LiHoxY1-xF4 likely inaccessible.

Instabilities in thermal gravity with a cosmological constant

It is shown that in quantum gravity at finite temperature, the effective potential evaluated in the tadpole approximation can have a local minimum below a certain critical temperature. However, when the leading higher order thermal loop corrections are included, one finds that no static solution exists at high temperature. (C) 2008 Elsevier B.V. All rights reserved.

Structural, electronic, and magnetic entropy contributions of the orbital order-disorder transition in LaMnO(3)

Measurements of X-ray diffraction, electrical resistivity, and magnetization are reported across the Jahn-Teller phase transition in LaMnO(3). Using a thermodynamic equation, we obtained the pressure derivative of the critical temperature (T(JT)), dT(JT)/dP = -28.3 K GPa(-1). This approach also reveals that 5.7(3)J(mol K)(-1) comes from the volume change and 0.8(2)J(mol K)(-1) from the magnetic exchange interaction change across the phase transition. Around T(JT), a robust increase in the electrical conductivity takes place and the electronic entropy change, which is assumed to be negligible for the majority of electronic systems, was found to be 1.8(3)J(mol K)(-1).

Ab initio calculation of the BCC Fe-Al-Mo (Iron-Aluminum-Molybdenum) phase diagram: Implications for the nature of the tau(2) phase

The metastable phase diagram of the BCC-based ordering equilibria in the Fe-Al-Mo system has been calculated via a truncated cluster expansion, through the combination of Full-Potential-Linear augmented Plane Wave (FP-LAPW) electronic structure calculations and of Cluster Variation Method (CVM) thermodynamic calculations in the irregular tetrahedron approximation. Four isothermal sections at 1750 K, 2000 K, 2250 K and 2500 K are calculated and correlated with recently published experimental data on the system. The results confirm that the critical temperature for the order-disorder equilibrium between Fe(3)Al-D0(3) and FeAl-B2 is increased by Mo additions, while the critical temperature for the FeAl-B2/A2 equilibrium is kept approximately invariant with increasing Mo contents. The stabilization of the Al-rich A2 phase in equilibrium with overstoichiometric B2-(Fe,Mo)Al is also consistent with the attribution of the A2 structure to the tau(2) phase, stable at high temperatures in overstoichiometric B2-FeAl. (C) 2009 Elsevier Ltd. All rights reserved.