Understanding high pressure effects on simple molecular system is of great interest for condensed matter science and geophysics. Accessing the static pressure and temperature regions found in planetary interiors is made possible by the development of the Diamond Anvil Cell technique. We developed a double sided resistive heating method for the membrane DAC operating in low pressure (<0.5 mTorr) pressure environment requiring only 175 W input power to reach sample temperatures up to 1300 K. We applied this technique successfully to study molecular solids at high temperatures, such as H2, N2 and CO2. We made an attempt to determine the melting line of hydrogen and present data up to 26 GPa in agreement with literature. Raman spectroscopy of Nitrogen indicates a high stability of the ε molecular phase, while θ phase is only accessible via certain P, T paths. Studies of solid CO2 at high pressure and temperature lead to the discovery of a six-fold coordinated stishovite-like phase VI, obtained by isothermal compression of associated CO2-II above 50 GPa at 530-650 K, or by isobaric heating of CO2-III above 55 GPa. From our X-ray diffraction experiment on isothermally compressed H2O we report a coexistence of ice VII and symmetric ice X from the start of the transition pressure 40GPa to just below 100 GPa and a volume change of 4% across the transition. Vanadium, a transition metal undergoes a phase transition upon compression unlike other elements (Nb, Ta) from its group. We confirm the bcc phase transition to rhombohedral structure at 62 GPa under quasi hydrostatic compression in Ne pressure medium. Compression without pressure medium results in a much lower 30 GPa transition pressure at room temperature and 37 GPa at 425 K, pointing to a positive phase line between the bcc and rhombohedral crystalline phases.

The goal of this work is to provide a detailed description of the freezing mechanism in gold clusters. This is accomplished by using constrained Monte Carlo simulations combined with parallel tempering algorithms to evaluate the free energy barriers for various temperatures with respect to crystalline order parameters on a 456 atom cluster. <p>Our simulation results help us to challenge the usual assumption of classic nucleation theory where nucleation starts at the center of a cluster, showing instead that nucleation is favored by freezing started at the surface. We study simplistic phenomenological models for surface freezing and find that the three phase contact line free energy term must be included in order to properly describe the features of the free energy barriers. <p>Furthermore, we propose an alternative free energy parameter with which we are able to identify a kinetic spinodal temperature where the nucleation barrier disappears and find that the critical cluster size remains finite at the limit of stability of the fluid phase. This result is supported by Molecular Dynamics simulations.

The goal of this work is to provide a detailed description of the freezing mechanism in gold clusters. This is accomplished by using constrained Monte Carlo simulations combined with parallel tempering algorithms to evaluate the free energy barriers for various temperatures with respect to crystalline order parameters on a 456 atom cluster. <p>Our simulation results help us to challenge the usual assumption of classic nucleation theory where nucleation starts at the center of a cluster, showing instead that nucleation is favored by freezing started at the surface. We study simplistic phenomenological models for surface freezing and find that the three phase contact line free energy term must be included in order to properly describe the features of the free energy barriers. <p>Furthermore, we propose an alternative free energy parameter with which we are able to identify a kinetic spinodal temperature where the nucleation barrier disappears and find that the critical cluster size remains finite at the limit of stability of the fluid phase. This result is supported by Molecular Dynamics simulations.

Magnetic nanocrystalline Eu2O3 (0.5 mol %) particles have been synthesized in a silica glass matrix by the sol-gel method at calcination temperatures of 700oC and above. X-ray and TEM studies reveal the nanocrystals with mean sizes in the range 4¡V8 nm, larger in the samples calcined at higher temperatures. The magnetization and magnetic hysteresis of Eu2O3 nanocrystals in the temperature range of 2-300K have demonstrated that the Eu2O3 nanocrystals in these glasses display superparamagnetic state. The temperature dependence of dielectric constant curves demonstrate a broad maximum around Tm ~ 270 K characteristic by diffuse phase transition (DPT). At the highest applied magnetic field 9 tesla, at superparamagnetic phase, the dielectric constant around Tm decreases almost ~ 1.5 (at 2.5 kHz) times compared with that at zero field for the sample calcined at 700¢J (~2 nm). The magnetodielectric effect observed in the glass composite is considered to be affected with the direct consequence of magnetoresistance changes which depends on the magnetic nanoparticle size and separation. Combustion mechanism is closely relate to the thermally activation oxygen vacancy. Such a material might be treated as a potential candidate for device miniaturization.

Band structures of ultrathin films of heavy elements, £\-Sn and Sb, were investigated using first-principle calculations with the inclusion of spin-orbit coupling. The band structures were gradually varied as the physical parameters were adjusted. The band inversion was obtained at the high symmetry point in Brillouin zone, making a topological phase transition. In this study, the band inversion at £F point of the Brillouin zone was predicted in single bi-layer of Sb(111) and single bi-layer and two bi-layers of £\-Sn(111). The topological phase transition is from trivial insulator to topological insulator for single bi-layer of Sb(111). Finally, the topological phase transition is from trivial semi-metal to topological semi-metal for single bi-layer of £\-Sn(111), whereas as it is from topological semi-metal to trivial metal for two bi-layers of £\-Sn(111).

Non-stoichiometric barium titanate (BaTiO3) powder of TiO2-excess compositions has been investigated using both reducing sintering and acceptor-doping. Crystalline phases were analysed by XRD. Attention has been paid to the analysis of the corresponding sintered microstructure by adopting scanning and transmission electron microcopy. Reducing sintering was in the low oxygen partial pressure, so as to dominate the oxygen-deficient. According to the defect chemistry, the purpose of acceptor-doping was the same as reducing sintering. We look out for phenomena which may be indicative that oxygen vacancies generated by acceptor-doping and reducing sintering have resulted in the metastable retention of high temperature hexagonal-BaTiO3 to an ambient temperature. In the Mg-doped study investigated the possibility that Mg2+ substitutes on Ti4+ site rather than the Ba2+ site, as expected from the radii. According to the unknown phase was indexed a supercell of MgTiO3, that showed evidence of Mg2+ dissolves in BaTiO3 and occupies the Ba2+ site. To reduce in a hydrogen atmosphere was a high dark conductivity. The Ti3+ content was determined via colorimetry. Because of the defect chemistry led to oxygen-deficient h-BaTiO3, i.e.BaTi1-xTixO3-x/2. The observed volume expansion behavior under Ar-H2 atmosphere demonstrates the possibility of having various microstructures via control of oxygen partial pressure. The transformation matrix described the relation between the two reciprocal lattices of the twinning. Investigation of reciprocal lattices was shown that ordering oxygen deficient on the BaO3 layer in the twin boundary. There was evidence of XRD patterns and surface energy that explained more and more twins in the microstructure via control of the low oxygen partial pressure. According to this theory, lamellae twins were generated by oxygen-deficient. The hexagonal phase might be also expressed as the cubic BaTiO3 containing twin boundary at BaO3 planes every three layers. That demonstrates the possibility of hexagonal phase retention in BaTiO3 was oxygen vacancies.

Abstract In this study, we employed the Monte Carlo simulation method to investigate the q-state coupled XY model based on the Landau free energy of couple hexatic order and herringbone order proposed by Bruinsma and Aeppli. On two-dimensional triangular lattices simulation results reveal that the q-state coupled XY model will generate a q-state clock phase transition and a XY transition. The unique generated q-state clock phase transition and XY transition will couple in some coupling parameter domain. The novel coupled transitions behavior agree with the phase transition of some kinds of liquid crystal. For example, the three-state Potts phase transition generated by a 3-state coupled XY model and the Sm-A ¡÷Hex-B transition of free ¡V standing two layers liquid crystal are matched. Their heat capacity anomaly is similar and the heat capacity exponent is both closed to £\¡Ü0.3. We also investigated the system of coupled ferromagnetic order and antiferromagnetic order. Adapted the positive coupling parameter on the Hamiltonian of 3-state coupled XY model, the simulation results reveal that the system generate an antiferromagnetic three-state Potts transition. In some parameter domain the antiferromagnetic three-state Potts transition and XY transition are coupled, and become a novel transition. The novel transition may explain the Sm-A ¡÷Hex-B transition of some kinds of liquid crystal which lack herringbone order.

The subject is to study two-dimension phase transition. The nano-magnetic particles dispersed in magnetic fluid can aggregate to form magnetic columns under external perpendicular magnetic field. At some appropriate condition, these magnetic columns are able to form two-dimension lattices. It is a novel mesoscopic system for studying two-dimensional melting transition. By controlling external magnetic field, we explore phenomena of phase transition and defect dynamics. This article can be divided to three parts. In the first part, we study the phase transition with two-dimension lattice forming with magnetic fluid, then we classify the crystal, hexatic and liquid phases in the melting process with translational correlation function and bond-orientational correlation function. In the second part, we analyze the defect dynamics within the lattice with Burgers Vector. In the third part of this article, the external DC magnetic field is replaced with the AC magnetic field, we find that the two-dimension lattices are also formed in low frequency region. The relationship between the two-dimension lattice forming and the AC frequency are studied.

The phase transitions of a newly proposed torsion X-Y model, are studied with molecular dynamics. ForJ3 >0, the influence from J1 term is similar to the term.We found the Torsion X-Y Model and X-Y Model or Coupled X-Y Model all have the Kosterlitz-Thouless transition. (KT) We also confirm that the KT transition is a second order transition , The distribution of angle £c start from dis-ordered to well-ordered. KT starts first before the randomly distributed £c shows preference and ends when the preference is merged. For the J3 <0 case, the KT is also found in J3=-0.0167 and J3=-0.0334 The temperature for negative J3 is lower then that for positive J3 due to the resistant force to £c ordering from the coupled interaction.

Many phenomena associated with nonlinear optics are produced by the light-matter interaction in liquid crystals. Nematic liquid crystals possess the properties of the birefringence and that refractive indexes of nematic liquid crystal vary with temperature. As a light beam propagates in liquid crystals, the light beam experiences changes of refractive indexes because the optical field reorientates molecules or the optical intensity changes the temperature of liquid crystal. Then, some interesting phenomena of optical nonlinearity produce in liquid crystals. This study investigates mainly the nonlinear behaviors in nematic liquid crystals. By etching ITO glasses to control distribution of electric field, we discuss applications in photo-electric field. These works are described as follows: First, a low voltage was applied to a planar nematic liquid crystal cell; the director field can be reoriented using a low intensity. Then, the self-focusing effect produces due to a variation of refractive indexes. The light beam in nematic liquid crystal forms a spatial soliton by producing the effect of the self-focusing to balance the diffraction. Additionally, we study the interaction between solitons. One soliton creates a potential well of refractive index, anther one will be attracted in the potential well. As the separated distance between two solitons and the pumping angle are appropriate, two solitons propagate in the form of spiral. Second, we study the behavior of light in a periodic refractive index medium. The director field of the nematic liquid crystal (NLC) is reorientated in a grating¡Vlike indium-tin-oxide electrode cell by applying a controllable-voltage. The variation of refractive index with voltage varied 0v to 10v was observed by a conoscopic method. Numerical simulations have reproduced the main features of the gradient distribution of refractive index in the waveguide. Several phenomena of a polarized laser beam that propagated in the waveguide with different incident angles and positions have observed by a CCD camera, including solitons, undulate beam, the total internal reflection and beam coupling. Third, at the temperature close to nematic-isotropic phase transition temperature, the variation of refractive index in the liquid crystal becomes obvious to the change of temperature. And, a laser beam can easily reorientate molecules. We changed beam intensity in liquid crystal cell, different nonlinear phenomena were observed. Besides, A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal.