Surface driven transition in a nematic liquid crystal cell

Surface driven reorientation effects in a nematic liquid crystal cell caused by light-induced changes of the anchoring parameters were studied. Theoretical consideration of one-dimensional flat distributions of the director has shown that the director can undergo threshold reorientation between hybrid, homeotropic, and planar alignments as the anchoring energy varies continuously. The threshold reorientation takes place when the reference and light-induced easy axes are perpendicular. In the one-elastic-constant approximation the light-induced transition was found to be of second order as shown by a critical increase of the director thermal fluctuations in the vicinity of the transition point. These effects were experimentally studied in the cells containing 5CB liquid crystal aligned by the photosensitive azo-containing polymer layer. Zh. éksp. Teor. Fiz. 112, 2045–2055 (December 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Creating and destroying vacancies in solids and non-equilibrium grain-boundary segregation

Determining how the vacancies in excess of equilibrium concentration are created and destroyed in solids is crucial for understanding many of their physical characteristics and processes. Grain boundaries are known to be sources and sinks for bulk vacancies, but the exchange that will occur between the grain boundary and the bulk under a stress is still obscure. In the present paper, we show that grain boundaries will work as sources to emit vacancies when a compressive stress is exerted on them and as sinks to absorb vacancies when a tensile stress is exerted. At the same time, this physical process will produce solute non-equilibrium grain-boundary segregation and dilution. A set of kinetic equations is established to describe this physical process. Additionally an attempt has been made to simulate the experimental data with the kinetic equations to justify the physical process.     

Advances in single crystal growth and annealing treatment of electron-doped HTSC

High quality electron-doped HTSC single crystals of Pr_2−xCe_xCuO_4+δ and Nd_2−xCe_xCuO_4+δ have been successfully grown by the container-free traveling solvent floating zone technique. The optimally doped Pr_2−xCe_xCuO_4+δ and Nd_2−xCe_xCuO_4+δ crystals have transition temperatures T _c of 25 K and 23.5 K, respectively, with a transition width of less than 1 K. We found a strong dependence of the optimal growth parameters on the Ce content x. We discuss the optimization of the post-growth annealing treatment of the samples, the doping extension of the superconducting dome for both compounds as well as the role of excess oxygen. The absolute oxygen content of the as-grown crystals is determined from thermogravimetric experiments and is found to be ≥ 4.0. This oxygen surplus is nearly completely removed by a post-growth annealing treatment. The reduction process is reversible as demonstrated by magnetization measurements. In as-grown samples the excess oxygen resides on the apical site O(3). This apical oxygen has nearly no doping effect, but rather influences the evolution of superconductivity by inducing additional disorder in the CuO₂ layers. The very high crystal quality of Nd_2−xCe_xCuO_4+δ is particularly manifest in magnetic quantum oscillations observed on several samples at different doping levels. They provide a unique opportunity of studying the Fermi surface and its dependence on the carrier concentration in the bulk of the crystals.     

About the kinetics of normal crystal growth

Usually, the kinetic coefficient of crystal growth, i.e. the coefficient of proportionality of the crystal growth velocity to the undercooled temperature, is supposed to be a constant parameter. In the present study we show that this assumption is valid only for low undercooling temperatures, and in general it is a function of temperature. The kinetic coefficient was found to be a linear function of the temperature of the crystal/melt interface, which becomes zero for the glass transition temperature. PACS 81.10.AJ     

Phase segregation in driven diffusive systems

Driven diffusive models describe an array of atoms in an external periodic potential, when the motion is damped due to energy exchange with the substrate. The systems of this class have wide application in modeling of charge and mass transport in solids. Recently, the driven diffusive models have been used in tribology, where the driving force emerges due to motion of one of two substrates, which are separated by a thin atomic layer. When a dc force is applied to the atoms, the system exhibits the locked-to-sliding transition. During the transition the system may split in domains of two kinds, the running domains where the atoms move with almost maximum velocity, and the immobile domains (“traffic jams”). We discuss a new model for a 1D chain, where the particles have a complex structure treated in a mean-field fashion: particle collisions are inelastic and also each particle is considered as having its own thermostat. This model exhibits a hysteresis and the “traffic jams” state even at high temperatures due to the clustering of atoms with the same velocity.     

The cooperative effect of electromigration and non-equilibrium vacancies on reactive phase growth

Phase formation under a direct current was analysed for a binary system with non-equilibrium vacancy distribution. It was shown that the non-equilibrium vacancy distribution diminishes the effect of electromigration on the phase formation process: if the electric current augments the phase growth, the resulting vacancy concentration gradient delays it; if the current opposes phase growth, the vacancy concentration gradient maintains it. A comparative analysis is presented involving the results obtained within Gösele and Tu theory (taking into consideration the finiteness of the reaction rate at interfaces).     

溶液中晶体生长动力学的MC计算机模拟研究

用蒙特卡罗计算机模拟方法研究了溶液中晶体表面的生长台阶和生长动力学,着重讨论了表面尺寸、表面粗糙度以及溶液过饱和度对晶体生长速率的影响以及相应的生长机制.     

The influence of silicon on the non-equilibrium surface segregation of carbon in an FeSi 6.3 at% single crystal

The non-equilibrium surface segregation of carbon to the (110) plane of an Fe-Si single crystal was investigated as a function of temperature using Auger electron spectroscopy. It was found that the segregation rates at high temperatures were lower than expected from an extrapolation of low temperature segregation rates. This discrepancy is explained in terms of an interaction between the carbon and silicon atoms on the surface.     

Surface segregation of chromium in a Fe72Cr28(110) crystal

Segregation of chromium to the surface of a Fe₇₂Cr₂₈(110) crystal has been characterized by X-ray photoelectron spectroscopy. Using XPS core level emissions with different electron mean free paths, the profile of chromium concentration from the surface to the bulk has been constructed and compared to results from Ar⁺ ion sputtering experiments.     

A correlation measure in non-equilibrium phase transitions

Bit-number cumulants of a probability distribution give ordered measures of correlations between subsystems. Especially the second one, the bit-number variance (which in equilibrium is connected with specific heat) is discussed with respect to its critical behaviour for special non-equilibrium phase transitions in chemical reaction systems. Also in non-equilibrium the critical behaviour indicates critical correlations.     

Amplitude and phase controlled absorption and dispersion of coherently driven five-level atom in double-band photonic crystal

The absorption–dispersion properties of a microwave-driven five-level atom embedded in an isotropic photonic bandgap(PBG) have been studied. Due to the singular density of modes(DOM) in the isotropic PBG and the dynamically coherence induced by the coupling fields, modified reservoir-induced transparency and quantum interference-induced transparency emerge simultaneously. Their interaction leads to ultra-narrow spectral structure. As a result of closed-loop configuration, these features can be manipulated by the amplitudes and relative phase of the coherently driven fields. The position and width of PBG also have an influence on the spectra. The theoretical studies can provide us with more efficient methods to control the atomic absorption–dispersion properties, which have applications in optical switching and slow light.     

Calculations of the complete morphological phase diagram for nonequilibrium growth of a spherical crystal under arbitrary surface kinetics

Complete morphological diagrams (with stable, metastable, and absolutely unstable regions) were calculated for the problem of morphology selection under the conditions of nonequilibrium growth of a spherical crystal taking into account arbitrary kinetic process rates at the boundary and a linear dependence of the growth rate on supersaturation. The consideration was performed by jointly using linear stability analysis and the principle of maximum entropy production. The principal difference between kinetically and diffusion-controlled crystal growth is the possibility of the coexistence of three or more morphological phases under the same conditions in the former case. It was shown that, at the transition point, the rate of accretion of the growing crystal mass increased in a jump. The jump value was studied as a function of the parameters of the problem.     

Monte Carlo studies of a driven lattice gas. I. Growth and asymmetry during phase segregation

We investigate the effects of an external field on the kinetics of phase segregation in systems with conservative diffusive dynamics. We find that, in contrast to the situation without a field, there are now qualitative differences between the results of microscopic simulations of a 2D lattice model with biased Kawasaki exchanges and those obtained from various modifications of the macroscopic Cahn-Hilliard equation (mCH). While both microscopic simulations and numerical solutions of MCH yield triangular domains, we find that in the former the triangles mainly pointopposite to the field, while in the latter and in new calculations with the mCH they pointalong the field. On the other hand, the rate of growth of the clusters and their final state, bands parallel to the field, are similar. This issue and the question of the mesoscopic behavior of cell dynamical systems is discussed but not resolved.     

Surface kinetics of GaN evaporation and growth by molecular-beam epitaxy

The kinetics of surface processes during the growth of GaN by molecular-beam epitaxy (MBE) with ammonia as the source of reactive nitrogen is studied theoretically and experimentally. A model of surface processes is developed taking into account specific effects of the blocking of NH₃ adsorption sites by Group III and Group V surface species. Parameters of the model (respective kinetic rate constants) are determined from comparison with experimental data. It is shown that the evaporation rate of GaN in ammonia atmosphere is much lower than that in vacuum. Kinetics of GaN growth under gallium-rich and nitrogen-rich conditions are compared. Under nitrogen-rich conditions the GaN surface is predicted to be enriched by NH_x surface radicals, in contrast to the case of growth under gallium-rich conditions or of free evaporation in vacuum. It is shown that use of the nitrogen-rich conditions allows one to increase the growth temperature by 80–90°C compared with the case of gallium-rich conditions or plasma-activated MBE. The increased growth temperature is favorable in improving the optical and electrical properties of the material grown.     

Nucleation kinetics,growth and characterization of guanidinium 3-nitrobenzoate single crystal

A potential organic nonlinear optical guanidinium 3-nitrobenzoate (Gu-3NB) was synthesized. Solubility of Gu-3NB was determined for various temperatures. Meta stable zone width and induction period values were determined in order to optimize the growth parameters. Nucleation parameters, such as interfacial tension, critical radius and free energy of formation of critical nucleus were evaluated. Optically good quality, bulk single crystal of Gu-3NB was successfully grown by slow evaporation method and slow cooling method with the optimized growth parameters. The etching study was performed to ascertain the growth quality of the crystal. The unit cell parameters and the morphology of Gu-3NB single crystal were determined by X-ray diffraction. The grown crystal was subjected to various characterization studies, such as optical, dielectric measurement and mechanical studies.     

Surface segregation in HAYNES 230 alloy

The surface segregation in the Ni-based alloy HAYNES 230 was studied by Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy between 400 and 1100 °C. The qualitative variations of the surface contents of S, P, W, Mo, N, Si, and Mn were determined as a function of annealing temperature and time. It was found that at 925 °C the maximum coverage of sulphur at the alloy surface is in the range 0.06-0.15 monolayers. Chromium evaporation from the HAYNES 230 surface under UHV conditions is clearly evidenced for annealing at 1100 °C.     

Superconductor-normal metal quantum phase transition in dissipative and non-equilibrium systems

Abstract

In physical systems, coupling to the environment gives rise to dissipation and decoherence. For nanoscopic materials, this may be a determining factor of their physical behaviour. However, even for macroscopic many-body systems, if the strength of this coupling is sufficiently strong, their ground-state properties and phase diagram may be severely modified. Also dissipation is essential to allow a system in the presence of a time-dependent perturbation to attain a steady, time-independent state. In this case, the non-equilibrium phase diagram depends on the intensity of the perturbation and on the strength of the coupling of the system to the outside world. In this paper, we investigate the effects of both dissipation and time-dependent external sources in the phase diagram of a many-body system at zero and finite temperatures. For concreteness, we consider the specific case of a superconducting layer under the action of an electric field and coupled to a metallic substrate. The former arises from a time dependent vector potential minimally coupled to the electrons in the layer. We introduce a Keldysh approach that allows to obtain the time dependence of the superconducting order parameter in an adiabatic regime. We study the phase diagram of this system as a function of the electric field, the coupling to the metallic substrate and temperature.