Cluster model of aluminum dense vapor plasma

The chemical model of aluminum vapor plasma, that take into account the formation of neutral and charged clusters, is suggested. Caloric and thermal equations of state and composition of plasma were received using the available information about properties of metal clusters. It is shown, that aluminum vapors are clusterized with decrease of temperature and with increase of density. Pressure dependence on internal energy is calculated and comparison with experimental data is made. The important role of aluminum clusters, especially in an initial phase of the metals vapor heating, is demonstrated. It is shown, that the region of plasma clusterization in gaseous phase agree with known literature data for binodal of vapor-liquid transition from gaseous region. Suggested cluster model may be used to forecast the location of metal vapors binodal. The conductivity of aluminum vapor plasma was calculated. The satisfactory agreement with available experimental data is received.     

Metal-insulator transition in hydrogen and in expanded alkali metals

Two methods are considered for testing the stability of an electron gas to formation of bound states round a pair of protons. In the first, the screened potential for the two protons is set up as a superposition, which is appropriate in a very high density electron gas. The condition for bound state formation is then examined in the two-centre problem. The density thus obtained is in the right density range to accord with the experiment of Hawke et al. for producing cold metallic hydrogen.This has encouraged us to attempt a more ambitious calculation, namely the investigation of the Heitler-London energy of a model H₂ molecule with screened electron-nuclear and electron-electron interactions, the screening being again through appropriate introduction of the Thomas-Fermi screening radius. The merit of this second model is that the theory contains the Heitler-London value of the dissociation energy of the free H₂ molecule in the limit when the density of the electron gas tends to zero. This feature, the binding energy of the diatomic and its importance in distinguishing the metal-insulator transition in hydrogen from those expected to occur in expanded alkali metals is stressed. The second point we stress is that, in both the models discussed above, there is a close connection with the one-centre criterion for bound state formation. Though we have not carried out detailed two-centre calculations for expanded alkali metals, nevertheless some discussion is given of the one-centre bound state criterion in these metals.Some remarks are also made on the dielectric function of molecular crystals, in relation to the insulator-metal transition.     

Critical local-moment fluctuations,anomalous exponents,and omega/T scaling in the Kondo problem with a pseudogap

Experiments in heavy-fermion metals and related theoretical work suggest that critical local-moment fluctuations can play an important role near a zero-temperature phase transition. We study such fluctuations at the quantum critical point of a Kondo impurity model in which the density of band states vanishes as /epsilon/(r) at the Fermi energy (epsilon=0). The local spin response is described by a set of critical exponents that vary continuously with r. For 0相似文献   

Dissociation and dissociative phase transition in dense hydrogen

A simple physical model is proposed for dissociating dense fluid hydrogen. We propose that free dissociated atoms interact via quantum electron-electron exchange analogously to the interaction in the liquid-metal phase of alkali metals. The density dependence of a hydrogen atom’s binding energy in such a quasi-liquid is calculated. It is shown that the transition from the molecular fluid to liquid hydrogen is a first-order phase transition. The critical parameters of the transition are determined: P _c = 72 GPa, T _c = 10500 K, and ρ _c = 0.5 g/cm³. The possibility of the metastable existence of atomic liquid hydrogen in a dissociated molecular fluid under decreased pressure is established.     

Atomic simulation of the vacancies in BCC metals with MAEAM

The formation energy of the mono-vacancy and both the formation energy and binding energy of the di-and tri-vacancy in BCC alkali metals and transition metals have been calculated by using the modified analytical embedded-atom method (MAEAM). The formation energy of each type of configuration of the vacancies in the alkali metals is much lower than that in the transition metals. From minimum of the formation energy or maximum of the binding energy, the favorable configuration of the di-vacancy and tri-vacancy respectively is the first-nearest-neighbor (FN) or second-nearest-neighbor (SN) di-vacancy and the [112] tri-vacancy constructed by two first-and one second-nearest-neighbor vacancies. It is indicated that there is a concentration tendency for vacancies in BCC metals.     

Optical properties and temperature dependence of critical transitions in ZnSe

The dielectric function of ZnSe has been deduced from ellipsometric measurements between 20 K and 380 K. is analysed around each critical point with the standard critical point model. The variations of the different parameters characterising each transition with temperature are presented and analysed. The temperature coefficients of the energies of the critical transitions are given. is essentially governed by the Coulomb interaction near the fundamental gap. Thanks to the high binding energy of the exciton and the low spectral width of the ellipsometer, the fundamental state of the exciton is found completely separated from the first excited states and the continuum at low temperature. In return the strong transition E₁ near the L points of the Brillouin zone can be described equally well with a 2D or an excitonic transition. Received 5 February 1999 and Received in final form 15 June 1999     

Alkali-metal-affected adsorption of molecules on metal surfaces

The study of coadsorption of alkali metals and simple molecules on transition metal surfaces has been a favored topic of research ever since the pioneering work by Langmuir in 1923. The main reasons for this continued interest are both of fundamental and applied nature. There are a number of interesting physical effects, such as work function changes, charge transfer, two-dimensional ordering, bond energy and molecule orientational changes, and altered surface reactive properties, which have been investigated by a large number of different surface analytical techniques. From an applied point of view, alkali metal covered surfaces are important in the areas of electron and ion emission and heterogenous catalysis, for example.In this paper we will give a short review of the adsorption of alkali metals on well-defined transition metal surfaces. The interaction of these adsorbed alkali metals with subsequently adsorbed atomic or molecular species will be treated more extensively. The emphasis will be on recent experiments dealing with well-characterized surfaces. In particular we will consider questions of adsorption energetics and kinetics, but also review in detail the vibrational, electronic, structural and reactive properties of the coadsorbed complex. Based on a wealth of experimental data, several models of the coadsorbed alkali metal-molecule complex will be introduced and discussed.     

Alkali metal and alkali metal hydroxide intercalates of 2s-tantalum disulfide

The alkali metal intercalates of the layered compound 2s-tantalum disulfide were prepared from the respective hexamethylphosphoric triamide solutions of the metals. The c-lattice parameters of the intercalates increased with increase in the crystallographic radii of the metals. All intercalates prepared were superconductors, and the transition temperatures increased as the crystallographic radii of the metals became larger. The intercalates reacted with water to produce hydrogen gas and changed to different intercalates. These had properties similar to those of the corresponding alkali metal hydroxide intercalates prepared from aqueous solutions of the metal hydroxides. The alkali metal hydroxide intercalates, on the other hand, were found to be classified into two groups in terms of the c-lattice parameters; one having c-lattice parameters around 23.8 Å and the other 18 Å. Lithium and sodium hydroxide intercalates belong to the former type, and potassium, rubidium and cesium hydroxide intercalates, including ammonium hydroxide, to the latter. Dried lithium and sodium hydroxide intercalates were also classified in the latter group. In the former case the disulfide was found to intercalate the cations, conserving the ice-like structure of the surrounding water molecules. In the latter, the cations were intercalated in their naked or primary hydrated states, and the interlayer distances were governed by cointercalated hydroxide ions. The observed superconducting transition temperatures were similar for the intercalates with c-lattice parameters around 18 Å irrespective of the particular cation.     

Simulation of phase boundaries using constrained cell models

Despite impressive advances, precise simulation of fluid-fluid and fluid-solid phase transitions still remains a challenging task. The present work focuses on the determination of the phase diagram of a system of particles that interact through a pair potential, ?(r), which is of the form ?(r)?=?4?[(σ/r)(2n)?-?(σ/r)(n)] with n?=?12. The vapor-liquid phase diagram of this model is established from constant-pressure simulations and flat-histogram techniques. The properties of the solid phase are obtained from constant-pressure simulations using constrained cell models. In the constrained cell model, the simulation volume is divided into Wigner-Seitz cells and each particle is confined to moving in a single cell. The constrained cell model is a limiting case of a more general cell model which is constructed by adding a homogeneous external field that controls the relative stability of the fluid and the solid phase. Fluid-solid coexistence at a reduced temperature of 2 is established from constant-pressure simulations of the generalized cell model. The previous fluid-solid coexistence point is used as a reference point in the determination of the fluid-solid phase boundary through a thermodynamic integration type of technique based on histogram reweighting. Since the attractive interaction is of short range, the vapor-liquid transition is metastable against crystallization. In the present work, the phase diagram of the corresponding constrained cell model is also determined. The latter is found to contain a stable vapor-liquid critical point and a triple point.     

Neutron enhancement from laser interaction with a critical fluid

We discuss experimentally and theoretically neutron production from the laser driven explosion of gas clusters prepared near the liquid-gas critical point. We let deuterated methane that was prepared very close to its critical temperature and pressure expand through a conical nozzle to create clusters, and then irradiated those clusters with a high intensity pulse from the Texas Petawatt Laser. After ionization, the clusters explode producing energetic ions, some of which fuse with resultant neutron emission. We show that the critical fluctuations present in the nozzle before the expansion influence the dynamics of neutron production. Neutron production near the critical point follows a power law, which is a signature of a second order phase transition and it is consistent with the Fisher model. This result might be relevant for energy production from fusion reactions.     

High temperature—high pressure neutron scattering experiments on expanded liquid alkali metals

Abstract

We present neutron diffraction investigation of the static structure factor S(Q) of liquid alkali metals expanded by heating towards conditions close to their liquid-vapour critical point. The experiments were designed to obtain the characteristic changes of the macroscopic structure as a function of temperature and density. The data give information on the density dependent changes in the interatomic forces. as the metal-nonmental transition is approached in the expanded liquid metal.     

A model for the metal-non-metal transition in metal-molten-salt solutions

Summary We outline a theory for the metal-non-metal transition occurring in solutions of alkali metals in molten alkali halides as governed by a balance between the binding of ionic clusters by localized electrons and the excess free energy of an ionic assembly screened by metallic electrons. In the model the transition is driven by the composition dependence of the screening length. The theory is amenable to an analytic solution within the mean spherical approximation when Thomas-Fermi screening is used and the ions are described by charged hard spheres.     

二维相变系统Collins模型的统计理论

Kawamura曾利用Collins模型,在忽略4-原子存在的情况下,研究了二维系统的相变。本文进一步考虑4-原子的存在,计算了有空格点存在时系统的吉布斯自由能,研究了4-原子所占百分数对三相点t_t,P_t值以及临界点t_c,p_c值的影响,获得了具有三相点和临界点的完整相图。本文还讨论了相变时各相的面积与温度之间的关系。所得结果与通常的三维物质非常相似。 关键词：     

Holographic insulator/superconductor phase transitions with excited states

We construct a family of solutions of the holographic insulator/superconductor phase transitions with the excited states in the AdS soliton background by using both the numerical and analytical methods. The interesting point is that the improved SturmLiouville method can not only analytically investigate the properties of the phase transition with the excited states, but also the distributions of the condensed fields in the vicinity of the critical point. We observe that, regardless of the type of the holographic model, the excited state has a higher critical chemical potential than the corresponding ground state, and the difference of the dimensionless critical chemical potential between the consecutive states is around 2.4, which is different from the finding of the metal/superconductor phase transition in the Ad S black hole background. Furthermore, near the critical point, we find that the phase transition of the systems is of the second order and a linear relationship exists between the charge density and chemical potential for all the excited states in both s-wave and p-wave insulator/superconductor models.     

The influence of quantum interference effects on the resonance fluorescence spectra of a degenerate three-level atom

The resonance fluorescence spectra of a degenerate three-level atom of the V-type in the field of an intense monochromatic wave with an arbitrary polarization composition are investigated. Analytical expressions are derived for the resonance fluorescence spectra, and the angular distribution of spontaneous fluorescence of atoms is analyzed for the D-line emitted by vapors of alkali atoms. It is shown that the number of lines in the spectrum may decrease in the case of the linear polarization of spontaneous radiation. The radiation relaxation operator is obtained for the D-line of alkali metals in the case when an atom is near the metal surface. Interference effects for such systems are analyzed.     

Temperature dependence of low angle structure factors of liquid alkali metals using electron-ion plasma model

The analytic solution of EIP model in the MSA via the OCP criterion yields temperature dependent results in RPA quite satisfactorily at high temperatures. The same set of parameters produces low and high angle structural properties. The model can work almost near to the critical point if the ion-core radius is allowed to vary systematically in the regionρ<2ρ _c,ρ _c being the critical density. The model can also accommodate the general scaling behaviour observed for the structure factors of liquid alkali metals.     

Determination of the vapor–liquid transition of square-well particles using a novel generalized-canonical-ensemble-based method

The square-well(SW) potential is one of the simplest pair potential models and its phase behavior has been clearly revealed, therefore it has become a benchmark for checking new theories or numerical methods. We introduce the generalized canonical ensemble(GCE) into the isobaric replica exchange Monte Carlo(REMC) algorithm to form a novel isobaric GCE-REMC method, and apply it to the study of vapor–liquid transition of SW particles. It is validated that this method can reproduce the vapor–liquid diagram of SW particles by comparing the estimated vapor–liquid binodals and the critical point with those from the literature. The notable advantage of this method is that the unstable vapor–liquid coexisting states,which cannot be detected using conventional sampling techniques, are accessed with a high sampling efficiency. Besides,the isobaric GCE-REMC method can visit all the possible states, including stable, metastable or unstable states during the phase transition over a wide pressure range, providing an effective pathway to understand complex phase transitions during the nucleation or crystallization process in physical or biological systems.     

Universal low-temperature properties of frustrated classical spin chain near the ferromagnet-helimagnet transition point

The thermodynamics of the classical frustrated spin chain near the transition point between the ferromagnetic and the helical phases is studied. The calculation of the partition and spin correlation functions at low temperature limit is reduced to the quantum mechanical problem of a particle in potential well. It is shown that the thermodynamic quantities are universal functions of the temperature normalized by the chiral domain wall energy. The obtained behavior of the static structure factor indicates that the short-range helical-type correlations existing at low temperatures on the helical side of the transition point disappear at some critical temperature, defining the Lifshitz point. It is also shown that the low-temperature susceptibility in the helical phase near the transition point has a maximum at some temperature. Such behavior is in agreement with that observed in several materials described by the quantum s = 1/2 version of this model.