Photosensitive anisotype n-ZnSe/p-InSe and n-ZnSe/p-GaSe heterojunctions

Anisotype n-ZnSe/p-InSe and n-ZnSe/p-GaSe heterojunctions are obtained for the first time. They are grown on layered crystalline GaSe and InSe substrates by annealing in Zn vapor. It is found that these heterojunctions are sensitive to light in the near-infrared and visible spectral ranges.     

Two-dimensional and layered structures in the discrete φ4 model

The properties of phase transitions in two-dimensional and layered systems are investigated on the basis of a discrete φ⁴ model by numerical and analytical methods. The only parameter a of the discrete φ⁴ model determines the behavior of the system and makes it possible to investigate phase transitions ranging from transitions of the displacement type (a → +0) to order-disorder type (a → +∞). The behavior of a two-dimensional system is investigated in a wide range of values of the parameter a. The temperature dependences of the squared order parameter η²(T) and the phase transition temperature T _c as a function of the thickness N of the system are obtained for three characteristic values of the parameter a using the Monte Carlo method. The properties of phase transitions in the discrete φ⁴ model are investigated on the basis of the mean-field approximation and the independent-mode approximation. The results obtained in the numerical experiments are compared with the analytical approximations. It is shown that the mean-field approximation qualitatively describes the behavior of the phase-transition temperature T _c as a function of the thickness N of the system for a wide range of values of the parameter a, and the independent-mode approximation describes quantitatively, to within 5%, the results of the numerical simulation for small values of a.     

First-principles study of structural and thermodynamic properties of osmium

We employ the first-principles plane wave pseudopotential density functional theory method to calculate the equilibrium lattice parameters of osmium and the thermodynamic properties of hcp structure osmium. The obtained lattice parameters are in good agreement with the experimental data investigated up to 58.2 GPa using radial X-ray diffraction (RXRD) together with lattice strain theory in a diamond-anvil cell and the available theoretical data of others. Through the quasi-harmonic Debye model, the dependencies of the normalized lattice parameters a/a₀ and c/c₀ on pressure P, the normalized primitive volume V/V₀ on pressure P, the Debye temperature Θ_D and the heat capacity C_V on pressure P and temperature T, as well as the variation of the thermal expansion α with temperature and pressure are obtained successfully.     

Phase diagram and exotic spin-spin correlations of anisotropic Ising model on the Sierpiński gasket

The anisotropic antiferromagnetic Ising model on the fractal Sierpiński gasket is intensively studied, and a number of exotic properties are disclosed. The ground state phase diagram in the plane of magnetic field-interaction of the system is obtained. The thermodynamic properties of the three plateau phases are probed by exploring the temperature-dependence of magnetization, specific heat, susceptibility and spin-spin correlations. No phase transitions are observed in this model. In the absence of a magnetic field, the unusual temperature dependence of the spin correlation length is obtained with 0 ≤ J_b/J_a< 1, and an interesting crossover behavior between different phases at J_b/J_a = 1 is unveiled, whose dynamics can be described by the J_b/J_a-dependence of the specific heat, susceptibility and spin correlation functions. The exotic spin-spin correlation patterns that share the same special rotational symmetry as that of the Sierpiński gasket are obtained in both the 1 / 3 plateau disordered phase and the 5/9 plateau partially ordered ferrimagnetic phase. Moreover, a quantum scheme is formulated to study the thermodynamics of the fractal Sierpiński gasket with Heisenberg interactions. We find that the unusual temperature dependence of the correlation length remains intact in a small quantum fluctuation.     

Strong coupling expansion of meson propagator in finite temperature lattice gauge theories

We consider Susskind fermions on a (d+1)-dimensional lattice interacting with aU(n) gauge field at finite temperature. We calculate the meson propagator in an expansion in 1/g ² and 1/d and determine the meson masses. To the order considered the results are identical to those obtained at zero temperature.     

Oscillator parameters of transverse phonons in <Emphasis Type="Italic">a</Emphasis>-domain lead titanate

The temperature behavior of polarized Raman spectra of a-domain PbTiO₃ single crystals has been investigated. The spectra obtained are fit by a damping harmonic oscillator model with allowance for a complex background structure. A full set of oscillator parameters of all phonon modes in the frequency range 12 < ν < 1200 cm^?1 is obtained and their temperature dependences in the range from 300 to 800 K are presented.     

Microscopic theory of second-order and third-order elastic constants for an NaCl crystal

Relations between the second-order and third-order symmetry-independent elastic constants and the energy of interatomic interactions dependent on the mutual arrangement of pairs and triplets of atoms are obtained for crystals belonging to the crystal class O _h. The derived relations and experimental data on the elastic constants are used to calculate four third-order elastic constants and the temperature dependence of the elastic anisotropy factor a(T) for an NaCl crystal. The calculated dependence a(T) is in qualitative agreement with the experimental dependence a _exp(T).     

Level-density parameters in the back-shifted Fermi gas model

The parameters a and δ_eff appearing in the back-shifted Fermi gas model are determined for about 3000 nuclei on the basis of modern estimated experimental data and the proposed systematics. For 272 of these nuclei, the parameters are deduced from experimental data on the cumulative numbers of low-lying levels and on mean spacings between S-wave neutron resonances at the neutron binding energy in the nuclei. For 952 nuclei, the parameter δ_eff is calculated by using the cumulative numbers of low-lying levels and values of the parameter a that were obtained via an interpolation from the points corresponding to the aforementioned 272 nuclei. For the remaining nuclei, the parameters a and δ_eff are obtained on the basis of the proposed systematics. An expression is constructed for taking into account the damping of shell effects with increasing excitation energy of nuclei. The results are compared with those from other studies.     

Folding/unfolding kinetics of lattice proteins studied using a simple statistical mechanical model for protein folding, I: Dependence on native structures and amino acid sequences

The folding/unfolding kinetics of a three-dimensional lattice protein was studied using a simple statistical mechanical model for protein folding that we developed earlier. We calculated a characteristic relaxation rate for the free energy profile starting from a completely unfolded structure (or native structure) that is assumed to be associated with a folding rate (or an unfolding rate). The chevron plot of these rates as a function of the inverse temperature was obtained for four lattice proteins, namely, proteins a1, a2, b1, and b2, in order to investigate the dependency of the folding and unfolding rates on their native structures and amino acid sequences. Proteins a1 and a2 fold to the same native conformation, but their amino acid sequences differ. The same is the case for proteins b1 and b2, but their native conformation is different from that of proteins a1 and a2. However, the chevron plots of proteins a1 and a2 are very similar to each other, and those of proteins b1 and b2 differ considerably. Since the contact orders of proteins b1 and b2 are identical, the differences in their kinetics should be attributed to the amino acid sequences and consequently to the interactions between the amino acid residues. A detailed analysis revealed that long-range interactions play an important role in causing the difference in the folding rates. The chevron plots for the four proteins exhibit a chevron rollover under both strongly folding and strongly unfolding conditions. The slower relaxation time on the broad and flat free energy surfaces of the unfolding conformations is considered to be the main origin of the chevron rollover, although the free energy surfaces have features that are rather complicated to be described in detail here. Finally, in order to concretely examine the relationship between changes in the free energy profiles and the chevron plots, we illustrate some examples of single amino acid substitutions that increase the folding rate.     

Bose–Einstein condensation of a relativistic Bose gas in a harmonic potential

Using semiclassical method, Bose–Einstein condensation (BEC) of a relativistic ideal Bose gas (RIBG) with and without antibosons in the three-dimensional (3D) harmonic potential is investigated. Analytical expressions for the BEC transition temperature, condensate fraction, specific heat and entropy of the system are obtained. Relativistic effects on the properties of the system are discussed and it is found that the relativistic effect decreases the transition temperature T_c but enlarges the gap of specific heat at T_c. We also study the influence of antibosons on a RIBG. Comparing with the system without antibosons, the system with antibosons has a higher transition temperature and a lower Helmholtz free energy. It implies that the system with antibosons is more stable.     

Optical absorption in tin monoselenide single crystal

Optical absorption in single crystals of tin monoselenide has been measured at room temperature with plane polarized light near the fundamental absorption edge. The electric field of the incident light was oriented parallel to the a- and b-crystallographic axes in the plane of cleavage. Results have been analyzed on basis of two- and three-dimensional models. Absorption near the fundamental edge was found to be due to indirect forbidden transition with two phonons involved in the process. The energy gap and phonon energies were found to be 0.948 eV and 54 and 21 meV for the a-axis and 0.902 eV and 41 and 20meV for the b-axis. The two-dimensional model gave results similar to those obtained from the three-dimensional model and could thus be used to describe the main optical properties of SnSe single crystal.     

Dimensionless heat transfer correlations for estimating edge heat loss in a flat plate absorber

Data obtained from heat transfer relations discretized with the finite element method were used in developing dimensionless correlations, which led to determining prediction equations for the average edge temperature of a flat plate absorber. For a prescribed flux, if parameters like the incident radiation intensity, edge insulation thermal conductivity and ambient temperature are known, the value of the edge temperature variable is immediately determined. A range of edge-to-absorptive area ratios is considered, as well as the effects of the edge insulation on enhancing thermal performance. Notably, the edge loss is high in absorbers with high edge-to-absorptive area ratios and ambient conditions with low h _a and T _a . In extreme operating conditions, however, the loss can be employed of a high proportion. As a result, prediction equations are obtained, which can be employed in design and simulation so as to minimize useful energy losses and thereby improve efficiency.     

Comparative investigations of the P-V-T relationship of NaCl at high pressure and temperature

Two different potential models of molecular dynamics (MD) simulations have been applied to investigate the pressure-volume-temperature (P-V-T) relationship and lattice parameter of NaCl under high pressure and temperature. The first one is the shell model (SM) potentials in which due to the short-range interaction pairs of ions are moved together as is the case in polarization of a crystal due to the motion of the positive and negative ions, and the second one is the two-body rigid-ion Born-Mayer-Huggins-Fumi-Tosi (BMHFT) potentials with full treatment of long-range Coulomb forces. The P-V relationship at 300 K, T-V relationship at zero pressure, and lattice parameter a, have been obtained and compared with the available experimental data and other theoretical results. Compared with SM potentials, the MD simulation with BMHFT potentials is very successful in reproducing accurately the measured volumes of NaCl. At an extended pressure and temperature ranges, P-V relationship under different isotherms at selected temperatures, T-V relationship under different pressures, and lattice parameter a have also been predicted. The properties of NaCl are summarized in the pressure range 0-30 GPa and the temperature up to 2000 K.     

Correlations of particle orientation in monolayer films

Two-dimensional and three-dimensional models of an extended layer that consists of orientation-ally ordered particles are used to describe the ordering in monolayer films. In both models, when the distance between particles of the layer increases, the correlation function decreases to zero (for the parameter of adhesion to the substrate a = 0) or remains constant (for a ≠ 0, i.e., when the energy of interaction between the particles and the substrate is taken into account). In the latter case, this means that the layers have a long-range orientation order. The proposed models of an extended monolayer can be used to interpret data obtained by the light scattering and molecular dynamics methods for Langmuir-Blodgett films with the adhesion parameter a = 0.05 and the particle-particle interaction parameter b = 0.6 in the three-dimensional model or b = 1 in the two-dimensional model. The smaller value of b in the three-dimensional layer model can be explained by a stronger effect of cooperation of particle-particle interactions as compared to the two-dimensional layer model.     

Anomalous states of the crystal structure of (Rb0.1(NH4)0.9)2SO4 solid solutions in the temperature range 4.2–300 K

Crystals of (Rb_0.1(NH₄)_0.9)₂SO₄ solid solutions are studied using x-ray diffractometry. It is revealed that the temperature dependence of the lattice parameter a exhibits an anomalous behavior, namely, the “invar effect” at temperatures above the ferroelectric phase transition point T _c and an anomalous increase in the temperature range from T _c to the liquid-helium temperature. An anomalous increase in the lattice parameter a and an increase in the intensity of Bragg reflections with a decrease in the temperature are interpreted within the model of the coexistence of two sublattices hypothetically responsible for the ferroelectric phase transition. A series of superstructure reflections observed along the basis axes corresponds to a sublattice formed in the matrix of the host structure. Analysis of the ratio between the lattice parameters of these structures allows the inference that, in the temperature range 4.2–300 K, the structure of the crystal under investigation can be considered an incommensurate single-crystal composite.     

An efficient model for laser surface hardening of hypo-eutectoid steels

This paper presents a model able to predict the austenization of hypo-eutectoid steels during very fast heat cycle such as laser hardening. Laser surface hardening is a process highly suitable for hypo-eutectoid carbon steels with carbon content below 0.6% or for low alloy steels where the critical cooling rate is reached by means of the thermal inertia of the bulk. As proposed by many authors, the severe heat cycle occurring in laser hardening leads to the pearlite to austenite microstructures transformation happening to a temperature much higher than the eutectoid temperature A_c1 and, afterwards, all the austenite predicted during the heating phase become martensite during quenching. Anyway, all these models usually generate a predicted hardness profile into the material depth with an on-off behavior or very complicated and time consumed software simulators. In this paper, a new austenization model for fast heating processes based on the austenite transformation time parameter I_p→a is proposed. By means of the I_p→a parameter it is possible to predict the typical hardness transition from the treated surface to the base material. At the same time, this new austenization model also reduces the calculation time. I_p→a was determined by experimental tests and it was postulated to be constant for low-medium carbon steels. Several experimental examples are proposed to validate the assumptions and to show the accuracy of the model.     

Thermal conductivity of liquid rubidium in the interval of 312–873 K

The thermal conductivity λ and the thermal diffusivity a of liquid rubidium were measured by the laser flash method in the temperature interval from the melting point up to 873 K. The measurement error was 4–6%. The data of this paper were compared with the results of other authors. Approximation equations and the table of reference values for the temperature dependence of λ and a have been obtained. The dependence of the Lorentz number on temperature has been calculated.     

p-type characteristics of ZnSe:Li3N grown by a closed Bridgman method

We intend to search a new method to prepare high-quality and large-size p-ZnSe single crystal. In this study, ZnSe:Li₃N single crystal is grown by a vertical Bridgman method using a closed double-crucible. The photoluminescence (PL) spectrum of the as-grown ZnSe:Li₃N crystal at 8 K shows very strong donor–acceptor pair (DAP) and very weak exciton emissions. In order to activate doped Li₃N, ZnSe:Li₃N single crystal is annealed at high temperature in Zn-saturated atmosphere. By selecting suitable annealing conditions, a very strong I₁ emission line related to shallow acceptor is observed. The capacitance–voltage (C–V) characteristics indicate that the annealed ZnSe:Li₃N single crystal is a p-type conduction. Furthermore, the acceptor concentration and ionization energy are estimated by examining the temperature dependences of the free-to-acceptor (FA) emission, the behaviors of Li and N are investigated, and the new emission at 2.34 eV is discussed.