Hydrostatic pressure and temperature dependence of correlation energy in a spherical quantum dot

The combined effects of hydrostatic pressure and temperature on the ground state binding energy of two electrons in a GaAs spherical quantum dot have been studied by using a perturbation approach within the effective-mass approximation. Our results show that an increment in temperature results in a decrease of the correlation energy while an increment in the pressure for the same temperature increases the correlation energy at a particular dot size. In all cases, it is observed that there is a decrease in the correlation energy due to an increase in the dot size with a given temperature and pressure. The combined effects of hydrostatic pressure and temperature affect the correlation energies appreciably for narrower dots only. The correlation contributes 10%–30% to the binding energy. All the calculations have been carried out with finite barriers, and good agreement is obtained with the existing literature values.     

Current correlation functions of ideal fermi gas at finite temperature

Expressions for transverse and longitudinal current-current correlation functions of an ideal Fermi gas describing the current fluctuations induced in the electron system by external probe perpendicular and parallel to the propagation of electron wave, have been obtained at finite temperature. The results obtained for transverse and longitudinal functions are presented for different values of wavelength and frequency at different temperatures. The diamagnetic susceptibility as a function of temperature has also been obtained from transverse current correlation function as its long wavelength and static limit, which smoothly cross over from known quantum values to the classical limit with increase in temperature.     

Adhesion studies of latex film surfaces on the meso- and nanoscale

In this paper the effects of surface roughness and annealing temperature (T) of latex coating films on adhesion are discussed for the different stages of the film formation process. The surface free energy of latex films was assessed in terms of practical work of adhesion (W) (or adherence) using a custom-built adhesion-testing device (ATD), atomic force microscopy (AFM), and contact angle measurements. For preannealed latex films surface roughness averages (R_a) were determined from AFM height images and were related to the values of W obtained from ATD measurements at room temperature. The results obtained using these tests exhibiting surface behavior on different length scales indicate a dependence of the measured adhesion on surface roughness and temperature, as well as on the length scale of the measurements.First preannealed samples were studied, which were obtained by heat treatment above the respective glass transition temperatures (T_g). Increasing the temperature of preannealing resulted in a decrease of the adherence observed in ATD experiments at room temperature. However, on the nanoscale, using AFM, no significant variation of the adherence was observed. This observation can be explained by roughness arguments. Preannealing decreases roughness which results in lower adherence values measured by ATD while for essentially single asperity AFM experiments roughness has an insignificant effect. Specimens were also annealed over a constant period of time (90 min) at different temperatures. At the end of the heat treatment, adhesion was measured at the treatment temperature by ATD. The amplified effect of temperature observed in this case on adherence is attributed to the combination of roughness decrease and increasing test temperature. In a third set of experiments completely annealed samples were studied by ATD as well as by AFM as a function of temperature. With increasing T values ATD showed a decrease in adherence, which is attributed to a decreasing surface free energy of the annealed films at elevated T values. AFM, on the other hand, showed an opposite trend which is assigned to increasing penetration of the tip into the tip/wetting polymer samples versus increasing temperature. Finally, annealing isotherms as a function of time were investigated by ATD in situ at different temperatures. This last set of experiments allowed us to optimize annealing time and temperature to achieve complete curing.     

Grain size dependence of the electric field gradient

Precise measurements of the electric field gradient (EFG) with the perturbed angular correlation method (PAC) in Cd and Zn show significant differences between single-crystalline and poly-crystalline samples. These differences (up to four percent at low temperatures) decrease with increasing temperature and vanish at about 2 60 K. In all samples the lowest EFG values are obtained for single crystals. In poly-crystalline samples the EFG as well as the width of the EFG distribution increase with decreasing grain size. This effect is also observed in semiconducting ZnO.This work was supported financially by the Bundesministerium für Forschung und Technologie.     

Fabrication of Poly-l-lactide Biomaterials with High Mechanical Properties Using Fiber Oriented Pressing

A new technique of dilute solution spinning to obtain poly-l-lactide (PLLA) fiber was presented. PLLA materials were fabricated by using fiber oriented pressing and the effects of pressing pressure and temperature on the mechanical properties and weight average molecular mass of PLLA were investigated. Good oriented fibers with porous structure (0.1 ～ 1 μm) were obtained by a dilute solution spinning method. Bending and shearing strength of PLLA samples first increase and then decrease with the increase of pressing temperature and pressure. The bending strength, shearing strength, and bending modulus reach 238.8 ± 6.5 MPa, 127.5 ± 3.5 MPa and 3.25 ± 0.25 GPa, respectively, if the pressing temperature is 185°C and the pressure is 130 MPa. These values are greater than conventional pressing methods. In addition, the decrease in weight average molecular mass (19.5%) that results from using oriented pressing is lower than that seen (30.0%) by using the conventional pressing method.     

Influence of temperature variations on the entropy and correlation of the Grey-Level Co-occurrence Matrix from B-Mode images

In this work, the feasibility of texture parameters extracted from B-Mode images were explored in quantifying medium temperature variation. The goal is to understand how parameters obtained from the gray-level content can be used to improve the actual state-of-the-art methods for non-invasive temperature estimation (NITE). B-Mode images were collected from a tissue mimic phantom heated in a water bath. The phantom is a mixture of water, glycerin, agar-agar and graphite powder. This mixture aims to have similar acoustical properties to in vivo muscle. Images from the phantom were collected using an ultrasound system that has a mechanical sector transducer working at 3.5 MHz. Three temperature curves were collected, and variations between 27 and 44 °C during 60 min were allowed. Two parameters (correlation and entropy) were determined from Grey-Level Co-occurrence Matrix (GLCM) extracted from image, and then assessed for non-invasive temperature estimation. Entropy values were capable of identifying variations of 2.0 °C. Besides, it was possible to quantify variations from normal human body temperature (37 °C) to critical values, as 41 °C. In contrast, despite correlation parameter values (obtained from GLCM) presented a correlation coefficient of 0.84 with temperature variation, the high dispersion of values limited the temperature assessment.     

One self-consistent closure for chains of equations for Green's functions. An anisotropic heisenberg model

This article present a self-consistent approach to computation of the correlation function in the method of Green's functions. The basis for the approach is representation of the desired Green's function in the form of a chain of fractions that is subsequently closed. The closure is based on the use of concrete relations imposed on the higher-order correlation functions. General expressions for the correlation functions and conditions for self-consistency of the computations are presented. The method has been tested by computing the magnetization and critical temperature in a Heisenberg model with arbitrary anisotropy parameter. We obtain general expressions for these quantities. The critical temperature obtained is less than the corresponding value given by the molecular-field approximation. The latter approximation also leads to an overestimate of the magnetization values. It is shown that no critical transition is possible for any value of the anisotropy parameter. The corresponding inequalities are obtained. The method is compared with a method for self-consistent computation of correlation functions that was proposed earlier by the author.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 1, pp. 46–52, January, 1996.     

Wave packet molecular dynamics–density functional theory method for non‐ideal plasma and warm dense matter simulations

A new wave packet molecular dynamics–density functional theory (WPMD‐DFT) method is proposed for atomistic simulations of non‐ideal plasma and warm dense matter. The method is based on the WPMD approach, where the electronic exchange and correlation effects are treated using an additional energy term taken from DFT. This term is calculated by integration over the mesh values of the wave packet density. The local density approximation is implemented so far. WPMD‐DFT is meant as a replacement for the anti‐symmetrized WPMD (AWPMD) method which is more time consuming and lacks electron correlation. In this paper, we compare the results obtained by WPMD‐DFT, WPMD, AWPMD, classical molecular dynamics, and path integral Monte Carlo methods for the internal energy of the hydrogen plasma in the temperature range 10–50 kK and electron number density from 10²⁰ to 10²⁴ cm^?3. We also demonstrate the ability to handle the simultaneous dynamics of electrons and ions by calculating the electron–ion temperature relaxation. The scalability of the WPMD‐DFT method with the number of electrons is shown for implementations in central processing unit and graphical processing unit.     

Room-temperature reversible spin Hall effect

Reversible spin Hall effect comprising the direct and inverse spin Hall effects was electrically detected at room temperature. A platinum wire with a strong spin-orbit interaction is used not only as a spin current absorber but also as a spin-current source in the specially designed lateral structure. The obtained spin Hall conductivities are 2.4 x 10(4) (Omega m)(-1) at room temperature, 10(4) times larger than the previously reported values of semiconductor systems. Spin Hall conductivities obtained from both the direct and inverse spin Hall effects are experimentally confirmed to be the same, demonstrating the Onsager reciprocal relations between spin and charge currents.     

Evidence from the de Haas-van Alphen effect

The amplitude of the de Haas-van Alphen effect is reduced by collision broadening of the Landau levels approximately as if the temperature were raised by an amountx (the Dingle temperature) which is inversely related to an appropriate relaxation timeτ. Following a discussion of what is involved in obtaining significant estimates ofτ from experimental measurements ofx, the rather meagre available results are reviewed. For fairly pure samples, values ofx are often found which are as much as 50 times what would be expected from the resistive relaxation time; the evidence suggests that such high values ofx are due to small-angle scattering by imperfections, though they may also be due partly to phase smearing effects not connected with collision broadening. The most significant results come from studies of the increases ofx due to controlled additions of small amount of impurity. These increases are generally of much the same order of magnitude as would be expected from the increase or residual resistance, though detailed correlation with resistivity is hardly possible where thex measurements refer to a small part of a complicated Fermi surface. Recent results on impurities in the noble metals are beginning to give some indication of the anisotropy of scattering and because of the relative simplicity of the Fermi surface, a more detailed correlation with resistivity is possible. Particularly interesting anomalous behaviour is obtained for transition metal impurities which give rise to the Kondo effect. The possibility of studying phonon scattering through its effect onx is briefly discussed.     

Carbon solubility and superconductivity in MgB2

Successful replacement of B by C in the series MgB_2−xC_x for values of x upto 0.3 is reported. Resistivity and ac susceptibility measurements have been carried out in the samples. Solubility of carbon, inferred from the observed change in the lattice parameter with carbon content indicates that carbon substitutes upto x=0.30 into the MgB₂ lattice. The superconducting transition temperature, T_c measured both by zero resistivity and the onset of the diamagnetic signal shows a systematic decrease with increase in carbon content upto x=0.30, beyond which the volume fraction decreases drastically. The temperature dependence of resistivity in the normal state fits to the Bloch–Gruneisen formula for all the carbon compositions studied. The Debye temperature, θ_D, extracted from the fit, is seen to decrease with carbon content from 900 to 525 K, whereas the electron–phonon interaction parameter, λ, obtained from the McMillan equation using the measured T_c and θ_D, is seen to increase monotonically from 0.8 in MgB₂ to 0.9 in the x=0.50 sample. The ratio of the resistivities between 300 and 40 K versus T_c is seen to follow the Testardi correlation for the C substituted samples. The decrease in T_c is argued to mainly arise due to large decrease in θ_D with C concentration and a decrease in the hole density of states at N(E_F).     

The effects of annealing on Au/pyronine-B/MD n-InP Schottky structure

Thin film of non-polymeric organic compound pyronine-B has been fabricated on moderately doped (MD) n-InP substrate as an interfacial layer using spin coating technique for the electronic modification of Au/MD n-InP Schottky contact. The electrical characteristics have been determined at room temperature. The barrier height and the ideality factor values for Au/pyronine-B/MD n-InP Schottky diode have been obtained from the forward bias I-V characteristics at room temperature as 0.60 eV and 1.041; 0.571 and 1.253 eV after annealing at 100 and 250 °C, respectively. An increase in annealing temperature at the Au/n-InP Schottky junction is shown to increase the reverse bias leakage current by about one order of magnitude and decrease the Schottky barrier height by 0.027 eV. Furthermore, the barrier height values for the Au/pyronine-B/MD n-InP Schottky diode have also been obtained from the C-V characteristics at room temperature as 1.001 and 0.709 eV after annealing at 100 and 250 °C, respectively. Finally, it was seen that the diode parameters changed with increase in the annealing temperature.     

Effects of sintering temperature on grain growth and the complex permeability of Co0.2Ni0.3Zn0.5Fe2O4 material prepared using mechanically alloyed nanoparticles

Nanoparticle-sized Co_0.2Ni_0.3Zn_0.5Fe₂O₄ was prepared using mechanical alloying and sintering. The starting raw materials were milled in air and subsequently sintered at various temperatures from 600 to 1300 °C. The effects of sintering temperature on physical, magnetic and electrical characteristics were studied. The complex permittivity and permeability were investigated in the frequency range 10 MHz to 1.0 GHz. The results show that single phase Co_0.2Ni_0.3Zn_0.5Fe₂O₄ could not be formed during milling alone and therefore requires sintering. The crystallization of the ferrite sample increases with increasing sintering temperature; which decrease the porosity and increase the density, crystallite size and the shrinkage of the material. The maximum magnetization value of 83.1 emu/g was obtained for a sample sintered at 1200 °C, while both the retentivity and the coercivity decrease with increasing the sintering temperature. The permeability values vary with both the sintering temperature and the frequency and the absolute value of the permeability decreased after the natural resonance frequency. The real part of the permittivity was constant within the measured frequency, while the loss tangent values decreased gradually with increasing frequency.     

Coercivity and squareness enhancement in ball-milled FeCo–MnO nanocomposites

FeCo–MnO nanocomposites were prepared by ball milling mixtures of Fe, Co and Mn, where MnO was obtained from the oxidation of Mn after or during the milling process. The coercivity and squareness of FeCo–MnO nanocomposites increase with increasing milling time. After annealing treatment, the coercivity and squareness increase and exhibit a maximum at 120 h milling time. The temperature dependence of the magnetic properties of FeCo–MnO was also studied and there is a distinct boundary at 120 K. The exchange-bias field and the coercivity decrease quickly with increasing temperature from 30 to 120 K. However, there are no shifted hysteresis loops and the coercivity decreases slightly with increasing temperature from 120 K to room temperature. The enhancement of the coercivity and squareness is mainly attributed to the exchange-bias effects and the reduced magnetic interactions between the FeCo particles by the efficient isolation in MnO matrix.     

Investigating the effect of operating parameters on the open circuit voltage of a passive DMFC

The variations of the open circuit voltages (OCVs) were studied in a passive air-breathing direct methanol fuel cell with an air-breathing cathode using Nafion 115 as the electrolyte membrane. The effects of some operating parameters such as cell temperature, cell orientation, and also methanol concentration on the OCV of fabricated fuel cell were investigated experimentally. The experimental results showed that the OCV values depend strongly on the cell orientation, cell temperature, and methanol concentration. The OCV values decrease with an increase in methanol concentration and cell temperature. Also, the OCV values in vertical orientation are lower than the OCV values in other orientations.     

The temperature-dependent elastic properties of B2-MgRE intermetallic compounds from first principles

The temperature-dependent elastic modulus of MgRE (RE=Y, Dy, Pr, Sc, Tb) intermetallics with B2-type structure are presented from first-principles quasistatic approach, in which the static volume-dependent elastic constants are obtained by the first-principles total-energy method within density functional theory and the thermal expansion is obtained from the quasiharmonic approach based on density-functional perturbation theory. The comparison between the predicted results and the available experimental data for a benchmark material NiAl provides good agreements. At T=0 K, our calculated values of lattice parameter and elastic moduli for MgRE intermetallics show excellent agreement with previous theoretical results and experimental data. With temperature increasing, we find that the elastic constants satisfy the stability conditions for B2 structures and follow a normal behavior with temperature, i.e., decrease and approach linearity at higher temperature and zero slope around zero temperature. In addition, the sound velocities as a function of temperature for the NiAl and MgRE intermetallics are calculated and the relations to phonon spectrums have also been discussed.     

Diagrammatic method for the calculation of short-range order effects in an ordering binary alloy

A statistical thermodynamic theory of short-range order effects in an ordering binary alloy with an arbitrary composition and an arbitrary radius of the interatomic interaction has been constructed in the approximation of ring diagrams for the free energy. For concreteness, a solid solution, which orders according to the Cu₃Au type, has been considered. A formula has been obtained for the Fourier components of the correlation parameters, which is valid for a wide range of variation of concentration and temperature of the alloy. Correlation parameters of the system Cu₃Au with a stoichiometric composition have been calculated according to this formula for given values of the displacement energy for various coordination spheres at several values of temperature and the interaction radius.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 62–67, August, 1988.The authors thank D. M. Sedrakyan and É. M. Kazaryan for a discussion of the results.     

Influence of temperature and frequency on the electrical parameters of thermally evaporated metal-free phthalocyanine H<Subscript>2</Subscript>Pc thin films

The ac electrical properties of metal-free phthalocyanine (H₂PC) thin films have been studied in the frequency range from 10² to 2×10⁴ Hz and in the temperature range from 150 to 475 K. The ac conductivity σ was found to vary as ω^s with the index s≤1. Although these general values of s appear to be consistent with a hopping process, the present σ values do not increase monotonically with temperature. At low frequency, the capacitance and loss tangent were found to be constant over the entire frequency range, in good qualitative agreement with the equivalent circuit model consisting of an inherent capacitance in parallel with a resistive element. Moreover, at constant frequency, the two parameters increased with increasing temperature up to approximately 300 K. Above this temperature, another sharp decrease in both capacitance and loss tangent was obtained. This type of behavior was interpreted in terms of nomadic (delocalized) polarization, which leads to an increase in the dielectric constant. The drastic decrease of the capacitance and loss tangent observed above room temperature is thought to be related to the decrease in the dielectric constant, which results from the inability of the domains to hold the increases in free charge carrier concentration due to the increase of temperature. Received: 6 December 2001 / Accepted: 7 January 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: +972-2/279-6960, E-mail: asaleh@science.alquds.edu     

Electrical conduction and dielectric studies of ZnO pellets

A series of Zinc Oxide pellets sintered at different temperatures was studied by means of dielectric spectroscopy in the wide frequency range of 1–10⁶ Hz and temperature interval from −100 °C to 30 °C. Electrical conductivity was analysed using Jonsher's universal power law, and the values of s were found to decrease with the increase in temperature, which agrees well with the correlation barrier hopping (CBH) model.     

The effects of temperature upon NiO formation and oxygen removal on Ni(110)

Oxygen chemisorption and NiO nucleation and growth on Ni(110) have been studied with low energy electron diffraction and Auger electron spectroscopy. Changes in the Auger peak energies and shapes were shown to occur only upon NiO formation. The effects of step-changes in temperature upon NiO nucleation and growth were studied and it was shown that temperature steps or annealing during the chemisorption regime did not significantly affect either chemisorption or NiO formation. During NiO growth, temperature steps to a higher temperature caused reduced growth rates, while steps to lower temperature caused increased growth rates. The reaction rate constant from the island growth model was calculated and shown to agree with literature data. The values obtained from temperature step measurements agreed within a factor of two with those obtained for reactions without temperature steps. Therefore, no systematic temperature effect upon the NiO nuclei density was observed for Ni(110). The activation energy for growth of NiO was found to be 5.5 kcal/mole. Dissolution of oxygen into bulk nickel was also studied and it was shown that bulk diffusion of oxygen in nickel was not rate controlling. Rather, surface phase transitions were observed which allowed incorporation of oxygen over the temperature range of 150°C to greater than 800°C, depending on the quantity of oxygen already incorporated.