Theoretical Temperature Dependence Research Articles

Influence of the transparency of tunnel barriers in Nb/Al 2O 3/Al/Al 2O 3/Nb junctions on transport properties

The transparency of the tunnel barriers in double-barrier junctions influences the critical current density and the form of the current–voltage characteristics (IVC). Moreover, the barrier asymmetry is an important parameter, which has to be controlled in the technological process. We have performed a systematic study of the influence of the barrier transparency on critical current, I C, and normal resistance, R N, by preparing SIS and SINIS junctions under identical technological conditions and comparing their transport properties. We have fabricated Nb/Al 2O 3/Nb and Nb/Al 2O 3/Al/Al 2O 3/Nb devices with different current densities using a conventional fabrication process, varying pressure and oxidation time. The thickness of the Al middle electrode in all Nb/Al 2O 3/Al/Al 2O 3/Nb junctions was 6 nm. Patterning of the multilayers was done using conventional photolithography and the selective niobium etching process. The current density of SIS junctions was changed in the range from 0.5 to 10 kA/cm 2. At the same conditions the current density of SINIS devices revealed 1–100 A/cm 2 with non-hysteretic IVC and characteristic voltages, I C R N, of up to 200 μV. By comparing the experimental and theoretical temperature dependence of the I C R N product we estimated the barrier transparency and its asymmetry. The comparison shows a good agreement of experimental data with the theoretical model of tunneling through double-barrier structures in the dirty limit and provides the effective barrier transparency parameter γ eff≈300. A theoretical framework is developed to study the influence of the barrier asymmetry on the current–phase relationship and it is proposed to determine the asymmetry parameter by measuring the critical current suppression as function of applied microwave power. The theoretical approach to determine the non-stationary properties of double-barrier junctions in the adiabatic regime is formulated and the results of calculations of the I– V characteristics are given in relevant limits. The existence and the magnitude of a current deficit are predicted as function of the barrier asymmetry.

Ab initio study on the rate constants of SiCl4 + H→SiCl3 + HCl

The reaction SiCl4 + H → SiCl3 + HCl is studied using an ab initio dynamic method. The direct chlorine abstraction of reaction processes ia a C3v symmetry transition state is calculated by the intrinsic reaction coordinate (IRC) method at the UMP2/6-311G(d,p) level. The forward and reverse barriers are refined by UMP4(SDTQ) and UQCISD(T) single point energy calculations using the same basis set of 6-311G(d,p). The forward barrier is calculated to be 24.34 kcal mol−1 at the UMP4(SDTQ) level and 22.62 kcal mol−1 at the UQCISD(T) level, while the reverse barrier is 22.46 and 21.90 kcal mol−1, respectively. The enthalpy of reaction is calculated to be 1.94 kcal mol−1 at the UMP4(SDTQ) level and 0.72 kcal mol−1 at the UQCISD(T) level. The forward rate constants in the temperature range 1500–1800 K are calculated by the conventional and variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The variational effect is important for the calculation of forward rate constants but the SCT correction is small. The CVT and CVT/SCT rate constants of the forward reaction at the UMP4(SDTQ) and UQCISD(T) levels are consistent with the lower limit of the experimental result. However, the theoretical activation energies in the temperature range 1500–1800 K, 28.43 kcal mol−1 at the UMP4(SDTQ) level and 27.10 kcal mol−1 at the UQCISD(T) level, are much higher than the experimental value, 9.54 ± 5.17 kcal mol−1. The theoretical temperature dependence of the rate constants differs from the experimental result.

Approach to the critical parameters calculation in ceramic Hg-containing high-temperature superconductors

Results of field and temperature dependencies investigation of ac-magnetic susceptibility χ ac for ceramic sample HgBa 2Ca 2Cu 3O 8+δ are shown. Measuring have been done using inductive Hartshorne bridge with the previous writing of output signal by computer. Such an approach allows to simplify distinguishing process of χ′ and χ″ components and simultaniously to receive information about the behaviour of several harmonics of ac-magnetic susceptibility. Theoretical temperature dependencies of ac-magnetic susceptibility have been calculated using the modefied critical state model with consideration of granules distribution by sizes.

Low-temperature pyroelectricity

The current state of the set of problems associated with the study and application of the pyroelectric effect at low (T<30 K) temperatures is outlined. The reasons for the qualitatively different temperature dependences of the total pyroelectric coefficient γσ(T) in linear pyroelectrics and ferroelectrics are discussed. An analysis is given of the reasons why the theoretical temperature dependences obtained for the primary pyroelectric coefficient γe(T) adequately describe the experimental γσ(T) dependences for all materials studied. In this connection, the correctness of determining the secondary pyroelectric coefficient γS(T) from the macroscopically measured coefficients of thermal expansion is considered. The potential of thermodynamically nonequilibrium polar media and low-temperature pyroelectric materials is substantiated. The review consists of the following sections: (1) an introduction; (2) the theory of low-temperature pyroelectricity; (3) experimental procedures; (4) discussion of the experimental data: (A) perfect single crystals, (B) the problem of the secondary pyroelectric coefficient, (C) imperfect single crystals; (5) pyroelectric materials for low-temperature applications; and (6) a conclusion.

Characterisation of undoped gallium antimonide grown by metalorganic chemical vapour deposition

Metalorganic chemical vapour deposition grown GaSb films on GaAs have been optimised for combined morphology, electrical and optical properties by variations in substrate temperature and V/III ratio. The transport properties of the films grown at 540°C depend on precursor ratios with a V/III ratio of 0.72 providing hole mobilities of around 500cm2V−1s−1 and hole concentration about 3×1016cm−3. For higher temperatures, the mobility decreases and the carrier concentration increases. Changing the V/III ratio for 540°C growth results in slight mobility loss in Sb-rich conditions and severe degradation in excess Ga conditions. Comparison of experimental and theoretical temperature dependencies of mobility shows good agreement, while differences between measured hole and inferred total impurity concentrations suggest the contribution of deeper compensating impurities. Films grown at lower temperatures were found to have very good optical quality and bandgaps equal to the bulk value of of 0.72eV. Above 500°C, the band tailing starts to develop, when substantial disorder becomes apparent at 560°C and above. This disorder is also observed in the Raman spectra of films grown at these temperatures. For the V/III ratio the highest optical quality was obtained for films grown with V/III=2.0. The excitonic features in absorption are observed for the first time in GaSb.

Physical Substantiation of the True-Strainndash;Temperature Diagram for Polycrystalline bcc Metals

We consider one of the possible versions of physical substantiation of the form of the true-strainndash;temperature diagram for polycrystalline bcc metals from the initial stages of plastic deformation to fracture within the temperature range from completely brittle fracture to the formation of a new grain structure as a result of dynamic recrystallization. We propose a model explaining the character of the temperature dependences of the critical strains corresponding to the transition from one type of dislocation structure to another under continuous loading. For the MChVP molybdenum alloy used as an example, we perform a comparative analysis of the theoretical and experimental temperature dependences of critical strains and demonstrate their good agreement. We also analyze the influence of factors determining the shape and location of the curves of critical strains in the true-strainndash;temperature diagram.

On the temperature dependence of the heterostructure period near the metal-semiconductor phase transition in a vanadium dioxide film

The effect of the nonlocality of the metallic-state instability upon the formation of semiconducting domains is considered theoretically in terms of the “strain” theory of the formation of superlattices consisting of alternating metallic and semiconducting phases in a vicinity of the critical point of the thermodynamically equilibrium phase transition metal-semiconductor in single-crystal vanadium oxide films. The above effect is shown to lead to the stabilization of a heterostructure in the region where the dimensions of the semiconducting and metallic domains are approximately equal. The obtained theoretical temperature dependence of the heterostructure period d is compared with the experimental data.

Coulometric total carbon dioxide analysis for marine studies: measurement and internal consistency of underway TCO2 concentrations

A coulometrically-based SOMMA system for the determination of total dissolved carbon dioxide (TCO2) in a continuous mode was designed and tested at sea. The new continuous technique approached the same high accuracy and reliability associated with prior discrete TCO2 measurements. During three cruises encompassing more than 19 weeks and 6000 continuous TCO2 measurements none of the three different systems tested exhibited any hardware-related failures. We found that coulometer cell lifetimes can greatly exceed prior expectations with many of the titration cells in the continuous mode remaining accurate for up to 72 h at carbon ages exceeding 50 mg C. We suggest a practical definition based on the CRM analyses for changing coulometer cells in the continuous mode. Systematic deviations of the SOMMA pipette volume from a theoretical temperature dependence were identified both from field data comparisons and pipette calibrations. Hence pipettes should be kept at constant temperature or they must be gravimetrically calibrated over the expected temperature range. Comparison of the continuous TCO2 data together with simultaneously measured additional CO2 system parameters showed that the refitted “Mehrbach” dissociation constants for carbonic acid best-represent fCO2 when calculated from TCO2 and alkalinity over a wide range of sea-surface temperatures and salinities. Some remaining systematic differences of calculated–measured fCO2 of up to 9 μatm likely reflect uncertainty in the temperature-dependence of the “Mehrbach” constants as well as possible uncertainty in the alkalinity–salinity relationship used to estimate alkalinity in the consistency checks.

Modelling of irradiation-induced reordering in Ni 3Al on the basis of the migration of point defects

The kinetics of reordering under irradiation was investigated using a chemical reaction model in the case of vacancy- or interstitial-controlled mechanisms. We have calculated the theoretical temperature dependence of the steady-state degree of order in L1 2 Ni 3Al using the results of recent investigations of point defect properties, in various regimes of point defect annihilation. In a given annihilation regime, the temperature dependence of the steady-state degree of order shows a clear difference between interstitial and vacancy mechanisms. An important result is that a significant reordering can only be related to vacancy diffusion, except when the mutual recombination is the main annihilation process.

Temperature dependence of the isotropic hyperfine coupling constants in 1,4-hydroquinone and 1,4-dihydroxynaphthalene cation radicals

The temperature dependences of hydroxyl group proton and ring proton isotropic hyperfine coupling constants (IHFC) of 1,4-hydroquinone (HQ) and 1,4-dihydroxynaphthalene (NQ) cation radicals were measured by electron paramagnetic resonance spectroscopy. The spectral parameters were obtained in the temperature range 150–300 K using various solvents. The hydroxyl group proton IHFC data obtained were fitted to the theoretical temperature dependence model, yielding the hydroxyl group rotation barrier height and completely planar hydroxyl proton IHFC value. Experimental results are discussed with reference to theoretical density functional calculations. The new experimental rotation barrier heights for HQ and NQ cation radicals are 59±4 and 37±4 kJ mol-1 in nitromethane, respectively. © 1998 John Wiley & Sons, Ltd.

Isotope and temperature effects on the 13C and 77Se nuclear shielding in carbon diselenide

A comprehensive theoretical and experimental study of the 13C and 77Se nuclear magnetic shieldings and their rovibrational corrections in carbon diselenide (CSe2) has been undertaken. The 13C and 77Se shielding tensors as well as all their first and second derivatives with respect to the internal displacement coordinates of the molecule have been calculated by several first principles gauge-including atomic orbital (GIAO) methods. Hartree-Fock (HF), multiconfiguration Hartree-Fock (MCHF), and density-functional (DFT) theories have been compared, the latter both in the local density approximation (LDA) and by using two gradient corrected exchange-correlation functionals. The shielding derivatives calculated with MCHF and DFT are very much smaller in magnitude than the derivatives obtained by using HF, being in reasonable mutual agreement. By using the theoretical shielding derivatives and the cubic anharmonic force constants calculated within LDA, together with an experimental harmonic force field, all the first and second order terms in the rovibrational contributions to the shielding constants and anisotropies have been worked out. The contributions to the shielding constants have been calculated for the various isotopomers of CSe2 at several temperatures, and the resulting theoretical temperature dependencies of the shielding constants, the isotope shifts and the temperature dependencies of the isotope shifts have been compared with the experimental results. There is excellent agreement between the theoretical and experimental results for 77Se. The agreement is not quite as good for the (anomalously small) shielding constant of 13C and its rovibrational corrections. Contrary to what has been frequently assumed, none of the first and second order terms in the rovibrational contributions to the shielding constants can safely be neglected. In particular, the first order isotope effect due to change in the bond not directly attached to the observed nucleus is very important. Furthermore, the second order terms — including the bending and even cross terms — are essential in order to give a correct description of the isotope shifts.

Magnetic linear birefringence of europium-gallium garnet

The magnetic linear birefringence of Eu3Ga5O12 in the 78–295 K temperature range is investigated experimentally and theoretically. Good qualitative agreement between the theoretical and experimental temperature dependences of the magnetic linear birefringence is obtained. The magneto-optical anisotropy of Eu3Ga5O12 is analyzed and it is shown that here there is only qualitative agreement between theory and experiment.

First-principles calculation of the phonon frequencies in γ Fe

The total energy, the equilibrium lattice constant, and the bulk modulus of the fcc phase of iron have been calculated by the full-potential LMTO method. The use of a generalized gradient approximation in the calculation of the electronic structure and lattice properties of γ-Fe is discussed. Next, the transverse phonon frequency at the W point of the Brillouin zone of a fcc lattice is calculated by the “frozen-phonon” method in the harmonic approximation. A local minimum has been found in the curve of the variation of the total energy of the system as a function of the amplitude of the atomic displacements corresponding to the chosen normal mode. To take account of anharmonic effects, a pseudoharmonic approximation is used and an effective potential that approximates the curve of the variation of the total energy of the system and depends on the temperature via the correlation function of the mean-square displacement of atoms from their equilibrium positions is constructed. The theoretical temperature dependence of the effective frequency of the phonon mode responsible for the structural phase transition corresponds qualitatively to the experimentally observed dependence. A new interpretation is given for the structural phase transition as a transition of the corresponding phonon mode from the excited to the ground state.

Temperature Dependences of Singlet Methylene Removal Rates

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, 1CH2 (a 1A1), in the temperature range 298−462 K. The temperature dependences of the removal rate constants for N2, CH4, C2H4, C2H6, H2O, H2CO, CH2CO, CH3F, C2F6, and CH3OH were measured and the Arrhenius parameters determined. For the reacting species, CH4, C2H4, C2H6, H2O, H2CO, CH2CO, CH3F, and CH3OH, the rate constants exhibit negative temperature dependences with activation energies covering the range −0.7 ± 0.7 kJ mol-1 for CH3F to −3.5 ± 1.0 kJ mol-1 for CH3OH. In contrast, positive temperature dependences are observed for the nonreacting species, N2 and C2F6, with activation energies of 1.8 ± 0.9 and 1.6 ± 2.0 kJ mol-1, respectively. Good agreement was obtained with previously reported measurements for the N2, CH4, CH2CO, C2H4, and C2H6 temperature dependences. For the nonreacting species, comparison is made with theoretical predicted temperature dependences for c...

Order parameter measurements from the Kossel diagrams of the liquid-crystal blue phases.

Scalar order parameter measurements of the liquid crystal blue phase II (BPII), taken using the Kossel diagram technique, are presented. The temperature dependence of the order parameter is found to follow a curve predicted by the Landau theory. The order parameter data, derived from the diffracted light intensity, are normalized such that the fitted theoretical temperature dependence has a value of unity at a temperature of 0 K, representing perfect order. This normalized order parameter is found to be approximately 0.1 within the blue phase. The theory of the temperature dependence of the blue phase order parameter is summarized, together with the measurement technique employed. The measurements were made possible by use of a high-resolution charge-coupled-device camera, frame grabber, and image-analysis computer software to capture the Kossel diagram images. Also, a high-stability temperature controller and reflection microscope were used.

Theoretical study of specific heat in ferroelastic calomel

The phenomenological model of phase transition in calomel is suggested. This model is constructed by using the singularities theory and the group theory methods. In the framework of the model a theoretical temperature dependence of specific heat is calculated and it is compared with experimental one. From comparison of theoretical and experimental data in anomalous part of specific heat it follows that the phase transition in calomel is the second-order phase transition that is far from the tricritical point.

Mobility of electrons in a quasi-one-dimensional conducting channel on the liquid-helium surface.

The mobility of electrons in a quasi-one-dimensional channel on the surface of liquid helium is studied theoretically at temperatures lower than 1.5 K. The influence on the mobility due to the electron-ripplon interaction and to the electron scattering from helium atoms in the vapor phase is investigated. The nonmonotonic temperature dependence of the mobility below 0.5 K is attributed to the contribution coming from the matrix elements of the scattering operators between different subbands due to the confinement of the electron motion along the channel. It is shown that the results taking electron-electron correlations into account in the complete control approximation differ significantly from the results in the usual one-electron approximation. This allows us to check the role of electron correlations in quasi-one-dimensional electron systems by comparing the theoretical temperature dependence of the mobility with the experimental data.

Effect of plastic deformation on the electrical resistivity of metals

Results from measurements of the electrical resistivity of copper and aluminum conductors under the action of a plane shock wave, and also analysis of the relationship between the electrical resistivity and the hydrostatic pressure and of the theoretical temperature dependence of the electrical resistivity, has made it possible to elucidate the role of defect formation due to the plastic deformation of the metal and to determine its contribution to the increase in electrical resisitivity of the metal. It has been established that the concentration of defects depends on the pressure gradient in a plane shock wave.

Ultrasonic attenuation in α-quartz as a result of E' defect centres

The ultrasonic attenuation (acoustic losses) in α-quartz as a result of relaxation processes at E' series defect centres is calculated. For this purpose use is made of the theoretical temperature dependences of the E' relaxation time, reported elsewhere, based on the pseudo-Jahn-Teller effect and the reaction rate method. A comparison is made of the temperature curve of the acoustic losses at 5 MHz with a literature experimental loss curve of an AT-cut quartz resonator measured following irradiation with neutrons.

Modelling heavy doping effects for low temperature device simulations

A new temperature dependent apparent bandgap narrowing model, appropriate for low temperature device simulations is proposed. The model is based on the decomposition of apparent bandgap narrowing in equilibrium into different contributions at 300K and on the theoretical temperature dependence of each phenomenon. Specific heavy doping effects that can be important at low temperatures are discussed. Temperature dependence of apparent bandgap narrowing in equilibrium is more significant at high concentration of ionized impurities, where apparent bandgap narrowing decreases with decreasing temperature, as a consequence of temperature dependent degeneracy