Dielectric and Pyroelectric Properties of （Pb0.50Sr0.50）TiO3 Ceramics

Lead strontium titanate （Pb0.50Sr0.50）TiO3 （PST） ceramics are prepared by the traditional ceramic processing. The dielectric constants and dielectric loss have been investigated in a temperature range from 25℃ to 300℃. The maximum dielectric constants for unpoled and poled samples are 9924 and 9683, respectively. The temperatures of phase transition for unpoled and poled samples are observed at 153℃ and 157℃, respectively. The phasetransition temperatures for unpoled and poled samples are not equal, which results from the polarization state of the domains. The remnant polarization and the coercive electric field are 18 uC/cm^2 and 6 kV/cm, respectively, from polarization-electric field （P - E） hysteresis loop. The temperature dependence of pyroelectric coefficients of the PST ceramics is measured by a dynamic technique. The dielectric constant and loss Lan δ of the poled PST ceramics are 813 and 0.010, respectively. The pyroelectric coefficients and figure of merit are 294 uC/cm^2 K and 13.6 × 10^-6 pa^-0.5, respectively, at room temperature 25℃and frequency lOOHz.     

Electron transport properties of ß-phase Co0.50Ti0.50 alloy films with various degrees of long range order

The influence of the structural disorder on the electron transport properties of Co_0.50Ti_0.50 alloy films in a temperature range of 4.2-300 K has been investigated at zero and 0.5 T of magnetic field. The disordered state in the alloy films was obtained by vapor quenching deposition onto substrates cooled by liquid nitrogen. The changes in the transport properties of the alloy films caused by the order-disorder structural transformation and the external magnetic field are explained by the analyses using various models for the electron transport in the disordered systems. Received 18 May 1999 and Received in final form 18 January 2000     

Opacity calculations for Non-Local-Thermodynamic-Equilibrium plasmas

In this paper, we presented a method to calculate the spectral-resolved opacity for Non-Local-Thermodynamic-Equilibrium (non-LTE) plasmas. By solving the rate equations, we get the population. In the rate equations, configuration-averaged rate coefficients are used and the cross sections are calculated based on the first-perturbation theory. Using the detailed configuration accounting with the term structures treated by the unresolved transition array model, we calculated the spectral-resolved opacity of Al plasmas. The results are compared with those of other theoretical models. From the comparison, we can see that the present results fit well with other models for low-Z plasmas. For high-Z plasmas, we will give detailed discussion in the future.     

Increase of grain boundary resistance with time by impedance spectroscopy in La0.50Ca0.50MnO3+δ at 77 K

In order to investigate the effects of grain boundaries on the electrical properties of La_0.50Ca_0.50MnO_3+δ impedance spectroscopic studies have been carried out at 77 K. With time there is irreversible formation of domains, which promotes the insulating behaviour of this charge ordered material. As time passes, the grain boundary resistance increases, showing the canted spin nature of the material. However, after 150 hours the grain boundary resistance becomes saturated, indicating that most of the metastable states have relaxed and transformed into stable charge ordered ones, most probably of antiferromagnetic nature. An equivalent circuit model, R₁(R₂C₂)(R₃C₃), i.e., a resistor–capacitor network, has been proposed to explain the impedance results.     

Diffusion Monte Carlo calculations ofthree-body systems

The application of the diffusion Monte Carlo algorithm in three-body systems is studied. We develop a program and use it to calculate the property of various three-body systems. Regular Coulomb systems such as atoms, molecules, and ions are investigated. The calculation is then extended to exotic systems where electrons are replaced by muons. Some nuclei with neutron halos are also calculated as three-body systems consisting of a core and two external nucleons. Our results agree well with experiments and others' work.     

Relativistically parameterized extended Hückel calculations

Spin-spin coupling tensors for one-bond coupling between main group elements are calculated using a sum-over-states of relativistically parameterized extended Hückel (REX) wavefunctions and the relativistic analogue of Ramsey's theory. All parameters, including the tabulated magnetic hyperfine integrals, come from relativistic (or non-relativistic) atomic Hartree-Fock calculations, affording a systematic comparison of these two cases.

The results reconfirm the strong relativistic increase of K(XH) in XH₄, X = C-Pb, and the rôle of the new, isotropic term K _ps . The relativistic increase of the relative anisotropy R = (K _‖-K _⊥)/K is found to be a rather general phenomenon. Its origin is traced to the s-tp _1/2 contribution and, ultimately, to a phase factor for p _1/2 orbitals. Thus, for example, the R(PbC) in Pb(CH₃)₄ is predicted to be larger than the corresponding R(SnC) in Sn(CH₃)₄. For molecules having lone pairs, both the increase of Z along a column and the relativistic effects diminish the ¹ K, making it large and negative for the heaviest members of the group, as found experimentally for (CH₃)₂Se and (CH₃)₂Te. The K _ss term, corresponding to the non-relativistic Fermi-contact term, is found to dominate almost invariably.     

X-ray calculations for a NLTE Ar plasma

A model is developed to calculate emission spectrum of non-local thermodynamic equilibrium(NLTE) plasmas. The Collisional-Radiative model is adopted for non-LTE population calculations. Configuration-averaged rate coefficients that needed in the rate equations are obtained based on the first order perturbation theory. The Hatree-Fock-Slater self-consistent-field method is used to calculate electron wave functions. The present model is applied to the calculation of emissivity from a Ar plasma. The features of the spectra are in good agreement with those calculated by other theoretical models, but the data of the integrated emissivity differ by a factor 2～8.     

NEXAFS multiple scattering calculations of KO2

Since many years the oxidation of alkali metals has being attracted much interest due to the catalytic properties of metal promoters and the simple electronic structure of alkali atoms.The alkali-oxides phase diagram indicates that the interaction of oxygen with alkali metals can lead to the formation of different atomic O^2 ions and molecular O2 and O2^2- ions.Potassium superoxide has been prepared in situ and high resolution O e-edge absorption NEXAFS spectra have been measured at the VUV beam-Line at ELETTRA facility.The experimental data have been analyzed by multiple scattering approach deriving many geometrical and electronic is of the KO2 type with an O-O distance of about 1.35A and that the transition involving singleπ molecular empty state of the superoxied O2 anion has a fine structure.Multiple Scattering self consistent calculation indicates that the bond between oxygen anion adn K atom is totally ionic and that the fine structure is essentially due to solid state effects.     

Green’s function calculations of light nuclei

The influence of short-range correlations in nuclei was investigated with realistic nuclear force. The nucleon-nucleon interaction was renormalized with V_lowk technique and applied to the Green’s function calculations. The Dyson equation was reformulated with algebraic diagrammatic constructions. We also analyzed the binding energy of ⁴He, calculated with chiral potential and CD-Bonn potential. The properties of Green’s function with realistic nuclear forces are also discussed.     

First-principles calculations of solute–vacancy interactions in aluminum

The interactions of solute atoms with vacancies play a key role in diffusion and precipitation of alloying elements,ultimately influencing the mechanical properties of aluminum alloys. In this study, first-principles calculations are systematically performed to quantify the solute–vacancy interactions for the 3 d–4 p series and the 4 d–5 p series. The solute–vacancy interaction gradually transforms from repulsion to attraction from left to right. The solute–vacancy binding energy is sensitive to the supercell size for elements at the beginning. These behaviors of the solute–vacancy binding energy can be understood in terms of the combination and competition between the elastic and electronic interactions. Overall, the electronic binding energy follows a similar trend to the total binding energy and plays a major role in the solute–vacancy interactions.     

Critical current calculations for long 0-π Josephson junctions

A zigzag boundary between a $d_{x^2 - y^2}$ and an s-wave superconductor is believed to behave like a long Josephson junction with alternating sections of 0 and π symmetry. We calculate the field-dependent critical current of such a junction, using a simple model. The calculation involves discretizing the partial differential equation for the phase difference across a long 0-π junction. In this form, the equations describe a hybrid ladder of inductively coupled small 0 and π resistively and capacitively shunted Josephson junctions (RCSJ's). The calculated critical critical current density J_c(H_a) is maximum at non-zero applied magnetic field H_a, and depends strongly on the ratio of Josephson penetration depth λ_J to facet length L_f. If λ_J/L_f ≫1 and the number of facets is large, there is a broad range of H_a where J_c(H_a) is less than 2% of the maximum critical current density of a long 0 junction. All of these features are in qualitative agreement with recent experiments. In the limit λ_J/L_f →∞, our model reduces to a previously-obtained analytical superposition result for J_c(H_a). In the same limit, we also obtain an analytical expression for the effective field-dependent quality factor Q_J(H_a), finding that . We suggest that measuring the field-dependence of Q_J(H_a) would provide further evidence that this RCSJ model applies to a long 0-π junction between a d-wave and an s-wave superconductor.     

Band parameters of α-LiBeN semiconductor from density functional calculations

The structural, elastic, electronic, optical and thermal properties of α phase in LiBeN semiconductor have been studied using pseudo-potential plane wave method based on the density functional theory. The computed lattice parameter agrees well with previous theoretical work. The elastic constants and their pressure dependence are predicted using the static finite strain technique. A set of isotropic elastic parameters and related properties, namely bulk and shear moduli, Young’s modulus, Poisson’s ratio, average sound velocity and Debye temperature are numerically estimated in the frame work of the Voigt–Reuss–Hill approximation for α-LiBeN polycrystalline aggregate. The assignments of the structures in the optical spectra and band structure transitions have been examined and discussed. The thermal effect on heat capacities is investigated by the quasi-harmonic Debye model. To the best of our knowledge, most of the studied properties of the material of interest are reported for the first time.     

First-principles calculations of structural and thermodynamic properties of β-PbO

We employed ab-initio calculations to investigate the structural and thermodynamic properties of Massicot or orthorhombic phase of PbO named β-PbO using the projector augmented-wave(PAW) method within the generalized gradient approximation(GGA). The temperature and pressure dependence of bulk modulus, heat capacity at constant pressure and constant volume, entropy, thermal expansion coefficient and Grüneisen parameter were discussed. Accuracy of two different models, the Debye and Debye-Grüneisen which are based on the quasi-harmonic approximation(QHA) for producing thermodynamic properties of material were compared. According to calculation results, these two models can be used to designate thermodynamic properties for β-PbO with sensible accuracy over a wide range of temperatures and pressures, and our work on the properties of this structure will be useful for more deeply understanding various properties of this structure.     

First-principles phonon calculations of Fe?? impurity in SrTiO?

The results of hybrid density functional theory calculations on phonons in Sr(Fe(x)Ti(1-x))O(3) solid solution within the formalism of a linear combination of atomic orbitals are presented. The phonon density of states (DOS) calculated for 6.25% Fe(4+) impurities is reported and defect-induced phonon modes are identified. Based on our calculations and group-theoretical analysis, we suggest for the first time an interpretation of experimentally observed Raman- and IR-active modes.     

Solid L-α-alanine: Spectroscopic properties and theoretical calculations

A spectroscopic study of L-α-alanine in different environments is presented, with special emphasis on the neutral to zwitterion transformation of this amino acid. Spectra of room temperature solids as deposited from the vapour and in KBr pellets are compared and discussed revealing some discrepancies. An assignment is proposed based on theoretical calculations of the solid structure. Vapour deposits at 25 K are prepared both of pure alanine and of mixtures with polar (H₂O) and non-polar (CO₂, CH₄) components. The spectra of the solids contain IR bands which can be individually assigned to the neutral and to the zwitterion, and whose relative intensity variation can be used to follow the neutral to zwitterion transformation. The assignment of the spectrum of the neutral species is proposed, again with help from theoretical calculations of the single neutral molecule. The neutral/zwitterion ratio in deposits at 25 K varies between ～60% for pure alanine and H₂O mixtures and ～90% for non-polar matrices. This ratio drops when the solid is heated until the ionic species only remains at 200 K. The conversion process depends on the environment surrounding the alanine molecules.     

Ab initio calculations of electronic structure of anatase TiO2

This paper presents the results of the self-consistent calculations on the electronic structure of anatase phase of TiO2. The calculations were performed using the full potential-linearized augmented plane wave method (FP-LAPW)in the framework of the density functional theory (DFT) with the generalized gradient approximation (GGA). The fullyoptimized structure, obtained by minimizing the total energy and atomic forces, is in good agreement with experiment.We also calculated the band structure and the density of states. In particular, the calculated band structure prefers an indirect transition between valence and conduction bands of anatase TiO2, which may be helpful for clarifying theambiguity in other theoretical works.     

First-principles calculations of structural and thermodynamic properties of BeB2 compound

The lattice parameter bulk modulus and pressure derivative of BeB2 are calculated by using the Cambridge Serial Total Energy Package （CASTEP） program in the frame of density function theory. The calculated results agree well with the average experimental data and other theoretical results. Through the quasi-harmonic Debye model, the dependences of the normalized lattice parameters a/ao, c/c0 and the normalized primitive cell volume V/Vo on pressure P, the variation of the thermal expansion coefficient ~ with pressure P and temperature T, as well as the dependences of the heat capacity Cv on pressure P and temperature T are obtained systematically.