The bipolaron theory of high-temperature superconductors

An outline is given for the bipolaron theory of high-temperature superconductivity, both for the cubic bismuth oxide and the copper oxide materials. It is emphasized that the BCS theory takes this form for strong interaction. The possible role of spin polarons in the latter is also discussed.     

The theory of ultrasonic attenuation in two-band superconductors

The ultrasonic attenuation coefficient for longitudinal waves is formulated for two-band superconductors. The resulting expression differs from that previously obtained by Fal'ko.     

The electrostatic potential in a crystalline slab

The electrostatic potential in a crystalline slab of point charges is expressed as a sum of three types of terms, an intrinsic term, microscopic fringing terms and a macroscopic term.     

Self-consistent theory of pairbreaking in kondo superconductors

A theory is presented for the calculation of equilibrium properties of superconductors containing magnetic impuritiesof finite concentration. The theory is based on the Nagaoka-Suhl approach to thes — d model and treats finite impurity concentrations within a self-consistentT-matrix-approximation. An integral equation is derived for the pairbreaking parameter which describes de-pairing of Cooper pairs by spin flip scattering. Numerical solutions of this equation are used to compute the phase diagram in the temperature-concentration-plane and the transition-temperature-dependence of the upper critical field. These calculations are in good agreement with experimental data on LaCeAl₂-samples. In addition, the recent discovery of three normal superconducting transitions in LaCeY is discussed quantitatively.Work performed within the research program of the Sonderforschungsbereich 125 Aachen/Jülich/Köln     

The electrostatic potential in the surface region of an ionic crystal

A method for the rapid evaluation of the electrostatic potential near the surface of an ionic crystal is described. The deviation of the potential from its behaviour in the crystal bulk is discussed with reference to X-ray photoelectron spectroscopy. Applications of the method include crystal faces where redistribution of charge and lattice reconstruction occur near the surface. The {100}, {110}, and {111} faces of the unreconstructed ionic NaCl crystal are studied. It is found that the potential changes, from its bulk behaviour to zero outside the surface, within a surface region of thickness compatible with the interionic spacing, although its detailed form varies considerably from face to face.     

The potential profile in LEED theory

The effect of variations in the inner potential at a crystal surface on low-energy electron diffraction (LEED) is investigated. It is shown in particular that surface structural determination using LEED is sensitive to the local value of the inner potential if energies roughly below 100 eV are used. It is however not sensitive to scattering effects at potential steps, except if intensities near beam emergences are considered, in which case LEED is shown to be a suitable tool for examining these potential steps.     

Nonlinear theory of electrostatic baryonic waves in ambiplasma

A collisionless nonmagnetized ambiplasma consisting of Maxwellian gases of protons, antiprotons, electrons, and positrons is considered. The dispersion relation for electrostatic baryonic waves is derived and analyzed and exact expressions for the linear wave phase velocities are obtained. Two types of such waves are shown to be possible in ambiplasma: acoustic and plasma ones. Analysis of the dispersion relation has allowed the ranges of parameters in which nonlinear solutions should be sought in the form of solitons to be found. A nonlinear theory of baryonic waves is developed and used to obtain and analyze the exact solution to the basic equations. The analysis is performed by the method of a fictitious potential. The ranges of phase velocities of periodic baryonic waves and soliton velocities (Mach numbers) are determined. It is shown that in the plasma under consideration, these ranges do not overlap and that the soliton velocity cannot be lower than the linear velocity of the corresponding wave. The profiles of physical quantities in a periodic wave and a soliton (wave scores) are plotted.     

Ideal focusing in the theory of electrostatic spectrographs

The problem of synthesis of field structures, in which the spectrographic principle of separating charged particle flows is implemented and ideal angular focusing is simultaneously observed in the symmetry plane for each energy value, is formulated. The search for spectrographic media is effectively carried out with the help of inverse mechanical problems. Two approaches are proposed, one of which involves the reduction to the integral Abel equation and the other is based on the application of the Goursat theorem. The algorithms for the synthesis of spectrographic media with ideal focusing of flows are described in detail, and possible versions of solutions corresponding to representatives of two structural classes (with potentials homogeneous in the Euler sense and with homogeneous potentials with a logarithmic singularity) are indicated. The prospects of developing high-transmission spectrographs for energy analysis are outlined.     

On the theory of layered high-temperature superconductors

Assuming that charge carriers form a Fermi liquid state, we study a model for layered high-temperature superconductors with unretarded intralayer and interlayer pairing. Guided by band structure calculations and inverse photoemission experiments, we adopt a tight binding band with nearest and next-nearest neighbors hopping within the sheets and weak interlayer hopping. The gap equations are solved numerically, without imposing a cutoff energy, characteristic to phonon mediated superconductivity. On this basis we calculate the gap parameters,T _c , the tunneling conductance, infrared absorption and the coherence length for various band fillings =1/2–x by introducing excess holes of concentrationx. Assuming the interlayer coupling strength to be smaller than the intralayer one, our main results are as follows:T _c is dominated by the intralayer properties, reaching a maximum atx0.3, where strong coupling features appear. In the presence of interlayer pairing, the gap becomes anisotropic perpendicular to the layers, and standard BCS-behavior is modified. In particular the BCS-square root singularity in the density of states and in the tunneling conductance is replaced by van Hove singularities characterizing the anisotropic gap. In particular, we investigate the anisotropy of the tunneling conductance for specular and diffuse tunneling for a junction parallel or perpendicular to the layers, infrared absorption, as well as the coherence length, parallel and perpendicular to the layers.     

Extension of Eliashberg's theory to type-II superconductors

Eliashberg's theory is extended to type-II superconductors in such a way that band structure effects and anisotropies of the phonon-induced electron-electron interaction are included. Thermodynamic potentials for the superconducting system are presented.     

3-Band theory of Fe pnictide superconductors

For the Fe pnictide superconductors, band features important for superconductivity (SC) in the neighborhood of ε_F are mimicked by tight-binding bands with bases of Fe d_xz, d_yz and d_xy orbitals. In terms of this 3-band model, effective pair transfer processes originating from Coulombic integrals are shown to be largely enhanced due to the nesting between electron and hole pockets by explicitly summing up ladder diagrams. Obtained coupled gap equations lead to s^±-type SC. This framework are argued to provide a scheme for discussing important elements working for the high T_c SC, although its validity is restricted to the case where Dirac cones are away from ε_F. Experimental results on Fe pnictides are discussed in terms of the present results. The pair transfer process explicitly due to the exchange-like integral is suggested to be considerably important for the high T_c in MFeAsO_1−xF_x to occur.