Superconductivity from finite-u valence fluctuations

The finite-U pairing mechanism of Newns is explored by an adaptation of our previous U = {infinity} variational treatment of the Anderson lattice. This pairing mechanism is found to be opposed by a magnetic tendency arising from Gutzwiller renormalization of the hybridization. This competition restricts superconductivity to the parameter regime {Delta} {approx gt} U, and this also reproduces the parabolic rise and fall of T{sub c} in La{sub 2-x}Sr{sub x}CuO{sub 4} with increasing x. Re-analysis of the photoemission and BIS data for CuO reveals that indeed {Delta} {approx gt} U for this compound, contrary to previous work. 12 refs

A valence-fluctuation mechanism with highly anisotropic S-wave pairing

The author has developed a fairly ab initio theory of cuprate superconductivity, based on the concept that the above-T{sub c} state is a normal Fermi liquid with strong electronic correlations of the type found in valence-fluctuation (VF) and heavy-fermion materials. The ground-state-correlation aspect is found to provide a satisfactory source or mechanism for the pairing. In addition to the high T{sub c}`s this explains many of the unusual features of cuprate superconductivity

Simple approximation for the starting-energy-independent two-body effective interaction with applications to 6Li

We apply the Lee-Suzuki iteration method to calculate the linked-folded diagram series for a new Nijmegen local NN potential. We obtain an exact starting-energy-independent effective two-body interaction for a multi-shell, no-core, harmonic-oscillator model space. It is found that the resulting effective-interaction matrix elements can be well approximated by the Brueckner G-matrix elements evaluated at starting energies selected in a simple way. These starting energies are closely related to the energies of the initial two-particle states in the ladder diagrams. The ``exact'' and approximate effective interactions are used to calculate the energy spectrum of 6Li in order to test the utility of the approximate form.Comment: 15 text pages and 2 PostScript figures (available upon request). University of Arizona preprint, Number unassigne

Metal insulator transition in TlSr2CoO5 from orbital degeneracy and spin disproportionation

To describe the metal insulator transition in the new oxide TlSr2CoO5 we investigate its electronic structure by LDA and model Hartree-Fock calculations. Within LDA we find a homogeneous metallic and ferromagnetic ground state, but when including the Coulomb interaction more explicitly within the Hartree-Fock approximation, we find an insulating state of lower energy with both spin and orbital order. We also interpret our results in terms of a simple model.Comment: 8 pages, 9 figure

Electronic dynamic Hubbard model: exact diagonalization study

A model to describe electronic correlations in energy bands is considered. The model is a generalization of the conventional Hubbard model that allows for the fact that the wavefunction for two electrons occupying the same Wannier orbital is different from the product of single electron wavefunctions. We diagonalize the Hamiltonian exactly on a four-site cluster and study its properties as function of band filling. The quasiparticle weight is found to decrease and the quasiparticle effective mass to increase as the electronic band filling increases, and spectral weight in one- and two-particle spectral functions is transfered from low to high frequencies as the band filling increases. Quasiparticles at the Fermi energy are found to be more 'dressed' when the Fermi level is in the upper half of the band (hole carriers) than when it is in the lower half of the band (electron carriers). The effective interaction between carriers is found to be strongly dependent on band filling becoming less repulsive as the band filling increases, and attractive near the top of the band in certain parameter ranges. The effective interaction is most attractive when the single hole carriers are most heavily dressed, and in the parameter regime where the effective interaction is attractive, hole carriers are found to 'undress', hence become more like electrons, when they pair. It is proposed that these are generic properties of electronic energy bands in solids that reflect a fundamental electron-hole asymmetry of condensed matter. The relation of these results to the understanding of superconductivity in solids is discussed.Comment: Small changes following referee's comment

Linear Field Dependence of the Normal-State In-Plane Magnetoresistance of Sr2RuO4

The transverse and longitudinal in-plane magnetoresistances in the normal state of superconducting Sr2RuO4 single crystals have been measured. At low temperatures, both of them were found to be positive with a linear magnetic-field dependence above a threshold field, a result not expected from electronic band theory. We argue that such behavior is a manifestation of a novel coherent state characterized by a spin pseudo gap in the quasi-particle excitation spectrum in Sr2RuO4.Comment: 4 pages + 5 figure

Correlation effects in ionic crystals: I. The cohesive energy of MgO

High-level quantum-chemical calculations, using the coupled-cluster approach and extended one-particle basis sets, have been performed for (Mg2+)n (O2-)m clusters embedded in a Madelung potential. The results of these calculations are used for setting up an incremental expansion for the correlation energy of bulk MgO. This way, 96% of the experimental cohesive energy of the MgO crystal is recovered. It is shown that only 60% of the correlation contribution to the cohesive energy is of intra-ionic origin, the remaining part being caused by van der Waals-like inter-ionic excitations.Comment: LaTeX, 20 pages, no figure

Computational Nuclear Physics and Post Hartree-Fock Methods

We present a computational approach to infinite nuclear matter employing Hartree-Fock theory, many-body perturbation theory and coupled cluster theory. These lectures are closely linked with those of chapters 9, 10 and 11 and serve as input for the correlation functions employed in Monte Carlo calculations in chapter 9, the in-medium similarity renormalization group theory of dense fermionic systems of chapter 10 and the Green's function approach in chapter 11. We provide extensive code examples and benchmark calculations, allowing thereby an eventual reader to start writing her/his own codes. We start with an object-oriented serial code and end with discussions on strategies for porting the code to present and planned high-performance computing facilities.Comment: 82 pages, to appear in Lecture Notes in Physics (Springer), "An advanced course in computational nuclear physics: Bridging the scales from quarks to neutron stars", M. Hjorth-Jensen, M. P. Lombardo, U. van Kolck, Editor

Highly anisotropic but nodeless gap from a valence-fluctuation pairing mechanism

We have refined and quantitatively explored a valence-fluctuation pairing mechanism, the finite-U mechanism introduced by Newns. This can provide an s-like (nodeless) gap. We use an Anderson-lattice Hamiltonian, with realistic parameter values derived from photoemission and BIS data. The Landau-Luttinger quasiparticle spectrum and the pairing interaction are obtained from a many-body variational formalism. Adequate pairing attraction is obtained, together with band narrowing (mass enhancement) and an extremely short coherence length, in reasonable agreement with experiment. These fully self-consistent results were obtained with an isotropic approximation to the band structure of a single CuO[sub 2] plane. Now, using a simplified treatment for a more realistic band structure, we find that the gap has strong in-plane anisotropy, although it should probably still remain nodeless. This conclusion is consistent with a variety of data

Re-analysis of photoemission data for CuO: Revision of the configuration-energy scheme for cuprate materials

The author has recently carried out a very careful analysis of photoemission and inverse photoemission (BIS) data for CuO, probably the most refined and thorough such analysis for any transition-metal compound to date. This has led to a configuration-energy scheme for cuprate materials which is quite different from a generally-accepted one, due largely to Sawatzky and co-workers. For example, it is commonly believed that the doping holes in La{sub 2-x}Sr{sub x}CuO{sub 4} are hosted almost entirely within oxygen 2p orbitals. He finds instead that these holes are shared roughly equally by 2p and 3d orbitals. In this report he shall sketch the motivations for this study the main results, and some of the lessons and surprises encountered along the way. 22 refs., 1 fig