469 research outputs found
Beyond the Nca: New Results for the Spectral Properties of the Anderson Model
In the framework of direct perturbation theory a fully self-consistent
approximation beyond the well known NCA will be presented for the Anderson
Model. The resummation of a class of skeleton diagrams up to infinite order in
includes all contribution up to the order ( = degeneracy of
the magnetic state). Qualitative improvements in maintaining local Fermi-Liquid
relations and one-particle spectral properties in comparison to the well known
NCA will be reported. The location and temperature dependence of the
AS-resonance for the case is found to be rather close to the chemical
potential in excellent agreement with Friedel's sum rule; the static magnetic
susceptibility exhibits the same -dependence as the exact {\em Bethe-Ansatz}
solution.Comment: 3 pages including 4 figures, Postscript file, self-extracting shell
script;(submitted to the SCES 94 Conference, Amsterdam
Fluctuation Exchange Analysis of Superconductivity in the Standard Three-Band CuO2 Model
The fluctuation exchange, or FLEX, approximation for interacting electrons is
applied to study instabilities in the standard three-band model for CuO2 layers
in the high-temperature superconductors. Both intra-orbital and near-neigbor
Coulomb interactions are retained. The filling dependence of the d(x2-y2)
transition temperature is studied in both the "hole-doped" and "electron-doped"
regimes using parameters derived from constrained-occupancy density-functional
theory for La2CuO4. The agreement with experiment on the overdoped hole side of
the phase diagram is remarkably good, i.e., transitions emerge in the 40 K
range with no free parameters. In addition the importance of the "orbital
antiferromagnetic," or flux phase, charge density channel is emphasized for an
understanding of the underdoped regime.Comment: REVTex and PostScript, 31 pages, 26 figures; to appear in Phys. Rev.
B (1998); only revised EPS figures 3, 4, 6a, 6b, 6c, 7 and 8 to correct
disappearance of some labels due to technical problem
Stability of self-consistent solutions for the Hubbard model at intermediate and strong coupling
We present a general framework how to investigate stability of solutions
within a single self-consistent renormalization scheme being a parquet-type
extension of the Baym-Kadanoff construction of conserving approximations. To
obtain a consistent description of one- and two-particle quantities, needed for
the stability analysis, we impose equations of motion on the one- as well on
the two-particle Green functions simultaneously and introduce approximations in
their input, the completely irreducible two-particle vertex. Thereby we do not
loose singularities caused by multiple two-particle scatterings. We find a
complete set of stability criteria and show that each instability, singularity
in a two-particle function, is connected with a symmetry-breaking order
parameter, either of density type or anomalous. We explicitly study the Hubbard
model at intermediate coupling and demonstrate that approximations with static
vertices get unstable before a long-range order or a metal-insulator transition
can be reached. We use the parquet approximation and turn it to a workable
scheme with dynamical vertex corrections. We derive a qualitatively new theory
with two-particle self-consistence, the complexity of which is comparable with
FLEX-type approximations. We show that it is the simplest consistent and stable
theory being able to describe qualitatively correctly quantum critical points
and the transition from weak to strong coupling in correlated electron systems.Comment: REVTeX, 26 pages, 12 PS figure
Towards analytic description of a transition from weak to strong coupling regime in correlated electron systems. I. Systematic diagrammatic theory with two-particle Green functions
We analyze behavior of correlated electrons described by Hubbard-like models
at intermediate and strong coupling. We show that with increasing interaction a
pole in a generic two-particle Green function is approached. The pole signals
metal-insulator transition at half filling and gives rise to a new vanishing
``Kondo'' scale causing breakdown of weak-coupling perturbation theory. To
describe the critical behavior at the metal-insulator transition a novel,
self-consistent diagrammatic technique with two-particle Green functions is
developed. The theory is based on the linked-cluster expansion for the
thermodynamic potential with electron-electron interaction as propagator.
Parquet diagrams with a generating functional are derived. Numerical
instabilities due to the metal-insulator transition are demonstrated on
simplifications of the parquet algebra with ring and ladder series only. A
stable numerical solution in the critical region is reached by factorization of
singular terms via a low-frequency expansion in the vertex function. We stress
the necessity for dynamical vertex renormalizations, missing in the simple
approximations, in order to describe the critical, strong-coupling behavior
correctly. We propose a simplification of the full parquet approximation by
keeping only most divergent terms in the asymptotic strong-coupling region. A
qualitatively new, feasible approximation suitable for the description of a
transition from weak to strong coupling is obtained.Comment: 17 pages, 4 figures, REVTe
Superconducting instability in the Holstein-Hubbard model: A numerical renormalization group study
We have studied the d-wave pairing-instability in the two-dimensional
Holstein-Hubbard model at the level of a full fluctuation exchange
approximation which treats both Coulomb and electron-phonon (EP) interaction
diagrammatically on an equal footing. A generalized numerical renormalization
group technique has been developed to solve the resulting self-consistent field
equations. The -wave superconducting phase diagram shows an optimal T_c at
electron concentration ~ 0.9 for the purely electronic Hubbard system. The
EP interaction suppresses the d-wave T_c which drops to zero when the
phonon-mediated on-site attraction becomes comparable to the on-site
Coulomb repulsion . The isotope exponent is negative in this model
and small compared to the classical BCS value or compared
to typical observed values in non-optimally doped cuprate superconductors.Comment: 4 pages RevTeX + 3 PS figures include
Reduction of Tc due to Impurities in Cuprate Superconductors
In order to explain how impurities affect the unconventional
superconductivity, we study non-magnetic impurity effect on the transition
temperature using on-site U Hubbard model within a fluctuation exchange (FLEX)
approximation. We find that in appearance, the reduction of Tc roughly
coincides with the well-known Abrikosov-Gor'kov formula. This coincidence
results from the cancellation between two effects; one is the reduction of
attractive force due to randomness, and another is the reduction of the damping
rate of quasi-particle arising from electron interaction. As another problem,
we also study impurity effect on underdoped cuprate as the system showing
pseudogap phenomena. To the aim, we adopt the pairing scenario for the
pseudogap and discuss how pseudogap phenomena affect the reduction of Tc by
impurities. We find that 'pseudogap breaking' by impurities plays the essential
role in underdoped cuprate and suppresses the Tc reduction due to the
superconducting (SC) fluctuation.Comment: 14 pages, 28 figures To be published in JPS
A model of semimetallic behavior in strongly correlated electron systems
Metals with values of the resistivity and the Hall coefficient much larger
than typical ones, e.g., of sodium, are called semimetals. We suggest a model
for semimetals which takes into account the strong Coulomb repulsion of the
charge carriers, especially important in transition-metal and rare-earth
compounds. For that purpose we extend the Hubbard model by coupling one
additional orbital per site via hybridization to the Hubbard orbitals. We
calculate the spectral function, resistivity and Hall coefficient of the model
using dynamical mean-field theory. Starting from the Mott-insulating state, we
find a transition to a metal with increasing hybridization strength
(``self-doping''). In the metallic regime near the transition line to the
insulator the model shows semimetallic behavior. We compare the calculated
temperature dependence of the resistivity and the Hall coefficient with the one
found experimentally for . The comparison demonstrates that the
anomalies in the transport properties of possibly can be
assigned to Coulomb interaction effects of the charge carriers not captured by
standard band structure calculations.Comment: 9 pages RevTeX with 7 ps figures, accepted by PR
Knight Shift Anomalies in Heavy Electron Materials
We calculate non-linear Knight Shift vs. susceptibility anomalies
for Ce ions possessing local moments in metals. The ions are modeled with the
Anderson Hamiltonian and studied within the non-crossing approximation (NCA).
The non-linearity diminishes with decreasing Kondo temperature
and nuclear spin- local moment separation. Treating the Ce ions as an
incoherent array in CeSn, we find excellent agreement with the observed Sn
data.Comment: 4 pages, Revtex, 3 figures available upon request from
[email protected]
Theory of Scanning Tunneling Spectroscopy of a Magnetic Adatom on a Metallic Surface
A comprehensive theory is presented for the voltage, temperature, and spatial
dependence of the tunneling current between a scanning tunneling microscope
(STM) tip and a metallic surface with an individual magnetic adatom. Modeling
the adatom by a nondegenerate Anderson impurity, a general expression is
derived for a weak tunneling current in terms of the dressed impurity Green
function, the impurity-free surface Green function, and the tunneling matrix
elements. This generalizes Fano's analysis to the interacting case. The
differential-conductance lineshapes seen in recent STM experiments with the tip
directly over the magnetic adatom are reproduced within our model, as is the
rapid decay, \sim 10\AA, of the low-bias structure as one moves the tip away
from the adatom. With our simple model for the electronic structure of the
surface, there is no dip in the differential conductance at approximately one
lattice spacing from the magnetic adatom, but rather we see a resonant
enhancement. The formalism for tunneling into small clusters of magnetic
adatoms is developed.Comment: 12 pages, 9 figures; to appear in Phys. Rev.
STUDIES IN PORPHYRIA
Porphyrin biosynthesis in mammalian skin and in skin obtained from patients with selected types of porphyria has been studied. Cutaneous porphyrinogenesis required the precursor δ-aminolevulinic acid (ALA) which, when added to murine, rat, and human skin in vitro, was rapidly converted to porphyrins. Total porphyrin content was quantitated by fluorescence assay, and spectral studies indicated that more than 80% of the porphyrin produced was protoporphyrin. The majority of skin porphyrinogenesis occurred in epidermis or in epidermal derivatives such as hair roots. Known inducers of hepatic δ-aminolevulinic acid synthetase (ALAS), the rate-limiting enzyme for heme biosynthesis, were not inducers when added to skin in vitro.Skin from patients with acute intermittent porphyria demonstrated a 43% decrease in cutaneous porphyrin production as compared to unaffected normals. This is consistent with the known deficiency of uroporphyrinogen synthetase that has been previously demonstrated in the liver and red blood cells of these patients. Porphyrinogenesis in skin of patients with porphyria cutanea tarda was not different from controls.These studies demonstrate that skin has the enzymatic capacity to synthesize porphyrins from added ALA and that cutaneous porphyrinogenesis from ALA is deficient in patients with acute intermittent porphyria
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