566 research outputs found
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
High temperature superconductivity in dimer array systems
Superconductivity in the Hubbard model is studied on a series of lattices in
which dimers are coupled in various types of arrays. Using fluctuation exchange
method and solving the linearized Eliashberg equation, the transition
temperature of these systems is estimated to be much higher than that of
the Hubbard model on a simple square lattice, which is a model for the high
cuprates. We conclude that these `dimer array' systems can generally
exhibit superconductivity with very high . Not only -electron systems,
but also -electron systems may provide various stages for realizing the
present mechanism.Comment: 4 pages, 9 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
Fourth Order Perturbation Theory for Normal Selfenergy in Repulsive Hubbard Model
We investigate the normal selfenergy and the mass enhancement factor in the
Hubbard model on the two-dimensional square lattice. Our purpose in this paper
is to evaluate the mass enhancement factor more quantitatively than the
conventional third order perturbation theory. We calculate it by expanding
perturbatively up to the fourth order with respect to the on-site repulsion
. We consider the cases that the system is near the half-filling, which are
similar situations to high- cuprates. As results of the calculations, we
obtain the large mass enhancement on the Fermi surface by introducing the
fourth order terms. This is mainly originated from the fourth order
particle-hole and particle-particle diagrams. Although the other fourth order
terms have effect of reducing the effective mass, this effect does not cancel
out the former mass enhancement completely and there remains still a large mass
enhancement effect. In addition, we find that the mass enhancement factor
becomes large with increasing the on-site repulsion and the density of
state (DOS) at the Fermi energy . According to many current reseaches,
such large and enhance the effective interaction between
quasiparticles, therefore the superconducting transition temperature
increases. On the other hand, the large mass enhancement leads the reduction of
the energy scale of quasiparticles, as a result, is reduced. When we
discuss , we have to estimate these two competitive effects.Comment: 6pages,8figure
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]
An electron correlation originated negative magnetoresistance in a system having a partly flat band
Inspired from an experimentally examined organic conductor, a novel mechanism
for negative magnetoresistance is proposed for repulsively interacting
electrons on a lattice whose band dispersion contains a flat portion (a flat
bottom below a dispersive part here). When the Fermi level lies in the flat
part, the electron correlation should cause ferromagnetic spin fluctuations to
develop with an enhanced susceptibility. A relatively small magnetic field will
then shift the majority-spin Fermi level to the dispersive part, resulting in a
negative magnetoresistance. We have actually confirmed the idea by calculating
the conductivity in magnetic fields, with the fluctuation exchange
approximation, for the repulsive Hubbard model on a square lattice having a
large second nearest-neighbor hopping.Comment: RevTex, 5 figures in Postscript, to be published in Phys. Rev.
Superconductivity in quantum-dot superlattices composed of quantum wire networks
Based on calculations using the local density approximation, we propose
quantum wire networks with square and plaquette type lattice structures that
form quantum dot superlattices. These artificial structures are well described
by the Hubbard model. Numerical analysis reveals a superconducting ground state
with transition temperatures of up to 90 mK for the plaquette, which is
more than double the value of 40 mK for the square lattice type and is
sufficiently high to allow for the experimental observation of
superconductivity.Comment: 10 pages, 4 figure
Andreev Scattering and the Kondo Effect
We examine the properties of an infinite- Anderson impurity coupled to
both normal and superconducting metals. Both the cases of a quantum dot and a
quantum point contact containing an impurity are considered; for the latter, we
study both one and two-channel impurities. Using a generalization of the
noncrossing approximation which incorporates multiple Andreev reflection, we
compute the impurity spectral function and the linear-response conductance of
these devices. We find generically that the Kondo resonance develops structure
at energies corresponding to the superconducting gap, and that the magnitude of
the resonance at the Fermi energy is altered. This leads to observable changes
in the zero-bias conductance as compared to the case with no superconductivity.Comment: 8 pages, 7 figures; expanded version to appear in PR
Theory for the excitation spectrum of High-T$_c superconductors : quasiparticle dispersion and shadows of the Fermi surface
Using a new method for the solution of the FLEX-equations, which allows the
determination of the self energy of the Hubbard
model on the real frequency axis, we calculate the doping dependence of the
quasi-particle excitations of High-T superconductors. We obtain new results
for the shadows of the Fermi surface, their dependence on the deformation of
the quasi particle dispersion, an anomalous -dependence of and a related violation of the Luttinger theorem.
This sheds new light on the influence of short range magnetic order on the low
energy excitations and its significance for photoemission experiments.Comment: 4 pages (REVTeX) with 3 figure
Effects of Spin Fluctuations in Quasi-One-Dimensional Organic Superconductors
We study the electronic states of quasi-one-dimensional organic conductors
using the single band Hubbard model at half-filling. We treat the effects of
the on-site Coulomb interaction by the fluctuation-exchange (FLEX) method, and
calculate the phase diagram and physical properties. The calculated pressure
dependence of the Neel temperature coincides well with the experimental one. We
also show that a pseudogap is formed in the density of states near the chemical
potential and that d-wave superconductivity appears next to the
antiferromagnetic state. Moreover the NMR relaxation rate increases on cooling
in the low-temperature region.Comment: 4 pages, 5 figures, to apprear in J. Phys. Soc. Jp
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