1,163 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
Toward a systematic 1/d expansion: Two particle properties
We present a procedure to calculate 1/d corrections to the two-particle
properties around the infinite dimensional dynamical mean field limit. Our
method is based on a modified version of the scheme of Ref.
onlinecite{SchillerIngersent}}. To test our method we study the Hubbard model
at half filling within the fluctuation exchange approximation (FLEX), a
selfconsistent generalization of iterative perturbation theory. Apart from the
inherent unstabilities of FLEX, our method is stable and results in causal
solutions. We find that 1/d corrections to the local approximation are
relatively small in the Hubbard model.Comment: 4 pages, 4 eps figures, REVTe
Spin Fluctuation-Induced Superconductivity in Organic Compounds
Spin fluctuation-induced superconductivity in two-dimensional organic
compounds such as \kappa-(ET)_2-X is investigated by using a simplified dimer
Hubbard model with right-angled isosceles triangular lattice (transfer matrices
-\tau, -\tau^\prime). The dynamical susceptiblity and the self-energy are
calculated self-consistently within the fluctuation exchange approximation and
the value for T_c as obtained by solving the linearized Eliashberg-type
equations is in good agreement with experiment. The pairing symmetry is of
d_{x^2-y^2} type. The calculated (U/\tau)-dependence of T_c compares
qualitatively well with the observed pressure dependence of T_c. Varying the
value for \tau^\prime/\tau from 0 to 1 we interpolate between the square
lattice and the regular triangular lattice and find firstly that values of T_c
for \kappa-(ET)_2-X and cuprates scale well and secondly that T_c tends to
decrease with increasing \tau^\prime/\tau and no superconductivity is found for
\tau^\prime/\tau=1, the regular triangular lattice.Comment: 4 pages, 6 eps figures, uses jpsj.st
Coefficient of normal restitution of viscous particles and cooling rate of granular gases
We investigate the cooling rate of a gas of inelastically interacting
particles. When we assume velocity dependent coefficients of restitution the
material cools down slower than with constant restitution. This behavior might
have large influence to clustering and structure formation processes.Comment: 3 figures, Phys. Rev. E (in press
Stability of condensate in superconductors
According to the BCS theory the superconducting condensate develops in a
single quantum mode and no Cooper pairs out of the condensate are assumed. Here
we discuss a mechanism by which the successful mode inhibits condensation in
neighboring modes and suppresses a creation of noncondensed Cooper pairs. It is
shown that condensed and noncondensed Cooper pairs are separated by an energy
gap which is smaller than the superconducting gap but large enough to prevent
nucleation in all other modes and to eliminate effects of noncondensed Cooper
pairs on properties of superconductors. Our result thus justifies basic
assumptions of the BCS theory and confirms that the BCS condensate is stable
with respect to two-particle excitations
Superconductivity and Pseudogap in Quasi-Two-Dimensional Metals around the Antiferromagnetic Quantum Critical Point
Spin fluctuations (SF) and SF-mediated superconductivity (SC) in
quasi-two-dimensional metals around the antiferrromagnetic (AF) quantum
critical point (QCP) are investigated by using the self-consistent
renormalization theory for SF and the strong coupling theory for SC. We
introduce a parameter y0 as a measure for the distance from the AFQCP which is
approximately proportional to (x-xc), x being the electron (e) or hole (h)
doping concentration to the half-filled band and xc being the value at the
AFQCP. We present phase diagrams in the T-y0 plane including contour maps of
the AF correlation length and AF and SC transition temperatures TN and Tc,
respectively. The Tc curve is dome-shaped with a maximum at around the AFQCP.
The calculated one-electron spectral density shows a pseudogap in the
high-density-of-states region near (pi,0) below around a certain temperature T*
and gives a contour map at the Fermi energy reminiscent of the Fermi arc. These
results are discussed in comparison with e- and h-doped high-Tc cuprates.Comment: 5 pages, 3 figure
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
Collective Excitations in High-Temperature Superconductors
Collective, low-energy excitations in quasi-two-dimensional d-wave
superconductors are analyzed. While the long-range Coulomb interaction shifts
the charge-density-wave and phase modes up to the plasma energy, the
spin-density-wave excitation that arises due to a strong local
electron-electron repulsion can propagate as a damped collective mode within
the superconducting energy gap. It is suggested that these excitations are
relevant to high-Tc superconductors, close to the antiferromagnetic phase
boundary, and may explain some of the exotic features of the experimentally
observed spectral-density and neutron-scattering data.Comment: 5 jolly page
s-wave superconductivity from antiferromagnetic spin-fluctuation model for bilayer materials
It is usually believed that the spin-fluctuation mechanism for
high-temperature superconductivity results in d-wave pairing, and that it is
destructive for the conventional phonon-mediated pairing. We show that in
bilayer materials, due to nearly perfect antiferromagnetic spin correlations
between the planes, the stronger instability is with respect to a
superconducting state whose order parameters in the even and odd plane-bands
have opposite signs, while having both two-dimensional -symmetry. The
interaction of electrons with Raman- (infrared-) active phonons enhances
(suppresses) the instability.Comment: Revtex, 3 figure
Superconductivity in the quasi-two-dimensional Hubbard model
On the basis of spin and pairing fluctuation-exchange approximation, we study
the superconductivity in quasi-two-dimensional Hubbard model. The integral
equations for the Green's function are self-consistently solved by numerical
calculation. Solutions for the order parameter, London penetration depth,
density of states, and transition temperature are obtained. Some of the results
are compared with the experiments for the cuprate high-temperature
superconductors. Numerical techniques are presented in details. With these
techniques, the amount of numerical computation can be greatly reduced.Comment: 17 pages, 13 figure
- …