83 research outputs found
Stripe formation in high-Tc superconductors
The non-uniform ground state of the two-dimensional three-band Hubbard model
for the oxide high-Tc superconductors is investigated using a variational Monte
Carlo method. We examine the effect produced by holes doped into the
antiferromagnetic (AF) background in the underdoped region. It is shown that
the AF state with spin modulations and stripes is stabilized du to holes
travelling in the CuO plane. The structures of the modulated AF spins are
dependent upon the parameters used in the model. The effect of the boundary
conditions is reduced for larger systems. We show that there is a region where
incommensurability is proportional to the hole density. Our results give a
consistent description of stripes observed by the neutron- scattering
experiments based on the three-band model for CuO plane.Comment: 8 pages, 9 figure
A crib-shaped triplet pairing gap function for an orthogonal pair of quasi-one dimensional Fermi surfaces in SrRuO
The competition between spin-triplet and singlet pairings is studied
theoretically for the tight-binding - bands in SrRuO,
which arise from two sets of quasi-one dimensional Fermi surfaces. Using
multiband FLEX approximation, where we incorporate an anisotropy in the spin
fluctuations as suggested from experiments, we show that (i) the triplet can
dominate over the singlet (which turns out to be extended s), and (ii) the
triplet gap function optimized in the Eliashberg equation has an unusual, very
non-sinusoidal form, whose time-reversal-broken combination exhibits a
crib-shaped amplitude with dips.Comment: 5 pages, RevTeX, to appear in Phys.Rev.B (Rapid Communications
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
Two-band Fluctuation Exchange Study on the Superconductivity of -(BEDT-TTF)ICl under High Pressure
We study the pressure dependence of the superconducting transition
temperature of an organic superconductor -(BEDT-TTF)ICl by
applying the fluctuation exchange method to the Hubbard model on the original
two-band lattice at 3/4-filling rather than the single band model in the strong
dimerization limit. Our study is motivated by the fact that hopping parameters
evaluated from a first-principles study suggest that the dimerization of the
BEDT-TTF molecules is not so strong especially at high pressure. Solving the
linearized Eliashberg's equation, a d-wave-like superconducting state
with realistic values of is obtained in a pressure regime somewhat higher
than the actual experimental result. These results are similar to those
obtained within the single band model in the previous study by Kino {\it et
al}. We conclude that the resemblance to the dimer limit is due to a
combination of a good Fermi surface nesting, a large density of states near the
Fermi level, and a moderate dimerization, which cooperatively enhance electron
correlation effects and also the superconducting .Comment: 6 pages, 8 figure
Effect of Umklapp Scattering on Magnetic Field Penetration Depth in High-Tc Cuprates
The renormalization of the magnetic field penetration depth owing
to the electron-electron correlation is discussed with its application to
high- cuprates. The formula for the current carried by quasiparticle
with the Umklapp scattering is derived, on the basis of which we investigate
how the value of deviates from that of where and
are the carrier density and the effective mass respectively. Although
this deviation is small in the case of weak momentum dependence of the vertex,
this is large and negative owing to the non-negligible value of the backflow in
the case of the strong antiferromagnetic spin fluctuation. The observed doping
dependence of in high- cuprates, specifically a peak
structure at the slightly overdoped region, is explained by the analytical
consideration and the numerical calculation based on the perturbation theory
and the spin fluctuation theory. The consistency between and
at absolute zero, which is the problem the
isotropic model fails to explain, is also obtained by our theory.Comment: 25 pages, 9 figures. Another version(11 pages longer) will appear in
J. Phys. Soc. Jpn (2002) No.
Spin-triplet superconductivity in repulsive Hubbard models with disconnected Fermi surfaces: a case study on triangular and honeycomb lattices
We propose that spin-fluctuation-mediated spin-triplet superconductivity may
be realized in repulsive Hubbard models with disconnected Fermi surfaces. The
idea is confirmed for Hubbard models on triangular (dilute band filling) and
honeycomb (near half-filling) lattices using fluctuation exchange
approximation, where triplet pairing order parameter with f-wave symmetry is
obtained. Possible relevance to real superconductors is suggested.Comment: 5 pages, 6 figures, RevTeX, uses epsf.sty and multicol.st
Spin-Dependent Mass Enhancement under Magnetic Field in the Periodic Anderson Model
In order to study the mechanism of the mass enhancement in heavy fermion
compounds in the presence of magnetic field, we study the periodic Anderson
model using the fluctuation exchange approximation. The resulting value of the
mass enhancement factor z^{-1} can become up to 10, which is significantly
larger than that in the single-band Hubbard model. We show that the difference
between the magnitude of the mass enhancement factor of up spin (minority spin)
electrons z^{-1}_up and that of down spin (majority spin) electrons z^{-1}_down
increases by the applied magnetic field B//z, which is consistent with de
Haas-van Alphen measurements for CeCoIn_5, CeRu_2Si_2 and CePd_2Si_2. We
predict that z^{-1}_up >z^{-1}_down in many Ce compounds, whereas z^{-1}_up <
z^{-1}_down in Yb compounds.Comment: 5 pages, 4 figure
Pseudogap Induced Antiferromagnetic Spin Correlation in High-Temperature Superconductors
The pseudogap phenomena observed on cuprate high temperature superconductors
are investigated based on the exact diagonalization method on the finite
cluster t-J model. The results show the presence of the gap-like behavior in
the temperature dependence of various magnetic properties; the NMR relaxation
rate, the neutron scattering intensity and the static susceptibility. The
calculated spin correlation function indicates that the pseudogap behavior
arises associated with the development of the antiferromagnetic spin
correlation with decreasing the temperature. The numerical results are
presented to clarify the model parameter dependence, that covers the realistic
experimental situation. The effect of the next-nearest neighbor hopping is
also studied.Comment: 7 pages, Revtex, with 10 eps figures, to appear in J. Phys. Soc. Jpn.
(Vol. 70, No. 1
Phase Diagram of Superconductivity on the Anisotropic Triangular Lattice Hubbard Model
We study the electronic states of the anisotropic triangular lattice Hubbard
model at half filling, which is a simple effective model for the organic
superconducting -BEDT-TTF compounds. We treat the effect of the Coulomb
interaction by the fluctuation exchange (FLEX) method, and obtain the phase
diagram of this model for various sets of parameters. It is shown that the
d-wave superconductivity is realized in the wide region of the phase diagram,
next to the antiferromagnetic states. The obtained phase diagram explains the
characters of the experimental results very well.Comment: 4 pages, 6 figs, submitted for publicatio
Theory of the beta-type Organic Superconductivity under Uniaxial Compression
We study theoretically the shift of the superconducting transition
temperature (Tc) under uniaxial compression in beta-type organic
superconductors, beta-(BEDT-TTF)2I3 and beta-(BDA-TTP)2X[X=SbF6,AsF6], in order
to clarify the electron correlation, the spin frustration and the effect of
dimerization. The transfer integrals are calculated by the extended Huckel
method assuming the uniaxial strain and the superconducting state mediated by
the spin fluctuation is solved using Eliashberg's equation with the
fluctuation-exchange approximation. The calculation is carried out on both the
dimerized (one-band) and nondimerized (two-band) Hubbard models. We have found
that (i) the behavior of Tc in beta-(BEDT-TTF)2I3 with a stronger dimerization
is well reproduced by the dimer model, while that in weakly dimerized
beta-BDA-TTP salts is rather well reproduced by the two-band model, and (ii)
the competition between the spin frustration and the effect induced by the
fluctuation is important in these materials, which causes nonmonotonic shift of
Tc against uniaxial compression.Comment: 18 pages, 16 figures, 2 tabl
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