150 research outputs found
The Isotope Effect in d-Wave Superconductors
Based on recently proposed anti-ferromagnetic spin fluctuation exchange
models for -superconductors, we show that coupling to harmonic
phonons {\it{cannot}} account for the observed isotope effect in the cuprate
high- materials, whereas coupling to strongly anharmonic multiple-well
lattice tunneling modes {\it{can}}. Our results thus point towards a strongly
enhanced {\it{effective}} electron-phonon coupling and a possible break-down of
Migdal-Eliashberg theory in the cuprates.Comment: 12 pages + 2 figures, Postscript files, all uuencoded Phys. Rev.
Lett. (1995, to be published
Screened-interaction expansion for the Hubbard model and determination of the quantum Monte Carlo Fermi surface
We develop a systematic self-consistent perturbative expansion for the self
energy of Hubbard-like models. The interaction lines in the Feynman diagrams
are dynamically screened by the charge fluctuations in the system. Although the
formal expansion is exact-assuming that the model under the study is
perturbative-only if diagrams to all orders are included, it is shown that for
large-on-site-Coulomb-repulsion-U systems weak-coupling expansions to a few
orders may already converge. We show that the screened interaction for the
large-U system can be vanishingly small at a certain intermediate electron
filling; and it is found that our approximation for the imaginary part of the
one-particle self energy agrees well with the QMC results in the low energy
scales at this particular filling. But, the usefulness of the approximation is
hindered by the fact that it has the incorrect filling dependence when the
filling deviates from this value. We also calculate the exact QMC Fermi
surfaces for the two-dimensional (2-D) Hubbard model for several fillings. Our
results near half filling show extreme violation of the concepts of the band
theory; in fact, instead of growing, Fermi surface vanishes when doped toward
the half-filled Mott-Hubbard insulator. Sufficiently away from half filling,
noninteracting-like Fermi surfaces are recovered. These results combined with
the Luttinger theorem might show that diagrammatic expansions for the
nearly-half-filled Hubbard model are unlikely to be possible; however, the
nonperturbative part of the solution seems to be less important as the filling
gradually moves away from one half. Results for the 2-D one-band Hubbard model
for several hole dopings are presented. Implications of this study for the
high-temperature superconductors are also discussed.Comment: 11 pages, 12 eps figures embedded, REVTeX, submitted to Phys. Rev. B;
(v2) minor revisions, scheduled for publication on November 1
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
An alternative approach for the dynamics of polarons in one dimension
We developed a new method based on functional integration to treat the
dynamics of polarons in one-dimensional systems. We treat the acoustical and
the optical case in an unified manner, showing their differences and
similarities. The mobility and diffusion coefficients are calculated in the
Markovian approximation in the strong coupling limit.Comment: 57 page
Raman Response in Doped Antiferromagnets
The resonant part of the electronic Raman scattering response is
calculated within the model on a planar lattice as a function of
temperature and hole doping, using a finite-temperature diagonalization method
for small systems. Results, directly applicable to experiments on cuprates,
reveal on doping a very pronounced increase of the width of the two-magnon
Raman peak, accompanied by a decrease of the total intensity. At the same time
the peak position does not shift substantially in the underdoped regime.Comment: 11 pages revtex, 3 postscript figures. Minor corrections and changes
from previous version, to be published in Phys. Rev.
Spectral weight function for the half-filled Hubbard model: a singular value decomposition approach
The singular value decomposition technique is used to reconstruct the
electronic spectral weight function for a half-filled Hubbard model with
on-site repulsion from Quantum Monte Carlo data. A two-band structure
for the single-particle excitation spectrum is found to persist as the lattice
size exceeds the spin-spin correlation length. The observed bands are flat in
the vicinity of the points in the Brillouin zone, in
accordance with experimental data for high-temperature superconducting
compounds.Comment: 4 pages, Revtex
Evidence concerning the mechanism of insulin-receptor interaction and the structure of the insulin receptor from biological properties of covalently linked insulin dimers
Numerical Calculations of the B1g Raman Spectrum of the Two-Dimensional Heisenberg Model
The B1g Raman spectrum of the two-dimensional S=1/2 Heisenberg model is
discussed within Loudon-Fleury theory at both zero and finite temperature. The
exact T=0 spectrum for lattices with up to 6*6 sites is computed using Lanczos
exact diagonalization. A quantum Monte Carlo (QMC) method is used to calculate
the corresponding imaginary-time correlation function and its first two
derivatives for lattices with up to 16*16 spins. The imaginary-time data is
continued to real frequency using the maximum-entropy method, as well as a fit
based on spinwave theory. The numerical results are compared with spinwave
calculations for finite lattices. There is a surprisingly large change in the
exact spectrum going from 4*4 to 6*6 sites. In the former case there is a
single dominant two-magnon peak at frequency w/J appr. 3.0, whereas in the
latter case there are two approximately equal-sized peaks at w/J appr. 2.7 and
3.9. This is in good qualitative agreement with the spinwave calculations
including two-magnon processes on the same lattices. Both the Lanczos and the
QMC results indicate that the actual infinite-size two-magnon profile is
broader than the narrow peak obtained in spinwave theory, but the positions of
the maxima agree to within a few percent. The higher-order contributions
present in the numerical results are merged with the two-magnon profile and
extend up to frequencies w/J appr. 7. The first three frequency cumulants of
the spectrum are in excellent agreement with results previously obtained from a
series expansion around the Ising limit. Typical experimental B1g$ spectra for
La2CuO4 are only slightly broader than what we obtain here. The exchange
constant extracted from the peak position is J appr. 1400K, in good agreement
with values obtained from neutron scattering and NMR experiments.Comment: 15 pages, Revtex, 13 PostScript figure
Role of Van Hove Singularities and Momentum Space Structure in High-Temperature Superconductivity
There is a great deal of interest in attributing the high critical
temperatures of the cuprates to either the proximity of the Fermi level to a
van Hove singularity or to structure of the superconducting pairing potential
in momentum space far from the Fermi surface. We examine these ideas by
calculating the critical temperature Tc for model Einstein-phonon- and
spin-fluctuation-mediated superconductors within both the standard,
Fermi-surface-restricted Eliashberg theory and the exact mean field theory,
which accounts for the full momentum structure of the pairing potential and the
energy dependence of the density of states. By using two models of
spin-fluctuation-mediated pairing in the cuprates, we demonstrate that our
results are independent of the details of the dynamical susceptibility, which
is taken to be the pairing potential. We also compare these two models against
available neutron scattering data, since these data provide the most direct
constraints on the susceptibility. We conclude that the van Hove singularity
does not drastically alter Tc from its value when the density of states is
constant and that the effect of momentum structure is significant but secondary
in importance to that of the energy dependence in the density of states.Comment: 23 pages, 6 figures upon request, revtex version 2, vHs-
Possible Odd-Frequency Superconductivity in Strong-Coupling Electron-Phonon Systems
A possibility of the odd-frequency pairing in the strong-coupling
electron-phonon systems is discussed. Using the Holstein-Hubbard model, we
demonstrate that the anomalously soft Einstein mode with the frequency
( is the order of the renormalized
bandwidth) mediates the s-wave odd-frequency triplet pairing against the
ordinary even-frequency singlet pairing. It is necessary for the emergence of
the odd-frequency pairing that the pairing interaction is strongly retarded as
well as the strong coupling, since the pairing interaction for the
odd-frequency pairing is effective only in the diagonal scattering channel,
with
. Namely, the odd-frequency
superconductivity is realized in the opposite limit of the original BCS theory.
The Ginzburg-Landau analysis in the strong-coupling region shows that the
specific-heat discontinuity and the slope of the temperature dependence of the
superfluid density can be quite small as compared with the BCS values,
depending on the ratio of the transition temperature and .Comment: 6 pages, 7 figures, submitted to J. Phys. Soc. Jp
- …