43 research outputs found
Comment on "Quantum Monte Carlo Evidence for Superconductivity in the Three-Band Hubbard Model in Two Dimensions"
In a recent Letter, Kuroki and Aoki [Phys. Rev. Lett. 76, 440 (1996)]
presented quantum Monte-Carlo (QMC) results for pairing correlations in the
three-band Hubbard model, which describes the Cu-d_{x^2-y^2} and O-p_{x,y}
orbitals present in the CuO_2 planes of high-T_c materials. In this comment we
argue that (i) the used parameter set is not appropriate for the description of
high-T_c materials since it does not satisfy the minimal requirement of a
charge-transfer gap at half-filling, and (ii) the observed increase in the
d_{x^2-y^2} channel is dominantly produced by the pair-field correlations
without the vertex part. Hence, the claim of evidence of ODLRO is not
justified.Comment: 1 page latex and 2 eps-figures, uses epsfig, submitted to PR
Oxide mediated spectral shifting in aluminum resonant optical antennas
As a key feature among metals showing good plasmonic behavior, aluminum extends the spectrum of achievable plasmon resonances of optical antennas into the deep ultraviolet. Due to degradation, a native oxide layer gives rise to a metal-core/oxide-shell nanoparticle and influences the spectral resonance peak position. In this work, we examine the role of the underlying processes by applying numerical nanoantenna models that are experimentally not feasible. Finite-difference time-domain simulations are carried out for a large variety of elongated single-arm and two-arm gap nanoantennas. In a detailed analysis, which takes into account the varying surface-to-volume ratio, we show that the overall spectral shift toward longer wavelengths is mainly driven by the higher index surrounding material rather than by the decrease of the initial aluminum volume. In addition, we demonstrate experimentally that this shifting can be minimized by an all-inert fabrication and subsequent proof-of-concept encapsulation. © 2015 Optical Society of America
Quantum Monte Carlo Study of Hole Binding and Pairing Correlations in the Three-Band Hubbard Model
We simulated the 3-band Hubbard model using the Constrained Path Monte Carlo
(CPMC) method in search for a possible superconducting ground state. The CPMC
is a ground state method which is free of the exponential scaling of computing
time with system size. We calculated the binding energy of a pair of holes for
systems up to unit cells. We also studied the pairing correlation
functions versus distance for both the d-wave and extended s-wave channels in
systems up to . We found that holes bind for a wide range of
parameters and that the binding increased as the system size is increased.
However, the pairing correlation functions decay quickly with distance.
For the extended s channel, we found that as the Coulomb interaction on
the Cu sites is increased, the long-range part of the correlation functions is
suppressed and fluctuates around zero. For the channel, we
found that the correlations decay rapidly with distance towards a small
positive value. However, this value becomes smaller as the interaction or
the system size is increased.Comment: 21 pages, 13 Postscript figures, Submitted to Phys. Rev.
Asymmetry in the effect of magnetic field on photon detection and dark counts in bended nanostrips
Current crowding in the bends of superconducting nano-structures not only
restricts measurable critical current in such structures but also redistributes
local probabilities for dark and light counts to appear. Using structures from
strips in the form of a square spiral which contain bends with the very same
curvature with respect to the directions of bias current and external magnetic
field, we have shown that dark counts as well as light counts at small photon
energies originate from areas around the bends. The minimum in the rate of dark
counts reproduces the asymmetry of the maximum critical current density as
function of the magnetic field. Contrary, the minimum in the rate of light
counts demonstrate opposite asymmetry. The rate of light counts become
symmetric at large currents and fields. Comparing locally computed absorption
probabilities for photons and the simulated threshold detection current we
found the approximate locations of areas near bends which deliver asymmetric
light counts. Any asymmetry is absent in Archimedean spiral structures without
bends
Superconductivity of the One-Dimensional d-p Model with p-p transfer
Using the numerical diagonalization method, we investigate the
one-dimensional - model, simulating a Cu-O linear chain with strong
Coulomb repulsions. Paying attention to the effect of the transfer energy
between the nearest neighbor oxygen-sites, we calculate the critical
exponent of correlation functions based on the Luttinger liquid
relations and the ground state energy as a function of an external
flux . We find that the transfer increases the charge
susceptibility and the exponent in cooperation with the repulsion
at Cu-site. We also show that anomalous flux quantization occurs for
. The superconducting region is presented on a phase diagram of
vs. plane.Comment: 4 pages, RevTex + 5 PS figures include
Magnetic and pair correlations of the Hubbard model with next-nearest-neighbor hopping
A combination of analytical approaches and quantum Monte Carlo simulations is
used to study both magnetic and pairing correlations for a version of the
Hubbard model that includes second-neighbor hopping as a
model for high-temperature superconductors. Magnetic properties are analyzed
using the Two-Particle Self-Consistent approach. The maximum in magnetic
susceptibility as a function of doping appears both at finite
and at but for two totally different physical reasons. When
, it is induced by antiferromagnetic correlations while at
it is a band structure effect amplified by interactions.
Finally, pairing fluctuations are compared with -matrix results to
disentangle the effects of van Hove singularity and of nesting on
superconducting correlations. The addition of antiferromagnetic fluctuations
increases slightly the -wave superconducting correlations despite the
presence of a van Hove singularity which tends to decrease them in the
repulsive model. Some aspects of the phase diagram and some subtleties of
finite-size scaling in Monte Carlo simulations, such as inverted finite-size
dependence, are also discussed.Comment: Revtex, 8 pages + 15 uuencoded postcript figure
Superconductivity in the Two-Band Hubbard Model in Infinite Dimensions
We study a two-band Hubbard model in the limit of infinite dimensions, using
a combination of analytical methods and Monte-Carlo techniques. The normal
state is found to display various metal to insulators transitions as a function
of doping and interaction strength. We derive self-consistent equations for the
local Green's functions in the presence of superconducting long-range order,
and extend previous algorithms to this case. We present direct numerical
evidence that in a specific range of parameter space, the normal state is
unstable against a superconducting state characterized by a strongly frequency
dependent order-parameter.Comment: 12 pages (14 figures not included, available upon request), Latex,
LPTENS Preprint 93/1
Ground state of the three-band Hubbard model
The ground state of the two-dimensional three-band Hubbard model in oxide
superconductors is investigated by using the variational Monte Carlo method.
The Gutzwiller-projected BCS and spin- density wave (SDW) functions are
employed in the search for a possible ground state with respect to dependences
on electron density. Antiferromagnetic correlations are considerably enhanced
near half-filling. It is shown that the d-wave state may exist away from
half-filling for both the hole and electron doping cases. The overall structure
of the phase diagram obtained by the calculations qualitatively agrees with
experimental indications. The superconducting condensation energy is in
reasonable agreement with the experimental value obtained from specific heat
and critical magnetic field measurements for optimally doped samples. The
inhomogeneous SDW state is also examined near 1/8-hole doping.Comment: 10 pages, 17 figure
Superconductivity in the Cuo Hubbard Model with Long-Range Coulomb Repulsion
A multiband CuO Hubbard model is studied which incorporates long-range (LR)
repulsive Coulomb interactions. In the atomic limit, it is shown that a
charge-transfer from copper to oxygen ions occurs as the strength of the LR
interaction is increased. The regime of phase separation becomes unstable, and
is replaced by a uniform state with doubly occupied oxygens. As the holes
become mobile a superfluid condensate is formed, as suggested by a numerical
analysis of pairing correlation functions and flux quantization. Although most
of the calculations are carried out on one dimensional chains, it isComment: LATEX, 14 pages, 4 figures available as postcript files or hard copy,
preprint ORNL-CCIP/93/1
Ginzburg-Landau theory of superconductors with short coherence length
We consider Fermions in two dimensions with an attractive interaction in the
singlet d-wave channel of arbitrary strength. By means of a
Hubbard-Stratonovich transformation a statistical Ginzburg-Landau theory is
derived, which describes the smooth crossover from a weak-coupling BCS
superconductor to a condensate of composite Bosons. Adjusting the interaction
strength to the observed slope of H_c2 at T_c in the optimally doped high-T_c
compounds YBCO and BSCCO, we determine the associated values of the
Ginzburg-Landau correlation length xi and the London penetration depth lambda.
The resulting dimensionless ratio k_F xi(0) approx 5-8 and the Ginzburg-Landau
parameter kappa=lambda xi approx 90-100 agree well with the experimentally
observed values. These parameters indicate that the optimally doped materials
are still on the weak coupling side of the crossover to a Bose regime.Comment: 12 pages, RevTeX, 6 postscript figures, resubmitted with minor
changes in section III, to appear in Physical Review