424 research outputs found
Hadron correlators with improved fermions
We investigate point-to-point correlation functions for various mesonic and
baryonic channels using the -improved Wilson action due to
Sheikholeslami and Wohlert. We consider propagators to both time slices 0 and
1. We find that discretisation effects are more pronounced than those reported
with unimproved Wilson fermions, but that the same procedure for removing
finite size effects is successful. Extrapolating to the chiral limit, we see
the notable features predicted phenomenologically: the ratio of interacting to
free correlators in the vector channel is roughly constant to about 1 fm, while
in the pseudoscalar channel the ratio increases markedly due to the strong
binding.Comment: Talk at Lattice '94: 3 pages, latex using espcrc2 and epsf with 4
tar'd and feathered PS figures appended in self-extracting shell archive.
PostScript file available on the WorldWide Web as
http://python.swan.ac.uk/~pypeters/p2pbiel.p
Synthesis effects on the magnetic and superconducting properties of RuSr2GdCu2O8
A systematic study on the synthesis of the Ru-1212 compound by preparing a
series of samples that were annealed at increasing temperatures and then
quenched has been performed. It results that the optimal temperature for the
annealing lies around 1060-1065 C; a further temperature increase worsens the
phase formation. Structural order is very important and the subsequent grinding
and annealing improves it. Even if from the structural point of view the
samples appear substantially similar, the physical characterization highlight
great differences both in the electrical and magnetic properties related to
intrinsic properties of the phase as well as to the connection between the
grains as inferred from the resistive and the Curie Weiss behaviour at high
temperature as well as in the visibility of ZFC anf FC magnetic signals.Comment: 17 pages, 12 figures. Proc. Int. Workshop " Ruthenate and
rutheno-cuprate materials: theory and experiments", Vietri, October 2001. To
be published on LNP Series, Springer Verlag, Berlin, C. Noce, A. Vecchione,
M. Cuoco, A. Romano Eds, 200
Underlying Pairing States in Cuprate Superconductors
In this Letter, we develop a microscopic theory to describe the close
proximity between the insulating antiferromagnetic (AF) order and the d-wave
superconducting (dSC) order in cuprates. We show that the cuprate ground states
form a configuration of coherent pairing states consisting of extended singlet
Cooper pairs and triplet pairs, which can simultaneously describe AF and
dSC orders.Comment: 4 papes, 1 figur
The Superconductivity, Intragrain Penetration Depth and Meissner Effect of RuSr2(Gd,Ce)2Cu2O10+delta
The hole concentration (p)(delta), the transition temperature Tc, the
intragrain penetration depth lambda, and the Meissner effect were measured for
annealed RuSr2(Gd,Ce)2Cu2O10+delta samples. The intragrain superconducting
transition temperature Tc} varied from 17 to 40 K while the p changed by only
0.03 holes/CuO2. The intragrain superfluid-density 1/lambda^2 and the
diamagnetic drop of the field-cooled magnetization across Tc (the Meissner
effect), however, increased more than 10 times. All of these findings are in
disagreement with both the Tc vs. p and the Tc vs. 1/lambda^2 correlations
proposed for homogeneous cuprates, but are in line with a possible
phase-separation and the granularity associated with it.Comment: 7 pages, 6 figures, accepted for publication in Phys. Rev. B (May 2,
2002
Inhibiting decoherence via ancilla processes
General conditions are derived for preventing the decoherence of a single
two-state quantum system (qubit) in a thermal bath. The employed auxiliary
systems required for this purpose are merely assumed to be weak for the general
condition while various examples such as extra qubits and extra classical
fields are studied for applications in quantum information processing. The
general condition is confirmed with well known approaches towards inhibiting
decoherence. A novel approach for decoherence-free quantum memories and quantum
operations is presented by placing the qubit into the center of a sphere with
extra qubits on its surface.Comment: pages 8, Revtex
Superconducting properties of RuSr2GdCu2O8 studied by SQUID magnetometry
For polycrystalline RuSr2GdCu2O8 (Ru-1212), distinct peaks have been reported
in d.c. magnetization in the superconducting state of the sample. Sr2GdRuO6
(Sr-2116), the precursor for the preparation of Ru-1212, shows similar peaks in
the same temperature regime. Based on measurements performed on both bulk and
powdered samples of Ru-1212 and Sr-2116, we exclude the possibility, that the
observed behavior of the magnetization of Ru-1212 is due to Sr-2116 impurities.
The effect is related to the superconductivity of Ru-1212, but it is not an
intrinsic property of this compound. We provide evidence that the observation
of magnetization peaks in the superconducting state of Ru-1212 is due to flux
motion generated by the movement of the sample in an inhomogeneous field,
during the measurement in the SQUID magnetometer. We propose several tests,
that help to decide, whether the features observed in a SQUID magnetization
measurement of Ru-1212 represent a property of the compound or not.Comment: 22 pages, 9 figure
The Hubbard model with smooth boundary conditions
We apply recently developed smooth boundary conditions to the quantum Monte
Carlo simulation of the two-dimensional Hubbard model. At half-filling, where
there is no sign problem, we show that the thermodynamic limit is reached more
rapidly with smooth rather than with periodic or open boundary conditions. Away
from half-filling, where ordinarily the simulation cannot be carried out at low
temperatures due to the existence of the sign problem, we show that smooth
boundary conditions allow us to reach significantly lower temperatures. We
examine pairing correlation functions away from half-filling in order to
determine the possible existence of a superconducting state. On a
lattice for , at a filling of and an inverse
temperature of , we did find enhancement of the -wave correlations
with respect to the non-interacting case, a possible sign of -wave
superconductivity.Comment: 16 pages RevTeX, 9 postscript figures included (Figure 1 will be
faxed on request
Improvement of Aerosol Optical Depth Retrieval over Hong Kong from a Geostationary Meteorological Satellite Using Critical Reflectance with Background Optical Depth Correction
Despite continuous efforts to retrieve aerosol optical depth (AOD) using a conventional 5-channelmeteorological imager in geostationary orbit, the accuracy in urban areas has been poorer than other areas primarily due to complex urban surface properties and mixed aerosol types from different emission sources. The two largest error sources in aerosol retrieval have been aerosol type selection and surface reflectance. In selecting the aerosol type from a single visible channel, the season-dependent aerosol optical properties were adopted from longterm measurements of Aerosol Robotic Network (AERONET) sun-photometers. With the aerosol optical properties obtained fromthe AERONET inversion data, look-up tableswere calculated by using a radiative transfer code: the Second Simulation of the Satellite Signal in the Solar Spectrum (6S). Surface reflectance was estimated using the clear sky composite method, awidely used technique for geostationary retrievals. Over East Asia, the AOD retrieved from the Meteorological Imager showed good agreement, although the values were affected by cloud contamination errors. However, the conventional retrieval of the AOD over Hong Kong was largely underestimated due to the lack of information on the aerosol type and surface properties. To detect spatial and temporal variation of aerosol type over the area, the critical reflectance method, a technique to retrieve single scattering albedo (SSA), was applied. Additionally, the background aerosol effect was corrected to improve the accuracy of the surface reflectance over Hong Kong. The AOD retrieved froma modified algorithmwas compared to the collocated data measured by AERONET in Hong Kong. The comparison showed that the new aerosol type selection using the critical reflectance and the corrected surface reflectance significantly improved the accuracy of AODs in Hong Kong areas,with a correlation coefficient increase from0.65 to 0.76 and a regression line change from MI [basic algorithm] = 0.41AERONET + 0.16 to MI [new algorithm] = 0.70AERONET + 0.01
Many-body theory of pump-probe spectra for highly excited semiconductors
We present a unified theory for pump-probe spectra in highly excited
semiconductors, which is applicable throughout the whole density regime
including the high-density electron-hole BCS state and the low-density
excitonic Bose-Einstein condensate (BEC). The analysis is based on the BCS-like
pairing theory combined with the Bethe-Salpeter (BS) equation, which first
enables us to incorporate the state-filling effect, the band-gap
renormalization and the strong/weak electron-hole pair correlations in a
unified manner. We show that the electron-hole BCS state is distinctly
stabilized by the intense pump-light, and this result strongly suggests that
the macroscopic quantum state can be observed under the strong photoexcitation.
The calculated spectra considerably deviate from results given by the BCS-like
mean field theory and the simple BS equation without electron-hole pair
correlation especially in the intermediate density states between the
electron-hole BCS state and the excitonic BEC state. In particular, we find the
sharp stimulated emission and absorption lines which originate from the optical
transition accompanied by the collective phase fluctuation mode in the
electron-hole BCS state. From the pump-probe spectral viewpoint, we show that
this fluctuation mode changes to the exciton mode with decreasing carrier
densityComment: RevTeX 11 pages, 10 figures. To appear in Phys.Rev.B1
Transverse Fresnel-Fizeau drag effects in strongly dispersive media
A light beam normally incident upon an uniformly moving dielectric medium is
in general subject to bendings due to a transverse Fresnel-Fizeau light drag
effect. In conventional dielectrics, the magnitude of this bending effect is
very small and hard to detect. Yet, it can be dramatically enhanced in strongly
dispersive media where slow group velocities in the m/s range have been
recently observed taking advantage of the electromagnetically induced
transparency (EIT) effect. In addition to the usual downstream drag that takes
place for positive group velocities, we predict a significant anomalous
upstream drag to occur for small and negative group velocities. Furthermore,
for sufficiently fast speeds of the medium, higher order dispersion terms are
found to play an important role and to be responsible for peculiar effects such
as light propagation along curved paths and the restoration of the spatial
coherence of an incident noisy beam. The physics underlying this new class of
slow-light effects is thoroughly discussed
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