146,879 research outputs found
Evolution of the gaps through the cuprate phase-diagram
The actual physical origin of the gap at the antinodes, and a clear
identification of the superconducting gap are fundamental open issues in the
physics of high- superconductors. Here, we present a systematic electronic
Raman scattering study of a mercury-based single layer cuprate, as a function
of both doping level and temperature. On the deeply overdoped side, we show
that the antinodal gap is a true superconducting gap. In contrast, on the
underdoped side, our results reveal the existence of a break point close to
optimal doping below which the antinodal gap is gradually disconnected from
superconductivity. The nature of both the superconducting and normal state is
distinctly different on each side of this breakpoint
Equation of state of strongly coupled Hamiltonian lattice QCD at finite density
We calculate the equation of state of strongly coupled Hamiltonian lattice
QCD at finite density by constructing a solution to the equation of motion
corresponding to an effective Hamiltonian using Wilson fermions. We find that
up to and beyond the chiral symmetry restoration density the pressure of the
quark Fermi sea can be negative indicating its mechanical instability. This
result is in qualitative agreement with continuum models and should be
verifiable by future numerical simulations.Comment: 14 pages, 2 EPS figures. Revised version - added discussion on the
equation of stat
About an alternative distribution function for fractional exclusion statistics
We show that it is possible to replace the actual implicit distribution
function of the fractional exclusion statistics by an explicit one whose form
does not change with the parameter . This alternative simpler
distribution function given by a generalization of Pauli exclusion principle
from the level of the maximal occupation number is not completely equivalent to
the distributions obtained from the level of state number counting of the
fractional exclusion particles. Our result shows that the two distributions are
equivalent for weakly bosonized fermions () at not very high
temperatures.Comment: 8 pages, 3 eps figures, TeX. Nuovo Cimento B (2004), in pres
New challenges for Adaptive Optics: Extremely Large Telescopes
The performance of an adaptive optics (AO) system on a 100m diameter ground
based telescope working in the visible range of the spectrum is computed using
an analytical approach. The target Strehl ratio of 60% is achieved at 0.5um
with a limiting magnitude of the AO guide source near R~10, at the cost of an
extremely low sky coverage. To alleviate this problem, the concept of
tomographic wavefront sensing in a wider field of view using either natural
guide stars (NGS) or laser guide stars (LGS) is investigated. These methods use
3 or 4 reference sources and up to 3 deformable mirrors, which increase up to
8-fold the corrected field size (up to 60\arcsec at 0.5 um). Operation with
multiple NGS is limited to the infrared (in the J band this approach yields a
sky coverage of 50% with a Strehl ratio of 0.2). The option of open-loop
wavefront correction in the visible using several bright NGS is discussed. The
LGS approach involves the use of a faint (R ~22) NGS for low-order correction,
which results in a sky coverage of 40% at the Galactic poles in the visible.Comment: 11 pages, 9 figures, 4 tables. Accepted for publication in MNRA
Critical Analysis of Theoretical Estimates for to Light Meson Form Factors and the Data
We point out that current estimates of form factors fail to explain the
non-leptonic decays and that the combination of data
on the semi-leptonic decays and on the non-leptonic
decays (in particular recent po\-la\-ri\-za\-tion
data) severely constrain the form (normalization and dependence) of the
heavy-to-light meson form factors, if we assume the factorization hypothesis
for the latter. From a simultaneous fit to \bpsi and \dk data we find that
strict heavy quark limit scaling laws do not hold when going from to
and must have large corrections that make softer the dependence on the masses.
We find that should increase slower with \qq than .
We propose a simple parametrization of these corrections based on a quark
model or on an extension of the \hhs laws to the \hl case, complemented with an
approximately constant . We analyze in the light of these data and
theoretical input various theoretical approaches (lattice calculations, QCD sum
rules, quark models) and point out the origin of the difficulties encountered
by most of these schemes. In particular we check the compatibility of several
quark models with the heavy quark scaling relations.Comment: 48 pages, DAPNIA/SPP/94-24, LPTHE-Orsay 94/1
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