599 research outputs found
High-frequency two-input CMOS OTA for continuous-time filter applications
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Catastrophe écologique, fragilité du mouvement vert : la voïvodie de Katowice
Expressions dâun processus de colonisation intĂ©rieure, lâhyperconcentration dâindustries anachroniques et le fonctionnement dĂ©voyĂ© du pouvoir ont eu dans la voĂŻvodie de Karowice des consĂ©quences dramatiques sur la situation Ă©cologique et lâĂ©tat de santĂ© de la population. Les partis et mouvements Ă©cologistes qui sây constituent depuis quelques annĂ©es prĂ©sentent un faible degrĂ© dâorganisation et sont fortement fractionnĂ©s.Ecological disaster, fragility of the Green movement : the VoĂŻvodie of Katowice Significant of a process of internal colonization, both hyperconcentration of anachronistic industries and longtime inconsequent political ruling have had in the voĂŻvodie of Katowice drastic consequences ont the ecological situation and on the health problems of the local population. Since a few years, ecological parties and movements which have set up in the area are still kknowing a low level of organisation and are divided into various trends
Catastrophe écologique, fragilité du mouvement vert : la voïvodie de Katowice
Expressions dâun processus de colonisation intĂ©rieure, lâhyperconcentration dâindustries anachroniques et le fonctionnement dĂ©voyĂ© du pouvoir ont eu dans la voĂŻvodie de Karowice des consĂ©quences dramatiques sur la situation Ă©cologique et lâĂ©tat de santĂ© de la population. Les partis et mouvements Ă©cologistes qui sây constituent depuis quelques annĂ©es prĂ©sentent un faible degrĂ© dâorganisation et sont fortement fractionnĂ©s.Ecological disaster, fragility of the Green movement : the VoĂŻvodie of Katowice Significant of a process of internal colonization, both hyperconcentration of anachronistic industries and longtime inconsequent political ruling have had in the voĂŻvodie of Katowice drastic consequences ont the ecological situation and on the health problems of the local population. Since a few years, ecological parties and movements which have set up in the area are still kknowing a low level of organisation and are divided into various trends
Anomalous Spin and Charge Dynamics of the 2D t-J Model at low doping
We present an exact diagonalization study of the dynamical spin and density
correlation function of the 2D t-J model for hole doping < 25%. Both
correlation functions show a remarkably regular, but completely different
scaling behaviour with both hole concentration and parameter values: the
density correlation function is consistent with that of bosons corresponding to
the doped holes and condensed into the lowest state of the noninteracting band
of width 8t, the spin correlation function is consistent with Fermions in a
band of width J. We show that the spin bag picture gives a natural explanation
for this unusual behaviour.Comment: Revtex-file, 4 PRB pages + 5 figures attached as uu-encoded ps-files
Hardcopies of figures (or the entire manuscript) can also be obtained by
e-mailing to: [email protected]
Interaction of a Magnetic Impurity with Strongly Correlated Conduction Electrons
We consider a magnetic impurity which interacts by hybridization with a
system of strongly correlated conduction electrons. The latter are described by
a Hubbard Hamiltonian. By means of a canconical transformation the charge
degrees of freedom of the magnetic impurity are eliminated. The resulting
effective Hamiltonian is investigated and various limiting cases
are considered. If the Hubbard interaction between the conduction electrons
is neglected reduces to a form obtained by the Schrieffer-Wolff
transformation, which is essentially the Kondo Hamiltonian. If is large and
the correlations are strong is changed. One modification concerns
the coefficient of the dominant exchange coupling of the magnetic impurity with
the nearest lattice site. When the system is hole doped, there is also an
antiferromagnetic coupling to the nearest neighbors of that site involving
additionally a hole. Furthermore, it is found that the magnetic impurity
attracts a hole. In the case of electron doping, double occupancies are
repelled by the impurity. In contrast to the hole-doped case, we find no
magnetic coupling which additionally involves a doubly occupied site.Comment: 16 pages, Revtex 3.
Green Function Monte Carlo with Stochastic Reconfiguration: an effective remedy for the sign problem disease
A recent technique, proposed to alleviate the ``sign problem disease'', is
discussed in details. As well known the ground state of a given Hamiltonian
can be obtained by applying the imaginary time propagator to a
given trial state for large imaginary time and sampling
statistically the propagated state . However
the so called ``sign problem'' may appear in the simulation and such
statistical propagation would be practically impossible without employing some
approximation such as the well known ``fixed node'' approximation (FN). This
method allows to improve the FN dynamic with a systematic correction scheme.
This is possible by the simple requirement that, after a short imaginary time
propagation via the FN dynamic, a number of correlation functions can be
further constrained to be {\em exact} by small perturbation of the FN
propagated state, which is free of the sign problem. By iterating this scheme
the Monte Carlo average sign, which is almost zero when there is sign problem,
remains stable and finite even for large . The proposed algorithm is
tested against the exact diagonalization results available on finite lattice.
It is also shown in few test cases that the dependence of the results upon the
few parameters entering the stochastic technique can be very easily controlled,
unless for exceptional cases.Comment: 44 pages, RevTeX + 5 encaplulated postscript figure
Propagation of a hole on a Neel background
We analyze the motion of a single hole on a N\'eel background, neglecting
spin fluctuations. Brinkman and Rice studied this problem on a cubic lattice,
introducing the retraceable-path approximation for the hole Green's function,
exact in a one-dimensional lattice. Metzner et al. showed that the
approximationalso becomes exact in the infinite-dimensional limit. We introduce
a new approach to this problem by resumming the Nagaoka expansion of the
propagator in terms of non-retraceable skeleton-paths dressed by
retraceable-path insertions. This resummation opens the way to an almost
quantitative solution of the problemin all dimensions and, in particular sheds
new light on the question of the position of the band-edges. We studied the
motion of the hole on a double chain and a square lattice, for which deviations
from the retraceable-path approximation are expected to be most pronounced. The
density of states is mostly adequately accounted for by the
retra\-ce\-able-path approximation. Our band-edge determination points towards
an absence of band tails extending to the Nagaoka energy in the spectrums of
the double chain and the square lattice. We also evaluated the spectral density
and the self-energy, exhibiting k-dependence due to finite dimensionality. We
find good agreement with recent numerical results obtained by Sorella et al.
with the Lanczos spectra decoding method. The method we employ enables us to
identify the hole paths which are responsible for the various features present
in the density of states and the spectral density.Comment: 26 pages,Revte
Hole dynamics in generalized spin backgrounds in infinite dimensions
We calculate the dynamical behaviour of a hole in various spin backgrounds in
infinite dimensions, where it can be determined exactly. We consider hypercubic
lattices with two different types of spin backgrounds. On one hand we study an
ensemble of spin configurations with an arbitrary spin probability on each
sublattice. This model corresponds to a thermal average over all spin
configurations in the presence of staggered or uniform magnetic fields. On the
other hand we consider a definite spin state characterized by the angle between
the spins on different sublattices, i.e a classical spin system in an external
magnetic field. When spin fluctuations are considered, this model describes the
physics of unpaired particles in strong coupling superconductors.Comment: Accepted in Phys. Rev. B. 18 pages of text (1 fig. included) in Latex
+ 2 figures in uuencoded form containing the 2 postscripts (mailed
separately
Hubbard model versus t-J model: The one-particle spectrum
The origin of the apparent discrepancies between the one-particle spectra of
the Hubbard and t-J models is revealed: Wavefunction corrections, in addition
to the three-site terms, should supplement the bare t-J. In this way a
quantitative agreement between the two models is obtained, even for the
intermediate- values appropriate for the high-Tc cuprate superconductors.
Numerical results for clusters of up to 20 sites are presented. The momentum
dependence of the observed intensities in the photoemission spectra of
Sr2CuO2Cl2 are well described by this complete strong-coupling approach.Comment: 4 two-column RevTeX pages, including 4 Postscript figures. Uses epsf.
Accepted for publication in Physical Review B, Rapid Communicatio
Theory of Dynamic Stripe Induced Superconductivity
Since the recently reported giant isotope effect on T* [1] could be
consistently explained within an anharmonic spin-charge-phonon interaction
model, we consider here the role played by stripe formation on the
superconducting properties within the same model. This is a two-component
scenario and we recast its basic elements into a BCS effective Hamiltonian. We
find that the stripe formation is vital to high-Tc superconductivity since it
provides the glue between the two components to enhance Tc to the unexpectedly
large values observed experimentally.Comment: 7 pages, 2 figure
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