5,535 research outputs found
First-Order Pairing Transition and Single-Particle Spectral Function in the Attractive Hubbard Model
A Dynamical Mean Field Theory analysis of the attractive Hubbard model is
carried out. We focus on the normal state upon restricting to solutions where
superconducting order is not allowed. Nevertheless a clear first-order pairing
transition as a function of the coupling takes place at all the electron
densities out of half-filling. The transition occurs between a Fermi liquid,
stable for . The
spectral function in the Fermi liquid phase is constituted by a low energy
structure around the Fermi level (similar to the Kondo resonance of the
repulsive half-filled model), which disappears discontinuously at , and
two high energy features (lower and upper Hubbard bands), which persist in the
insulating phase.Comment: 5 pages, 3 figures, accepted for publication in Physical Review
Letter
Electron-phonon interaction in Strongly Correlated Systems
The Hubbard-Holstein model is a simple model including both electron-phonon
interaction and electron-electron correlations. We review a body of theoretical
work investigating the effects of strong correlations on the electron-phonon
interaction. We focus on the regime, relevant to high-T_c superconductors, in
which the electron correlations are dominant. We find that the electron-phonon
interaction can still have important signatures, even if many anomalies appear,
and the overall effect is far from conventional. In particular in the
paramagnetic phase the effects of phonons are much reduced in the low-energy
properties, while the high-energy physics can be strongly affected by phonons.
Moreover, the electron-phonon interaction can still give rise to important
effects, like phase separation and charge-ordering, and it assumes a
predominance of forward scattering even if the bare interaction is assumed to
be local (momentum independent). Antiferromagnetic correlations reduce the
screening effects due to electron-electron interactions and revive the
electron-phonon effects.Comment: 15 pages, 12 figure
On localization effects in underdoped cuprates
We comment on transport experiments in underdoped LaSrCuO in the
non-superconducting phase. The temperature dependence of the resistance
strongly resembles what is expected from standard localization theory. However
this theory fails, when comparing with experiments in more detail.Comment: 8 pages, to be published in J. of Superconductivit
Electron-phonon Interaction close to a Mott transition
The effect of Holstein electron-phonon interaction on a Hubbard model close
to a Mott-Hubbard transition at half-filling is investigated by means of
Dynamical Mean-Field Theory. We observe a reduction of the effective mass that
we interpret in terms of a reduced effective repulsion. When the repulsion is
rescaled to take into account this effect, the quasiparticle low-energy
features are unaffected by the electron-phonon interaction. Phonon features are
only observed within the high-energy Hubbard bands. The lack of electron-phonon
fingerprints in the quasiparticle physics can be explained interpreting the
quasiparticle motion in terms of rare fast processes.Comment: 4 pages, 3 color figures. Slightly revised text and references. Kondo
effect result added in Fig. 2 for comparison with DMFT dat
R-Matrix Formulation of the Quantum Inhomogeneous Groups Iso_qr(N) and Isp_qr(N)
The quantum commutations and the orthogonal (symplectic) conditions
for the inhomogeneous multiparametric -groups of the type are
found in terms of the -matrix of . A consistent
Hopf structure on these inhomogeneous -groups is constructed by means of a
projection from . Real forms are discussed: in
particular we obtain the -groups , including the quantum
Poincar\'e group.Comment: 14 pages, latex, no figure
Stellar Evolutionary Models for Magellanic Clouds
We supplement current evolutionary computations concerning Magellanic Cloud
stars by exploring the evolutionary behavior of canonical stellar models
(i.e.,with inefficient core overshooting) with metallicities suitable for stars
in the Clouds. After discussing the adequacy of the adopted evolutionary
scenario, we present evolutionary sequences as computed following a selected
sample of stellar models in the mass range 0.8-8 Mo from the Main Sequence till
the C ignition or the onset of thermal pulses in the advanced Asymptotic Giant
Branch phase. On this basis, cluster isochrones covering the range of ages from
100 Myr to 15 Gyr are presented and discussed. To allow a comparison with
evolutionary investigations appeared in the recent literature, we computed
additional sets of models which take into account moderate core overshooting
during the H burning phase, discussing the comparison in terms of current
uncertainties in the stellar evolutionary models. Selected predictions
constraining the cluster ages are finally discussed, presenting a calibration
of the difference in magnitude between the luminous MS termination and the He
burning giants in terms of cluster age. Both evolutionary tracks and isochrones
have been made available at the node http://gipsy.cjb.net as a first step of a
planned ``Pisa Evolutionary Library''.Comment: 11 pages, 9 eps figures, A&A accepted, evolutionary tracks and
isochrones available at http://gipsy.cjb.net at the link ``Pisa Evolutionary
Library'
Theoretical Zero Age Main Sequences revisited
Zero Age Main Sequence (ZAMS) models with updated physical inputs are
presented for selected assumptions about the chemical composition, covering the
ranges 0.6 < M/Mo < 1.2, 0.0001 < Z < 0.04, 0.23 < Y < 0.34.The HR diagram
location of the ZAMS as a function of Y and Z is discussed both in the
theoretical and in the observational HR diagrams, showing that the V magnitude
presents an increased dependence on Z to be taken into account when discussing
observational evidences. Analytical relations quantifying both these
dependences are derived. Implications for the galactic helium to heavier
elements enrichment are finally discussed.Comment: 4 pages, 4 postscript figures, accepted for publication on Astronomy
& Astrophysic
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