820 research outputs found
VLA observations of a sample of galaxies with high far-infrared luminosities
Preliminary results are presented from a radio survey of galaxies detected by the IRAS minisurvey. It was found that the main difference between galaxies selected in the far infrared and those selected in the optical is that the former have higher radio luminosities and that the radio emission is more centrally concentrated. There is some evidence that the strong central radio sources in the galaxies selected in the infrared are due to star formation, the star formation rate divided by the volume in which the star formation is occuring is 100 to 1000 times greater in the galaxies selected in the infrared than in the disks of normal galaxies
Resonating bipolarons
Electrons coupled to local lattice deformations end up in selftrapped
localized molecular states involving their binding into bipolarons when the
coupling is stronger than a certain critical value. Below that value they exist
as essentially itinerant electrons. We propose that the abrupt crossover
between the two regimes can be described by resonant pairing similar to the
Feshbach resonance in binary atomic collision processes. Given the
intrinsically local nature of the exchange of pairs of itinerant electrons and
localized bipolarons, we demonstrate the occurrence of such a resonance on a
finite-size cluster made out of metallic atoms surrounding a polaronic ligand
center.Comment: 7 pages, 4 figures, to be published in Europhysics Letter
Optical conductivity of polaronic charge carriers
The optical conductivity of charge carriers coupled to quantum phonons is
studied in the framework of the one-dimensional spinless Holstein model. For
one electron, variational diagonalisation yields exact results in the
thermodynamic limit, whereas at finite carrier density analytical
approximations based on previous work on single-particle spectral functions are
obtained. Particular emphasis is put on deviations from weak-coupling,
small-polaron or one-electron theories occurring at intermediate coupling
and/or finite carrier density. The analytical results are in surprisingly good
agreement with exact data, and exhibit the characteristic polaronic excitations
observed in experiments on manganites.Comment: 23 pages, 11 figure
Klein-Gordon Equation for Quark Pairs in Color Superconductor
The wave equation is derived for quark pairs in color superconductor in the
regime of low density / strong coupling.Comment: 4 pages, no figure
Many-body large polaron optical conductivity in SrTiNbO
Recent experimental data on the optical conductivity of niobium doped
SrTiO are interpreted in terms of a gas of large polarons with effective
coupling constant . The {theoretical approach takes into
account} many-body effects, the electron-phonon interaction with multiple
LO-phonon branches, and the degeneracy and the anisotropy of the Ti t
conduction band. {Based on the Fr\"{o}hlich interaction, the many-body
large-polaron theory} provides an interpretation for the essential
characteristics, except -- interestingly -- for the unexpectedly large
intensity of a peak at meV, of the observed optical conductivity
spectra of SrTiNbO \textit{without} any adjustment of
material parameters.Comment: to appear in Phys. Rev.
Coboson formalism for Cooper pairs used to derive Richardson's equations
We propose a many-body formalism for Cooper pairs which has similarities to
the one we recently developed for composite boson excitons (coboson in short).
Its Shiva diagram representation evidences that Cooper pairs differ from
single pairs through electron exchange only: no direct coupling exists due
to the very peculiar form of the BCS potential. As a first application, we here
use this formalism to derive Richardson's equations for the exact eigenstates
of Cooper pairs. This gives hints on why the dependence of the
-pair ground state energy we recently obtained by solving Richardson's
equations analytically in the low density limit, stays valid up to the dense
regime, no higher order dependence exists even under large overlap, a
surprising result hard to accept at first. We also briefly question the BCS
wave function ansatz compared to Richardson's exact form, in the light of our
understanding of coboson many-body effects
Mobile small polaron
Extending the Froehlich polaron problem to a discrete ionic lattice we study
a polaronic state with a small radius of the wave function but a large size of
the lattice distortion. We calculate the energy dispersion and the effective
mass of the polaron with the 1/\lambda perturbation theory and with the exact
Monte Carlo method in the nonadiabatic and adiabatic regimes, respectively. The
``small'' Froehlich polaron is found to be lighter than the small Holstein
polaron by one or more orders of magnitude.Comment: 4 pages, 4 figures, published versio
Optical conductivity of CuO_2 infinite-layer films
The infrared conductivity of CaCuO_2, SrCuO_{2-y}, and
Sr_{0.85}Nd_{0.15}CuO_2 infinite-layer films is obtained from reflectivity
measurements by taking into account the substrate contribution. SrCuO_{2-y} and
Sr_{0.85}Nd_{0.15}CuO_2 exhibit extra-phonon modes and structured bands in the
midinfrared, not found in stoichiometric CaCuO_2. These features mirror those
observed in the perovskitic cuprates, thus showing that the polaronic
properties of high-T_c superconductors are intrinsic to the CuO_2 planes.Comment: File latex, 5 p. incl. 4 fig. in epsf. Submitted to Solid State Com
Generic Finite Size Enhancement of Pairing in Mesoscopic Fermi Systems
The finite size dependent enhancement of pairing in mesoscopic Fermi systems
is studied under the assumption that the BCS approach is valid and that the two
body force is size independent. Different systems are investigated such as
superconducting metallic grains and films as well atomic nuclei. It is shown
that the finite size enhancement of pairing in these systems is in part due to
the presence of a surface which accounts quite well for the data of nuclei and
explains a good fraction of the enhancement in Al grains.Comment: Updated version 17/02/0
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