3,601 research outputs found
Boson-fermion model beyond mean-field approximation
A model of hybridized bosons and fermions is studied beyond the mean field
approximation. The divergent boson self-energy at zero temperature makes the
Cooper pairing of fermions impossible.The frequency and momentum dependence of
the self- energy and the condensation temperature of initially
localized bosons are calculated analytically. The value of the boson
condensation temperature is below which rules out the
boson-fermion model with the initially localized bosons as a phenomenological
explanation of high-temperature superconductivity. The intra-cell
density-density fermion-boson interaction dominates in the fermion self-energy.
The model represents a normal metal with strongly damped bosonic excitations.
The latter play the role of normal impurities.Comment: 16 pages, Latex, 5 figures available upon reques
Comment on `Experimental and Theoretical Constraints of Bipolaronic Superconductivity in High Materials: An Impossibility'
We show that objections raised by Chakraverty (Phys. Rev. Lett. 81,
433 (1998)) to the bipolaron model of superconducting cuprates are the result
of an incorrect approximation for the bipolaron energy spectrum and misuse of
the bipolaron theory. The consideration, which takes into account the multiband
energy structure of bipolarons and the unscreened electron-phonon interaction
clearly indicates that cuprates are in the Bose-Einstein condensation regime
with mobile charged bosons.Comment: 1 page, no figure
An infinitesimally nonrigid polyhedron with nonstationary volume in the Lobachevsky 3-space
We give an example of an infinitesimally nonrigid polyhedron in the
Lobachevsky 3-space and construct an infinitesimal flex of that polyhedron such
that the volume of the polyhedron isn't stationary under the flex.Comment: 10 pages, 2 Postscript figure
Key pairing interaction in layered doped ionic insulators
A controversial issue on whether the electron-phonon interaction (EPI) is
crucial for high-temperature superconductivity or it is weak and inessential
has remained one of the most challenging problems of contemporary condensed
matter physics. We employ a continuum RPA approximation for the dielectric
response function allowing for a selfconsistent semi-analytical evaluation of
the EPI strength, electron-electron attractions, and the carrier mass
renormalisation in layered high-temperature superconductors. We show that the
Fr\"{o}hlich EPI with high-frequency optical phonons in doped ionic lattices is
the key pairing interaction, which is beyond the BCS-Migdal-Eliashberg
approximation in underdoped superconductors, and it remains a significant
player in overdoped compounds.Comment: 4 pages, 4 figure
Green's and spectral functions of the small Frolich polaron
According to recent Quantum Monte Carlo simulations the small polaron theory
is practically exact in a wide range of the long-range (Frohlich)
electron-phonon coupling and adiabatic ratio. We apply the Lang-Firsov
transformation to convert the strong-coupling term in the Hamiltonian into the
form of an effective hopping integral and derive the single-particle Green's
function describing propagation of the small Frohlich polaron. One and two
dimensional spectral functions are studied by expanding the Green's function
perturbatively. Numerical calculations of the spectral functions are produced.
Remarkably, the coherent spectral weight (Z) and effective mass (Z')
renormalisation exponents are found to be different with Z'>>Z, which can
explain a small coherent spectral weight and a relatively moderate mass
enhancement in oxides.Comment: RevTeX, 5 pages, 2 postscript figures, LaTeX processing problems
correcte
Theory of SIS tunnelling in cuprates
We show that the single-particle polaron Green's function describes SIS
tunnelling in cuprates, including the absence of Ohm's law at high voltages,
the dip/hump features in the first derivative of the current, a substantial
incoherent spectral weight beyond quasiparticle peaks and unusual shape of the
peaks.
The theory allows us to determine the characteristic phonon frequencies,
normal and superconducting gaps, impurity scattering rate, and the
electron-phonon coupling from the tunnelling data.Comment: 10 pages, 2 figure
Semi-classical theory of magnetic quantum oscillations in a two-dimensional multiband canonical Fermi liquid
The semi-classical Lifshitz-Kosevich (LK) description of quantum oscillations
is extended to a multiband two-dimensional Fermi liquid with a constant number
of electrons. The amplitudes of novel oscillations with combination
frequencies, recently predicted and observed experimentally, are analytically
derived and compared with the single-band amplitudes. The combination
amplitudes decay with temperature exponentially faster than the standard
harmonics, and this provides a valuable tool for their experimental
identification.Comment: 3 pages, REVTeX 3.0, one eps-figure included in the tex
Parameter-free expression for superconducting Tc in cuprates
A parameter-free expression for the superconducting critical temperature of
layered cuprates is derived which allows us to express Tc in terms of
experimentally measured parameters. It yields Tc values observed in about 30
lanthanum, yttrium and mercury-based samples for different levels of doping.
This remarkable agreement with the experiment as well as the unusual critical
behaviour and the normal-state gap indicate that many cuprates are close to the
Bose-Einstein condensation regime.Comment: 5 pages, 2 figures. Will be published in Physical Review
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