907 research outputs found
Untersuchungen über die Ausscheidung des durch infusion in das Blut gebrachten phosphorsauren Natrons durch die Nieren
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The Electron-Phonon Interaction in the Presence of Strong Correlations
We investigate the effect of strong electron-electron repulsion on the
electron-phonon interaction from a Fermi-liquid point of view: the strong
interaction is responsible for vertex corrections, which are strongly dependent
on the ratio. These corrections generically lead to a strong
suppression of the effective coupling between quasiparticles mediated by a
single phonon exchange in the limit. However, such effect
is not present when . Analyzing the Landau stability
criterion, we show that a sizable electron-phonon interaction can push the
system towards a phase-separation instability. A detailed analysis is then
carried out using a slave-boson approach for the infinite-U three-band Hubbard
model. In the presence of a coupling between the local hole density and a
dispersionless optical phonon, we explicitly confirm the strong dependence of
the hole-phonon coupling on the transferred momentum versus frequency ratio. We
also find that the exchange of phonons leads to an unstable phase with negative
compressibility already at small values of the bare hole-phonon coupling. Close
to the unstable region, we detect Cooper instabilities both in s- and d-wave
channels supporting a possible connection between phase separation and
superconductivity in strongly correlated systems.Comment: LateX 3.14, 04.11.1994 Preprint no.101
Polaronic optical absorption in electron-doped and hole-doped cuprates
Polaronic features similar to those previously observed in the photoinduced
spectra of cuprates have been detected in the reflectivity spectra of
chemically doped parent compounds of high-critical-temperature superconductors,
both -type and -type. In NdCuO these features, whose
intensities depend both on doping and temperature, include local vibrational
modes in the far infrared and a broad band centered at 1000 cm.
The latter band is produced by the overtones of two (or three) local modes and
is well described in terms of a small-polaron model, with a binding energy of
about 500 cm. Most of the above infrared features are shown to survive
in the metallic phase of NdCeCu0, BiSrCuO, and
YBaCuO, where they appear as extra-Drude peaks. The occurrence
of polarons is attributed to local modes strongly coupled to carriers, as shown
by a comparison with tunneling results.Comment: File latex, 31 p., submitted to Physical Review B. Figures may be
faxed upon reques
Polaron Formation in the Three-Band Peierls-Hubbard Model for Cuprate Superconductors
Exact diagonalization calculations show a continuous transition from
delocalized to small polaron behavior as a function of intersite
electron-lattice coupling. A transition, found previously at Hartree-Fock level
[Yonemitsu et al., Phys. Rev. Lett. {\bf 69}, 965 (1992)], between a magnetic
and a non magnetic state does not subsist when fluctuations are included. Local
phonon modes become softer close to the polaron and by comparison with optical
measurements of doped cuprates we conclude that they are close to the
transition region between polaronic and non-polaronic behavior. The barrier to
adiabatically move a hole vanishes in that region suggesting large mobilities.Comment: 7 pages + 3 poscript figures, Revtex 3.0, MSC-199
A Precision Calculation of the Next-to-Leading Order Energy-Energy Correlation Function
The O(alpha_s^2) contribution to the Energy-Energy Correlation function (EEC)
of e+e- -> hadrons is calculated to high precision and the results are shown to
be larger than previously reported. The consistency with the leading logarithm
approximation and the accurate cancellation of infrared singularities exhibited
by the new calculation suggest that it is reliable. We offer evidence that the
source of the disagreement with previous results lies in the regulation of
double singularities.Comment: 6 pages, uuencoded LaTeX and one eps figure appended Complete paper
as PostScript file (125 kB) available at:
http://www.phys.washington.edu/~clay/eecpaper1/paper.htm
Infrared response of ordered polarons in layered perovskites
We report on the infrared absorption spectra of three oxides where charged
superlattices have been recently observed in diffraction experiments. In
LaSrNiO, polaron localization is found to suppress the
low-energy conductivity through the opening of a gap and to split the
- vibrational manifold of the oxygen octahedra. Similar effects
are detected in SrLaMnO and in LaNiO, with
peculiar differences related to the type of charge ordering.Comment: File latex, 11 p. + 3 Figures, to appear on Phys. Rev. B (Rapid
Commun.), 1 Oct. 1996. The figures will be faxed upon request.
E-mail:[email protected] Fax: +39-6-446315
Small and large polarons in nickelates, manganites, and cuprates
By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO),
Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and
Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of
polarons in this cuprate family. While in LSNO and SLMO small polarons localize
into ordered structures below a transition temperature, in those cuprates the
polarons appear to be large, and at low T their binding energy decreases. This
reflects into an increase of the polaron radius, which may trigger coherent
transport.Comment: File latex, 15 p. incl. 4 Figs. epsf, to appear on the Journal of
Superconductivity - Proc. "Stripes 1996" - Roma Dec 199
Optical conductivity of the nonsuperconducting cuprate La(8-x)Sr(x)Cu(8)O(20)
La(8-x)Sr(x)Cu(8)O(20) is a non-superconducting cuprate, which exhibits a
doubling of the elementary cell along the c axis. Its optical conductivity
sigma (omega) has been first measured here, down to 20 K, in two single
crystals with x = 1.56 and x = 2.24. Along c, sigma (omega) shows, in both
samples, bands due to strongly bound charges, thus confirming that the cell
doubling is due to charge ordering. In the ab plane, in addition to the Drude
term one observes an infrared peak at 0.1 eV and a midinfrared band at 0.7 eV.
The 0.1 eV peak hardens considerably below 200 K, in correspondence of an
anomalous increase in the sample dc resistivity, in agreement with its
polaronic origin. This study allows one to establish relevant similarities and
differences with respect to the spectrum of the ab plane of the superconducting
cuprates.Comment: Revised version submitted to Phys. Rev. B, including the elimination
of Fig. 1 and changes to Figs. 4 and
Involution and Constrained Dynamics I: The Dirac Approach
We study the theory of systems with constraints from the point of view of the
formal theory of partial differential equations. For finite-dimensional systems
we show that the Dirac algorithm completes the equations of motion to an
involutive system. We discuss the implications of this identification for field
theories and argue that the involution analysis is more general and flexible
than the Dirac approach. We also derive intrinsic expressions for the number of
degrees of freedom.Comment: 28 pages, latex, no figure
Spectral properties of the t-J model in the presence of hole-phonon interaction
We examine the effects of electron-phonon interaction on the dynamics of the
charge carriers doped in two-dimensional (2D) Heisenberg antiferromagnet. The
- model Hamiltonian with a Fr\"ohlich term which couples the holes to a
dispersionless (optical) phonon mode is considered for low doping
concentration. The evolution of the spectral density function, the density of
states, and the momentum distribution function of the holes with an increase of
the hole-phonon coupling constant is studied numerically. As the coupling
to a phonon mode increases the quasiparticle spectral weight decreases and a
``phonon satellite'' feature close to the quasi-particle peak becomes more
pronounced. Furthermore, strong electron-phonon coupling smears the
multi-magnon resonances (``string states'') in the incoherent part of the
spectral function. The jump in the momentum distribution function at the Fermi
surface is reduced without changing the hole pocket volume, thereby providing a
numerical verification of Luttinger theorem for this strongly interacting
system. The vertex corrections due to electron- phonon interaction are
negligible in spite of the fact that the ratio of the phonon frequency to the
effective bandwidth is not small.Comment: REVTeX, 20 pages, 9 figures, to be published in Phys. Rev. B (Nov. 1,
1996
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