939 research outputs found
On the electron-energy loss spectra and plasmon resonance in cuprates
The consequences of the non-Drude charge response in the normal state of
cuprates and the effect of the layered structure on electron-energy loss
spectra are investigated, both for experiments in the transmission and the
reflection mode. It is shown that in the intermediate doping regime the plasmon
resonance has to be nearly critically damped as a result of the anomalous
frequency dependence of the relaxation rate. This also implies an unusual
low-energy dependence of the loss function. Both facts are consistent with
experiments in cuprates. Our study based on the t-J model shows good agreement
with measured plasmon frequencies.Comment: LaTeX, 4 pages with 2 figures. submitted to PR
Charge dynamics of t-J model and anomalous bond-stretching phonons in cuprates
The density response of a doped Mott-Hubbard insulator is discussed starting
from the t-J model in a slave boson 1/N representation. In leading order O(1)
the density fluctuation spectra are determined by an
undamped collective mode at large momentum transfer, in striking disagreement
with results obtained by exact diagonalization, which reveal a very broad
dispersive peak, reminescent of strong spin-charge coupling. The 1/N
corrections introduce the polaron character of the bosonic holes moving in a
uniform RVB background. The resulting captures all features
observed in diagonalization studies, fulfills the appropriate sum rules, and
apart from the broadening of the collective mode shows a new low energy feature
at the energy related to the polaron motion in the spinon
background. It is further shown that the low energy structure, which is
particularly pronounced in direction, describes the strong
renormalization and anomalous damping of the highest bond-stretching phonons in
LaSrCuO.Comment: Presented at the meeting "Highlights in Condensed Matter Physics" in
honor of the 60th birthday of Prof. Ferdinando Mancini, May 9-11, 2003,
Salerno, Ital
Electron-phonon coupling and spin-charge separation in one-dimensional Mott insulators
We examine the single-particle excitation spectrum in the one-dimensional
Hubbard-Holstein model at half-filling by performing the dynamical density
matrix renormalization group (DDMRG) calculation. The DDMRG results are
interpreted as superposition of spectra for a spinless carrier dressed with
phonons. The superposition is a consequence of robustness of the spin-charge
separation against electron-phonon coupling. The separation is in contrast to
the coupling between phonon and spin degrees of freedom in two-dimensional
systems. We discuss implication of the results of the recent angle-resolved
photoemission spectroscopy measurements on SrCuO.Comment: 5 pages, 4 figures. submitted to the Physical Review Letter
Density response of the t-J model and renormalization of breathing and half-breathing phonon modes: A slave-fermion calculation
The density fluctuation spectrum is calculated for the
t-J model in the low-doping regime using a slave-fermion method for the
constrained fermions. The obtained results for are in good
agreement with diagonalization results. The density response is characterized
by incoherent, momentum dependent spectral functions reaching up to energies
and a low-energy structure at energy due to transitions in
the quasiparticle band. is shown to lead to a strong
renormalization of planar bond-streching and breathing phonon modes with a
large phonon linewidth at intermediate momenta caused by the low-energy
response. Our results are consistent with recent neutron scattering data,
showing the peculiar behavior of these modes.Comment: 4 pages, 3 figures, `Materials and Mechanisms of Superconductivity
VI' conference, Houston, USA, Febr. 20-25, 200
Magnetism of one-dimensional Wigner lattices and its impact on charge order
The magnetic phase diagram of the quarter-filled generalized Wigner lattice
with nearest- and next-nearest-neighbor hopping t_1 and t_2 is explored. We
find a region at negative t_2 with fully saturated ferromagnetic ground states
that we attribute to kinetic exchange. Such interaction disfavors
antiferromagnetism at t_2 <0 and stems from virtual excitations across the
charge gap of the Wigner lattice, which is much smaller than the Mott-Hubbard
gap proportional to U. Remarkably, we find a strong dependence of the charge
structure factor on magnetism even in the limit U to infinity, in contrast to
the expectation that charge ordering in the Wigner lattice regime should be
well described by spinless fermions. Our results, obtained using the
density-matrix renormalization group and exact diagonalization, can be
transparently explained by means of an effective low-energy Hamiltonian
Magnetic properties of spin-orbital polarons in lightly doped cobaltates
We present a numerical treatment of a spin-orbital polaron model for
Na_xCoO_2 at small hole concentration (0.7 < x < 1). We demonstrate how the
polarons account for the peculiar magnetic properties of this layered compound:
They explain the large susceptibility; their internal degrees of freedom lead
both to a negative Curie-Weiss temperature and yet to a ferromagnetic
intra-layer interaction, thereby resolving a puzzling contradiction between
these observations. We make specific predictions on the momentum and energy
location of excitations resulting from the internal degrees of freedom of the
polaron, and discuss their impact on spin-wave damping.Comment: 4+ pages, 6 figures, accepted for publication in Phys. Rev. Let
Critical behavior of the S=3/2 antiferromagnetic Heisenberg chain
Using the density-matrix renormalization-group technique we study the
long-wavelength properties of the spin S=3/2 nearest-neighbor Heisenberg chain.
We obtain an accurate value for the spin velocity v=3.8+- 0.02, in agreement
with experiment. Our results show conclusively that the model belongs to the
same universality class as the S=1/2 Heisenberg chain, with a conformal central
charge c=1 and critical exponent eta=1Comment: RevTeX (version 3.0), 4 twocolumn pages with 4 embedded figure
A new view of the electronic structure of the spin-Peierls compound alpha'-NaV2O5
The present understanding of the electronic and magnetic properties of
-NaVO is based on the hypothesis of strong charge
disproportionation into V and V, which is assumed to lead to a
spin-1/2 Heisenberg chain system. A recent structure analysis shows, however,
that the V-ions are in a mixed valence state and indistiguishable. We propose
an explanation for the insulating state, which is not based on charge
modulation, and show that strong correlations together with the Heitler-London
character of the relevant intermediate states naturally lead to
antiferromagnetic Heisenberg chains. The interchain coupling is weak and
frustrated, and its effect on the uniform susceptibility is found to be small.Comment: EPJ-style, 7 pages with 5 eps figure
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