2,689 research outputs found
Stoner-Wohlfart model applied to bicrystal magnetoresistance hysteresis
We calculate numerically the magnetization direction as function of magnetic
field in the Stoner-Wohlfart theory and are able to reproduce the shape of the
low-field magnetoresistance hysteresis observed in manganite grain boundary
junctions. Moreover, we show that it is necessary to include biaxial
magnetocrystalline anisotropy to fully describe the grain boundary
magnetoresistance in LaSrMnO/SrTiO bicrystal tunnel
junctions.Comment: 5 pages, 4 figure
Metal-insulator transitions: Influence of lattice structure, Jahn-Teller effect, and Hund's rule coupling
We study the influence of the lattice structure, the Jahn-Teller effect and
the Hund's rule coupling on a metal-insulator transition in AnC60 (A= K, Rb).
The difference in lattice structure favors A3C60 (fcc) being a metal and A4C60
(bct) being an insulator, and the coupling to Hg Jahn-Teller phonons favors
A4C60 being nonmagnetic. The coupling to Hg (Ag) phonons decreases (increases)
the value Uc of the Coulomb integral at which the metal-insulator transition
occurs. There is an important partial cancellation between the Jahn-Teller
effect and the Hund's rule coupling.Comment: 4 pages, RevTeX, 3 eps figure, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Two Component Heat Diffusion Observed in CMR Manganites
We investigate the low-temperature electron, lattice, and spin dynamics of
LaMnO_3 (LMO) and La_0.7Ca_0.3MnO_3 (LCMO) by resonant pump-probe reflectance
spectroscopy. Probing the high-spin d-d transition as a function of time delay
and probe energy, we compare the responses of the Mott insulator and the
double-exchange metal to the photoexcitation. Attempts have previously been
made to describe the sub-picosecond dynamics of CMR manganites in terms of a
phenomenological three temperature model describing the energy transfer between
the electron, lattice and spin subsystems followed by a comparatively slow
exponential decay back to the ground state. However, conflicting results have
been reported. Here we first show clear evidence of an additional component in
the long term relaxation due to film-to-substrate heat diffusion and then
develop a modified three temperature model that gives a consistent account for
this feature. We confirm our interpretation by using it to deduce the bandgap
in LMO. In addition we also model the non-thermal sub-picosecond dynamics,
giving a full account of all observed transient features both in the insulating
LMO and the metallic LCMO.Comment: 6 pages, 5 figures http://link.aps.org/doi/10.1103/PhysRevB.81.064434
v2: Abstract correcte
Screening, Coulomb pseudopotential, and superconductivity in alkali-doped Fullerenes
We study the static screening in a Hubbard-like model using quantum Monte
Carlo. We find that the random phase approximation is surprisingly accurate
almost up to the Mott transition. We argue that in alkali-doped Fullerenes the
Coulomb pseudopotential is not very much reduced by retardation
effects. Therefore efficient screening is important in reducing
sufficiently to allow for an electron-phonon driven superconductivity. In this
way the Fullerides differ from the conventional picture, where retardation
effects play a major role in reducing the electron-electron repulsion.Comment: 4 pages RevTeX with 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Important role of alkali atoms in A4C60
We show that hopping via the alkali atoms plays an important role for the t1u
band of A4C60 (A=K, Rb), in strong contrast to A3C60. Thus the t1u band is
broadened by more than 40 % by the presence of the alkali atoms. The difference
between A4C60 and A3C60 is in particular due to the less symmetric location of
the alkali atoms in A4C60.Comment: 5 pages, revtex, 2 figures, submitted to Phys. Rev. B more
information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
Lensing magnification of supernovae in the GOODS-fields
Gravitational lensing of high-redshift supernovae is potentially an important
source of uncertainty when deriving cosmological parameters from the measured
brightness of Type Ia supernovae, especially in deep surveys with scarce
statistics. Photometric and spectroscopic measurements of foreground galaxies
along the lines-of-sight of 33 supernovae discovered with the Hubble Space
Telescope, both core-collapse and Type Ia, are used to model the magnification
probability distributions of the sources. Modelling galaxy halos with SIS or
NFW-profiles and using M/L scaling laws provided by the Faber-Jackson and
Tully-Fisher relations, we find clear evidence for supernovae with lensing
(de)magnification. However, the magnification distribution of the Type Ia
supernovae used to determine cosmological distances matches very well the
expectations for an unbiased sample, i.e.their mean magnification factor is
consistent with unity. Our results show that the lensing distortions of the
supernova brightness can be well understood for the GOODS sample and that
correcting for this effect has a negligible impact on the derived cosmological
parameters.Comment: 22 pages, 9 figures, accepted for publication by Ap
Corrections for gravitational lensing of supernovae: better than average?
We investigate the possibility of correcting for the magnification due to
gravitational lensing of standard candle sources, such as Type Ia supernovae.
Our method uses the observed properties of the foreground galaxies along the
lines-of-sight to each source and the accuracy of the lensing correction
depends on the quality and depth of these observations as well as the
uncertainties in translating the observed luminosities to the matter
distribution in the lensing galaxies. The current work is limited to cases
where the matter density is dominated by the individual galaxy halos. However,
it is straightforward to generalize the method to include also gravitational
lensing from cluster scale halos. We show that the dispersion due to lensing
for a standard candle source at z=1.5 can be reduced from about 7% to ~< 3%,
i.e. the magnification correction is useful in reducing the scatter in the Type
Ia Hubble diagram, especially at high redshifts where the required long
exposure times makes it hard to reach large statistics and the dispersion due
to lensing becomes comparable to the intrinsic Type Ia scatter.Comment: Matches accepted version, includes clarifications and additional
issues. 28 pages, 7 figures, accepted for publication in Ap
Pauli susceptibility of A3C60 (A=K, Rb)
The Pauli paramagnetic susceptibility of A3C60 (A= K, Rb) compounds is
calculated. A lattice quantum Monte Carlo method is applied to a multi-band
Hubbard model, including the on-site Coulomb interaction U. It is found that
the many-body enhancement of the susceptibility is of the order of a factor of
three. This reconciles estimates of the density of states from the
susceptibility with other estimates. The enhancement is an example of a
substantial many-body effect in the doped fullerenes.Comment: 4 pages, revtex, 2 figures, submitted to Phys. Rev. B more
information at http://www.mpi-stuttgart.mpg.de/dokumente/andersen/fullerene
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