3,414 research outputs found
Bond-Bending and Bond-Stretching Phonons in Ferromagnetic La_0.7Sr_0.3MnO_3
Longitudinal optical phonons with oxygen character were measured in
La_0.7Sr_0.3MnO_3 by inelastic neutron scattering in the (1 0 0) cubic
direction and results were compared with shell model predictions. Measurements
were performed in several Brillouin zones, which enabled us to identify the
eigenvectors independent of the shell model. All major disagreements between
model predictions and experimental results are primarily due to the anomalous
downward dispersion of the bond-stretching vibration. The main new result is
that the rhombohedral distortion of the cubic lattice makes the bond-stretching
vibrations interact strongly with bond-bending modes folded into the cubic
Brillouin zone
Postselection threshold against biased noise
The highest current estimates for the amount of noise a quantum computer can
tolerate are based on fault-tolerance schemes relying heavily on postselecting
on no detected errors. However, there has been no proof that these schemes give
even a positive tolerable noise threshold. A technique to prove a positive
threshold, for probabilistic noise models, is presented. The main idea is to
maintain strong control over the distribution of errors in the quantum state at
all times. This distribution has correlations which conceivably could grow out
of control with postselection. But in fact, the error distribution can be
written as a mixture of nearby distributions each satisfying strong
independence properties, so there are no correlations for postselection to
amplify.Comment: 13 pages, FOCS 2006; conference versio
Phonon Mechanism of the Ferromagnetic Transition in La1-xSrxMnO3
Temperature dependence of longitudinal optical phonons with oxygen character
was measured in La1-xSrxMnO3 (x=0.2, 0.3) by inelastic neutron scattering in
the (1 0 0) cubic direction. The zone center mode intensity is consistent with
the Debye-Waller factor. However the intensity of the bond-stretching phonons
half way to the zone boundary and near the zone boundary decreases dramatically
as the temperature increases through the ferromagnetic (FM) transition. We
found evidence that the lost phonon spectral weight might shift into polaron
scattering at the same wavevectors. The temperature evolution starts well below
the onset of the FM transition suggesting that the transition is driven by
phonon renormalization rather than by magnetic fluctuations
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