1,232 research outputs found
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
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
Photoemission kinks and phonons in cuprates
One of the possible mechanisms of high Tc superconductivity is Cooper pairing
with the help of bosons, which change the slope of the electronic dispersion as
observed by photoemission. Giustino et al. calculated that in the high
temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations
(phonons) should have a negligible effect on photoemission spectra and
concluded that phonons do not play an important role. We show that the
calculations employed by Giustino et al. fail to reproduce huge influence of
electron-phonon coupling on important phonons observed in experiments. Thus one
would expect these calculations to similarly fail in explaining the role of
electron-phonon coupling for the electronic dispersion.Comment: To appear in Nature as a Brief Communiction Arisin
Giant electron-phonon anomaly in doped La2CuO4 and other cuprates
Since conventional superconductivity is mediated by phonons, their role in
the mechanism of high temperature superconductivity has been considered very
early after the discovery of the cuprates. The initial consensus was that
phonons could not produce transition temperatures near 100K, and the main
direction of research focused on nonphononic mechanisms. Subsequent work last
reviewed by L. Pintschovius in 2005 showed that electron-phonon coupling in the
cuprates is surprisingly strong for some phonons and its role is controversial.
Experiments performed since then identified anomalous behavior of certain Cu-O
bond-stretching phonons in cuprates as an important phenomenon that is somehow
related to the mechanism of superconductivity. A particularly big advance was
made in the study of doped La2CuO4. This work is reviewed here.Comment: Review article. To appear in Advances in Condensed Matter Physics,
Special Issue: Phonons and Electron Correlations in High-Temperature and
Other Novel Superconductors (HTS
Inelastic Neutron and X-ray Scattering from Incommensurate Magnetic Systems
Neutrons and X-rays are powerful probes for studying magnetic and lattice
excitations in strongly correlated materials over very wide ranges of momentum
and energy transfers. In the focus of the present work are the incommensurate
magnetic systems MnSi and Cr. Under application of a magnetic field, helically
ordered MnSi transforms into a weak itinerant ferromagnet. Using polarized
neutrons we demonstrate that the Stoner excitations are spin flip excitations.
The amplitude (longitudinal) fluctuations associated with the magnon modes are
already strong far away from T_C. Interestingly, even the non spin flip
excitations associated with the Stoner modes are observable. In Cr, we have
observed Kohn anomalies in the phonon spectrum at those incommensurate
positions in reciprocal space, where the spin density wave is observed. The
corresponding phonon and magnon modes are not coupled. In addition, an
anomalous softening of a transverse phonon branch along the N-H zone boundary
line is observed that is caused by strong electron phonon coupling. High
resolution neutron scattering indicate that the low energy Fincher-Burke
excitations may rather correspond to localized modes in momentum and energy and
not to propagating collective modes. Finally, we demonstrate that in the near
future it may become feasible to investigate excitations in very small samples
thus allowing to measure the dynamics of strongly correlated materials under
extreme conditions and in the vicinity of quantum phase transitions
A probabilistic and information theoretic interpretation of quantum evolutions
In quantum mechanics, outcomes of measurements on a state have a
probabilistic interpretation while the evolution of the state is treated
deterministically. Here we show that one can also treat the evolution as being
probabilistic in nature and one can measure `which unitary' happened. Likewise,
one can give an information-theoretic interpretation to evolutions by defining
the entropy of a completely positive map. This entropy gives the rate at which
the informational content of the evolution can be compressed. One cannot
compress this information and still have the evolution act on an unknown state,
but we demonstrate a general scheme to do so probabilistically. This allows one
to generalize super-dense coding to the sending of quantum information. One can
also define the ``interaction-entanglement'' of a unitary, and concentrate this
entanglement.Comment: 9 page
Inelastic neutron and x-ray scattering as probes of the sign structure of the Fe-pnictide superconducting gap
Neutron spin-flip scattering observations of a resonance in the
superconducting state is often taken as evidence of an unconventional
superconducting state in which the gap changes sign
for momentum transfers which play an important role in the pairing.
Recently questions regarding this identification for the Fe-pnictide
superconductors have been raised and it has been suggested that
. Here we propose that inelastic neutron or x-ray
scattering measurements of the spectral weight of a phonon of momentum can
distinguish between these two pairing scenarios.Comment: 4 pages, 4 figure
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