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

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 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

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 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 $\Delta(k+Q)=-\Delta(k)$ for momentum transfers $Q$ 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 $\Delta(k+Q)=\Delta(k)$. Here we propose that inelastic neutron or x-ray scattering measurements of the spectral weight of a phonon of momentum $Q$ can distinguish between these two pairing scenarios.Comment: 4 pages, 4 figure