6,998 research outputs found
Cooperative dynamics in doped manganite films: phonon anomalies in the ferromagnetic state
We present optical measurements of phononic excitations in
LaCaMnO (LCMO) and LaSrMnO (LSMO)
thin films covering the full temperature range from the metallic ferromagnetic
to the insulating paramagnetic phase. All eight phonons expected for the
Rc symmetry in LSMO and 17 out of the expected 25 phonons for the Pnma
symmetry in LCMO have been determined. Close to the
ferromagnetic-to-paramagnetic transition both compounds reveal an anomalous
behavior but with different characteristics. Anomalies in the phononic spectra
are a manifestation of the coupling of lattice degrees of freedom (DOF) to
electronic DOF. Specifically, the low-frequency external group proves to be an
indicator for lattice modifications induced by electronic correlations. The
enhanced electron-phonon coupling in LCMO is responsible for Fano-like
interference effects of distinct phonon modes with electronic continuum
excitations: we observe asymmetric phonon line shapes, mode splitting and
spectral weight transfer between modes.Comment: 10 pages, 10 figure
Pitch-angle scattering in magnetostatic turbulence. I. Test-particle simulations and the validity of analytical results
Context. Spacecraft observations have motivated the need for a refined
description of the phase-space distribution function. Of particular importance
is the pitch-angle diffusion coefficient that occurs in the Fokker-Planck
transport equation. Aims. Simulations and analytical test-particle theories are
compared to verify the diffusion description of particle transport, which does
not allow for non-Markovian behavior. Methods. A Monte-Carlo simulation code
was used to trace the trajectories of test particles moving in turbulent
magnetic fields. From the ensemble average, the pitch-angle Fokker-Planck
coefficient is obtained via the mean square displacement. Results. It is shown
that, while excellent agreement with analytical theories can be obtained for
slab turbulence, considerable deviations are found for isotropic turbulence. In
addition, all Fokker-Planck coefficients tend to zero for high time values.Comment: 8 pages, 10 figures, accepted for publication in Astron. Astrophy
Large and Small Polaron Excitations in La2/3(Sr/Ca)1/3MnO3 Films
We present detailed optical measurements of the mid-infrared (MIR)
excitations in thin films of La2/3Sr1/3MnO3 (LSMO) and La2/3Ca1/3MnO3 (LCMO)
across the magnetic transition. The shape of the excitation at about 0.2 eV in
both samples is analyzed in terms of polaron models. We propose to identify the
MIR resonance in LSMO as the excitation of large polarons and that in LCMO as a
small polaron excitation. A scaling behavior for the low-energy side of the
polaronic MIR resonance in LSMO is established
Supercurrent through grain boundaries in the presence of strong correlations
Strong correlations are known to severely reduce the mobility of charge
carriers near half-filling and thus have an important influence on the current
carrying properties of grain boundaries in the high- cuprates. In this
work we present an extension of the Gutzwiller projection approach to treat
electronic correlations below as well as above half-filling consistently. We
apply this method to investigate the critical current through grain boundaries
with a wide range of misalignment angles for electron- and hole-doped systems.
For the latter excellent agreement with experimental data is found. We further
provide a detailed comparison to an analogous weak-coupling evaluation.Comment: 4 pages, 3 figure
Phonon-Metamorphosis in Ferromagnetic Manganite Films: Probing the Evolution of an Inhomogeneous State
The analysis of phonon anomalies provides valuable information about the
cooperative dynamics of lattice, spin and charge degrees of freedom.
Significant is the anomalous temperature dependence of the external modes
observed in LaSrMnO (LSMO) films. The two external modes
merge close to the ferromagnetic to paramagnetic transition at and,
moreover, two new modes evolve in this temperature range with strong resonances
at slightly higher frequencies. We propose that this observed phonon
metamorphosis probes the inhomogeneous Jahn-Teller distortion, manifest on the
temperature scale . The analysis is based on the first observation of all
eight phonon modes in the metallic phase of LSMO and on susceptibility
measurements which identify a Griffiths-like phase.Comment: 4 pages, 4 figure
Crossover from hc/e to hc/2e current oscillations in rings of s-wave superconductors
We analyze the crossover from an hc/e-periodicity of the persistent current
in flux threaded clean metallic rings towards an hc/2e-flux periodicity of the
supercurrent upon entering the superconducting state. On the basis of a model
calculation for a one-dimensional ring we identify the underlying mechanism,
which balances the hc/e versus the hc/2e periodic components of the current
density. When the ring circumference exceeds the coherence length of the
superconductor, the flux dependence is strictly hc/2e periodic. Further, we
develop a multi-channel model which reduces the Bogoliubov - de Gennes
equations to a one-dimensional differential equation for the radial component
of the wave function. The discretization of this differential equation
introduces transverse channels, whose number scales with the thickness of the
ring. The periodicity crossover is analyzed close the critical temperature
Two experiments for the price of one? -- The role of the second oscillation maximum in long baseline neutrino experiments
We investigate the quantitative impact that data from the second oscillation
maximum has on the performance of wide band beam neutrino oscillation
experiments. We present results for the physics sensitivities to standard three
flavor oscillation, as well as results for the sensitivity to non-standard
interactions. The quantitative study is performed using an experimental setup
similar to the Fermilab to DUSEL Long Baseline Neutrino Experiment (LBNE). We
find that, with the single exception of sensitivity to the mass hierarchy, the
second maximum plays only a marginal role due to the experimental difficulties
to obtain a statistically significant and sufficiently background-free event
sample at low energies. This conclusion is valid for both water Cherenkov and
liquid argon detectors. Moreover, we confirm that non-standard neutrino
interactions are very hard to distinguish experimentally from standard
three-flavor effects and can lead to a considerable loss of sensitivity to
\theta_{13}, the mass hierarchy and CP violation.Comment: RevTex 4.1, 23 pages, 10 figures; v2: Typos corrected, very minor
clarifications; matches published version; v3: Fixed a typo in the first
equation in sec. III
NR2A at CA1 synapses is obligatory for the susceptibility of hippocampal plasticity to sleep loss.
A loss in the necessary amount of sleep alters expression of genes and proteins implicated in brain plasticity, but key proteins that render neuronal circuits sensitive to sleep disturbance are unknown. We show that mild (4-6 h) sleep deprivation (SD) selectively augmented the number of NR2A subunits of NMDA receptors on postsynaptic densities of adult mouse CA1 synapses. The greater synaptic NR2A content facilitated induction of CA3-CA1 long-term depression in the theta frequency stimulation range and augmented the synaptic modification threshold. NR2A-knock-out mice maintained behavioral response to SD, including compensatory increase in post-deprivation resting time, but hippocampal synaptic plasticity was insensitive to sleep loss. After SD, the balance between synaptically activated and slowly recruited NMDA receptor pools during temporal summation was disrupted. Together, these results indicate that NR2A is obligatory for the consequences of sleep loss on hippocampal synaptic plasticity. These findings could advance pharmacological strategies aiming to sustain hippocampal function during sleep restriction
- …