35 research outputs found
Stripes and Superconductivity in Cuprates
Holes doped into the CuO2 planes of cuprate parent compounds frustrate the
antiferromagnetic order. The development of spin and charge stripes provides a
compromise between the competing magnetic and kinetic energies. Static stripe
order has been observed only in certain particular compounds, but there are
signatures which suggest that dynamic stripe correlations are common in the
cuprates. Though stripe order is bad for superconducting phase coherence,
stripes are compatible with strong pairing. Ironically, magnetic-field-induced
stripe order appears to enhance the stability of superconducting order within
the planes.Comment: 6 pages, submitted to proceedings of ECRYS-201
Effects of impurities and vortices on the low-energy spin excitations in high-Tc materials
We review a theoretical scenario for the origin of the spin-glass phase of
underdoped cuprate materials. In particular it is shown how disorder in a
correlated d-wave superconductor generates a magnetic phase by inducing local
droplets of antiferromagnetic order which eventually merge and form a
quasi-long range ordered state. When correlations are sufficiently strong,
disorder is unimportant for the generation of static magnetism but plays an
additional role of pinning disordered stripe configurations. We calculate the
spin excitations in a disordered spin-density wave phase, and show how disorder
and/or applied magnetic fields lead to a slowing down of the dynamical spin
fluctuations in agreement with neutron scattering and muon spin rotation (muSR)
experiments.Comment: 4 pages, 3 figures, submitted for SNS2010 conference proceeding
Giant phonon anomalies and central peak due to charge density wave formation in YBaCuO
The electron-phonon interaction is a major factor influencing the competition
between collective instabilities in correlated-electron materials, but its role
in driving high-temperature superconductivity in the cuprates remains poorly
understood. We have used high-resolution inelastic x-ray scattering to monitor
low-energy phonons in YBaCuO (superconducting
K), which is close to a charge density wave (CDW) instability. Phonons in a
narrow range of momentum space around the CDW ordering vector exhibit extremely
large superconductivity-induced lineshape renormalizations. These results imply
that the electron-phonon interaction has sufficient strength to generate
various anomalies in electronic spectra, but does not contribute significantly
to Cooper pairing. In addition, a quasi-elastic "central peak" due to CDW
nanodomains is observed in a wide temperature range above and below ,
suggesting that the gradual onset of a spatially inhomogeneous CDW domain state
with decreasing temperature is a generic feature of the underdoped cuprates
Progress in Neutron Scattering Studies of Spin Excitations in High-Tc Cuprates
Neutron scattering experiments continue to improve our knowledge of spin
fluctuations in layered cuprates, excitations that are symptomatic of the
electronic correlations underlying high-temperature superconductivity.
Time-of-flight spectrometers, together with new and varied single crystal
samples, have provided a more complete characterization of the magnetic energy
spectrum and its variation with carrier concentration. While the spin
excitations appear anomalous in comparison with simple model systems, there is
clear consistency among a variety of cuprate families. Focusing initially on
hole-doped systems, we review the nature of the magnetic spectrum, and
variations in magnetic spectral weight with doping. We consider connections
with the phenomena of charge and spin stripe order, and the potential
generality of such correlations as suggested by studies of magnetic-field and
impurity induced order. We contrast the behavior of the hole-doped systems with
the trends found in the electron-doped superconductors. Returning to hole-doped
cuprates, studies of translation-symmetry-preserving magnetic order are
discussed, along with efforts to explore new systems. We conclude with a
discussion of future challenges.Comment: revised version, to be published in JPSJ, 20 pages, 21 figure
A Laser Intensity Modulation Method for the Evaluation of the Polarization State of Embedded Piezoceramics
Entwurf und Auslegung Hochfliegender Plattformen
Hochfliegende Plattformen, welche mithilfe effizienter Solarpaneele und Energiespeicher dauerhaft in der Stratosphäre stationiert werden können, versprechen eine Vielzahl möglicher Anwendungsszenarien. Die technische Realisierung solcher Plattformen stellt jedoch große Herausforderungen an die unterschiedlichen Disziplinen des Flugzeugbaus. Im Rahmen des Vorhabens Hochfliegende Plattformen wurden Konzepte und Methoden für den anwendungsorientierten Entwurf hochfliegender Plattformen erarbeitet. Als zentrale Problemstellungen wurden der multidisziplinäre Auslegungsprozess, sowie die Optimierung und Bewertung von Plattformkonzepten identifiziert. Der vorliegende Bericht gibt einen Überblick über die Forschungsaktivitäten in den Bereichen aerodynamische Optimierung, Vorentwurf und Strukturauslegung, Energiegewinnungs- und Speichersysteme sowie missionsspezifische Bewertungsmethoden für hochfliegende Plattformen. Die dargestellten Ergebnisse bilden die Grundlage für einen automatisierten, multidisziplinären und anwendungsorientierten Entwurfsprozess für hochfliegende Plattformen