8,437 research outputs found
Climate model simulation of winter warming and summer cooling following the 1991 Mount Pinatubo volcanic eruption
We simulate climate change for the 2-year period following the eruption of Mount Pinatubo in the Philippines on June 15, 1991, with the ECHAM4 general circulation model (GCM). The model was forced by realistic aerosol spatial-time distributions and spectral radiative characteristics calculated using Stratospheric Aerosol, and Gas Experiment II extinctions and Upper Atmosphere Research Satellite-retrieved effective radii. We calculate statistical ensembles of GCM simulations with and without volcanic aerosols for 2 years after the eruption for three different sea surface temperatures (SSTs): climatological SST, El Nino-type SST of 1991-1993, and La Nina-type SST of 1984-1986. We performed detailed comparisons of calculated fields with observations, We analyzed the atmospheric response to Pinatubo radiative forcing and the ability of the GCM to reproduce it with different SSTs. The temperature of the tropical lower stratosphere increased by 4 K because of aerosol absorption of terrestrial longwave and solar near-infrared radiation. The heating is larger than observed, but that is because in this simulation we did not account for quasi-biennial oscillation (QBO) cooling and the cooling effects of volcanically induced ozone depletion. We estimated that both QBO and ozone depletion decrease the stratospheric temperature by about 2 K. The remaining 2 K stratospheric warming is in good agreement with observations. By comparing the runs with the Pinatubo aerosol forcing with those with no aerosols, we find that the model calculates a general cooling of the global troposphere, but with a clear winter warming pattern of surface air temperature over Northern Hemisphere continents. This pattern is consistent with the observed temperature patterns. The stratospheric heating and tropospheric summer cooling are directly caused by aerosol radiative effects, but the winter warming is indirect, produced by dynamical responses to the enhanced stratospheric latitudinal temperature gradient. The aerosol radiative forcing, stratospheric thermal response, and summer tropospheric cooling do not depend significantly on SST. The stratosphere-troposphere dynamic interactions and tropospheric climate response in winter are sensitive to SST
Weak magnetism phenomena in heavy-fermion superconductors: selected SR studies
The behavior of the so-called weak moment antiferromagnetic states, observed
in the heavy-fermion superconductors UPt and URuSi, is discussed in
view of recent SR results obtained as function of control parameters like
chemical substitution and external pressure. In UPt, the Pd substitution
for Pt reveals the dynamical character of the weak moment order. On the other
hand, SR measurements performed on samples in which Th substitutes U
suggest that crystallographic disorder on the magnetic sites deeply affects the
fluctuation timescale. In URuSi, a phase separation between the
so-called hidden order state, present at ambient pressure, and an
antiferromagnetic state, occurring under pressure, is observed. In view of the
pressure-temperature phase diagram obtained by SR, it is deduced that the
respective order parameters have different symmetries.Comment: To appear in: J. Phys.: Cond. Matte
Functional requirements for onboard management of space shuttle consumables, volume 1
A study was conducted to determine the functional requirements for onboard management of space shuttle consumables. A generalized consumable management concept was developed for application to advanced spacecraft. The subsystems and related consumables selected for inclusion in the consumables management system are: (1) propulsion, (2) power generation, and (3) environmental and life support
The new versatile general purpose surface-muon instrument (GPS) based on silicon photomultipliers for SR measurements on a continuous-wave beam
We report on the design and commissioning of a new spectrometer for muon-spin
relaxation/rotation studies installed at the Swiss Muon Source (SS) of the
Paul Scherrer Institute (PSI, Switzerland). This new instrument is essentially
a new design and replaces the old general-purpose surface-muon instrument (GPS)
which has been for long the workhorse of the SR user facility at PSI. By
making use of muon and positron detectors made of plastic scintillators read
out by silicon photomultipliers (SiPMs), a time resolution of the complete
instrument of about 160 ps (standard deviation) could be achieved. In addition,
the absence of light guides, which are needed in traditionally built SR
instrument to deliver the scintillation light to photomultiplier tubes located
outside magnetic fields applied, allowed us to design a compact instrument with
a detector set covering an increased solid angle compared to the old GPS.Comment: 11 pages, 11 figure
Fermi-liquid effects in the Fulde-Ferrell-Larkin-Ovchinnikov state of two-dimensional d-wave superconductors
We study the effects of Fermi-liquid interactions on quasi-two-dimensional
d-wave superconductors in a magnetic field. The phase diagram of the
superconducting state, including the periodic Fulde-Ferrell-Larkin-Ovchinnikov
(FFLO) state in high magnetic fields, is discussed for different strengths of
quasiparticle many-body interactions within Landau's theory of Fermi liquids.
Decreasing the Fermi-liquid parameter causes the magnetic spin
susceptibility to increase, which in turn leads to a reduction of the FFLO
phase. It is shown that a negative results in a first-order phase
transition from the normal to the uniform superconducting state in a finite
temperature interval. Finally, we discuss the thermodynamic implications of a
first-order phase transition for CeCoIn.Comment: published version; removed direct comparison with experiment for the
upper critical field, as required by the referee
Phase-field-crystal model for liquid crystals
Based on static and dynamical density functional theory, a
phase-field-crystal model is derived which involves both the translational
density and the orientational degree of ordering as well as a local director
field. The model exhibits stable isotropic, nematic, smectic A, columnar,
plastic crystalline and orientationally ordered crystalline phases. As far as
the dynamics is concerned, the translational density is a conserved order
parameter while the orientational ordering is non-conserved. The derived
phase-field-crystal model can serve for efficient numerical investigations of
various nonequilibrium situations in liquid crystals
The elimination of surface cross-hatch from relaxed, limited-area Si1 – xGex buffer layers
The influence of lateral dimensions on the relaxation and surface topography of linearly graded Si1 – xGex buffer layers has been investigated. A dramatic change in the relaxation mechanism has been observed for depositions on Si mesa pillars of lateral dimensions 10 µm and below. Misfit dislocations are able to extend unhindered and terminate at the edges of the growth zone, yielding a surface free of cross-hatch. For lateral dimensions in excess of 10 µm orthogonal misfit interactions occur and relaxation is dominated by the modified Frank–Read (MFR) mechanism. The stress fields associated with the MFR dislocation pile-ups result in a pronounced cross-hatch topography
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