1,698 research outputs found
Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systems Yb2Ti2O7 and Yb2Sn2O7
We report a positive muon spin relaxation and rotation (\muSR) study of the
quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field
response. In agreement with earlier reports, data recorded in small
longitudinal fields evidence anomalously slow spin dynamics in the microsecond
range below the temperature T_c at which the specific heat displays an intense
peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We
found that slow dynamics extends above T_c up to at least 0.7 K for both
compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the
\muSR spectra recorded above T_c. At lower temperatures a published analytical
extension of the Gaussian Kubo-Toyabe model provides a good description,
consistent with the existence of short-range magnetic correlations. While the
physical response of the two systems is qualitatively the same, Yb2Ti2O7
exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c.
Considering previously reported ac susceptibility, neutron scattering and \muSR
results, we suggest the existence of anomalously slow spin dynamics to be a
common physical property of pyrochlore magnetic materials. The possibility of
molecular spin substructures to be associated to the slow dynamics and
therefore the short-range correlations is mentioned. The slow spin dynamics
observed under field does not exclude the presence of much faster dynamics
detected in extremely low or zero field.Comment: 11 pages, 10 figure
Exploring the spatial, temporal, and vertical distribution of methane in Pluto's atmosphere
High-resolution spectra of Pluto in the 1.66 um region, recorded with the
VLT/CRIRES instrument in 2008 (2 spectra) and 2012 (5 spectra), are analyzed to
constrain the spatial and vertical distribution of methane in Pluto's
atmosphere and to search for mid-term (4 year) variability. A sensitivity study
to model assumptions (temperature structure, surface pressure, Pluto's radius)
is performed. Results indicate that (i) no variation of the CH4 atmospheric
content (column-density or mixing ratio) with Pluto rotational phase is present
in excess of 20 % (ii) CH4 column densities show at most marginal variations
between 2008 and 2012, with a best guess estimate of a ~20 % decrease over this
time frame. As stellar occultations indicate that Pluto's surface pressure has
continued to increase over this period, this implies a concomitant decrease of
the methane mixing ratio (iii) the data do not show evidence for an
altitude-varying methane distribution; in particular, they imply a roughly
uniform mixing ratio in at least the first 22-27 km of the atmosphere, and high
concentrations of low-temperature methane near the surface can be ruled out.
Our results are also best consistent with a relatively large (> 1180 km) Pluto
radius. Comparison with predictions from a recently developed global climate
model GCM indicates that these features are best explained if the source of
methane occurs in regional-scale CH4 ice deposits, including both low latitudes
and high Northern latitudes, evidence for which is present from the rotational
and secular evolution of the near-IR features due to CH4 ice. Our "best guess"
predictions for the New Horizons encounter in 2015 are: a 1184 km radius, a 17
ubar surface pressure, and a 0.44 % CH4 mixing ratio with negligible
longitudinal variations.Comment: 21 pages, 6 figure
BaCu3O4: High Temperature Magnetic Order in One-Dimensional S=1/2 Diamond-Chains
The magnetic properties of the alkaline earth oxocuprate BaCu3O4 are
investigated. We show that the characteristic Cu3O4 layers of this material can
be described with diamond chains of antiferromagnetically coupled Cu 1/2 spins
with only a weak coupling between two adjacent chains. These Cu3O4 layers seem
to represent a so far unique system of weakly coupled one-dimensional magnetic
objects where the local AF ordering of the Cu2+ ions leads to an actual net
magnetic moment of an isolated diamond chain. We demonstrate a magnetic
transition at a high N\'eel temperature T_{N}=336 K
Highly efficient multilayer organic pure-blue-light emitting diodes with substituted carbazoles compounds in the emitting layer
Bright blue organic light-emitting diodes (OLEDs) based on
1,4,5,8,N-pentamethylcarbazole (PMC) and on dimer of N-ethylcarbazole
(N,N'-diethyl-3,3'-bicarbazyl) (DEC) as emitting layers or as dopants in a
4,4'-bis(2,2'-diphenylvinyl)-1,1'-biphenyl (DPVBi) matrix are described. Pure
blue-light with the C.I.E. coordinates x = 0.153 y = 0.100, electroluminescence
efficiency \eta_{EL} of 0.4 cd/A, external quantum efficiency \eta_{ext.} of
0.6% and luminance L of 236 cd/m2 (at 60 mA/cm2) were obtained with PMC as an
emitter and the 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline (BCP) as a
hole-blocking material in five-layer emitting devices. The highest efficiencies
\eta_{EL.} of 4.7 cd/A, and \eta_{ext} = 3.3% were obtained with a four-layer
structure and a DPVBi DEC-doped active layer (CIE coordinates x = 0.158,
y=0.169, \lambda_{peak} = 456 nm). The \eta_{ext.} value is one the highest
reported at this wavelength for blue OLEDs and is related to an internal
quantum efficiency up to 20%
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Atmospheric Mars Entry and Landing Investigations & Analysis (AMELIA) by ExoMars 2016 Schiaparelli Entry Descent Module
Opioid prescribing for acute postoperative pain : an overview of systematic reviews related to two consensus statements relevant at patient, prescriber, system and public health levels
Peer reviewedPublisher PD
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