2,654 research outputs found
Protection of densely populated excited triplet state ensembles against deactivation by molecular oxygen
Hexa-μ-chlorido-hexachlorido(η6-hexamethylbenzene)trialuminium(III)lanthanum(III) benzene solvate
In the title compound, [Al3LaCl12(C12H18)]·C6H6, all molecules are located on a mirror plane. Three chloridoaluminate groups and a hexamethylbenzene molecule are bound to the central lanthanum(III) ion, forming a distorted pentagonal bipyramid with the η6-coordinated arene located at the apical position. The hexamethylbenzene ligand disordered between two orientations in a 1:1 ratio is also involved in parallel-slipped π–π stacking intermolecular interactions with a benzene solvent molecule [centroid–centroid distance 3.612 (4) Å]
Electronic structure and magnetic properties of the spin-1/2 Heisenberg system CuSe2O5
A microscopic magnetic model for the spin-1/2 Heisenberg chain compound
CuSe2O5 is developed based on the results of a joint experimental and
theoretical study. Magnetic susceptibility and specific heat data give evidence
for quasi-1D magnetism with leading antiferromagnetic (AFM) couplings and an
AFM ordering temperature of 17 K. For microscopic insight, full-potential DFT
calculations within the local density approximation (LDA) were performed. Using
the resulting band structure, a consistent set of transfer integrals for an
effective one-band tight-binding model was obtained. Electronic correlations
were treated on a mean-field level starting from LDA (LSDA+U method) and on a
model level (Hubbard model). In excellent agreement of experiment and theory,
we find that only two couplings in CuSe2O5 are relevant: the nearest-neighbour
intra-chain interaction of 165 K and a non-frustrated inter-chain coupling of
20 K. From a comparison with structurally related systems (Sr2Cu(PO4)2,
Bi2CuO4), general implications for a magnetic ordering in presence of
inter-chain frustration are made.Comment: 20 pages, 8 figures, 3 table
Remote atomic clock synchronization via satellites and optical fibers
In the global network of institutions engaged with the realization of
International Atomic Time (TAI), atomic clocks and time scales are compared by
means of the Global Positioning System (GPS) and by employing telecommunication
satellites for two-way satellite time and frequency transfer (TWSTFT). The
frequencies of the state-of-the-art primary caesium fountain clocks can be
compared at the level of 10e-15 (relative, 1 day averaging) and time scales can
be synchronized with an uncertainty of one nanosecond. Future improvements of
worldwide clock comparisons will require also an improvement of the local
signal distribution systems. For example, the future ACES (atomic clock
ensemble in space) mission shall demonstrate remote time scale comparisons at
the uncertainty level of 100 ps. To ensure that the ACES ground instrument will
be synchronized to the local time scale at PTB without a significant
uncertainty contribution, we have developed a means for calibrated clock
comparisons through optical fibers. An uncertainty below 50 ps over a distance
of 2 km has been demonstrated on the campus of PTB. This technology is thus in
general a promising candidate for synchronization of enhanced time transfer
equipment with the local realizations of UTC . Based on these experiments we
estimate the uncertainty level for calibrated time transfer through optical
fibers over longer distances. These findings are compared with the current
status and developments of satellite based time transfer systems, with a focus
on the calibration techniques for operational systems
Parameter selection for the supply system in a Pilger mill
The braking chamber in the supply system of a Pilger pipe mill is modernized. The influence of constant and variable parameters of the supply system on braking is studied. © 2013 Allerton Press, Inc
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