1,992 research outputs found
Evidence of several dipolar quasi-invariants in Liquid Crystals
In a closed quantum system of N coupled spins with magnetic quantum number I,
there are about (2I + 1)^N constants of motion. However, the possibility of
observing such quasi-invariant (QI) states in solid-like spin systems in
Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of
this work is to provide experimental evidence of several QI, in the proton NMR
of small spin clusters, besides those already known Zeeman, and dipolar orders
(strong and weak). We explore the spin states prepared with the
Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by
this sequence as a function of the preparation times, in a variety of dipolar
networks. We observe that the signals can be explained with two dipolar QIs
only within a range of short preparation times. At longer times the time-domain
signals have an echo-like behaviour. We study their multiple quantum coherence
content on a basis orthogonal to the z-basis and see that such states involve a
significant number of correlated spins. Then we show that the whole preparation
time-scale can only be reconstructed by assuming the occurrence of multiple QI
which we isolate experimentally
Development of dry coal feeders
Design and fabrication of equipment of feed coal into pressurized environments were investigated. Concepts were selected based on feeder system performance and economic projections. These systems include: two approaches using rotating components, a gas or steam driven ejector, and a modified standpipe feeder concept. Results of development testing of critical components, design procedures, and performance prediction techniques are reviewed
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Hippocampus-dependent emergence of spatial sequence coding in retrosplenial cortex.
Retrosplenial cortex (RSC) is involved in visuospatial integration and spatial learning, and RSC neurons exhibit discrete, place cell-like sequential activity that resembles the population code of space in hippocampus. To investigate the origins and population dynamics of this activity, we combined longitudinal cellular calcium imaging of dysgranular RSC neurons in mice with excitotoxic hippocampal lesions. We tracked the emergence and stability of RSC spatial activity over consecutive imaging sessions. Overall, spatial activity in RSC was experience-dependent, emerging gradually over time, but, as seen in the hippocampus, the spatial code changed dynamically across days. Bilateral but not unilateral hippocampal lesions impeded the development of spatial activity in RSC. Thus, the emergence of spatial activity in RSC, a major recipient of hippocampal information, depends critically on an intact hippocampus; the indirect connections between the dysgranular RSC and the hippocampus further indicate that hippocampus may exert such influences polysynaptically within neocortex
Measurement of dynamic Stark polarizabilities by analyzing spectral lineshapes of forbidden transitions
We present a measurement of the dynamic scalar and tensor polarizabilities of
the excited state 3D1 in atomic ytterbium. The polarizabilities were measured
by analyzing the spectral lineshape of the 408-nm 1S0->3D1 transition driven by
a standing wave of resonant light in the presence of static electric and
magnetic fields. Due to the interaction of atoms with the standing wave, the
lineshape has a characteristic polarizability-dependent distortion. A
theoretical model was used to simulate the lineshape and determine a
combination of the polarizabilities of the ground and excited states by fitting
the model to experimental data. This combination was measured with a 13%
uncertainty, only 3% of which is due to uncertainty in the simulation and
fitting procedure. The scalar and tensor polarizabilities of the state 3D1 were
measured for the first time by comparing two different combinations of
polarizabilities. We show that this technique can be applied to similar atomic
systems.Comment: 13 pages, 7 figures, submitted to PR
Attosecond tracking of light absorption and refraction in fullerenes
The collective response of matter is ubiquitous and widely exploited, e.g. in
plasmonic, optical and electronic devices. Here we trace on an attosecond time
scale the birth of collective excitations in a finite system and find distinct
new features in this regime. Combining quantum chemical computation with
quantum kinetic methods we calculate the time-dependent light absorption and
refraction in fullerene that serve as indicators for the emergence of
collective modes. We explain the numerically calculated novel transient
features by an analytical model and point out the relevance for ultra-fast
photonic and electronic applications. A scheme is proposed to measure the
predicted effects via the emergent attosecond metrology.Comment: 11 pages, 3 figures, accepted in Phys. Rev.
State-insensitive trapping of Rb atoms: linearly versus circularly polarized lights
We study the cancellation of differential ac Stark shifts in the 5s and 5p
states of rubidium atom using the linearly and circularly polarized lights by
calculating their dynamic polarizabilities. Matrix elements were calculated
using a relativistic coupled-cluster method at the single, double and important
valence triple excitations approximation including all possible non-linear
correlation terms. Some of the important matrix elements were further optimized
using the experimental results available for the lifetimes and static
polarizabilities of atomic states. "Magic wavelengths" are determined from the
differential Stark shifts and results for the linearly polarized light are
compared with the previously available results. Possible scope of facilitating
state-insensitive optical trapping schemes using the magic wavelengths for
circularly polarized light are discussed. Using the optimized matrix elements,
the lifetimes of the 4d and 6s states of this atom are ameliorated.Comment: 13 pages, 13 tables and 4 figure
Quasi-equilibrium states in thermotropic liquid crystals studied by multiple quantum NMR
We study the nature of the quasiinvariants in nematic 5CB and measure their
relaxation times by encoding the multiple quantum coherences of the states
following the JB pulse pair on two orthogonal bases, Z and X. The experiments
were also performed in powder adamantane at 301 K which is used as a reference
compound having only one dipolar quasiinvariant. We show that the evolution of
the quantum states during the build up of the quasi-equilibrium state in 5CB
prepared under the S condition is similar to the case of adamantane and that
their quasi-equilibrium density operators have the same tensor structure. In
contrast, the second constant of motion, whose explicit operator form is not
known, involves a richer composition of multiple quantum coherences on the X
basis of even order, in consistency with the truncation inherent in its
definition. We exploited the exclusive presence coherences 4, 6, 8, besides 0
and 2 under the W condition to measure the spin-lattice relaxation time T_{W}
accurately, so avoiding experimental difficulties that usually impair dipolar
order relaxation measurement such as Zeeman contamination at high fields, and
also superposition of the different quasiinvariants. This procedure opens the
possibility of measuring the spin-lattice relaxation of a quasiinvariant
independent of the Zeeman and S reservoirs, so incorporating a new relaxation
parameter useful for studying the complex molecular dynamics in mesophases. In
fact, we report the first measurement of T_{W} in a liquid crystal at high
magnetic fields. The comparison of the obtained value with the one
corresponding to a lower field (16 MHz) points out that the relaxation of the
W-order strongly depends on the intensity of the external magnetic field,
similarly to the case of the S reservoir, indicating that the relaxation of the
W-quasiinvariant is also governed by the cooperative molecular motions.Comment: 7 figures. http://www.famaf.unc.edu.ar/series/AFis2005.ht
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