6,693 research outputs found
High hydrogen yield and purity from palm empty fruit bunch and pine pyrolysis oils
The benefits of CO 2 sorption enhanced steam reforming using calcined dolomite were demonstrated for the production of hydrogen from highly oxygenated pyrolysis oils of the agricultural waste palm empty fruit bunches (PEFB) and pine wood. At 1 atm in a down-flow packed bed reactor at 600 °C, the best molar steam to carbon ratios were between 2 and 3 using a Ni catalyst. After incorporating steam-activated calcined dolomite as the CO 2 sorbent in the reactor bed, the H 2 yield from the moisture free PEFB oil increased from 9.5 to 10.4 wt.% while that of the pine oil increased from 9.9 to 13.9 wt.%. The hydrogen purity also rose from 68 to 96% and from 54 to 87% for the PEFB and pine oils respectively, demonstrating very substantial sorption enhancement effects
Autonomous control procedures for shuttle rendezvous proximity operations
The results are presented of a study which uses fuzzy sets to model a Space Shuttle pilot's reasoning and actions while performing rendezvous proximity operation maneuvers. In this model fuzzy sets are used to simulate smooth and continuous actions as would be expected from an experienced pilot and to simulate common sense reasoning in the decision process. The present model assumes visual information available to the Shuttle pilot from the Shuttle Crew Optical Alignment Sighting (COAS) device and the overhead window and rendezvous radar sensor information available to him from an onboard display. This model will be used in a flight analysis simulator to perform studies requiring a large number of runs, each of which currently needs an engineer in the loop to supply the piloting decisions. This work has much broader implications in control of robots such as the Flight Telerobotic Servicer, in automated pilot control and attitude control, and in advisory and evaluation functions that could be used for flight data monitoring or for testing of various rule sets in flight preparation
Supporting Emerging Multilingual Newcomer Students and their Teachers in California Public High Schools
This study focuses supporting emerging multilingual newcomer students and their teachers. The study examines research regarding deficit mindset that has led to student labels that perpetuate negative school and teacher views of culturally and linguistically diverse students. Additional research used for this study includes studies on what has been determined to benefit emerging multilingual students, as well as studies about the role of globalization and politics in the education of emerging multilingual and other culturally and linguistically diverse students. The study seeks to further the research on what changes in practice and mindset need to take place in the system of education, as well as what can be done at the district, school, and classroom level to meet the needs to emerging multilingual students. The study was done in a public high school district in the state of California, and carried out by reviewing comments and feedback from participants - teachers and bilingual instructional assistants - during professional development sessions regarding emerging multilingual students. The feedback and comments were used to determine what teachers need to effectively teach emerging multilingual students. Additional data was collected from the school data system. The results of the study show that the teachers have a desire to teach their emerging multilingual students, but that their view of how to meet student needs is clouded by deficit mindset. Additionally, the study determines that teachers and students need proper materials, teachers, administrators, and school staff need professional learning that is based on current research, and teachers need time to collaborate with colleagues and plan for instruction
Dynamical control of electron spin coherence in a quantum dot
We investigate the performance of dynamical decoupling methods at suppressing
electron spin decoherence from a low-temperature nuclear spin reservoir in a
quantum dot. The controlled dynamics is studied through exact numerical
simulation, with emphasis on realistic pulse delays and long-time limit. Our
results show that optimal performance for this system is attained by a periodic
protocol exploiting concatenated design, with control rates substantially
slower than expected from the upper spectral cutoff of the bath. For a known
initial electron spin state, coherence can saturate at long times, signaling
the creation of a stable ``spin-locked'' decoherence-free subspace. Analytical
insight on saturation is obtained for a simple echo protocol, in good agreement
with numerical results.Comment: 4 pages, 4 figures with 3 of them in colo
Suppression of electron spin decoherence in a quantum dot
The dominant source of decoherence for an electron spin in a quantum dot is
the hyperfine interaction with the surrounding bath of nuclear spins. The
decoherence process may be slowed down by subjecting the electron spin to
suitable sequences of external control pulses. We investigate the performance
of a variety of dynamical decoupling protocols using exact numerical
simulation. Emphasis is given to realistic pulse delays and the long-time
limit, beyond the domain where available analytical approaches are guaranteed
to work. Our results show that both deterministic and randomized protocols are
capable to significantly prolong the electron coherence time, even when using
control pulse separations substantially larger than what expected from the {\em
upper cutoff} frequency of the coupling spectrum between the electron and the
nuclear spins. In a realistic parameter range, the {\em total width} of such a
coupling spectrum appears to be the physically relevant frequency scale
affecting the overall quality of the decoupling.Comment: 8 pages, 3 figures. Invited talk at the XXXVII Winter Colloquium on
the Physics of Quantum Electronics, Snowbird, Jan 2007. Submitted to J. Mod.
Op
Domain wall dynamics in integrable and chaotic spin-1/2 chains
We study the time evolution of correlation functions, spin current, and local
magnetization in an isolated spin-1/2 chain initially prepared in a sharp
domain wall state. The results are compared with the level of spatial
delocalization of the eigenstates of the system which is measured using the
inverse participation ratio. Both integrable and non-integrable regimes are
considered. Non-integrability is introduced to the integrable Hamiltonian with
nearest neighbor couplings by adding a single site impurity field or by adding
next-nearest-neighbor couplings. A monotonic correspondence between the
enhancement of the level of delocalization, spin current and magnetization
dynamics occurs in the integrable domain. This correspondence is however lost
for chaotic models with weak Ising interactions.Comment: 9 pages, 5 figures, 1 tabl
Overwintering Habitat of American Dipper, Cinclus mexicanus, Observed in an Arctic Groundwater Spring Feeding on Dolly Varden, Salvelinus malma
Perennial groundwater springs along the Alaska and Yukon North Slope provide overwintering habitat for various organisms, including birds and fishes. We observed an American Dipper, Cinclus mexicanus, in the open water of a perennial spring situated in Fish Creek, Yukon, in Ivvavik National Park on 8 March 2018. The observation at Fish Creek was among the most northern documented sightings of an American Dipper during the winter in North America. Moreover, the observation was approximately 650 km farther north than where American Dippers have been documented overwintering in Yukon, making this the most northern Canadian observation documented for this species in any season. Additionally, the American Dipper was photographed feeding on a juvenile Dolly Varden, Salvelinus malma. Although American Dippers are known to feed on small fish, our observation was a novel documentation of a trophic interaction between both species during winter. The open-water habitat in Fish Creek, which is important for both species and has not been previously described, was short (~730 m long), shallow (mean = 20 cm deep), narrow (mean = 2.8 m wide), and cold (mean water temperature = 0.34ºC). While there is little information regarding the ecological interactions of American Dipper overwintering in the Arctic, we note that all observations in the North Slope area during winter occurred in river systems also used by Dolly Varden, which indicates that juvenile Dolly Varden could be an important source of food for American Dipper in winter.Les sources d’eau souterraine pérennes le long du versant nord de l’Alaska et du Yukon procurent un habitat d’hivernage à divers organismes, y compris les oiseaux et les poissons. Le 8 mars 2018, nous avons observé un cincle d’Amérique (Cinclus mexicanus) dans l’eau libre d’une source pérenne située au ruisseau Fish, dans le parc national Ivvavik, au Yukon. L’observation faite au ruisseau Fish figurait parmi les observations hivernales les plus nordiques du cincle d’Amérique à avoir été répertoriées en Amérique du Nord. Cette observation a été faite à environ 650 km plus au nord que l’endroit où l’habitat d’hivernage des cincles d’Amérique a été documenté au Yukon, ce qui représente l’observation la plus nordique au Canada à avoir été consignée pour cette espèce à n’importe quelle saison. Par surcroît, le cincle d’Amérique a été photographié en train de se nourrir d’un omble malma juvénile (Salvelinus malma). Bien que l’on sache que les cincles d’Amérique se nourrissent de petits poissons, l’observation que nous avons documentée constituait un nouveau cas d’interaction trophique entre les deux espèces pendant l’hiver. L’habitat en eau libre du ruisseau Fish, qui est important pour les deux espèces et n’a pas encore été décrit, était court (environ 730 m de long), peu profond (moyenne de 20 cm de profondeur), étroit (moyenne de 2,8 m de largeur) et froid (moyenne de la température de l’eau = 0,34 ºC). Bien qu’il existe peu d’information sur les interactions écologiques du cincle d’Amérique hivernant dans l’Arctique, notons qu’en hiver, toutes les observations faites dans la région du versant nord ont eu lieu dans des réseaux hydrographiques où vit également l’omble malma, ce qui laisse croire que l’omble malma juvénile pourrait représenter une source de nourriture importante pour le cincle d’Amérique en hiver
Atomic clocks with suppressed blackbody radiation shift
We develop a nonstandard concept of atomic clocks where the blackbody
radiation shift (BBRS) and its temperature fluctuations can be dramatically
suppressed (by one to three orders of magnitude) independent of the
environmental temperature. The suppression is based on the fact that in a
system with two accessible clock transitions (with frequencies v1 and v2) which
are exposed to the same thermal environment, there exists a "synthetic"
frequency v_{syn} (v1-e12 v2) largely immune to the BBRS. As an example, it is
shown that in the case of ion 171Yb+ it is possible to create a clock in which
the BBRS can be suppressed to the fractional level of 10^{-18} in a broad
interval near room temperature (300\pm 15 K). We also propose a realization of
our method with the use of an optical frequency comb generator stabilized to
both frequencies v1 and v2. Here the frequency v_{syn} is generated as one of
the components of the comb spectrum and can be used as an atomic standard.Comment: 5 pages, 2 figure
Localization and the effects of symmetries in the thermalization properties of one-dimensional quantum systems
We study how the proximity to an integrable point or to localization as one
approaches the atomic limit, as well as the mixing of symmetries in the chaotic
domain, may affect the onset of thermalization in finite one-dimensional
systems. We consider systems of hard-core bosons at half-filling with nearest
neighbor hopping and interaction, and next-nearest neighbor interaction. The
latter breaks integrability and induces a ground-state superfluid to insulator
transition. By full exact diagonalization, we study chaos indicators and
few-body observables. We show that when different symmetry sectors are mixed,
chaos indicators associated with the eigenvectors, contrary to those related to
the eigenvalues, capture the onset of chaos. The results for the complexity of
the eigenvectors and for the expectation values of few-body observables confirm
the validity of the eigenstate thermalization hypothesis in the chaotic regime,
and therefore the occurrence of thermalization. We also study the properties of
the off-diagonal matrix elements of few-body observables in relation to the
transition from integrability to chaos and from chaos to localization.Comment: 12 pages, 13 figures, as published (Fig.09 was corrected in this
final version
Magneto-shear modes and a.c. dissipation in a two-dimensional Wigner crystal
The a.c. response of an unpinned and finite 2D Wigner crystal to electric
fields at an angular frequency has been calculated in the dissipative
limit, , where is the scattering rate. For
electrons screened by parallel electrodes, in zero magnetic field the
long-wavelength excitations are a diffusive longitudinal transmission line mode
and a diffusive shear mode. A magnetic field couples these modes together to
form two new magneto-shear modes. The dimensionless coupling parameter where and are the
speeds of transverse and longitudinal sound in the collisionless limit and
and are the tensor components of the
magnetoconductivity. For , both the coupled modes contribute
to the response of 2D electrons in a Corbino disk measurement of
magnetoconductivity. For , the electron crystal rotates rigidly in
a magnetic field. In general, both the amplitude and phase of the measured a.c.
currents are changed by the shear modulus. In principle, both the
magnetoconductivity and the shear modulus can be measured simultaneously.Comment: REVTeX, 7 pp., 4 eps figure
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