394 research outputs found
Accidental lean: Performance improvement in an NHS hospital and reflections on the role of operations strategy
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Phase-dependent light propagation in atomic vapors
Light propagation in an atomic medium whose coupled electronic levels form a
diamond-configuration exhibits a critical dependence on the input conditions.
In particular, the relative phase of the input fields gives rise to
interference phenomena in the electronic excitation whose interplay with
relaxation processes determines the stationary state. We integrate numerically
the Maxwell-Bloch equations and observe two metastable behaviors for the
relative phase of the propagating fields corresponding to two possible
interference phenomena. These phenomena are associated to separate types of
response along propagation, minimize dissipation, and are due to atomic
coherence. These behaviors could be studied in gases of isotopes of
alkali-earth atoms with zero nuclear spin, and offer new perspectives in
control techniques in quantum electronics.Comment: 16 pages, 11 figures, v2: typos corrected, v3: final version, to
appear in Phys. Rev.
JHK Observations of Faint Standard Stars in the Mauna Kea Near-Infrared Photometric System
JHK photometry in the Mauna Kea Observatory (MKO) near-IR system is presented
for 115 stars. Of these, 79 are UKIRT standards and 42 are LCO standards. The
average brightness is 11.5 mag, with a range of 10 to 15. The average number of
nights each star was observed is 4, and the average of the internal error of
the final results is 0.011 mag. These JHK data agree with those reported by
other groups to 0.02 mag. The measurements are used to derive transformations
between the MKO JHK photometric system and the UKIRT, LCO and 2MASS systems.
The 2MASS-MKO data scatter by 0.05 mag for redder stars: 2MASS-J includes H2O
features in dwarfs and MKO-K includes CO features in giants. Transformations
derived for stars whose spectra contain only weak features cannot give accurate
transformations for objects with strong absorption features within a filter
bandpasses. We find evidence of systematic effects at the 0.02 mag level in the
photometry of stars with J<11 and H,K<10.5. This is due to an underestimate of
the linearity correction for stars observed with the shortest exposure times;
very accurate photometry of stars approaching the saturation limits of infrared
detectors which are operated in double-read mode is difficult to obtain. Four
stars in the sample, GSPC S705-D, FS 116 (B216-b7), FS 144 (Ser-EC84) and FS 32
(Feige 108), may be variable. 84 stars in the sample have 11< J< 15 and
10.5<H,K<15, are not suspected to be variable, and have magnitudes with an
estimated error <0.027 mag; 79 of these have an error of <0.020 mag. These
represent the first published high-accuracy JHK stellar photometry in the MKO
photometric system; we recommend these objects be employed as primary standards
for that system [abridged].Comment: Accepted for publication in MNRAS, 14 pages, 5 Figure
Structural, magnetic, and electrical properties of NiMnSb thin films grown on InSb by pulsed-laser deposition
Nonlinear Effects in Pulse Propagation through Doppler-Broadened Closed-Loop Atomic Media
Nonlinear effects in pulse propagation through a medium consisting of
four-level double--type systems are studied theoretically. We apply
three continous-wave driving fields and a pulsed probe field such that they
form a closed interaction loop. Due to the closed loop and the finite frequency
width of the probe pulses the multiphoton resonance condition cannot be
fulfilled, such that a time-dependent analysis is required. By identifying the
different underlying physical processes we determine the parts of the solution
relevant to calculate the linear and nonlinear response of the system. We find
that the system can exhibit a strong intensity dependent refractive index with
small absorption over a range of several natural linewidths. For a realistic
example we include Doppler and pressure broadening and calculate the nonlinear
selfphase modulation in a gas cell with Sodium vapor and Argon buffer gas. We
find that a selfphase modulation of is achieved after a propagation of
few centimeters through the medium while the absorption in the corresponding
spectral range is small.Comment: 4 figure
Optical Microscopy as a probe of the rate limiting transport lifetime in InSb/Al1-xInxSb quantum wells
Recent reports of magnetotransport measurements of InSb/Al 1-x In x Sb quantum well structures at low temperature (3 K) have shown the need for inclusion of a new scattering mechanism not present in traditional transport lifetime models. Observations and analysis of characteristic surface structures using differential interference contrast DIC (Nomarski) optical imaging have extracted representative average grain feature sizes for this surface structure and shown these features to be the limiting low temperature scattering mechanism. We have subsequently modelled the potential profile of these surface structures using Landauer-Büttiker tunnelling calculations and a combination of a Monte-Carlo simulation and Drude model for mobility. This model matches experimentally measured currents and mobilities at low temperatures, giving a range of possible barrier heights and widths, as well modelling the theoretical trend in mobility with temperature
High-affinity DNA binding sites for H-NS provide a molecular basis for selective silencing within proteobacterial genomes
The global transcriptional regulator H-NS selectively silences bacterial genes associated with pathogenicity and responses to environmental insults. Although there is ample evidence that H-NS binds preferentially to DNA containing curved regions, we show here that a major basis for this selectivity is the presence of a conserved sequence motif in H-NS target transcriptons. We further show that there is a strong tendency for the H-NS binding sites to be clustered, both within operons and in genes contained in the pathogenicity-associated islands. In accordance with previously published findings, we show that these motifs occur in AT-rich regions of DNA. On the basis of these observations, we propose that H-NS silences extensive regions of the bacterial chromosome by binding first to nucleating high-affinity sites and then spreading along AT-rich DNA. This spreading would be reinforced by the frequent occurrence of the motif in such regions. Our findings suggest that such an organization enables the silencing of extensive regions of the genetic material, thereby providing a coherent framework that unifies studies on the H-NS protein and a concrete molecular basis for the genetic control of H-NS transcriptional silencing
Physical properties of dense cores in Orion B9
We aim to determine the physical and chemical properties of dense cores in
Orion B9. We observed the NH3(1,1) and (2,2), and the N2H+(3-2) lines towards
the submm peak positions. These data are used in conjunction with our LABOCA
870 micron dust continuum data. The gas kinetic temperature in the cores is
between ~9.4-13.9 K. The non-thermal velocity dispersion is subsonic in most of
the cores. The non-thermal linewidth in protostellar cores appears to increase
with increasing bolometric luminosity. The core masses are very likely drawn
from the same parent distribution as the core masses in Orion B North. Starless
cores in the region are likely to be gravitationally bound, and thus
prestellar. Some of the cores have a lower radial velocity than the systemic
velocity of the region, suggesting that they are members of the "low-velocity
part" of Orion B. The observed core-separation distances deviate from the
corresponding random-like model distributions. The distances between the
nearest-neighbours are comparable to the thermal Jeans length. The fractional
abundances of NH3 and N2H+ in the cores are ~1.5-9.8x10^{-8} and
~0.2-5.9x10^{-10}, respectively. The NH3 abundance appears to decrease with
increasing H2 column and number densities. The NH3/N2H+ column density ratio is
larger in starless cores than in cores with embedded protostars. The core
population in Orion B9 is comparable in physical properties to those in nearby
low-mass star-forming regions. It is unclear if the origin of cores could be
explained by turbulent fragmentation. On the other hand, many of the core
properties conform with the picture of dynamic core evolution. The Orion B9
region has probably been influenced by the feedback from the nearby Ori OB 1b
group, and the fragmentation of the parental cloud into cores could be caused
by gravitational instability.Comment: 17 pages, 11 figures, 7 tables. Accepted for publication in Astronomy
and Astrophysics. Version 2: minor language corrections adde
Four small puzzles that Rosetta doesn't solve
A complete macromolecule modeling package must be able to solve the simplest
structure prediction problems. Despite recent successes in high resolution
structure modeling and design, the Rosetta software suite fares poorly on
deceptively small protein and RNA puzzles, some as small as four residues. To
illustrate these problems, this manuscript presents extensive Rosetta results
for four well-defined test cases: the 20-residue mini-protein Trp cage, an even
smaller disulfide-stabilized conotoxin, the reactive loop of a serine protease
inhibitor, and a UUCG RNA tetraloop. In contrast to previous Rosetta studies,
several lines of evidence indicate that conformational sampling is not the
major bottleneck in modeling these small systems. Instead, approximations and
omissions in the Rosetta all-atom energy function currently preclude
discriminating experimentally observed conformations from de novo models at
atomic resolution. These molecular "puzzles" should serve as useful model
systems for developers wishing to make foundational improvements to this
powerful modeling suite.Comment: Published in PLoS One as a manuscript for the RosettaCon 2010 Special
Collectio
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