3,832 research outputs found
Stimulated Raman spin coherence and spin-flip induced hole burning in charged GaAs quantum dots
High-resolution spectral hole burning (SHB) in coherent nondegenerate
differential transmission spectroscopy discloses spin-trion dynamics in an
ensemble of negatively charged quantum dots. In the Voigt geometry, stimulated
Raman spin coherence gives rise to Stokes and anti-Stokes sidebands on top of
the trion spectral hole. The prominent feature of an extremely narrow spike at
zero detuning arises from spin population pulsation dynamics. These SHB
features confirm coherent electron spin dynamics in charged dots, and the
linewidths reveal spin spectral diffusion processes.Comment: 5 pages, 5 figure
Polarization dependence of four-wave mixing in a degenerate two-level system
Nearly degenerate four-wave mixing (NDFWM) within a closed degenerate
two-level atomic transition is theoretically and experimentally examined. Using
the model presented by A. Lezama et al [Phys. Rev. A 61, 013801 (2000)] the
NDFWM spectra corresponding to different pump and probe polarization cases are
calculated and discussed. The calculated spectra are compared to the
observation of NDFWM within the transition of
cesium in a phase conjugation experiment using magneto optically cooled atomsComment: 10 pages, 13 figures; submitted to Phys. Rev.
The Josephson plasmon as a Bogoliubov quasiparticle
We study the Josephson effect in alkali atomic gases within the two-mode
approximation and show that there is a correspondence between the Bogoliubov
description and the harmonic limit of the phase representation. We demonstrate
that the quanta of the Josephson plasmon can be identified with the Bogoliubov
excitations of the two-site Bose fluid. We thus establish a mapping between the
Bogoliubov approximation for the many-body theory and the linearized pendulum
Hamiltonian.Comment: 9 pages, LaTeX, submitted to J. Phys.
Systematic effects and a new determination of the primordial abundance of 4He and dY/dZ from observations of blue compact galaxies
We use spectroscopic observations of a sample of 82 HII regions in 76 blue
compact galaxies to determine the primordial helium abundance Yp and the slope
dY/dZ from the Y-O/H linear regression. To improve the accuracy of the dY/dZ
measurement, we have included new spectrophotometric observations of 33 HII
regions which span a large metallicity range, with oxygen abundance 12+log(O/H)
varying between 7.43 and 8.30 (Zsun/30<Z<Zsun/4). For a subsample of 7 HII
regions, we derive the He mass fraction taking into account known systematic
effects, including collisional and fluorescent enhancements of HeI emission
lines, collisional excitation of hydrogen emission, underlying stellar HeI
absorption and the difference between the temperatures Te(HeII) in the He^+
zone and Te(OIII) derived from the collisionally excited [OIII] lines. We find
that the net result of all the systematic effects combined is small, changing
the He mass fraction by less than 0.6%. By extrapolating the Y vs. O/H linear
regression to O/H=0 for 7 HII regions of this subsample, we obtain
Yp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds to dY/dZ=3.7+/-1.2,
assuming the oxygen mass fraction to be O=0.66Z. In the framework of the
standard Big Bang nucleosynthesis theory, this Yp corresponds to Omega_b h^2 =
0.012^+0.003_-0.002, where h is the Hubble constant in units of 100 km/s/Mpc.
This is smaller at the 2sigma level than the value obtained from recent
deuterium abundance and microwave background radiation measurements. The linear
regression slope dY/dO=4.3+/-0.7 (corresponding to dY/dZ=2.8+/-0.5) for the
whole sample of 82 HII regions is similar to that derived for the subsample of
7 HII regions, although it has a considerably smaller uncertainty.Comment: 53 pages, 3 Postscript figures, accepted for publication in the
Astrophysical Journa
Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots
We report on the coherent optical excitation of electron spin polarization in
the ground state of charged GaAs quantum dots via an intermediate charged
exciton (trion) state. Coherent optical fields are used for the creation and
detection of the Raman spin coherence between the spin ground states of the
charged quantum dot. The measured spin decoherence time, which is likely
limited by the nature of the spin ensemble, approaches 10 ns at zero field. We
also show that the Raman spin coherence in the quantum beats is caused not only
by the usual stimulated Raman interaction but also by simultaneous spontaneous
radiative decay of either excited trion state to a coherent combination of the
two spin states.Comment: 4 pages, 3 figures. Minor modification
Clemastine/tamoxifen hybrids as easily accessible antileishmanial drug leads
A library of hybrid molecules is developed based on the common chemical features shared by clemastine and tamoxifen both of which are well known for their antileishmanial activities. In the initial screening against Leishmania major and L. amazonensis promastigotes, as well as cytotoxicity assays using HepG2 cells, several hybrids showed submicromolar activity against the parasite and no toxicity against human cells. The compounds with an EC50 10 were further characterized against intracellular amastigotes as well as promastigotes of species that cause both visceral and cutaneous leishmaniasis, such as L. infantum and L. braziliensis, respectively. These sequential screenings revealed the high pan-activity of this class of molecules against these species, with several compounds displaying an EC50 †2 ΌM against both promastigotes and intracellular amastigotes. Two of them were identified as the potential templates for lead optimization of this series having shown the highest activities against all species in both stages of parasite. The present findings can serve as a good starting point in the search for novel antileishmanial compounds that are easy to access and highly active
Manipulating the Speed of Sound in a Two-Component Bose-Einstein Condensate
We consider a two-component weakly interacting Bose-Einstein condensate in
the presence of an external field which couples the two components. We express
the Hamiltonian in terms of the energy eigenstates of the single-body part of
the Hamiltonian. These eigenstates are the atomic dressed states of quantum
optics. When the energy difference between the two dressed states is much
larger than the mean-field interactions, two-body interactions in the dressed
state basis that do not conserve the number of atoms in each of the two dressed
states are highly suppressed. The two-body interactions then take on a
simplified form in the dressed basis with effective coupling constants that
depend on the intensity and frequency of the external field. This implies that
the chemical potential as well as the quasiparticle spectrum may be controlled
experimentally in a simple manner. We demonstrate this by showing that one may
achieve significant variations in the speed of sound in the condensate, a
quantity which has been measured experimentally.Comment: 20 pages, 3 figures. Accepted for publication in Phys. Rev.
Search for Magnetic Monopoles Trapped in Matter
There have been many searches for magnetic monopoles in flight, but few for
monopoles in matter. We have searched for magnetic monopoles in meteorites,
schists, ferromanganese nodules, iron ores and other materials. The detector
was a superconducting induction coil connected to a SQUID (Superconducting
Quantum Interference Device) with a room temperature bore 15 cm in diameter. We
tested a total of more than 331 kg of material including 112 kg of meteorites.
We found no monopole and conclude the overall monopole/nucleon ratio in the
samples is with a 90\% confidence level.Comment: 6 pages, rev tex, no figure
Folding and unfolding phylogenetic trees and networks
Phylogenetic networks are rooted, labelled directed acyclic graphs which are commonly used to represent reticulate evolution. There is a close relationship between phylogenetic networks and multi-labelled trees (MUL-trees). Indeed, any phylogenetic network can be "unfolded" to obtain a MUL-tree and, conversely, a MUL-tree can in certain circumstances be "folded" to obtain a phylogenetic network that exhibits . In this paper, we study properties of the operations and in more detail. In particular, we introduce the class of stable networks, phylogenetic networks for which is isomorphic to , characterise such networks, and show that they are related to the well-known class of tree-sibling networks.We also explore how the concept of displaying a tree in a network can be related to displaying the tree in the MUL-tree . To do this, we develop a phylogenetic analogue of graph fibrations. This allows us to view as the analogue of the universal cover of a digraph, and to establish a close connection between displaying trees in and reconcilingphylogenetic trees with networks
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