1,119 research outputs found
Optical control of coherent interactions between quantum dot electron spins
Coherent interactions between spins in quantum dots are a key requirement for
quantum gates. We have performed pump-probe experiments in which pulsed lasers
emitting at different photon energies manipulate two distinct subsets of
electron spins within an inhomogeneous InGaAs quantum dot ensemble. The spin
dynamics are monitored through their precession about an external magnetic
field. These measurements demonstrate spin precession phase shifts and
modulations of the magnitude of one subset of oriented spins after optical
orientation of the second subset. The observations are consistent with results
from a model using a Heisenberg-like interaction with microeV-strength.Comment: 5 pages, 4 figure
Identification of a Protein in Several Borrelia Species which is Related to OspC of the Lyme Disease Spirochetes.
Using oligonucleotide probes which have previously been shown to be specific for the ospC gene found in the Lyme disease spirochete species Borrelia burgdorferi, B. garinii, and group VS461, we detected an ospC homolog in other Borrelia species including B. coriaceae, B. hermsii, B. anserina, B. turicatae, and B. parkeri. In contrast to the Lyme disease spirochetes, which carry the ospC gene on a 26-kb circular plasmid, we mapped the gene in other Borrelia species to linear plasmids which varied in size among the isolates tested. Some isolates carry multiple copies of the gene residing on linear plasmids of different sizes. The analyses conducted here also demonstrate that these Borrelia species contain a linear chromosome. Northern (RNA) blot analyses demonstrated that the gene is transcriptionally expressed in all species examined. High levels of transcriptional expression were observed in some B. hermsii isolates. Transcriptional start site analyses revealed that the length of the untranslated leader sequence was identical to that observed in the Lyme disease spirochete species. By Western blotting (immunoblotting) with antiserum (polyclonal) raised against the OspC protein of B. burgdorferi, we detected an immunoreactive protein of the same molecular weight as the OspC found in Lyme disease spirochete species. The results presented here demonstrate the presence of a protein that is genetically and antigenically related to OspC which is expressed in all species of the genus Borrelia tested
The prolate-to-oblate shape transition of phospholipid vesicles in response to frequency variation of an AC electric field can be explained by the dielectric anisotropy of a phospholipid bilayer
The external electric field deforms flaccid phospholipid vesicles into
spheroidal bodies, with the rotational axis aligned with its direction.
Deformation is frequency dependent: in the low frequency range (~ 1 kHz), the
deformation is typically prolate, while increasing the frequency to the 10 kHz
range changes the deformation to oblate. We attempt to explain this behaviour
with a theoretical model, based on the minimization of the total free energy of
the vesicle. The energy terms taken into account include the membrane bending
energy and the energy of the electric field. The latter is calculated from the
electric field via the Maxwell stress tensor, where the membrane is modelled as
anisotropic lossy dielectric. Vesicle deformation in response to varying
frequency is calculated numerically. Using a series expansion, we also derive a
simplified expression for the deformation, which retains the frequency
dependence of the exact expression and may provide a better substitute for the
series expansion used by Winterhalter and Helfrich, which was found to be valid
only in the limit of low frequencies. The model with the anisotropic membrane
permittivity imposes two constraints on the values of material constants:
tangential component of dielectric permittivity tensor of the phospholipid
membrane must exceed its radial component by approximately a factor of 3; and
the membrane conductivity has to be relatively high, approximately one tenth of
the conductivity of the external aqueous medium.Comment: 17 pages, 6 figures; accepted for publication in J. Phys.: Condens.
Matte
Efficient Photon Upconversion Enabled by Strong Coupling Between Organic Molecules and Quantum Dots
Hybrid structures formed between organic molecules and inorganic quantum dots
can accomplish unique photophysical transformations by taking advantage of
their disparate properties. The electronic coupling between these materials is
typically weak, leading photoexcited charge carriers to spatially localize to a
dot or a molecule at its surface. However, we show that by converting a
chemical linker that covalently binds anthracene molecules to silicon quantum
dots from a carbon-carbon single bond to a double bond, we access a
strong-coupling regime where excited carriers spatially delocalize across both
anthracene and silicon. By pushing the system to delocalize, we design a photon
upconversion system with a higher efficiency (17.2%) and lower threshold
intensity (0.5 W/cm^2) than that of a corresponding weakly-coupled system. Our
results show that strong coupling between molecules and nanostructures achieved
through targeted linking chemistry provides a new route for tailoring
properties in materials for light-driven applications.Comment: 33 pages (20 in main text, 13 in supporting information), 12 figures
(5 in main text, 7 in supporting information
Brown dwarfs in the Hyades and beyond?
We have used both the Low-Resolution Imaging Spectrograph and the HIRES
echelle spectrograph on the Keck telescopes to obtain spectra of twelve
candidate members of the Hyades cluster identified by Leggett and Hawkins
(1988, 1989). All of the objects are chromospherically-active, late-type
M-dwarfs, with H equivalent widths varying from 1 to 30\AA. Based on
our measured radial velocities, the level of stellar activity and other
spectroscopic features, only one of the twelve stars has properties consistent
with cluster membership. We consider how this result affects estimates of the
luminosity and mass function of the Hyades cluster. Five of the eleven field
stars have weak K I 7665/7699\AA and CaH absorption as compared with M-dwarf
standards of the same spectral type, suggesting a lower surface gravity. Two of
these sources, LH0416+14 and LH0419+15, exhibit significant lithium 6708 \AA
absorption. Based partly on parallax measurements by the US Naval Observatory
(Harris et al, 1998), we identify all five as likely to be young, pre-main
sequence objects in or near the Taurus-Auriga association at distances of
between 150 and 250 parsecs. A comparison with theoretical models of pre-main
sequence stars indicates masses of less than 0.05 M.Comment: to appear in AJ, January 1999; 34 pages, (Latex format), including 10
embedded postscript figures and two table
The Helium content and age of the Hyades: Constraints from five binary systems and Hipparcos parallaxes
We compare the accurate empirical mass-luminosity (M-L) relation based on
five Hyades binary systems to predictions of stellar models calculated with
various input parameters (helium, metallicity, age) or physics (mixing-length
ratio, model atmosphere, equation of state, microscopic diffusion). Models
based on a helium content Ysim0.28 inferred from the dydz enrichment law are
more than 3sigma beyond the observations, suggesting that the initial helium
abundance is lower than expected from its supersolar metallicity. With the
photometric metallicity (FeH=0.144pm0.013 dex, Grenon (2000) we derive
Y=0.255\pm0.009. Because of the (Y,FeH) degeneracy in the M-L plane, the
uncertainty grows to Delta Y=0.013 if the metallicity from spectroscopy is
adopted (FeH=0.14pm0.05 dex, Cayrel de Strobel et al 1997). We use these
results to discuss the Hertzsprung-Russell (HR) diagram of the Hyades, in the
(Mv,B-V) plane, based on the very precise Hipparcos dynamical parallaxes.
Present models fit the tight observed sequence very well except at low
temperatures. In the low mass region of the HR diagram sensitive to the
mixing-length parameter (aMLT), the slope of the main sequence (MS) suggests
that aMLT could decrease from a solar (or even supersolar) value at higher mass
to subsolar values at low mass, which is also supported by the modeling of the
vB22 M-L relation. We find that the discrepancy at low temperatures (B-V\gtrsim
1.2) remains, even if an improved equation of state or better model atmospheres
are used. Finally, we discuss the positions of the stars at turn-off in the
light of their observed rotation rates and we deduce that the maximum age of
the Hyades predicted by the present models is sim650 Myr.Comment: 14 pages, 13 figures, accepted for publication in A&
On the determination of oxygen abundances in chromospherically active stars
We discuss oxygen abundances derived from [O I] 6300 and the O I triplet in
stars spanning a wide range in chromospheric activity level, and show that
these two indicators yield increasingly discrepant results with higher
chromospheric/coronal activity measures. While the forbidden and permitted
lines give fairly consistent results for solar-type disk dwarfs, spuriously
high O I triplet abundances are observed in young Hyades and Pleiades stars, as
well as in individual components of RS CVn binaries (up to 1.8 dex). The
distinct behaviour of the [O I]-based abundances which consistently remain
near-solar suggests that this phenomenon mostly results from large departures
from LTE affecting the O I triplet at high activity level that are currently
unaccounted for, but also possibly from a failure to adequately model the
atmospheres of K-type stars. These results suggest that some caution should be
exercised when interpreting oxygen abundances in active binaries or young open
cluster stars.Comment: 8 pages, accepted for publication in A&
Dimethyl 2-[23-oxo-22,24-diphenyl-8,11,14-trioxa-25-azatetracyclo[19.3.1.02,7.015,20]pentacosa-2,4,6,15(20),16,18-hexaen-25-yl]but-2-enedioate
The title compound, C39H37NO8, is a product of the Michael addition of the cyclic secondary amine subunit of aza-14-crown-4 ether to dimethyl acetylenedicarboxylate. The piperidinone ring exhibits a distorted chair conformation and the dimethyl acetylenedicarboxylate fragment has a cis configuration with a dihedral angle of 56.61 (5)° between the two carboxylate groups. The crystal packing is stabilized by the weak C—H⋯O hydrogen bonds
- …