88,266 research outputs found
Mapping the Dirac point in gated bilayer graphene
We have performed low temperature scanning tunneling spectroscopy
measurements on exfoliated bilayer graphene on SiO2. By varying the back gate
voltage we observed a linear shift of the Dirac point and an opening of a band
gap due to the perpendicular electric field. In addition to observing a shift
in the Dirac point, we also measured its spatial dependence using spatially
resolved scanning tunneling spectroscopy. The spatial variation of the Dirac
point was not correlated with topographic features and therefore we attribute
its shift to random charged impurities.Comment: 3 pages, 3 figure
Direct Measurement of Quantum Dot Spin Dynamics using Time-Resolved Resonance Fluorescence
We temporally resolve the resonance fluorescence from an electron spin
confined to a single self-assembled quantum dot to measure directly the spin's
optical initialization and natural relaxation timescales. Our measurements
demonstrate that spin initialization occurs on the order of microseconds in the
Faraday configuration when a laser resonantly drives the quantum dot
transition. We show that the mechanism mediating the optically induced
spin-flip changes from electron-nuclei interaction to hole-mixing interaction
at 0.6 Tesla external magnetic field. Spin relaxation measurements result in
times on the order of milliseconds and suggest that a magnetic field
dependence, due to spin-orbit coupling, is sustained all the way down to 2.2
Tesla.Comment: An additional EPAPS file in PDF format is available for download at
the publications section of our website
http://www.amop.phy.cam.ac.uk/amop-ma
Magnitude bias of microlensed sources towards the Large Magellanic Cloud
There are lines of evidence suggesting that some of the observed microlensing
events in the direction of the Large Magellanic Cloud (LMC) are caused by
ordinary star lenses as opposed to dark Machos in the Galactic halo. Efficient
lensing by ordinary stars generally requires the presence of one or more
additional concentrations of stars along the line of sight to the LMC disk. If
such a population behind the LMC disk exists, then the source stars (for
lensing by LMC disk objects) will be drawn preferentially from the background
population and will show systematic differences from LMC field stars. One such
difference is that the (lensed) source stars will be farther away than the
average LMC field stars, and this should be reflected in their apparent
baseline magnitudes. We focus on red clump stars: these should appear in the
color-magnitude diagram at a few tenths of a magnitude fainter than the field
red clump. Suggestively, one of the two near-clump confirmed events,
MACHO-LMC-1, is a few tenths of magnitude fainter than the clump.Comment: To appear in ApJ Letters. Shortened to match the accepted version, 8
pages plus 1 ps figur
An ecological approach to problems of Dark Energy, Dark Matter, MOND and Neutrinos
Modern astronomical data on galaxy and cosmological scales have revealed
powerfully the existence of certain dark sectors of fundamental physics, i.e.,
existence of particles and fields outside the standard models and inaccessible
by current experiments. Various approaches are taken to modify/extend the
standard models. Generic theories introduce multiple de-coupled fields A, B, C,
each responsible for the effects of DM (cold supersymmetric particles), DE
(Dark Energy) effect, and MG (Modified Gravity) effect respectively. Some
theories use adopt vanilla combinations like AB, BC, or CA, and assume A, B, C
belong to decoupled sectors of physics. MOND-like MG and Cold DM are often
taken as opposite frameworks, e.g. in the debate around the Bullet Cluster.
Here we argue that these ad hoc divisions of sectors miss important clues from
the data. The data actually suggest that the physics of all dark sectors is
likely linked together by a self-interacting oscillating field, which governs a
chameleon-like dark fluid, appearing as DM, DE and MG in different settings. It
is timely to consider an interdisciplinary approach across all semantic
boundaries of dark sectors, treating the dark stress as one identity, hence
accounts for several "coincidences" naturally.Comment: 12p, Proceedings to the 6-th Int. Conf. of Gravitation and Cosmology.
Neutrino section expande
Giant spin-dependent photo-conductivity in GaAsN dilute nitride semiconductor
A theoretical and experimental study of the spin-dependent photoconductivity
in dilute Nitride GaAsN is presented. The non linear transport model we develop
here is based on the rate equations for electrons, holes, deep paramagnetic and
non paramagnetic centers both under CW and pulsed optical excitation. Emphasis
is given to the effect of the competition between paramagnetic centers and non
paramagnetic centers which allows us to reproduce the measured characteristics
of the spin-dependent recombination power dependence. Particular attention is
paid to the role of an external magnetic field in Voigt geometry. The
photoconductivity exhibits a Hanle-type curve whereas the spin polarization of
electrons shows two superimposed Lorentzian curves with different widths,
respectively related to the recombination of free and trapped electrons. The
model is capable of reproducing qualitatively and quantitatively the most
important features of photoluminescence and photocurrent experiments and is
helpful in providing insight on the various mechanisms involved in the electron
spin polarization and filtering in GaAsN semiconductors.Comment: 10 pages, 5 figure
Many-body Systems Interacting via a Two-body Random Ensemble (I): Angular Momentum distribution in the ground states
In this paper, we discuss the angular momentum distribution in the ground
states of many-body systems interacting via a two-body random ensemble.
Beginning with a few simple examples, a simple approach to predict P(I)'s,
angular momenta I ground state (g.s.) probabilities, of a few solvable cases,
such as fermions in a small single-j shell and d boson systems, is given. This
method is generalized to predict P(I)'s of more complicated cases, such as even
or odd number of fermions in a large single-j shell or a many-j shell, d-boson,
sd-boson or sdg-boson systems, etc. By this method we are able to tell which
interactions are essential to produce a sizable P(I) in a many-body system. The
g.s. probability of maximum angular momentum is discussed. An
argument on the microscopic foundation of our approach, and certain matrix
elements which are useful to understand the observed regularities, are also
given or addressed in detail. The low seniority chain of 0 g.s. by using the
same set of two-body interactions is confirmed but it is noted that
contribution to the total 0 g.s. probability beyond this chain may be more
important for even fermions in a single-j shell. Preliminary results by taking
a displaced two-body random ensemble are presented for the I g.s.
probabilities.Comment: 39 pages and 8 figure
Decolorization of dyes by recombinase CotA from Escherichia coli BL21 (DE3) and characterization of the purified enzyme
Dyes are usually difficult to be decolorized due to their complex chemical structures. In this work, recombinant CotA laccase was purified from Escherichia coli to evaluate its application in dye decolorization. Factors influencing laccase expression, such as induction temperature, phosphate buffer (pH), copper concentration and isopropyl ÎČ-D-1-thiogalactopyranoside (IPTG) concentration, were investigated. The recombinant laccase was purified to electrophoretic homogeneity, and was estimated to have a molecular mass about 67.5 kDa. The CotA protein was stained red by syringaldazine after Native-PAGE. The purified enzyme showed a similar behaviour to the spores laccase produced by Bacillus subtilis WD23. Using syringaldazine as the substrate to determine the CotA laccase activity, the optimum pH and temperature were 7.2 and 45°C, respectively. High laccase activity was maintained in a pH range from 6.0~7.6. The temperature half-life of the CotA laccases was 95 min at 80°C. The pH half-life was 8 h at pH 9.0 he CotA laccase was strongly inhibited by ethylenediaminetetraacetic acid (EDTA) and methyl alcohol. The CotA laccase could efficiently decolorize anthraquinone and azo dyes in 24 h. The decolourization capacity of this recombinant laccase suggested that it could be a useful biocatalyst for the treatment of dye-containing effluents.Key words: Recombinant CotA laccase, Escherichia coli, purification, dye decolorization
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