642 research outputs found
CARS spectroscopy of the () band in
Molecular hydrogen is a benchmark system for bound state quantum calculation
and tests of quantum electrodynamical effects. While spectroscopic measurements
on the stable species have progressively improved over the years, high
resolution studies on the radioactive isotopologues , and
have been limited. Here we present an accurate determination of
transition energies in the fundamental vibrational
band of the ground electronic state, by means of high resolution Coherent
Anti-Stokes Raman Spectroscopy. With the present experimental uncertainty of
, which is a fivefold improvement over previous
measurements, agreement with the latest theoretical calculations is
demonstrated.Comment: 9 pages, 3 figure
Optimisation of on-line principal component analysis
Different techniques, used to optimise on-line principal component analysis,
are investigated by methods of statistical mechanics. These include local and
global optimisation of node-dependent learning-rates which are shown to be very
efficient in speeding up the learning process. They are investigated further
for gaining insight into the learning rates' time-dependence, which is then
employed for devising simple practical methods to improve training performance.
Simulations demonstrate the benefit gained from using the new methods.Comment: 10 pages, 5 figure
Site-directed mutagenesis of conserved inverted repeat sequences in the xylanase C promoter region from Streptomyces sp EC3
Streptomyces sp. EC3, a strain which was originally isolated from cattle manure compost, was shown to possess a strong xylanolytic activity. One of the genes responsible for this activity, xlnC, encodes a secreted xylanase. In the native strain, as in the heterologous host S. lividans, expression of xlnC was detectable in the presence of xylan but not in the presence of glucose. Induction by xylan was shown to take place at the transcriptional level. The transcriptional start site of xlnC was mapped and likely -35 (5'-TTGACA-3') and -10 (5'-GAGAAC-3') motifs were identified. In order to localise putative conserved regulatory sequences, the promoter regions of xylanase-encoding genes from various Streptomyces species were aligned. This alignment revealed the existence of three sets of quite well conserved palindromic AT rich sequences called boxes 1, 2 and 3. Box 3 (5'-CGAAA N TTTCG-3') is the farthest away from the promoter region (150-200 bp). A shorter version of this palindrome (5'-GAAA NN TTTC-3') or (5'-CGAAA-3') constitutes box 1, which is located just upstream of the putative -35 promoter sequence. Box 2, located 5-7 bp upstream of box 1, comprises a shorter palindrome than box 3, with inverted polarity [5'-(G/C)TTTC (N) GAAA(G/C)-3']. The putative regulatory role of the conserved inverted repeats in boxes 2 and 3 in the promoter region of the xlnC gene from Streptomyces sp. EC3, was assessed. These boxes were modified by site-directed mutagenesis, and the mutant promoter regions, as well as the wild-type promoter region, were separately fused to a beta-lactamase reporter gene. Analysis of the expression patterns of these fusions in cultures grown in the presence of glucose, xylan or both carbon sources demonstrated that these motifs were cis -acting negative regulatory elements, each playing a specific role in the regulation of xlnC expression. Box 3 was shown to be critical for the establishment of repression of xlnC expression by glucose, whereas box 2 was shown to play an important role in the induction of xlnC expression by xylan.Peer reviewe
Relativistic and QED effects in the fundamental vibration of T
The hydrogen molecule has become a test ground for quantum electrodynamical
calculations in molecules. Expanding beyond studies on stable hydrogenic
species to the heavier radioactive tritium-bearing molecules, we report on a
measurement of the fundamental T vibrational splitting for rotational levels. Precision frequency metrology is performed
with high-resolution coherent anti-Stokes Raman spectroscopy at an experimental
uncertainty of ~MHz, where sub-Doppler saturation features are exploited
for the strongest transition. The achieved accuracy corresponds to a fifty-fold
improvement over a previous measurement, and allows for the extraction of
relativistic and QED contributions to T transition energies.Comment: 5 pages, 5 figure
Sheared bioconvection in a horizontal tube
The recent interest in using microorganisms for biofuels is motivation enough
to study bioconvection and cell dispersion in tubes subject to imposed flow. To
optimize light and nutrient uptake, many microorganisms swim in directions
biased by environmental cues (e.g. phototaxis in algae and chemotaxis in
bacteria). Such taxes inevitably lead to accumulations of cells, which, as many
microorganisms have a density different to the fluid, can induce hydrodynamic
instabilites. The large-scale fluid flow and spectacular patterns that arise
are termed bioconvection. However, the extent to which bioconvection is
affected or suppressed by an imposed fluid flow, and how bioconvection
influences the mean flow profile and cell transport are open questions. This
experimental study is the first to address these issues by quantifying the
patterns due to suspensions of the gravitactic and gyrotactic green
biflagellate alga Chlamydomonas in horizontal tubes subject to an imposed flow.
With no flow, the dependence of the dominant pattern wavelength at pattern
onset on cell concentration is established for three different tube diameters.
For small imposed flows, the vertical plumes of cells are observed merely to
bow in the direction of flow. For sufficiently high flow rates, the plumes
progressively fragment into piecewise linear diagonal plumes, unexpectedly
inclined at constant angles and translating at fixed speeds. The pattern
wavelength generally grows with flow rate, with transitions at critical rates
that depend on concentration. Even at high imposed flow rates, bioconvection is
not wholly suppressed and perturbs the flow field.Comment: 19 pages, 9 figures, published version available at
http://iopscience.iop.org/1478-3975/7/4/04600
Phase transitions in soft-committee machines
Equilibrium statistical physics is applied to layered neural networks with
differentiable activation functions. A first analysis of off-line learning in
soft-committee machines with a finite number (K) of hidden units learning a
perfectly matching rule is performed. Our results are exact in the limit of
high training temperatures. For K=2 we find a second order phase transition
from unspecialized to specialized student configurations at a critical size P
of the training set, whereas for K > 2 the transition is first order. Monte
Carlo simulations indicate that our results are also valid for moderately low
temperatures qualitatively. The limit K to infinity can be performed
analytically, the transition occurs after presenting on the order of N K
examples. However, an unspecialized metastable state persists up to P= O (N
K^2).Comment: 8 pages, 4 figure
Ultra-stable implanted 83Rb/83mKr electron sources for the energy scale monitoring in the KATRIN experiment
The KATRIN experiment aims at the direct model-independent determination of
the average electron neutrino mass via the measurement of the endpoint region
of the tritium beta decay spectrum. The electron spectrometer of the MAC-E
filter type is used, requiring very high stability of the electric filtering
potential. This work proves the feasibility of implanted 83Rb/83mKr calibration
electron sources which will be utilised in the additional monitor spectrometer
sharing the high voltage with the main spectrometer of KATRIN. The source
employs conversion electrons of 83mKr which is continuously generated by 83Rb.
The K-32 conversion line (kinetic energy of 17.8 keV, natural line width of 2.7
eV) is shown to fulfill the KATRIN requirement of the relative energy stability
of +/-1.6 ppm/month. The sources will serve as a standard tool for continuous
monitoring of KATRIN's energy scale stability with sub-ppm precision. They may
also be used in other applications where the precise conversion lines can be
separated from the low energy spectrum caused by the electron inelastic
scattering in the substrate.Comment: 30 pages, 10 figures, 1 table, minor revision of the preprint,
accepted by JINST on 5.2.201
Quantum Hall effect in a p-type heterojunction with a lateral surface quantum dot superlattice
The quantization of Hall conductance in a p-type heterojunction with lateral
surface quantum dot superlattice is investigated. The topological properties of
the four-component hole wavefunction are studied both in r- and k-spaces. New
method of calculation of the Hall conductance in a 2D hole gas described by the
Luttinger Hamiltonian and affected by lateral periodic potential is proposed,
based on the investigation of four-component wavefunction singularities in
k-space. The deviations from the quantization rules for Hofstadter "butterfly"
for electrons are found, and the explanation of this effect is proposed. For
the case of strong periodic potential the mixing of magnetic subbands is taken
into account, and the exchange of the Chern numbers between magnetic subands is
discussed.Comment: 12 pages, 5 figures; reported at the 15th Int. Conf. on High Magnetic
Fields in Semicond. Phys. (Oxford, UK, 2002
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