1,744 research outputs found
Response of Bose gases in time-dependent optical superlattices
The dynamic response of ultracold Bose gases in one-dimensional optical
lattices and superlattices is investigated based on exact numerical time
evolutions in the framework of the Bose-Hubbard model. The system is excited by
a temporal amplitude modulation of the lattice potential, as it was done in
recent experiments. For regular lattice potentials, the dynamic signatures of
the superfluid to Mott-insulator transition are studied and the position and
the fine-structure of the resonances is explained by a linear response
analysis. Using direct simulations and the perturbative analysis it is shown
that in the presence of a two-colour superlattice the excitation spectrum
changes significantly when going from the homogeneous Mott-insulator the quasi
Bose-glass phase. A characteristic and experimentally accessible signature for
the quasi Bose-glass is the appearance of low-lying resonances and a
suppression of the dominant resonance of the Mott-insulator phase.Comment: 20 pages, 9 figures; added references and corrected typo
Do balances of opinion summarize at the best firms answers to business surveys?
Business surveys aim at getting, as quickly and as simply as possible, the recent and probable changes in economic activity. Answers to most of the questions are qualitative with three modalities (qualification of an evolution: up / stable / down). The qualitative indicators provided by these surveys are summed up in the form of balances of opinion equal to the difference between the percentage of answers in the upper modality and those in the lower one. However, such indicators do really summarise firms answers only if the percentage of answers to the modality stable are almost constant. The purpose of this paper is to sum up the answers to business surveys in a single indicator which takes into account the percentage of answers to the modality stable. This indicator is constructed from a principal component analysis which gives the linear combination of the three percentages capturing most variability between the sucessive surveys. Very close for some questions, the two summaries of answers appreciably diverge for other questions and can sometimes lead to a different short-term economic analysis.Business surveys, balance of opinion, principal component analysis, Granger causality, short-term forecast, VAR models
French inflation forecasts
In this paper, we estimate and analyse a set of equations of French inflation for forecasting purpose at the horizon of three months, six months and one year. A different equation is associated to each horizon. This approach has the advantage of modeling directly the variable of interest at the desired horizon without resorting to assumed forecast paths of the exogeneous variables outside the sampling period: given the history at time t, one forecasts the value of inflation at time t+h. The study of historical properties of those equations with real-time datatsets when possible shows that the forecast errors, for the last four years, are about 0.25 point for the 3-month and the 6-month horizon and about 0.5 point for the 12-month horizon.Inflation, short-term forecast, cointegration
Bose-Fermi mixtures in 1D optical superlattices
The zero temperature phase diagram of binary boson-fermion mixtures in
two-colour superlattices is investigated. The eigenvalue problem associated
with the Bose-Fermi-Hubbard Hamiltonian is solved using an exact numerical
diagonalization technique, supplemented by an adaptive basis truncation scheme.
The physically motivated basis truncation allows to access larger systems in a
fully controlled and very flexible framework. Several experimentally relevant
observables, such as the matter-wave interference pattern and the
condensatefraction, are investigated in order to explore the rich phase
diagram. At symmetric half filling a phase similar to the Mott-insulating phase
in a commensurate purely bosonic system is identified and an analogy to recent
experiments is pointed out. Furthermore a phase of complete localization of the
bosonic species generated by the repulsive boson-fermion interaction is
identified. These localized condensates are of a different nature than the
genuine Bose-Einstein condensates in optical lattices.Comment: 18 pages, 9 figure
Review of the Laguerre-Gauss mode technology research program at Birmingham
Gravitational wave detectors from the advanced generation onwards are
expected to be limited in sensitivity by thermal noise of the optics, making
the reduction of this noise a key factor in the success of such detectors. A
proposed method for reducing the impact of this noise is to use higher-order
Laguerre-Gauss (LG) modes for the readout beam, as opposed to the currently
used fundamental mode. We present here a synopsis of the research program
undertaken by the University of Birmingham into the suitability of LG mode
technology for future gravitational wave detectors. This will cover our
previous and current work on this topic, from initial simulations and table-top
LG mode experiments up to implementation in a prototype scale suspended cavity
and high-power laser bench
Charge redistribution in electrochemically-actuated mechanical sensors
International audienceMany proofs of concept studies have established the mechanical sensitivity of functionalized microcantilevers to a large spectrum of target molecules. However, moving to real-life applications also requires the monitored mechanical effect to be highly specific. Moving towards more specificity in cantilever-based sensing, monitoring the mechanical response of electrochemically actuated microcantilevers is then thought to provide a fast, reliable and complementary experimental information to the long-time cantilever bending measurement for the detection of target molecules. Full-field measurements are therefore used to investigate the way the electro-elastic coupling is altered when a microcantilever undergoes decane-thiol adsorption. The proposed technique reveals that the latter results in a charge density redistribution along the cantilever in addition to the local surface passivation. Focusing on the cantilever tip displacement under electrochemical actuation, this redistribution partially compensates the electro-elastic coupling alteration due to the surface passivation, therefore possibly yielding an ambiguous detection result. This effect should be taken into account for the optimal design of specific electrochemically actuated mechanical sensors
Photon pressure induced test mass deformation in gravitational-wave detectors
A widely used assumption within the gravitational-wave community has so far
been that a test mass acts like a rigid body for frequencies in the detection
band, i.e. for frequencies far below the first internal resonance. In this
article we demonstrate that localized forces, applied for example by a photon
pressure actuator, can result in a non-negligible elastic deformation of the
test masses. For a photon pressure actuator setup used in the gravitational
wave detector GEO600 we measured that this effect modifies the standard
response function by 10% at 1 kHz and about 100% at 2.5 kHz
The upgrade of GEO600
The German / British gravitational wave detector GEO 600 is in the process of
being upgraded. The upgrading process of GEO 600, called GEO-HF, will
concentrate on the improvement of the sensitivity for high frequency signals
and the demonstration of advanced technologies. In the years 2009 to 2011 the
detector will undergo a series of upgrade steps, which are described in this
paper.Comment: 9 pages, Amaldi 8 conference contributio
3D Digital Volume Correlation of Synchrotron Radiation Laminography images of ductile crack initiation: An initial feasibility study
International audienceA feasibility study of measuring 3D displacement fields in the bulk during ductile crack initiation via combined Synchrotron Radiation Computed Laminography (SRCL) and Digital Volume Correlation (DVC) is performed. In contrast to Tomography, SRCL is a technique that is particularly adapted to obtain in 3D reconstructed volumes of objects that are laterally extended (i.e., in 2 directions) and thin in the third direction, i.e. sheet-like objects. In-situ laminography data of an initiating crack ahead of a machined notch are used with a voxel size of 0.7 ÎĽm. The natural contrast of the observed 2XXX Al-alloy caused by intermetallic particles and initial porosity is used to measure displacement fields via a global DVC technique assuming a continuous displacement field. An initial performance study is carried out on data of the same undeformed material but after a substantial shift of the laminography rotation axis with respect to the imaged specimen. Volume correlations between different loading steps provide displacement fields that are qualitatively consistent with the remote loading conditions. Computed strain fields display a strain concentration close to the notch tip
DC-readout of a signal-recycled gravitational wave detector
All first-generation large-scale gravitational wave detectors are operated at
the dark fringe and use a heterodyne readout employing radio frequency (RF)
modulation-demodulation techniques. However, the experience in the currently
running interferometers reveals several problems connected with a heterodyne
readout, of which phase noise of the RF modulation is the most serious one. A
homodyne detection scheme (DC-readout), using the highly stabilized and
filtered carrier light as local oscillator for the readout, is considered to be
a favourable alternative. Recently a DC-readout scheme was implemented on the
GEO 600 detector. We describe the results of first measurements and give a
comparison of the performance achieved with homodyne and heterodyne readout.
The implications of the combined use of DC-readout and signal-recycling are
considered.Comment: 11 page
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