3,324 research outputs found
A Bose-Einstein Condensate in a Uniform Light-induced Vector Potential
We use a two-photon dressing field to create an effective vector gauge
potential for Bose-condensed Rb atoms in the F=1 hyperfine ground state. The
dressed states in this Raman field are spin and momentum superpositions, and we
adiabatically load the atoms into the lowest energy dressed state. The
effective Hamiltonian of these neutral atoms is like that of charged particles
in a uniform magnetic vector potential, whose magnitude is set by the strength
and detuning of Raman coupling. The spin and momentum decomposition of the
dressed states reveals the strength of the effective vector potential, and our
measurements agree quantitatively with a simple single-particle model. While
the uniform effective vector potential described here corresponds to zero
magnetic field, our technique can be extended to non-uniform vector potentials,
giving non-zero effective magnetic fields.Comment: 5 pages, submitted to Physical Review Letter
Developments in two dimensional arrays
A two dimensional array of individual millimeter wave antennas with detectors will be described. The array is placed on a substrate lens [1] in the focal plane of a primary lens to form an imaging system (Fig. 1.). Calculations which predict ideal efficiencies of over 90% will be presented. Fabrication of the array and preliminary measurements will also be discussed
Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM)
Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101Ì
4) cleavage surface (from natural âIceland Sparâ) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved âmirrorâ surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 ÎŒm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (± 1.3) Ă 10â4 m sâ1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites
Literacy practices of primary education children in Andalusia (Spain): a family-based perspective
Primary school children develop literacy practices in various domains and situations in everyday life.
This study focused on the analysis of literacy practices of children aged 8â12 years from the perspec-
tive of their families. 1,843 families participated in the non-experimental explanatory study. The
children in these families speak Spanish as a first language and are schooled in this language. The
instrument used was a self-report questionnaire about childrenâs home-literacy practices. The data
obtained were analysed using categorical principal components analysis (CATPCA) and analysis of
variance (ANOVA). The results show the complex relationship between literacy practices developed
by children in the domains of home and school and the limited development of a literacy-promoting
âthird spaceâ. In conclusion, the families in our study had limited awareness of their role as literacy-
promoting agents and thought of literacy learning as restricted to formal or academic spaces
Variability of M giant stars based on Kepler photometry: general characteristics
M giants are among the longest-period pulsating stars which is why their
studies were traditionally restricted to analyses of low-precision visual
observations, and more recently, accurate ground-based data. Here we present an
overview of M giant variability on a wide range of time-scales (hours to
years), based on analysis of thirteen quarters of Kepler long-cadence
observations (one point per every 29.4 minutes), with a total time-span of over
1000 days. About two-thirds of the sample stars have been selected from the
ASAS-North survey of the Kepler field, with the rest supplemented from a
randomly chosen M giant control sample.
We first describe the correction of the light curves from different quarters,
which was found to be essential. We use Fourier analysis to calculate multiple
frequencies for all stars in the sample. Over 50 stars show a relatively strong
signal with a period equal to the Kepler-year and a characteristic phase
dependence across the whole field-of-view. We interpret this as a so far
unidentified systematic effect in the Kepler data. We discuss the presence of
regular patterns in the distribution of multiple periodicities and amplitudes.
In the period-amplitude plane we find that it is possible to distinguish
between solar-like oscillations and larger amplitude pulsations which are
characteristic for Mira/SR stars. This may indicate the region of the
transition between two types of oscillations as we move upward along the giant
branch.Comment: 12 pages, 13 figures, accepted for publication in MNRAS. The
normalized light curves are available upon reques
Deeply Virtual Compton Scattering
We study in QCD the physics of deeply-virtual Compton scattering (DVCS)---the
virtual Compton process in the large s and small t kinematic region. We show
that DVCS can probe a new type of off-forward parton distributions. We derive
an Altarelli-Parisi type of evolution equations for these distributions. We
also derive their sum rules in terms of nucleon form-factors of the twist-two
quark and gluon operators. In particular, we find that the second sum rule is
related to fractions of the nucleon spin carried separately by quarks and
gluons. We estimate the cross section for DVCS and compare it with the
accompanying Bethe-Heitler process at CEBAF and HERMES kinematics.Comment: 20 pages, 2 figures, replaced with the version to appear in Phys.
Rev.
Rapid production of Rb BECs in a combined magnetic and optical potential
We describe an apparatus for quickly and simply producing \Rb87
Bose-Einstein condensates. It is based on a magnetic quadrupole trap and a red
detuned optical dipole trap. We collect atoms in a magneto-optical trap (MOT)
and then capture the atom in a magnetic quadrupole trap and force rf
evaporation. We then transfer the resulting cold, dense cloud into a spatially
mode-matched optical dipole trap by lowering the quadrupole field gradient to
below gravity. This technique combines the efficient capture of atoms from a
MOT into a magnetic trap with the rapid evaporation of optical dipole traps;
the approach is insensitive to the peak quadrupole gradient and the precise
trapping beam waist. Our system reliably produces a condensate with
atoms every 16\second
First upper limit analysis and results from LIGO science data: stochastic background
I describe analysis of correlations in the outputs of the three LIGO
interferometers from LIGO's first science run, held over 17 days in August and
September of 2002, and the resulting upper limit set on a stochastic background
of gravitational waves. By searching for cross-correlations between the LIGO
detectors in Livingston, LA and Hanford, WA, we are able to set a 90%
confidence level upper limit of h_{100}^2 Omega_0 < 23 +/- 4.6.Comment: 7 pages; 1 eps figures; proceeding from 2003 Edoardo Amaldi Meeting
on Gravitational Wave
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