2,108 research outputs found
Ginzburg-Landau theory of crystalline anisotropy for bcc-liquid interfaces
The weak anisotropy of the interfacial free-energy is a crucial
parameter influencing dendritic crystal growth morphologies in systems with
atomically rough solid-liquid interfaces. The physical origin and quantitative
prediction of this anisotropy are investigated for body-centered-cubic (bcc)
forming systems using a Ginzburg-Landau theory where the order parameters are
the amplitudes of density waves corresponding to principal reciprocal lattice
vectors. We find that this theory predicts the correct sign,
, and magnitude, , of this anisotropy in good agreement
with the results of MD simulations for Fe. The results show that the
directional dependence of the rate of spatial decay of solid density waves into
the liquid, imposed by the crystal structure, is a main determinant of
anisotropy. This directional dependence is validated by MD computations of
density wave profiles for different reciprocal lattice vectors for
crystal faces. Our results are contrasted with the prediction of the reverse
ordering from an earlier formulation of
Ginzburg-Landau theory [Shih \emph{et al.}, Phys. Rev. A {\bf 35}, 2611
(1987)].Comment: 9 pages, 5 figure
Direct excitation of the forbidden clock transition in neutral 174Yb atoms confined to an optical lattice
We report direct single-laser excitation of the strictly forbidden
(6s^2)^1S_0 -(6s6p)^3P_0 clock transition in the even 174Yb isotope confined to
a 1D optical lattice. A small (~1.2 mT) static magnetic field was used to
induce a nonzero electric dipole transition probability between the clock
states at 578.42 nm. Narrow resonance linewidths of 20 Hz (FHWM) with high
contrast were observed, demonstrating a record neutral-atom resonance quality
factor of 2.6x10^13. The previously unknown ac Stark shift-canceling (magic)
wavelength was determined to be 759.35+/-0.02 nm. This method for using the
metrologically superior even isotope can be easily implemented in current Yb
and Sr lattice clocks, and can create new clock possibilities in other alkaline
earth-like atoms such as Mg and Ca.Comment: Submitted to Physics Review Letter
Weight Beliefs and Messages: Mindsets Predict Body-Shame and Anti-Fat Attitudes via Attributions
In two samples (N=247, N= 291), we examined the link between beliefs and messages about the changeable (incremental theory) vs. fixed (entity theory) nature of weight, attributions for weight, and body shame. We recruited participants using online sampling, employing a correlational design in Study 1 and an experimental design in Study 2. Across both studies, we found evidence for the stigma-asymmetry effect—incremental, relative to entity beliefs/messages of weight predicted both (a) stronger onset responsibility attributions, indirectly increasing body shame and (b) stronger offset efficacy attributions, indirectly decreasing body shame. Study 2 replicated the stigma-asymmetry effect with anti-fat attitudes. We discuss implications for public health obesity messages with the goal of reducing stigma
Mice expressing HLA-DQ6α8β transgenes develop polychondritis spontaneously
Relapsing polychondritis (RP) is a human autoimmune disease of unknown etiology in which cartilaginous sites are destroyed by cyclic inflammatory episodes beginning, most commonly, during the fourth or fifth decade of life. We have previously described collagen-induced polychondritis that closely mirrors RP occurring in young (6–8 weeks old) HLA-DQ6αβ8αβ transgenic Aβ0 mice, following immunization with heterologous type II collagen (CII). We present evidence here that transgenic strains expressing the DQ6α8β transgene develop spontaneous polychondritis (SP) at the mouse equivalent of human middle age (4.5–6 months and 40–50 years old, respectively) and display polyarthritis, auricular chondritis and nasal chondritis – three of the most common sites affected in RP. Auricular chondritis in SP, like RP but unlike CII-induced polychondritis, exhibited a relapsing/remitting phenotype, requiring several inflammatory cycles before the cartilage is destroyed. Elevated serum levels of total IgG corresponded with the onset of disease in SP, as in RP and CII-induced polychondritis. No CII-specific immune response was detected in SP, however – more closely mirroring RP, in which as few as 30% of RP patients have been reported to have CII-specific IgG. CII-induced polychondritis displays a strong CII-specific immune response. SP also demonstrated a strong female preponderance, as some workers have reported in RP but has not observed in CII-induced polychondritis. These characteristics of SP allow for the examination of the immunopathogenesis of polychondritis in the absence of an overwhelming CII-specific immune response and the strong adjuvant-induced immunostimulatory influence in CII-induced polychondritis. This spontaneous model of polychondritis provides a new and unique tool to investigate both the initiatory events as well as the immunopathogenic mechanisms occurring at cartilaginous sites during the cyclic inflammatory assaults of polychondritis
Capillary force-induced structural instability in liquid infiltrated elastic circular tubes
The capillary-induced structural instability of an elastic circular tube
partially filled by a liquid is studied by combining theoretical analysis and
molecular dynamics simulations. The analysis shows that, associated with the
instability, there is a well-defined length scale (elasto-capillary length),
which exhibits a scaling relationship with the characteristic length of the
tube, regardless of the interaction details. We validate this scaling
relationship for a carbon nanotube partially filled by liquid iron. The
capillary-induced structural transformation could have potential applications
for nano-devices
Invisible sunspots and rate of solar magnetic flux emergence
Aims.
We study the visibility of sunspots and its influence on observed values of sunspot region parameters.
Methods.
We use Virtual Observatory tools provided by AstroGrid to analyse a sample of 6862 sunspot regions. By studying the distributions of locations where sunspots were first and last observed on the solar disk, we derive the visibility function of sunspots, the rate of magnetic flux emergence and the ratio between the durations of growth and decay phases of solar active regions.
Results.
We demonstrate that the visibility of small sunspots has a strong centre-to-limb variation, far larger than would be expected from geometrical (projection) effects. This results in a large number of young spots being invisible: 44% of new regions emerging in the west of the Sun go undetected. For sunspot regions that are detected, large differences exist between actual locations and times of flux emergence, and the apparent ones derived from sunspot data. The duration of the growth phase of solar regions has been, up to now, underestimated
Optical Lattice Induced Light Shifts in an Yb Atomic Clock
We present an experimental study of the lattice induced light shifts on the
1S_0-3P_0 optical clock transition (v_clock~518 THz) in neutral ytterbium. The
``magic'' frequency, v_magic, for the 174Yb isotope was determined to be 394
799 475(35)MHz, which leads to a first order light shift uncertainty of 0.38 Hz
on the 518 THz clock transition. Also investigated were the hyperpolarizability
shifts due to the nearby 6s6p 3P_0 - 6s8p 3P_0, 6s8p 3P_2, and 6s5f 3F_2
two-photon resonances at 759.708 nm, 754.23 nm, and 764.95 nm respectively. By
tuning the lattice frequency over the two-photon resonances and measuring the
corresponding clock transition shifts, the hyperpolarizability shift was
estimated to be 170(33) mHz for a linear polarized, 50 uK deep, lattice at the
magic wavelength. In addition, we have confirmed that a circularly polarized
lattice eliminates the J=0 - J=0 two-photon resonance. These results indicate
that the differential polarizability and hyperpolarizability frequency shift
uncertainties in a Yb lattice clock could be held to well below 10^-17.Comment: Accepted to PR
Observation and absolute frequency measurements of the 1S0 - 3P0 optical clock transition in ytterbium
We report the direct excitation of the highly forbidden (6s^2) 1S0 - (6s6p)
3P0 optical transition in two odd isotopes of ytterbium. As the excitation
laser frequency is scanned, absorption is detected by monitoring the depletion
from an atomic cloud at ~70 uK in a magneto-optical trap. The measured
frequency in 171Yb (F=1/2) is 518,295,836,593.2 +/- 4.4 kHz. The measured
frequency in 173Yb (F=5/2) is 518,294,576,850.0 +/- 4.4 kHz. Measurements are
made with a femtosecond-laser frequency comb calibrated by the NIST cesium
fountain clock and represent nearly a million-fold reduction in uncertainty.
The natural linewidth of these J=0 to J=0 transitions is calculated to be ~10
mHz, making them well-suited to support a new generation of optical atomic
clocks based on confinement in an optical lattice.Comment: 4 pages, 3 figure
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