8,636 research outputs found
An atomic clock with instability
Atomic clocks have been transformational in science and technology, leading
to innovations such as global positioning, advanced communications, and tests
of fundamental constant variation. Next-generation optical atomic clocks can
extend the capability of these timekeepers, where researchers have long aspired
toward measurement precision at 1 part in . This milestone will
enable a second revolution of new timing applications such as relativistic
geodesy, enhanced Earth- and space-based navigation and telescopy, and new
tests on physics beyond the Standard Model. Here, we describe the development
and operation of two optical lattice clocks, both utilizing spin-polarized,
ultracold atomic ytterbium. A measurement comparing these systems demonstrates
an unprecedented atomic clock instability of after
only hours of averaging
Precision measurement of light shifts at two off-resonant wavelengths in a single trapped Ba+ ion and determination of atomic dipole matrix elements
We define and measure the ratio (R) of the vector ac-Stark effect (or light
shift) in the 6S_1/2 and 5D_3/2 states of a single trapped barium ion to 0.2%
accuracy at two different off-resonant wavelengths. We earlier found R =
-11.494(13) at 514.531nm and now report the value at 1111.68nm, R = +0.4176(8).
These observations together yield a value of the matrix element,
previously unknown in the literature. Also, comparison of our results with an
ab initio calculation of dynamic polarizability would yield a new test of
atomic theory and improve the understanding of atomic structure needed to
interpret a proposed atomic parity violation experiment.Comment: 12 pages, 11 figures, in submission to PR
Anomalous electronic Raman scattering in Na_xCoO_2 H_2O
Raman scattering experiments on Na_{x}CoO_2 yH_2O single crystals show a
broad electronic continuum with a pronounced peak around 100 cm-1 and a cutoff
at approximately 560 cm-1over a wide range of doping levels. The electronic
Raman spectra in superconducting and non-superconducting samples are similar at
room temperature, but evolve in markedly different ways with decreasing
temperature. For superconducting samples, the low-energy spectral weight is
depleted upon cooling below T* sim 150K, indicating a opening of a pseudogap
that is not present in non-superconducting materials. Weak additional phonon
modes observed below T* suggest that the pseudogap is associated with charge
ordering.Comment: 5 pages, 4 figures, for further information see www.peter-lemmens.d
Linearized model Fokker-Planck collision operators for gyrokinetic simulations. II. Numerical implementation and tests
A set of key properties for an ideal dissipation scheme in gyrokinetic
simulations is proposed, and implementation of a model collision operator
satisfying these properties is described. This operator is based on the exact
linearized test-particle collision operator, with approximations to the
field-particle terms that preserve conservation laws and an H-Theorem. It
includes energy diffusion, pitch-angle scattering, and finite Larmor radius
effects corresponding to classical (real-space) diffusion. The numerical
implementation in the continuum gyrokinetic code GS2 is fully implicit and
guarantees exact satisfaction of conservation properties. Numerical results are
presented showing that the correct physics is captured over the entire range of
collisionalities, from the collisionless to the strongly collisional regimes,
without recourse to artificial dissipation.Comment: 13 pages, 8 figures, submitted to Physics of Plasmas; typos fixe
Approximate Analytic Solution for the Spatiotemporal Evolution of Wave Packets undergoing Arbitrary Dispersion
We apply expansion methods to obtain an approximate expression in terms of
elementary functions for the space and time dependence of wave packets in a
dispersive medium. The specific application to pulses in a cold plasma is
considered in detail, and the explicit analytic formula that results is
provided. When certain general initial conditions are satisfied, these
expressions describe the packet evolution quite well. We conclude by employing
the method to exhibit aspects of dispersive pulse propagation in a cold plasma,
and suggest how predicted and experimental effects may be compared to improve
the theoretical description of a medium's dispersive properties.Comment: 17 pages, 4 figures, RevTe
Evaluating the Potential Efficacy of Invasive Lionfish (Pterois volitans) Removals
The lionfish, Pterois volitans (Linnaeus) and Pterois miles (Bennett), invasion of the Western Atlantic Ocean, Caribbean Sea and Gulf of Mexico has the potential to alter aquatic communities and represents a legitimate ecological concern. Several local removal programs have been initiated to control this invasion, but it is not known whether removal efforts can substantially reduce lionfish numbers to ameliorate these concerns. We used an age-structured population model to evaluate the potential efficacy of lionfish removal programs and identified critical data gaps for future studies. We used high and low estimates for uncertain parameters including: length at 50% vulnerability to harvest (Lvul), instantaneous natural mortality (M), and the Goodyear compensation ratio (CR). The model predicted an annual exploitation rate between 35 and 65% would be required to cause recruitment overfishing on lionfish populations for our baseline parameter estimates for M and CR (0.5 and 15). Lionfish quickly recovered from high removal rates, reaching 90% of unfished biomass six years after a 50-year simulated removal program. Quantifying lionfish natural mortality and the size-selective vulnerability to harvest are the most important knowledge gaps for future research. We suggest complete eradication of lionfish through fishing is unlikely, and substantial reduction of adult abundance will require a long-term commitment and may be feasible only in small, localized areas where annual exploitation can be intense over multiple consecutive years
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