479 research outputs found
High-accuracy optical clock based on the octupole transition in 171Yb+
We experimentally investigate an optical frequency standard based on the 467
nm (642 THz) electric-octupole reference transition 2S1/2(F=0) -> F7/2(F=3) in
a single trapped 171Yb+ ion. The extraordinary features of this transition
result from the long natural lifetime and from the 4f136s2 configuration of the
upper state. The electric quadrupole moment of the 2F7/2 state is measured as
-0.041(5) e(a0)^2, where e is the elementary charge and a0 the Bohr radius. We
also obtain information on the differential scalar and tensorial components of
the static polarizability and of the probe light induced ac Stark shift of the
octupole transition. With a real-time extrapolation scheme that eliminates this
shift, the unperturbed transition frequency is realized with a fractional
uncertainty of 7.1x10^(-17). The frequency is measured as 642 121 496 772
645.15(52) Hz.Comment: 5 pages, 4 figure
Chemical and ecological health of white sucker (Catostomus commersoni) in Rock Creek Park, Washington, D.C., 2003-04 / by Cherie V. Miller ... [et al.] ; prepared in copoeration with the National Park Service
Several classes of chemicals that are known or suspected contaminants were found in bed sediment in Rock Creek, including polyaromatic hydrocarbons (PAHs), phthalate esters, organochlorine pesticides, dioxins and furans, trace metals and metalloids (mercury, arsenic, cadmium, chromium, cobalt, copper, lead, nickel, silver, and zinc), and polychlorinated biphenyls (total PCBs and selected aroclors). Concentrations of many of these chemicals consistently exceeded thresholdor chronic-effects guidelines for the protection of aquatic life and often exceeded probable effects levels (PELs). Exceedance of PELs was dependent on the amount of total organic carbon in the sediments. Concurrent with the collection of sediment-quality data, white sucker (Catostomus commersoni) were evaluated for gross-external and internal-organ anomalies, whole-body burdens of chemical contaminants, and gut contents to determine prey. The histopathology of internal tissues of white sucker was compared to contaminant levels in fish tissue and bed sediment. Gut contents were examined to determine preferential prey and thus potential pathways for the bioaccumulation of chemicals from bed sediments. Male and female fish were tested separately. Lesions and other necroses were observed in all fish collected during both years of sample collection, indicating that fish in Rock Creek have experienced some form of environmental stress. No direct cause and effect was determined for chemical exposure and compromised fish health, but a substantial weight of evidence indicates that white sucker, which are bottom-eeding fish and low-order consumers in Rock Creek, are experiencing some reduction in vitality, possibly due to immunosuppression. Abnormalities observed in gonads of both sexes of white sucker and observations of abnormal behavior during spawning indicated some interruption in reproductive success
Cancellation of the collisional frequency shift in caesium fountain clocks
We have observed that the collisional frequency shift in primary caesium
fountain clocks varies with the clock state population composition and, in
particular, is zero for a given fraction of the |F = 4, mF = 0> atoms,
depending on the initial cloud parameters. We present a theoretical model
explaining our observations. The possibility of the collisional shift
cancellation implies an improvement in the performance of caesium fountain
standards and a simplification in their operation. Our results also have
implications for test operation of fountains at multiple pi/2 pulse areas
Implications of an arithmetical symmetry of the commutant for modular invariants
We point out the existence of an arithmetical symmetry for the commutant of
the modular matrices S and T. This symmetry holds for all affine simple Lie
algebras at all levels and implies the equality of certain coefficients in any
modular invariant. Particularizing to SU(3)_k, we classify the modular
invariant partition functions when k+3 is an integer coprime with 6 and when it
is a power of either 2 or 3. Our results imply that no detailed knowledge of
the commutant is needed to undertake a classification of all modular
invariants.Comment: 17 pages, plain TeX, DIAS-STP-92-2
Characterisation of HTSC ceramics from their resistive transition
The resistivity vs. temperature relation in bulk ceramic HTSC under
self-field conditions as well as in weak external magnetic fields is modelled
by local Lorentz force induced fluxon motion with temperature dependent
pinning. A pinning force density and two viscous drag coefficients in
intergrain and intragrain regions, respectively, can be used as characteristic
parameters describing the temperature, current, and external field dependences
of the sample resistance.Comment: 12 pages, LaTeX2e, 6 figures (epsfig), to be published in Supercond.
Sci. and Techno
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Origin of a-plane (Al,Ga)N formation on patterned c-plane AIN/sapphire templates
a-plane (Al,Ga)N layers can be grown on patterned c-plane AlN/sapphire templates with a ridge direction along [1bar 100]Al2O3. Scanning nanobeam diffraction reveals that the formation of a-plane layers can be explained by nucleation of c-plane (Al,Ga)N with [11bar 20](Al,Ga)N[0001]Al2O3 at the ridge sidewalls. Faster growth of the top (11bar 20)(Al,Ga)N facet in the vertical direction leads to the overgrowth of c-plane (Al,Ga)N nucleated on the horizontal ridge and trench surfaces. Phase separation into binary GaN and AlN takes place during the first growth stages. However, this fades out and does not influence the composition of the final thick a-plane (Al,Ga)N layer
Precision determination of band offsets in strained InGaAs/GaAs quantum wells by C-V-profiling and Schroedinger-Poisson self-consistent simulation
The results of measurements and numerical simulation of charge carrier
distribution and energy states in strained quantum wells In_xGa_{1-x}As/GaAs
(0.06 < x < 0.29) by C-V-profiling are presented. Precise values of conduction
band offsets for these pseudomorphic QWs have been obtained by means of
self-consistent solution of Schroedinger and Poisson equations and following
fitting to experimental data. For the conduction band offsets in strained
In_xGa_{1-x}As/GaAs - QWs the expression DE_C(x) = 0.814x - 0.21x^2 has been
obtained.Comment: 9 pages, 12 figures, RevTeX
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