Reflection Asymmetric Relativistic Mean Field Approach and Its Application to the Octupole Deformed Nucleus ^226Ra

A reflection asymmetric relativistic mean field （RAS-RMF） approach is developed by expanding the equations of motion for both the nucleons and the mesons on the eigenfunctions of the two-centre harmonic-oscillator potential. The efficiency and reliability of the RAS-RMF approach are demonstrated in its application to the well-known octupole deformed nucleus 226Ra and the available data, including the binding energy and the deformation parameters, are well reproduced.     

Instabilities developed in stratified flows over pronounced obstacles

In the present work we study numerical and experimentally the flow of a two-layer stratified fluid over a topographic obstacle. The problem reflects a wide number of oceanographic and meteorological situations, where the stratification plays an important role. We identify the different instabilities developed by studying the pycnocline deformation due to a pronounced obstacle. The numerical simulations were made using the model caffa3D.MB which works with a numerical model of Navier-Stokes equations with finite volume elements in curvilinear meshes. The experimental results are contrasted with numerical simulations. Linear stability analysis predictions are checked with particle image velocimetry (PIV) measurements.     

Improvement of the fractional uncertainty of a neutral-atom calcium optical frequency standard to 2×10-14

We have investigated the two major effects that limit the accuracy of an optical frequency standard based on laser-cooled neutral calcium atoms, i.e. the residual Doppler shift and atomic collisions. A new correction method was applied to reduce the contribution of the residual Doppler effect to the total fractional uncertainty to 1×10^-14. Measurements of the shift of the clock transition frequency due to cold collisions allowed us to reduce their contribution to 4×10^-15. With these improvements we have reduced the total fractional frequency uncertainty of the standard by nearly an order of magnitude to 2×10^-14. Received: 9 August 2002 / Revised version: 16 November 2002 / Published online: 26 February 2003 RID="*" ID="*"Permanent address: Russian Academy of Sciences, P.N. Lebedev Physical Institute, Samara Branch, Novo-Sadovaya st. 221, Samara 443011, Russia RID="**" ID="**"Corresponding author. Fax: +49-531/592-4305, E-mail: uwe.sterr@ptb.de     

Absolute-frequency measurement of the iodine-based length standard at 514.67 nm

The absolute frequency of the length standard at 514.67 nm based on the molecular iodine (¹²⁷ I ₂) transition of the P(13) 43-0 component a₃ is measured using a self-referenced femtosecond optical comb. This frequency-based technique improves measurement precision more than 100 times compared with previous wavelength-based results. Power- and pressure-related frequency shifts have been carefully studied. The measured absolute frequency is 71.8±1.5 kHz higher than the internationally accepted value of 582490603.37±0.15 MHz, adopted by the Comité International des Poids et Mesures (CIPM) in 1997. Received: 7 March 2002 / Published online: 24 April 2002     

High-resolution spectroscopy with laser-cooled and trapped calcium atoms

We report on high-resolution spectroscopy with two different samples of calcium atoms, in a laser-cooled and deflected beam and in a magneto-optical trap. The atomic beam was excited by spatially separated laser fields. For spectroscopy with stored atoms in a magneto-optical trap we used a multiple-pulse excitation scheme. The resolution as low as 2.5 kHz was limited by residual frequency fluctuations of our dye-laser spectrometer. The results should allow to establish a frequency standard with a relative uncertainty below 10^–14.     

Cesium saturation spectroscopy revisited: How to reverse peaks and observe narrow resonances

The complex magnetic structure of the cesium atom is responsible for the interesting behaviour of its saturated absorption spectra, e.g., a two-fold sign reversal of a crossover resonance, under various polarization configurations with and without applied magnetic fields. We show that this morphology is a result of optical pumping processes including coherent population trapping which, under normal laboratory conditions, prevent the atoms from reaching an equilibrium situation. Our interpretation is useful for an intuitive and rapid understanding of this important tool in high-resolution spectroscopy.     

Diamagnetism in quasicrystals

A reliable explanation of diamagnetism in quasicrystals is given. We show that the weak diamagnetism in perfect icosahedral quasicrystals is due to an atomic-like diamagnetic contribution of tightly bound conduction electrons in electron pockets of a multiconnected Fermi surface. The Landau-Peierls diamagnetic term is small due to large effective masses. At temperatures above the Debye temperature the intervalley electron-phonon scattering makes the electrons ‘free’, and the temperature dependence of the Pauli paramagnetism related to a pseudogap in the density of states at the Fermi level becomes important.     

Rotational bands in 169Re

Based on the systematic investigation of the data available for nuclei with A≥ 40, a Z ^1/3-dependence for the nuclear charge radii is shown to be superior to the generally accepted A ^1/3 law. A delicate scattering of data around R _c/Z ^1/3 is inferred as owing to the isospin effect and a linear dependence of R _c/Z ^1/3 on N/Z (or (N - Z)/2) is found. This inference is well supported by the microscopic Relativistic Continuum Hartree-Bogoliubov (RCHB) calculation conducted for the proton magic Ca, Ni, Zr, Sn and Pb isotopes including the exotic nuclei close to the neutron drip line. With the linear isospin dependence provided by the data and RCHB theory, a new isospin-dependent Z ^1/3 formula for the nuclear charge radii is proposed. Received: 23 September 2001 / Accepted: 21 January 2002     

Genetically controlled MRI contrast mechanisms and their prospects in systems neuroscience research

Application of MRI contrast agents to neural systems research is complicated by the need to deliver agents past the blood-brain barrier or into cells, and the difficulty of targeting agents to specific brain structures or cell types. In the future, these barriers may be wholly or partially overcome using genetic methods for producing and directing MRI contrast. Here we review MRI contrast mechanisms that have used gene expression to manipulate MRI signal in cultured cells or in living animals. We discuss both fully genetic systems involving endogenous biosynthesis of contrast agents, and semi-genetic systems in which expressed proteins influence the localization or activity of exogenous contrast agents. We close by considering which contrast-generating mechanisms might be most suitable for applications in neuroscience, and we ask how genetic control machinery could be productively combined with existing molecular agents to enable next-generation neuroimaging experiments.     

Absolute frequency measurements and comparisons in iodine at 735 nm and 772 nm

We report frequency measurements at the rovibronic transition P(42)1-14 (772 nm) and R(114)2-11 (735 nm) from the electronic transition of the iodine molecule ¹²⁷I₂ with the help of a frequency comb as a reference. By using Doppler-free saturation spectroscopy a frequency precision in the 7 × 10⁻¹⁰ region is reached and two iodine cells both operated at 550-600 °C are compared. To relate our results to other measurements, the absolute transition frequency of the hyperfine structure line P(148)1-14 a₁ at 780 nm with an already known transition frequency was also determined.