Electronic correlations in double ionization of atoms in pump-probe experiments

The ionization dynamics of a two-electron atom in an attosecond XUV-infrared pump-probe experiment is simulated by solving the time-dependent two-electron Schr\"odinger equation. A dramatic change of the double ionization (DI) yield with variation of the pump-probe delay is reported and the governing role of electron-electron correlations is shown. The results allow for a direct control of the DI yield and of the relative strength of double and single ionization

Collisional shifts in optical-lattice atom clocks

We theoretically study the effects of elastic collisions on the determination of frequency standards via Ramsey fringe spectroscopy in optical-lattice atom clocks. Interparticle interactions of bosonic atoms in multiply-occupied lattice sites can cause a linear frequency shift, as well as generate asymmetric Ramsey fringe patterns and reduce fringe visibility due to interparticle entanglement. We propose a method of reducing these collisional effects in an optical lattice by introducing a phase difference of $\pi$ between the Ramsey driving fields in adjacent sites. This configuration suppresses site to site hopping due to interference of two tunneling pathways, without degrading fringe visibility. Consequently, the probability of double occupancy is reduced, leading to cancellation of collisional shifts.Comment: 15 pages, 11 figure

Significance of variability in Turborotalita quinqueloba (Natland) test size and abundance for paleoceanographic interpretations in the Norwegian-Greenland Sea

Biometric analyses on shells of Turborotalita quinqueloba (Natland) reflect the paleoceanographic conditions in the Norwegian-Greenland Sea (NGS). Both median and mean size variations exhibit a steady increase after the Last Glacial Maximum (LGM). After Termination I the size eventually reaches a constant level with only minor fluctuation. In contrast, peak abundances of specimens occur somewhat later, during the Holocene climatic optimum and rapidly decrease again in the youngest sediments. Test sizes are larger at the Vøring Plateau in the vicinity of incoming Atlantic water than in water further to the west. Turborotalita quinqueloba first appeared in the southwestern part of the NGS with a preliminary major peak in abundance and size well below the Younger Dryas. This species was present in sizes < 125 μm in this area during almost entire oxygen isotopic Stage 2. This was not observed in the more easterly located cores. It seems likely that size variations as a ‘tool’ for NGS paleoceanographic interpretations are not only valuable for the time since the LGM, but can also be applied to older isotopic stages where abundances of T. quinqueloba are low or absent in the > 125 μm size-fractions but are high in the 63–125 μm fraction (e.g. Stages 7, 9 and 11)

Monitoring Termination II at high latitude: anomalies in the planktic foraminiferal record

Microfaunal studies of planktic foraminifera carried out on 21 sediment cores from the Norwegian-Greenland Sea (NGS) reveal the spatial and lateral distribution as well as meltwater implication of today's non-polar/subpolar species Beella megastoma (Earland). Previous findings are verified in that this foraminifera is characteristic only of the deglaciation phases of Termination II, III, and VI and not the ensuing interglacial optima, thus, rendering this species a ‘meltwater’ indicator. Its distribution is restricted to cores from the central, i.e., more ‘pelagic’, part of the NGS covering an area as far north as 77 ° latitude. A detailed investigation of Termination II indicates that B. megastoma first appeared in the southwest of the NGS at ~131 ka and then about 6 kyr later in the eastern and northern parts of the NGS. For the entire duration B. megastoma always coincided with the deposition of distinct ice-rafted detritus (IRD) suggesting the presence of drifting icebergs during this period. Two different oceanographic models, each with a two-stepped evolution of the post-Saalian surface water circulation, are proposed to account for this time transgressive character. The mechanism of brine formation as possible oceanic phenomenon forcing Atlantic water northwards is suggested for being responsible for the occurrence of B. megastoma in the NGS during early Termination II. The presence of B. megastoma always ceased with the culmination of the interglacial optimum, oxygen isotopic Substage 5.51 (Eemian), when the subpolar foraminiferal fauna reached highest abundances and a general lack of IRD is observed

School-Community Partnerships in Rural Schools: Leadership, Renewal, and a Sense of Place

Rural schools are vulnerable to imitating the reform standards of national and urban school. Urban schools, to which much of the research on current reform efforts has been directed, are not rural schools writ large. Neither are rural communities like urban neighborhood communities. Hodgkinson and Obarakpor (1994) declared rural poverty is not the same as urban poverty in a different setting (p. 2). Rather, the context of rural has its own set of community identifiers that make rural schools dramatically different from their metropolitan counterparts. The goals and purposes of schooling and educational renewal processes appropriate for urban and suburban schools may be inappropriate for rural schools. As aptly expressed by Theobald and Nachtigal (1995), The work of the rural school is no longer to emulate the urban or suburban school, but to attend to its own place (p. 132). Rural students face many challenges in gaining a sound education, but one of the advantages they have is that their schools are set in a community context that values a sense of place and offers a unique set of conditions for building the social capital important for helping students succeed in school

Comparison of Josephson vortex flow transistors with different gate line configurations

We performed numerical simulations and experiments on Josephson vortex flow transistors based on parallel arrays of YBa2Cu3O(7-x) grain boundary junctions with a cross gate-line allowing to operate the same devices in two different modes named Josephson fluxon transistor (JFT) and Josephson fluxon-antifluxon transistor (JFAT). The simulations yield a general expression for the current gain vs. number of junctions and normalized loop inductance and predict higher current gain for the JFAT. The experiments are in good agreement with simulations and show improved coupling between gate line and junctions for the JFAT as compared to the JFT.Comment: 3 pages, 6 figures, accept. for publication in Appl. Phys. Let

Arctic Siberian shelf environments : an introduction

1. Why study the Arctic? 2. The Siberian shelf seas as sensitive “environmental recorder” 3. Outloo