Spin flip scattering at Al surfaces

Non-local measurements are performed on a multi terminal device to $in-situ$ determine the spin diffusion length and in combination with resistivity measurements also the spin relaxation time in Al films. By varying the thickness of Al we determine the contribution to spin relaxation from surface scattering. From the temperature dependence of the spin diffusion length it is established that the spin relaxation is impurity dominated at low temperature. A comparison of the spin and momentum relaxation lengths for different thicknesses reveals that the spin flip scattering at the surfaces is weak compared to that within the bulk of the Al films.Comment: 11 pages, 5 figure

Diel Vertical Migration of an Invasive Calanoid Copepod, Eurytemora affinis, in Little Sturgeon Bay, Wisconsin

Eurytemora affinis, a calanoid copepod, is known to be a versatile, prolific invader of freshwater ecosystems across the globe. It has recently been documented in the Laurentian Great Lakes, including in Little Sturgeon Bay, an embayment of Lake Michigan. One survival mechanism that could make E. affinis a successful invader is diel vertical migration (DVM), a behavior in which animals move to different lakes depths at different times of day in order to avoid predation. Much is known about DVM of E. affinis, but primarily from studies in marine and brackish systems. Our goal was to investigate how E. affinis responds to its new, non-native freshwater environment, and to make inferences about its invasive success. During the summer of 2014, samples were taken at Little Sturgeon Bay twice on four days—once at noon and again at night. Samples were collected at one-meter intervals from one nearshore site and one offshore site. Body size and darkness of different life-stages of E. affinis were evaluated to determine stage-dependent differences in visual predation risk. Abundance of E. affinis was determined at each depth of each site to describe diel patterns of movement through the water column. Results show significant differences among life-stages in both length and visual area, but not our measure of darkness. Magnitude of DVM was greater near shore than in the offshore habitat. This may be a result of greater predation pressure near shore. The magnitude of DVM was also stage-dependent, with adults performing a more drastic migration than copepodites. This stage-dependency could be a result of differing visual predation risk, since copepodites are smaller than adults. The variety of DVM magnitudes exhibited for different life stages and environmental conditions support the notion that E. affinis is highly phenotypically plastic, making it a successful invader

Long-lived Bloch oscillations with bosonic Sr atoms and application to gravity measurement at micrometer scale

We report on the observation of Bloch oscillations on the unprecedented time scale of severalseconds. The experiment is carried out with ultra-cold bosonic strontium-88 loaded into a vertical optical standing wave. The negligible atom-atom elastic cross section and the absence of spin makes $^{88}$Sr an almost ideal Bose gas insensitive to typical mechanisms of decoherence due to thermalization and to external stray fields. The small size enables precision measurements of forces at micrometer scale. This is a challenge in physics for studies of surfaces, Casimir effects, and searches for deviations from Newtonian gravity predicted by theories beyond the standard model

CSM-365 - Using schema theory to explore interactions of multiple operators

In the last two years the schema theory for Genetic Programming (GP) has been applied to the problem of understanding the length biases of a variety of crossover and mutation operators on variable length linear structures. In these initial papers, operators were studied in isolation. In practice, however, they are typically used in various combinations, and in this paper we present the first schema theory analysis of the complex interactions of multiple operators. In particular we apply the schema theory to the use of standard subtree crossover, full mutation, and grow mutation (in varying proportions) to variable length linear structures in the one-then-zeros problem. We then show how the results can be used to guide choices about the relative proportion of these operators in order to achieve certain structural goals during a run

Enhanced spin accumulation in a superconductor

A lateral array of ferromagnetic tunnel junctions is used to inject and detect non-equilibrium quasi-particle spin distribution in a superconducting strip made of Al. The strip width and thickness is kept below the quasi particle spin diffusion length in Al. Non-local measurements in multiple parallel and antiparallel magnetic states of the detectors are used to in-situ determine the quasi-particle spin diffusion length. A very large increase in the spin accumulation in the superconducting state compared to that in the normal state is observed and is attributed to a diminishing of the quasi-particle population by opening of the gap below the transition temperature.Comment: 6 pages, 4 figures; accepted for publication in Journal of Applied Physic

Precision measurement of gravity with cold atoms in an optical lattice and comparison with a classical gravimeter

We report on a high precision measurement of gravitational acceleration using ultracold strontium atoms trapped in a vertical optical lattice. Using amplitude modulation of the lattice intensity, an uncertainty $\Delta g /g \approx 10^{-7}$ was reached by measuring at the 5$^{th}$ harmonic of the Bloch oscillation frequency. After a careful analysis of systematic effects, the value obtained with this microscopic quantum system is consistent with the one we measured with a classical absolute gravimeter at the same location. This result is of relevance for the recent interpretation of related experiments as tests of gravitational redshift and opens the way to new tests of gravity at micrometer scale.Comment: 4 pages, 4 figure

A transportable strontium optical lattice clock

We report on a transportable optical clock, based on laser-cooled strontium atoms trapped in an optical lattice. The experimental apparatus is composed of a compact source of ultra-cold strontium atoms including a compact cooling laser set-up and a transportable ultra-stable laser for interrogating the optical clock transition. The whole setup (excluding electronics) fits within a volume of less than 2 m$^3$. The high degree of operation reliability of both systems allowed the spectroscopy of the clock transition to be performed with 10 Hz resolution. We estimate an uncertainty of the clock of $7\times10^{-15}$.Comment: 12 pages, 9 figures, to be published in Appl. Phys.