223 research outputs found
Optical waveguide manipulation of micro- and nano-spheres
Optical tweezers are well-established as a tool for non-contact, non-destructive handling of biological materials [1] and of inorganic nanospheres attached to biological molecules [2]. Recently, interest has grown in optical manipulation at surfaces [3] potentially as part of the toolbox of the "lab-on-a-chip". In particular, advances have been made in trapping and propulsion of metallic and dielectric micro- and nano-particles in the evanescent fields of optical waveguides [4,5], which may form part of a planar microsystem into which optical detection and spectroscopy of separated species could also be integrated. Optical waveguides embedded in surfaces represent a powerful means of controlling the distribution of optical intensity and intensity gradient at such surfaces, for particle control. In this paper, the design of optical waveguides and waveguide devices for trapping, propulsion and sorting of gold nanospheres and latex microspheres [6,7] will be described and recent experimental results presented and compared with theoretical models. The implications of these results for some proposed applications in the biosciences will be discussed
Late Neogene tectonically driven crustal exhumation of the Sikkim Himalaya : Insights from inversion of multithermochronologic data
Apatite fission track and zircon (U-Th)/He data are reported for 34 bedrock samples distributed between the foothills and the topographic crest of the Darjeeling-Sikkim Himalaya. The pattern of observed cooling ages does not correlate with topography, rainfall distribution, and the deeply incised high-relief Tista window, indicating that tectonic processes are mainly responsible for their spatial distribution. Inversion of this thermochronometric data set using 3-D thermokinematic modeling constrained by independent geological and geophysical observations was performed to evaluate the contribution of slip partitioning, duplex development, and relief growth on the evolution of the thermal structure of the Himalaya during the last 12Ma. Models involving significant relief growth do not show a substantial influence of topography evolution on the cooling age distribution, while models involving duplex growth demonstrate that tectonic processes exert a dominant influence on their distribution. In concert with equivalent studies in Bhutan, central Nepal, and NW India, our results attest that the lateral variation of the geometry and kinematics of the Himalayan basal decollement locally associated with duplex formation exert a leading influence on lateral variations of middle to upper crustal long-term exhumation rates documented along the strike of the Himalaya.Peer reviewe
Electron Impact Ionization Close to the Threshold: Classical Calculations
In this paper we present Classical Trajectory Monte Carlo (CTMC) calculations
for single and multiple electron ionization of Argon atoms and ions in the
threshold region. We are able to recover the Wannier exponents a for the
power-law behavior of the cross section s versus excess energy: the exact value
of the exponent as well as the existence of its saturation for multiple
ionization appear to be related to how the total binding energy is shared
between target electrons.Comment: 9 pages. To be published in Journal of Physics
Non-equilibrium supercurrent through mesoscopic ferromagnetic weak links
We consider a mesoscopic normal metal, where the spin degeneracy is lifted by
a ferromagnetic exchange field or Zeeman splitting, coupled to two
superconducting reservoirs. As a function of the exchange field or the distance
between the reservoirs, the supercurrent through this device oscillates with an
exponentially decreasing envelope. This phenomenon is similar to the tuning of
a supercurrent by a non-equilibrium quasiparticle distribution between two
voltage-biased reservoirs. We propose a device combining the exchange field and
non-equilibrium effects, which allows us to observe a range of novel phenomena.
For instance, part of the field-suppressed supercurrent can be recovered by a
voltage between the additional probes.Comment: 7 pages, 8 figures, Europhys. Lett., to be published, corrected two
reference
Threshold detachment of negative ions by electron impact
The description of threshold fragmentation under long range repulsive forces
is presented. The dominant energy dependence near threshold is isolated by
decomposing the cross section into a product of a back ground part and a
barrier penetration probability resulting from the repulsive Coulomb
interaction. This tunneling probability contains the dominant energy variation
and it can be calculated analytically based on the same principles as Wannier's
description for threshold ionization under attractive forces. Good agreement is
found with the available experimental cross sections on detachment by electron
impact from , and .Comment: 4 pages, 4 figures (EPS), to appear in Phys.Rev.Lett, Feb. 22nd, 199
A highly stable atomic vector magnetometer based on free spin precession
We present a magnetometer based on optically pumped Cs atoms that measures
the magnitude and direction of a 1 T magnetic field. Multiple circularly
polarized laser beams were used to probe the free spin precession of the Cs
atoms. The design was optimized for long-time stability and achieves a scalar
resolution better than 300 fT for integration times ranging from 80 ms to 1000
s. The best scalar resolution of less than 80 fT was reached with integration
times of 1.6 to 6 s. We were able to measure the magnetic field direction with
a resolution better than 10 rad for integration times from 10 s up to 2000
s
Constraining interactions mediated by axion-like particles with ultracold neutrons
We report a new limit on a possible short range spin-dependent interaction
from the precise measurement of the ratio of Larmor precession frequencies of
stored ultracold neutrons and Hg atoms confined in the same volume. The
measurement was performed in a 1 T vertical magnetic holding field
with the apparatus searching for a permanent electric dipole moment of the
neutron at the Paul Scherrer Institute. A possible coupling between freely
precessing polarized neutron spins and unpolarized nucleons of the wall
material can be investigated by searching for a tiny change of the precession
frequencies of neutron and mercury spins. Such a frequency change can be
interpreted as a consequence of a short range spin-dependent interaction that
could possibly be mediated by axions or axion-like particles. The interaction
strength is proportional to the CP violating product of scalar and pseudoscalar
coupling constants . Our result confirms limits from complementary
experiments with spin-polarized nuclei in a model-independent way. Limits from
other neutron experiments are improved by up to two orders of magnitude in the
interaction range of m
Fabrication and operation of a two-dimensional ion-trap lattice on a high-voltage microchip
Microfabricated ion traps are a major advancement towards scalable quantum computing with trapped ions. The development of more versatile ion-trap designs, in which tailored arrays of ions are positioned in two dimensions above a microfabricated surface, will lead to applications in fields as varied as quantum simulation, metrology and atom–ion interactions. Current surface ion traps often have low trap depths and high heating rates, because of the size of the voltages that can be applied to them, limiting the fidelity of quantum gates. Here we report on a fabrication process that allows for the application of very high voltages to microfabricated devices in general and use this advance to fabricate a two-dimensional ion-trap lattice on a microchip. Our microfabricated architecture allows for reliable trapping of two-dimensional ion lattices, long ion lifetimes, rudimentary shuttling between lattice sites and the ability to deterministically introduce defects into the ion lattice
Reconciling Himalayan midcrustal discontinuities: The Main Central thrust system
The occurrence of thrust-sense tectonometamorphic discontinuities within the exhumed Himalayan metamorphic core can be explained as part of the Main Central thrust system. This imbricate thrust structure, which significantly thickened the orogenic midcrustal core, comprises a series of thrust-sense faults that all merge into a single detachment. The existence of these various structures, and their potential for complex overprinting along the main detachment, may help explain the contention surrounding the definition, mapping, and interpretation of the Main Central thrust. The unique evolution of specific segments of the Main Central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position, and structural level of exposure of the mid-crust. This helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt
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