16,600 research outputs found
Microfluidic immunomagnetic multi-target sorting – a model for controlling deflection of paramagnetic beads
We describe a microfluidic system that uses a magnetic field to sort paramagnetic beads by deflecting them in the direction normal to the flow. Our experiments systematically study the dependence of the beads’ deflection on: bead size and susceptibility, magnet strength, fluid speed and viscosity, and device geometry. We also develop a design parameter that can aid in the design of microfluidic devices for immunomagnetic multi-target sorting
Optical probes of the quantum vacuum: The photon polarization tensor in external fields
The photon polarization tensor is the central building block of an effective
theory description of photon propagation in the quantum vacuum. It accounts for
the vacuum fluctuations of the underlying theory, and in the presence of
external electromagnetic fields, gives rise to such striking phenomena as
vacuum birefringence and dichroism. Standard approximations of the polarization
tensor are often restricted to on-the-light-cone dynamics in homogeneous
electromagnetic fields, and are limited to certain momentum regimes only. We
devise two different strategies to go beyond these limitations: First, we aim
at obtaining novel analytical insights into the photon polarization tensor for
homogeneous fields, while retaining its full momentum dependence. Second, we
employ wordline numerical methods to surpass the constant-field limit.Comment: 13 pages, 4 figures; typo in Eq. (5) corrected (matches journal
version
THE INFLUENCE OF MANUALLY ADJUSTING THE RUNNING SPEED ON THE IMPACT ACCELERATION OF THE TIBIA DURING TREADMILL RUNNING
The purpose of this study was to investigate the influence of pressing the speed key on the treadmill console to change the running speed on the impact acceleration of the tibia during treadmill running. Twenty-seven subjects were asked to run on the treadmill and increase the speed gradually until their preferred speed within two minutes. Then the subjects were required to adjust their running speed manually every two minutes. The peak impact accelerations of the right and left tibia were measured for 30 seconds at the 2nd, 4th, 6th, 8th, and 10thminutes. The results showed that there are greater peaks of impact acceleration of the tibia when some subjects pressed the speed key on the treadmill console to change their running speed. It is suggested that the position of the speed key of the treadmill be close to the runner
Revealing common artifacts due to ferromagnetic inclusions in highly-oriented pyrolytic graphite
We report on an extensive investigation to figure out the origin of
room-temperature ferromagnetism that is commonly observed by SQUID magnetometry
in highly-oriented pyrolytic graphite (HOPG). Electron backscattering and X-ray
microanalysis revealed the presence of micron-size magnetic clusters
(predominantly Fe) that are rare and would be difficult to detect without
careful search in a scanning electron microscope in the backscattering mode.
The clusters pin to crystal boundaries and their quantities match the amplitude
of typical ferromagnetic signals. No ferromagnetic response is detected in
samples where we could not find such magnetic inclusions. Our experiments show
that the frequently reported ferromagnetism in pristine HOPG is most likely to
originate from contamination with Fe-rich inclusions introduced presumably
during crystal growth.Comment: 8 pages, 7 figure
Metamaterial with polarization and direction insensitive resonant transmission response mimicking electromagnetically induced transparency
We report on a planar metamaterial, the resonant transmission frequency of which does not depend on the polarization and angle of incidence of electromagnetic waves. The resonance results from the excitation of high-Q antisymmetric trapped current mode and shows sharp phase dispersion characteristic to Fano-type resonances of the electromagnetically induced transparency phenomenon
Tunneling of correlated electrons in ultra high magnetic field
Effects of the electron-electron interaction on tunneling into a metal in
ultra-high magnetic field (ultra-quantum limit) are studied. The range of the
interaction is found to have a decisive effect both on the nature of the
field-induced instability of the ground state and on the properties of the
system at energies above the corresponding gap. For a short-range repulsive
interaction, tunneling is dominated by the renormalization of the coupling
constant, which leads eventually to the charge-density wave instability. For a
long-range interaction, there exists an intermediate energy range in which the
conductance obeys a power-law scaling form, similar to that of a 1D Luttinger
liquid. The exponent is magnetic-field dependent, and more surprisingly, may be
positive or negative, i. e., interactions may either suppress or enhance the
tunneling conductance compared to its non-interacting value. At energies near
the gap, scaling breaks down and tunneling is again dominated by the
instability, which in this case is an (anisotropic) Wigner crystal instability.Comment: 4 pages, 2 .eps figure
A microscopic investigation of the transition form factor in the region of collective multipole excitations of stable and unstable nuclei
We have used a self-consistent Skyrme-Hartree-Fock plus Continuum-RPA model
to study the low-multipole response of stable and neutron/proton-rich Ni and Sn
isotopes. We focus on the momentum-transfer dependence of the strength
distribution, as it provides information on the structure of excited nuclear
states and in particular on the variations of the transition form factor (TFF)
with the energy. Our results show, among other things, that the TFF may show
significant energy dependence in the region of the isoscalar giant monopole
resonance and that the TFF corresponding to the threshold strength in the case
of neutron-rich nuclei is different compared to the one corresponding to the
respective giant resonance. Perspectives are given for more detailed future
investigations.Comment: 13 pages, incl. 9 figures; to appear in J.Phys.G,
http://www.iop.org/EJ/jphys
Spontaneous Crystallization of Skyrmions and Fractional Vortices in the Fast-rotating and Rapidly-quenched Spin-1 Bose-Einstein Condensates
We investigate the spontaneous generation of crystallized topological defects
via the combining effects of fast rotation and rapid thermal quench on the
spin-1 Bose-Einstein condensates. By solving the stochastic projected
Gross-Pitaevskii equation, we show that, when the system reaches equilibrium, a
hexagonal lattice of skyrmions, and a square lattice of half-quantized vortices
can be formed in a ferromagnetic and antiferromagnetic spinor BEC, respetively,
which can be imaged by using the polarization-dependent phase-contrast method
Selection of the ground state for nonlinear Schroedinger equations
We prove for a class of nonlinear Schr\"odinger systems (NLS) having two
nonlinear bound states that the (generic) large time behavior is characterized
by decay of the excited state, asymptotic approach to the nonlinear ground
state and dispersive radiation. Our analysis elucidates the mechanism through
which initial conditions which are very near the excited state branch evolve
into a (nonlinear) ground state, a phenomenon known as {\it ground state
selection}.
Key steps in the analysis are the introduction of a particular linearization
and the derivation of a normal form which reflects the dynamics on all time
scales and yields, in particular, nonlinear Master equations.
Then, a novel multiple time scale dynamic stability theory is developed.
Consequently, we give a detailed description of the asymptotic behavior of the
two bound state NLS for all small initial data. The methods are general and can
be extended to treat NLS with more than two bound states and more general
nonlinearities including those of Hartree-Fock type.Comment: Revision of 2001 preprint; 108 pages Te
Electron waves in chemically substituted graphene
We present exact analytical and numerical results for the electronic spectra
and the Friedel oscillations around a substitutional impurity atom in a
graphene lattice. A chemical dopant in graphene introduces changes in the
on-site potential as well as in the hopping amplitude. We employ a T-matrix
formalism and find that disorder in the hopping introduces additional
interference terms around the impurity that can be understood in terms of
bound, semi-bound, and unbound processes for the Dirac electrons. These
interference effects can be detected by scanning tunneling microscopy.Comment: 4 pages, 7 figure
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