1,013 research outputs found
No anomalous scaling in electrostatic calibrations for Casimir force measurements
In a recent paper (Phys.Rev.A78, 020101(R) (2008)), Kim at al. have reported
a large anomaly in the scaling law of the electrostatic interaction between a
sphere and a plate, which was observed during the calibration of their Casimir
force set-up. Here we experimentally demonstrate that in proper electrostatic
calibrations the scaling law follows the behavior expected from elementary
electrostatic arguments, even when the electrostatic voltage that one must
apply to minimize the force (typically ascribed to contact potentials) depends
on the separation between the surfaces.Comment: Final versio
Long range polarization attraction between two different likely charged macroions
It is known that in a water solution with multivalent counterions (Z-ions),
two likely charged macroions can attract each other due to correlations of
Z-ions adsorbed on their surfaces. This "correlation" attraction is
short-ranged and decays exponentially with increasing distance between
macroions at characteristic distance A/2\pi, where A is the average distance
between Z-ions on the surfaces of macroions. In this work, we show that an
additional long range "polarization" attraction exists when the bare surface
charge densities of the two macroions have the same sign, but are different in
absolute values. The key idea is that with adsorbed Z-ions, two insulating
macroions can be considered as conductors with fixed but different electric
potentials. Each potential is determined by the difference between the entropic
bulk chemical potential of a Z-ion and its correlation chemical potential at
the surface of the macroion determined by its bare surface charge density. When
the two macroions are close enough, they get polarized in such a way that their
adjacent spots form a charged capacitor, which leads to attraction. In a salt
free solution this polarization attractive force is long ranged: it decays as a
power of the distance between the surfaces of two macroions, d. The
polarization force decays slower than the van der Waals attraction and
therefore is much larger than it in a large range of distances. In the presence
of large amount of monovalent salt, when A/2\pi<< d<< r_s (r_s is the
Debye-H\"{u}ckel screening radius), this force is still much stronger than the
van der Waals attraction and the correlation attraction mentioned above.Comment: 12 pages, 7 figures. Small change in the text, no change in result
Counterion-Mediated Weak and Strong Coupling Electrostatic Interaction between Like-Charged Cylindrical Dielectrics
We examine the effective counterion-mediated electrostatic interaction
between two like-charged dielectric cylinders immersed in a continuous
dielectric medium containing neutralizing mobile counterions. We focus on the
effects of image charges induced as a result of the dielectric mismatch between
the cylindrical cores and the surrounding dielectric medium and investigate the
counterion-mediated electrostatic interaction between the cylinders in both
limits of weak and strong electrostatic couplings (corresponding, e.g., to
systems with monovalent and multivalent counterions, respectively). The results
are compared with extensive Monte-Carlo simulations exhibiting good agreement
with the limiting weak and strong coupling results in their respective regime
of validity.Comment: 19 pages, 10 figure
Ion exchange phase transitions in "doped" water--filled channels
Ion transport through narrow water--filled channels is impeded by a high
electrostatic barrier. The latter originates from the large ratio of the
dielectric constants of the water and a surrounding media. We show that
``doping'', i.e. immobile charges attached to the walls of the channel,
substantially reduces the barrier. This explains why most of the biological ion
channels are ``doped''. We show that at rather generic conditions the channels
may undergo ion exchange phase transitions (typically of the first order). Upon
such a transition a finite latent concentration of ions may either enter or
leave the channel, or be exchanged between the ions of different valences. We
discuss possible implications of these transitions for the Ca-vs.-Na
selectivity of biological Ca channels. We also show that transport of divalent
Ca ions is assisted by their fractionalization into two separate excitations.Comment: 16 pages, 27 figure
Nanomechanical displacement detection using coherent transport in ordered and disordered graphene nanoribbon resonators
Graphene nanoribbons provide an opportunity to integrate phase-coherent
transport phenomena with nanoelectromechanical systems (NEMS). Due to the
strain induced by a deflection in a graphene nanoribbon resonator, coherent
electron transport and mechanical deformations couple. As the electrons in
graphene have a Fermi wavelength \lambda ~ a_0 = 1.4 {\AA}, this coupling can
be used for sensitive displacement detection in both armchair and zigzag
graphene nanoribbon NEMS. Here it is shown that for ordered as well as
disordered ribbon systems of length L, a strain \epsilon ~ (w/L)^2 due to a
deflection w leads to a relative change in conductance \delta G/G ~ (w^2/a_0L).Comment: 4 Pages, 4 figure
Electronic states and optical properties of PbSe nanorods and nanowires
A theory of the electronic structure and excitonic absorption spectra of PbS
and PbSe nanowires and nanorods in the framework of a four-band effective mass
model is presented. Calculations conducted for PbSe show that dielectric
contrast dramatically strengthens the exciton binding in narrow nanowires and
nanorods. However, the self-interaction energies of the electron and hole
nearly cancel the Coulomb binding, and as a result the optical absorption
spectra are practically unaffected by the strong dielectric contrast between
PbSe and the surrounding medium. Measurements of the size-dependent absorption
spectra of colloidal PbSe nanorods are also presented. Using room-temperature
energy-band parameters extracted from the optical spectra of spherical PbSe
nanocrystals, the theory provides good quantitative agreement with the measured
spectra.Comment: 35 pages, 12 figure
The effects of metals and inhibitors on thermal oxidative degradation reactions of unbranched perfluoroalkylethers
Thermal oxidative degradation studies were performed on unbranched perfluoroalkylethers at 288 C in oxygen. Metals and alloys studied included Ti, Al, and Ti (4 Al, 4 Mn). The mechanism of degradation was by chain scission. Ti and Al promoted less degradation than Ti (4 Al, 4 Mn). The two inhibitors investigated (a perfluorophenyl phosphine and a phosphatriazine) reduced degradation rates by several orders of magnitude. Both inhibitors were effective for the same duration (75 to 100 hours). The phosphatriazine appeared to provide more surface protection
Interaction of vortices in thin superconducting films and Berezinskii-Kosterlitz-Thouless transition
The precondition for the BKT transition in thin superconducting films, the
logarithmic intervortex interaction, is satisfied at distances short relative
to , is the London penetration depth of the
bulk material and is the film thickness. For this reason, the search for
the transition has been conducted in samples of the size . It is
argued below that film edges turn the interaction into near exponential
(short-range) thus making the BKT transition impossible. If however the
substrate is superconducting and separated from the film by an insulated layer,
the logarithmic intervortex interaction is recovered and the BKT transition
should be observable.Comment: 4 pages, no figure
Force on a neutral atom near conducting microstructures
We derive the non-retarded energy shift of a neutral atom for two different
geometries. For an atom close to a cylindrical wire we find an integral
representation for the energy shift, give asymptotic expressions, and
interpolate numerically. For an atom close to a semi-infinite halfplane we
determine the exact Green's function of the Laplace equation and use it derive
the exact energy shift for an arbitrary position of the atom. These results can
be used to estimate the energy shift of an atom close to etched microstructures
that protrude from substrates.Comment: 7 pages, 5 figure
Effects of magnetic fields on magnetohydrodynamic cylindrical and spherical Richtmyer-Meshkov instability
The effects of seed magnetic fields on the Richtmyer-Meshkov instability driven by converging cylindrical and spherical implosions in ideal magnetohydrodynamics are investigated. Two different seed field configurations at various strengths are applied over a cylindrical or spherical density interface which has a single-dominant-mode perturbation. The shocks that excite the instability are generated with appropriate Riemann problems in a numerical formulation and the effect of the seed field on the growth rate and symmetry of the perturbations on the density interface is examined. We find reduced perturbation growth for both field configurations and all tested strengths. The extent of growth suppression increases with seed field strength but varies with the angle of the field to interface. The seed field configuration does not significantly affect extent of suppression of the instability, allowing it to be chosen to minimize its effect on implosion distortion. However, stronger seed fields are required in three dimensions to suppress the instability effectively
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