1,733 research outputs found
Melting temperature of screened Wigner crystal on helium films by molecular dynamics
Using molecular dynamics (MD) simulation, we have calculated the melting
temperature of two-dimensional electron systems on \AA-\AA helium
films supported by substrates of dielectric constants
at areal densities varying from cm to cm. Our results are in good agreement with the available
theoretical and experimental results.Comment: 4 pages and 4 figure
Electrostatics of Edge States of Quantum Hall Systems with Constrictions: Metal--Insulator Transition Tuned by External Gates
The nature of a metal--insulator transition tuned by external gates in
quantum Hall (QH) systems with point constrictions at integer bulk filling, as
reported in recent experiments of Roddaro et al. [1], is addressed. We are
particularly concerned here with the insulating behavior--the phenomena of
backscattering enhancement induced at high gate voltages. Electrostatics
calculations for QH systems with split gates performed here show that
observations are not a consequence of interedge interactions near the point
contact. We attribute the phenomena of backscattering enhancement to a
splitting of the integer edge into conducting and insulating stripes, which
enable the occurrence of the more relevant backscattering processes of
fractionally charged quasiparticles at the point contact. For the values of the
parameters used in the experiments we find that the conducting channels are
widely separated by the insulating stripes and that their presence alters
significantly the low-energy dynamics of the edges. Interchannel impurity
scattering does not influence strongly the tunneling exponents as they are
found to be irrelevant processes at low energies. Exponents of backscattering
at the point contact are unaffected by interchannel Coulomb interactions since
all channels have same chirality of propagation.Comment: 19 pages; To appear in Phys. Rev.
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
On the uniqueness of the surface sources of evoked potentials
The uniqueness of a surface density of sources localized inside a spatial
region and producing a given electric potential distribution in its
boundary is revisited. The situation in which is filled with various
metallic subregions, each one having a definite constant value for the electric
conductivity is considered. It is argued that the knowledge of the potential in
all fully determines the surface density of sources over a wide class of
surfaces supporting them. The class can be defined as a union of an arbitrary
but finite number of open or closed surfaces. The only restriction upon them is
that no one of the closed surfaces contains inside it another (nesting) of the
closed or open surfaces.Comment: 16 pages, 5 figure
Modifying the Casimir force between indium tin oxide film and Au sphere
We present complete results of the experiment on measuring the Casimir force
between an Au-coated sphere and an untreated or, alternatively, UV-treated
indium tin oxide film deposited on a quartz substrate. Measurements were
performed using an atomic force microscope in a high vacuum chamber. The
measurement system was calibrated electrostatically. Special analysis of the
systematic deviations is performed, and respective corrections in the
calibration parameters are introduced. The corrected parameters are free from
anomalies discussed in the literature. The experimental data for the Casimir
force from two measurement sets for both untreated and UV-treated samples are
presented. The experimental errors are determined at a 95% confidence level. It
is demonstrated that the UV treatment of an I TO plate results in a significant
decrease in the magnitude of the Casimir force (from 21% to 35% depending on
separation). However, ellipsometry measurements of the imaginary parts of
dielectric permittivities of the untreated and UV-treated samples did not
reveal any significant differences. The experimental data are compared with
computations in the framework of the Lifshitz theory. It is found that the data
for the untreated sample are in a very good agreement with theoretical results
taking into account the free charge carriers in an ITO film. For the UV-treated
sample the data exclude the theoretical results obtained with account of free
charge carriers. These data are in a very good agreement with computations
disregarding the contribution of free carriers. According to the explanation
provided, this is caused by the phase transition of the ITO film from metallic
to dielectric state caused by the UV treatment. Possible applications of the
discovered phenomenon in nanotechnology are discussed.Comment: 30 pages, 19 figures, 1 tabl
Gradient of the Casimir force between Au surfaces of a sphere and a plate measured using atomic force microscope in a frequency shift technique
We present measurement results for the gradient of the Casimir force between
an Au-coated sphere and an Au-coated plate obtained by means of an atomic force
microscope operated in a frequency shift technique. This experiment was
performed at a pressure of 3x10^{-8} Torr with hollow glass sphere of 41.3 mcm
radius. Special attention is paid to electrostatic calibrations including the
problem of electrostatic patches. All calibration parameters are shown to be
separation-independent after the corrections for mechanical drift are included.
The gradient of the Casimir force was measured in two ways with applied
compensating voltage to the plate and with different applied voltages and
subsequent subtraction of electric forces. The obtained mean gradients are
shown to be in mutual agreement and in agreement with previous experiments
performed using a micromachined oscillator. The obtained data are compared with
theoretical predictions of the Lifshitz theory including corrections beyond the
proximity force approximation. An independent comparison with no fitting
parameters demonstrated that the Drude model approach is excluded by the data
at a 67% confidence level over the separation region from 235 to 420 nm. The
theoretical approach using the generalized plasma-like model is shown to be
consistent with the data over the entire measurement range. Corrections due to
the nonlinearity of oscillator are calculated and the application region of the
linear regime is determined. A conclusion is made that the results of several
performed experiments call for a thorough analysis of the basics of the theory
of dispersion forces.Comment: 35 pages, 14 figures, 1 table; to appear in Phys. Rev.
Equation of the field lines of an axisymmetric multipole with a source surface
Optical spectropolarimeters can be used to produce maps of the surface magnetic fields of stars and hence to determine how stellar magnetic fields vary with stellar mass, rotation rate, and evolutionary stage. In particular, we now can map the surface magnetic fields of forming solar-like stars, which are still contracting under gravity and are surrounded by a disk of gas and dust. Their large scale magnetic fields are almost dipolar on some stars, and there is evidence for many higher order multipole field components on other stars. The availability of new data has renewed interest in incorporating multipolar magnetic fields into models of stellar magnetospheres. I describe the basic properties of axial multipoles of arbitrary degree â and derive the equation of the field lines in spherical coordinates. The spherical magnetic field components that describe the global stellar field topology are obtained analytically assuming that currents can be neglected in the region exterior to the star, and interior to some fixed spherical equipotential surface. The field components follow from the solution of Laplaceâs equation for the magnetostatic potential
Steplike electric conduction in a classical two-dimensional electron system through a narrow constriction in a microchannel
Using molecular dynamics simulation, we investigate transport properties of a
classical two-dimensional electron system confined in a microchannel with a
narrow constriction. As a function of the confinement strength of the
constriction, the calculated conductance in the simulations exhibits steplike
increases as reported in a recent experiment [D. G. Rees et al., Phys. Rev.
Lett. 106, 026803 (2011)]. It is confirmed that the number of the steps
corresponds to the number of stream lines of electrons through the
constriction. We verify that density fluctuation plays a major role in
smoothing the steps in the conductance.Comment: 11 pages, 9 figure
Addition-Deletion Networks
We study structural properties of growing networks where both addition and
deletion of nodes are possible. Our model network evolves via two independent
processes. With rate r, a node is added to the system and this node links to a
randomly selected existing node. With rate 1, a randomly selected node is
deleted, and its parent node inherits the links of its immediate descendants.
We show that the in-component size distribution decays algebraically, c_k ~
k^{-beta}, as k-->infty. The exponent beta=2+1/(r-1) varies continuously with
the addition rate r. Structural properties of the network including the height
distribution, the diameter of the network, the average distance between two
nodes, and the fraction of dangling nodes are also obtained analytically.
Interestingly, the deletion process leads to a giant hub, a single node with a
macroscopic degree whereas all other nodes have a microscopic degree.Comment: 8 pages, 5 figure
Diffraction of light by a planar aperture in a metallic screen
We present a complete derivation of the formula of Smythe [Phys.Rev.72, 1066
(1947)] giving the electromagnetic field diffracted by an aperture created in a
perfectly conducting plane surface. The reasoning, valid for any excitating
field and any hole shape, makes use only of the free scalar Green function for
the Helmoltz equation without any reference to a Green dyadic formalism. We
compare our proof with the one previously given by Jackson and connect our
reasoning to the general Huygens Fresnel theorem.Comment: J. Math. Phys. 47, 072901 (2006
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