37,055 research outputs found
Hofstadter-type energy spectra in lateral superlattices defined by periodic magnetic and electrostatic fields
We calculate the energy spectrum of an electron moving in a two-dimensional
lattice which is defined by an electric potential and an applied perpendicular
magnetic field modulated by a periodic surface magnetization. The spatial
direction of this magnetization introduces complex phases into the Fourier
coefficients of the magnetic field. We investigate the effect of the relative
phases between electric and magnetic modulation on band width and internal
structure of the Landau levels.Comment: 5 LaTeX pages with one gif figure to appear in Phys. Rev.
Planar cyclotron motion in unidirectional superlattices defined by strong magnetic and electric fields: Traces of classical orbits in the energy spectrum
We compare the quantum and the classical description of the two-dimensional
motion of electrons subjected to a perpendicular magnetic field and a
one-dimensional lateral superlattice defined by spatially periodic magnetic and
electric fields of large amplitudes. We explain in detail the complicated
energy spectra, consisting of superimposed branches of strong and of weak
dispersion, by the correspondence between the respective eigenstates and the
``channeled'' and ``drifting'' orbits of the classical description.Comment: 11 pages, 11 figures, to appear in Physical Review
Far Ultraviolet Spectroscopic Explorer Observations of a Supernova Remnant in the Line of Sight to HD 5980 in the Small Magellanic Cloud
We report a detection of far ultraviolet absorption from the supernova
remnant SNR 0057 - 7226 in the Small Magellanic Cloud (SMC). The absorption is
seen in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the
LBV/WR star HD 5980. Absorption from O VI 1032 and C III 977 is seen at a
velocity of +300 km/s with respect to the Galactic absorption lines, +170 km/s
with respect to the SMC absorption. The O VI 1038 line is contaminated by H_2
absorption, but is present. These lines are not seen in the FUSE spectrum of
Sk80, only ~1' (~17 pc) away from HD 5980. No blue-shifted O VI 1032 absorption
from the SNR is seen in the FUSE spectrum. The O VI 1032 line in the SNR is
well described by a Gaussian with FWHM=75 km/s. We find log N(O
VI)=14.33-14.43, which is roughly 50% of the rest of the O VI column in the SMC
(excluding the SNR) and greater than the O VI column in the Milky Way halo
along this sight line. The N(C IV)/N(O VI) ratio for the SNR absorption is in
the range of 0.12-0.17, similar to the value seen in the Milky Way disk, and
lower than the halo value, supporting models in which SNRs produce the highly
ionized gas close to the plane of the Galaxy, while other mechanisms occur in
the halo. The N(C IV)/N(O VI) ratio is also lower than the SMC ratio along this
sight line, suggesting that other mechanisms contribute to the creation of the
global hot ionized medium in the SMC. The O VI, C IV, and Si IV apparent column
density profiles suggest the presence of a multi-phase shell followed by a
region of higher temperature gas.Comment: 7 pages, 3 figures, 2 tables, uses emulateapj5.sty. Accepted for
publication in ApJ Letter
Negative Link Prediction in Social Media
Signed network analysis has attracted increasing attention in recent years.
This is in part because research on signed network analysis suggests that
negative links have added value in the analytical process. A major impediment
in their effective use is that most social media sites do not enable users to
specify them explicitly. In other words, a gap exists between the importance of
negative links and their availability in real data sets. Therefore, it is
natural to explore whether one can predict negative links automatically from
the commonly available social network data. In this paper, we investigate the
novel problem of negative link prediction with only positive links and
content-centric interactions in social media. We make a number of important
observations about negative links, and propose a principled framework NeLP,
which can exploit positive links and content-centric interactions to predict
negative links. Our experimental results on real-world social networks
demonstrate that the proposed NeLP framework can accurately predict negative
links with positive links and content-centric interactions. Our detailed
experiments also illustrate the relative importance of various factors to the
effectiveness of the proposed framework
Acoustic Attenuation by Two-dimensional Arrays of Rigid Cylinders
In this Letter, we present a theoretical analysis of the acoustic
transmission through two-dimensional arrays of straight rigid cylinders placed
parallelly in the air. Both periodic and completely random arrangements of the
cylinders are considered. The results for the sound attenuation through the
periodic arrays are shown to be in a remarkable agreement with the reported
experimental data. As the arrangement of the cylinders is randomized, the
transmission is significantly reduced for a wider range of frequencies. For the
periodic arrays, the acoustic band structures are computed by the plane-wave
expansion method and are also shown to agree with previous results.Comment: 4 pages, 3 figure
Expected Sensitivity to Galactic/Solar Axions and Bosonic Super-WIMPs based on the Axio-electric Effect in Liquid Xenon Dark Matter Detectors
We present systematic case studies to investigate the sensitivity of axion
searches by liquid xenon detectors, using the axio-electric effect (analogue of
the photoelectric effect) on xenon atoms. Liquid xenon is widely considered to
be one of the best target media for detection of WIMPs (Weakly Interacting
Massive Particles which may form the galactic dark matter) using nuclear
recoils. Since these detectors also provide an extremely low radioactivity
environment for electron recoils, very weakly-interacting low-mass particles (<
100 keV/c^2), such as the hypothetical axion, could be detected as well - in
this case using the axio-electric effect. Future ton-scale liquid Xe detectors
will be limited in sensitivity only by irreducible neutrino background
(pp-chain solar neutrino and the double beta decay of 136Xe) in the mass range
between 1 and 100 keV/c^2. Assuming one ton-year of exposure, galactic axions
(as non-relativistic dark matter) could be detected if the axio-electric
coupling g_Ae is greater than 10^-14 at 1 keV/c^2 (or $10^-13 at 100 keV/c^2).
Below a few keV/c^2, and independent of the mass, a solar axion search would be
sensitive to a coupling g_Ae ~ 10^-12. This limit will set a stringent upper
bound on axion mass for the DFSV and KSVZ models for the mass ranges m_A < 0.1
eV/c^2 and < 10 eV/c^2, respectively. Vector-boson dark matter could also be
detected for a coupling constant alpha'/alpha > 10^-33 (for mass 1 keV/c^2) or
> 10^-27 (for mass 100 keV/c^2).Comment: 17 pages, 10 figure
The Deformation of an Elastic Substrate by a Three-Phase Contact Line
Young's classic analysis of the equilibrium of a three-phase contact line
ignores the out-of-plane component of the liquid-vapor surface tension. While
it has long been appreciated that this unresolved force must be balanced by
elastic deformation of the solid substrate, a definitive analysis has remained
elusive because conventional idealizations of the substrate imply a divergence
of stress at the contact line. While a number of theories of have been
presented to cut off the divergence, none of them have provided reasonable
agreement with experimental data. We measure surface and bulk deformation of a
thin elastic film near a three-phase contact line using fluorescence confocal
microscopy. The out-of-plane deformation is well fit by a linear elastic theory
incorporating an out-of-plane restoring force due to the surface tension of the
gel. This theory predicts that the deformation profile near the contact line is
scale-free and independent of the substrate elastic modulus.Comment: 4 pages, 3 figure
Narrow Line Photoassociation in an Optical Lattice
With ultracold Sr in a 1D magic wavelength optical lattice, we
performed narrow line photoassociation spectroscopy near the SP intercombination transition. Nine least-bound vibrational molecular
levels associated with the long-range and potential energy surfaces
were measured and identified. A simple theoretical model accurately describes
the level positions and treats the effects of the lattice confinement on the
line shapes. The measured resonance strengths show that optical tuning of the
ground state scattering length should be possible without significant atom
loss.Comment: 4 pages, 4 figure
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