1,954 research outputs found
Hopping magneto-transport via nonzero orbital momentum states and organic magnetoresistance
In hopping magnetoresistance of doped insulators, an applied magnetic field
shrinks the electron (hole) s-wave function of a donor or an acceptor and this
reduces the overlap between hopping sites resulting in the positive
magnetoresistance quadratic in a weak magnetic field, B. We extend the theory
of hopping magnetoresistance to states with nonzero orbital momenta. Different
from s-states, a weak magnetic field expands the electron (hole) wave functions
with positive magnetic quantum numbers, m > 0, and shrinks the states with
negative m in a wide region outside the point defect. This together with a
magnetic-field dependence of injection/ionization rates results in a negative
weak-field magnetoresistance, which is linear in B when the orbital degeneracy
is lifted. The theory provides a possible explanation of a large low-field
magnetoresistance in disordered pi-conjugated organic materials (OMAR).Comment: 4 pages, 3 figure
On Classification of QCD defects via holography
We discuss classification of defects of various codimensions within a
holographic model of pure Yang-Mills theories or gauge theories with
fundamental matter. We focus on their role below and above the phase transition
point as well as their weights in the partition function. The general result is
that objects which are stable and heavy in one phase are becoming very light
(tensionless) in the other phase. We argue that the dependence of the
partition function drastically changes at the phase transition point, and
therefore it correlates with stability properties of configurations. Some
possible applications for study the QCD vacuum properties above and below phase
transition are also discussed.Comment: 21 pages, 2 figure
Localization of Classical Waves in Weakly Scattering Two-Dimensional Media with Anisotropic Disorder
We study the localization of classical waves in weakly scattering 2D systems
with anisotropic disorder. The analysis is based on a perturbative
path-integral technique combined with a spectral filtering that accounts for
the first-order Bragg scattering only. It is shown that in the long-wavelength
limit the radiation is always localized, and the localization length is
independent of the direction of propagation, the latter in contrast to the
predictions based on an anisotropic tight-binding model. For shorter
wavelengths that are comparable to the correlation scales of the disorder, the
transport properties of disordered media are essentially different in the
directions along and across the correlation ellipse. There exists a
frequency-dependent critical value of the anisotropy parameter, below which
waves are localized at all angles of propagation. Above this critical value,
the radiation is localized only within some angular sectors centered at the
short axis of the correlation ellipse and is extended in other directions.Comment: 10 pages, 5 figure
HST observations of the blue compact dwarf SBS 0335-052: a probable young galaxy
We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the
spectral regions around Ly and OI 1302 of the extremely metal-deficient
(Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star
formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4
Myr. Dust is clearly present and mixed spatially with the SSCs. There is a
supershell of radius ~380 pc, delineating a large supernova cavity. The
instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly
emission is not observed. Rather there is low intensity broad (FWZI = 20 A)
Ly emission superposed on even broader Ly absorption by the HI
envelope. This broad low-intensity emission is probably caused by resonant
scattering of Ly photons. The BCD appears to be a young galaxy,
undergoing its very first burst of star formation. This conclusion is based on
the following evidence: 1) the underlying extended low-surface-brightness
component is very irregular and filamentary, suggesting that a significant part
of the emission comes from ionized gas; 2) it has very blue colors (-0.34 =<
(V-I) =< 0.16), consistent with gaseous emission colors; 3) the OI 1302
line is not detected in absorption in the GHRS spectrum, setting an upper limit
for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller
than the value in Orion.Comment: 20 pages and 6 Postscript figures. Submitted to Astrophysical Journa
Universal limiting shape of worn profile under multiple-mode fretting conditions: theory and experimental evidence
We consider multiple-mode fretting wear in a frictional contact of elastic bodies subjected to a small-amplitude oscillation, which may include in-plane and out-of-plane translation, torsion and tilting (“periodic rolling”). While the detailed kinetics of wear depends on the particular loading history and wear mechanism, the final worn shape, under some additional conditions, occurs to be universal for all types and loading and wear mechanisms. This universal form is determined solely by the radius of the permanent stick region and the maximum indentation depth during the loading cycle. We provide experimental evidence for the correctness of the theoretically predicted limiting shape. The existence of the universal limiting shape can be used for designing joints which are resistant to fretting wear
Optical conductivity of metal nanofilms and nanowires: The rectangular-box model
The conductivity tensor is introduced for the low-dimensional electron
systems. Within the particle-in-a-box model and the diagonal response
approximation, components of the conductivity tensor for a quasi-homogeneous
ultrathin metal film and wire are calculated under the assumption (where is the characteristic small dimension of the
system, is the Fermi wavelength for bulk metal). We find the
transmittance of ultrathin films and compare these results with available
experimental data. The analytical estimations for the size dependence of the
Fermi level are presented, and the oscillations of the Fermi energy in
ultrathin films and wires are computed. Our results demonstrate the strong size
and frequency dependences of the real and imaginary parts of the conductivity
components in the infrared range. A sharp distinction of the results for Au and
Pb is observed and explained by the difference in the relaxation time of these
metals.Comment: 13 pages, 8 figure
Invariants of pseudogroup actions: Homological methods and Finiteness theorem
We study the equivalence problem of submanifolds with respect to a transitive
pseudogroup action. The corresponding differential invariants are determined
via formal theory and lead to the notions of k-variants and k-covariants, even
in the case of non-integrable pseudogroup. Their calculation is based on the
cohomological machinery: We introduce a complex for covariants, define their
cohomology and prove the finiteness theorem. This implies the well-known
Lie-Tresse theorem about differential invariants. We also generalize this
theorem to the case of pseudogroup action on differential equations.Comment: v2: some remarks and references addee
SBS 0335-052W - an Extremely Low Metallicity Dwarf Galaxy
We present Multiple Mirror Telescope (MMT) and Keck II telescope
spectrophotometry and 3.5m Calar Alto telescope R, I photometry of the western
component of the extremely low-metallicity blue compact galaxy SBS 0335-052.
The components, separated by 22 kpc, appear to be members of a unique,
physically connected system. It is shown that SBS 0335-052W consists of at
least three stellar clusters and has the same redshift as SBS 0335-052. The
oxygen abundance in its two brightest knots is extremely low, 12+log(O/H)=
7.22+/-0.03 and 7.13+/-0.08, respectively. These values are lower than in SBS
0335-052 and are nearly the same as those in I Zw 18. The (R-I) color profiles
are very blue in both galaxies due to the combined effects of ionized gas and a
young stellar population emission. We argue that SBS 0335-052W is likely to be
a nearby, young dwarf galaxy.Comment: 18 pages, 4 EPS figures, to appear in ApJ, 1 July 199
A new field line tracer for the study of coronal magnetic topologies
We present a new code for the tracing of magnetic field lines and calculation of related quantities such as the squashing factor in the solar corona. The Universal Fieldline Tracer (UFiT) is an open-source package that can currently take inputs directly from four well-established coronal models, with additional models planned to be made directly accessible in the future. This package contains tools to make use of large-scale three-dimensional field line maps to calculate volumetric quantities, such as the total volume of the open corona, or the fraction that maps to regions on the solar surface within some distance of a coronal hole boundary, which may be relevant to phenomenological models of solar wind speed such as the Wang–Sheeley–Arge model. Synthetic coronagraphs can also be produced rapidly by this package. We have postprocessed long-term magnetofrictional simulations to demonstrate that the separatrix web occupies a larger fraction of the corona during solar maximum than solar minimum
The final NO-WEAR state due to dual-mode fretting: Numerical prediction and experimental validation
We study fretting wear due to superimposed oscillations in the normal and tangential directions with respect to the contact plane (dual mode fretting wear). In the limit of infinite time, the profile of the indenter tends to a limiting shape, which does not change further. For axisymmetric profiles, the limiting shape is found analytically for the general case of two different frequencies and amplitudes of oscillations in normal and tangential direction. The dependence of worn volume on the frequency ratio is strongly singular showing sharp minima for small rational ratios of frequencies. For the special case of coinciding frequencies, the fretting process was studied both analytically and experimentally. Comparison of experimental results with theoretical predictions showed a good qualitative and quantitative agreement (discrepancy of the order of experimental noise)
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