14,685 research outputs found
Origin of positive magnetoresistance in small-amplitude unidirectional lateral superlattices
We report quantitative analysis of positive magnetoresistance (PMR) for
unidirectional-lateral-superlattice samples with relatively small periods
(a=92-184 nm) and modulation amplitudes (V_0=0.015-0.25 meV). By comparing
observed PMR's with ones calculated using experimentally obtained mobilities,
quantum mobilities, and V_0's, it is shown that contribution from streaming
orbits (SO) accounts for only small fraction of the total PMR. For small V_0,
the limiting magnetic field B_e of SO can be identified as an inflection point
of the magnetoresistance trace. The major part of PMR is ascribed to drift
velocity arising from incompleted cyclotron orbits obstructed by scatterings.Comment: 12 pages, 9 figures, REVTe
Surface effects on nanowire transport: numerical investigation using the Boltzmann equation
A direct numerical solution of the steady-state Boltzmann equation in a
cylindrical geometry is reported. Finite-size effects are investigated in large
semiconducting nanowires using the relaxation-time approximation. A nanowire is
modelled as a combination of an interior with local transport parameters
identical to those in the bulk, and a finite surface region across whose width
the carrier density decays radially to zero. The roughness of the surface is
incorporated by using lower relaxation-times there than in the interior.
An argument supported by our numerical results challenges a commonly used
zero-width parametrization of the surface layer. In the non-degenerate limit,
appropriate for moderately doped semiconductors, a finite surface width model
does produce a positive longitudinal magneto-conductance, in agreement with
existing theory. However, the effect is seen to be quite small (a few per cent)
for realistic values of the wire parameters even at the highest practical
magnetic fields. Physical insights emerging from the results are discussed.Comment: 15 pages, 7 figure
The (In)Stability of Planetary Systems
We present results of numerical simulations which examine the dynamical
stability of known planetary systems, a star with two or more planets. First we
vary the initial conditions of each system based on observational data. We then
determine regions of phase space which produce stable planetary configurations.
For each system we perform 1000 ~1 million year integrations. We examine
upsilon And, HD83443, GJ876, HD82943, 47UMa, HD168443, and the solar system
(SS). We find that the resonant systems, 2 planets in a first order mean motion
resonance, (HD82943 and GJ876) have very narrow zones of stability. The
interacting systems, not in first order resonance, but able to perturb each
other (upsilon And, 47UMa, and SS) have broad regions of stability. The
separated systems, 2 planets beyond 10:1 resonance, (we only examine HD83443
and HD168443) are fully stable. Furthermore we find that the best fits to the
interacting and resonant systems place them very close to unstable regions. The
boundary in phase space between stability and instability depends strongly on
the eccentricities, and (if applicable) the proximity of the system to perfect
resonance. In addition to million year integrations, we also examined stability
on ~100 million year timescales. For each system we ran ~10 long term
simulations, and find that the Keplerian fits to these systems all contain
configurations which may be regular on this timescale.Comment: 37 pages, 49 figures, 13 tables, submitted to Ap
Breakdown of weak-field magnetotransport at a metallic quantum critical point
We show how the collapse of an energy scale in a quantum critical metal can
lead to physics beyond the weak-field limit usually used to compute transport
quantities. For a density-wave transition we show that the presence of a finite
magnetic field at the critical point leads to discontinuities in the transport
coefficients as temperature tends to zero. The origin of these discontinuities
lies in the breakdown of the weak field Jones-Zener expansion which has
previously been used to argue that magneto-transport coefficients are
continuous at simple quantum critical points. The presence of potential
scattering and magnetic breakdown rounds the discontinuities over a window
determined by tau Delta < 1 where Delta is the order parameter and tau is the
quasiparticle elastic lifetime.Comment: 4 pages, 3 figures RevTeX forma
Determining the phonon DOS from specific heat measurements via maximum entropy methods
The maximum entropy and reverse Monte-Carlo methods are applied to the
computation of the phonon density of states (DOS) from heat capacity data. The
approach is introduced and the formalism is described. Simulated data is used
to test the method, and its sensitivity to noise. Heat capacity measurements
from diamond are used to demonstrate the use of the method with experimental
data. Comparison between maximum entropy and reverse Monte-Carlo results shows
the form of the entropy used here is correct, and that results are stable and
reliable. Major features of the DOS are picked out, and acoustic and optical
phonons can be treated with the same approach. The treatment set out in this
paper provides a cost-effective and reliable method for studies of the phonon
properties of materials.Comment: Reprint to improve access. 10 pages, 6 figure
Symmetry Scheme for Amino Acid Codons
Group theoretical concepts are invoked in a specific model to explain how
only twenty amino acids occur in nature out of a possible sixty four. The
methods we use enable us to justify the occurrence of the recently discovered
twenty first amino acid selenocysteine, and also enables us to predict the
possible existence of two more, as yet undiscovered amino acids.Comment: 18 pages which include 4 figures & 3 table
Lunar lander conceptual design
This paper is a first look at the problems of building a lunar lander to support a small lunar surface base. A series of trade studies was performed to define the lander. The initial trades concerned choosing number of stages, payload mass, parking orbit altitude, and propellant type. Other important trades and issues included plane change capability, propellant loading and maintenance location, and reusability considerations. Given a rough baseline, the systems were then reviewed. A conceptual design was then produced. The process was carried through only one iteration. Many more iterations are needed. A transportation system using reusable, aerobraked orbital transfer vehicles (OTV's) is assumed. These OTV's are assumed to be based and maintained at a low Earth orbit (LEO) space station, optimized for transportation functions. Single- and two-stage OTV stacks are considered. The OTV's make the translunar injection (TLI), lunar orbit insertion (LOI), and trans-Earth injection (TEI) burns, as well as midcourse and perigee raise maneuvers
-Stars or On Extending a Drawing of a Connected Subgraph
We consider the problem of extending the drawing of a subgraph of a given
plane graph to a drawing of the entire graph using straight-line and polyline
edges. We define the notion of star complexity of a polygon and show that a
drawing of an induced connected subgraph can be extended with at
most bends per edge, where is the
largest star complexity of a face of and is the size of the
largest face of . This result significantly improves the previously known
upper bound of [5] for the case where is connected. We also show
that our bound is worst case optimal up to a small additive constant.
Additionally, we provide an indication of complexity of the problem of testing
whether a star-shaped inner face can be extended to a straight-line drawing of
the graph; this is in contrast to the fact that the same problem is solvable in
linear time for the case of star-shaped outer face [9] and convex inner face
[13].Comment: Appears in the Proceedings of the 26th International Symposium on
Graph Drawing and Network Visualization (GD 2018
Supersoft X-rays reveal a classical nova in the M 31 globular cluster Bol 126
[Abridged] Classical novae (CNe) represent the main class of supersoft X-ray
sources (SSSs) in the central region of our neighbouring galaxy M 31. Only
three confirmed novae and three SSSs have been discovered in globular clusters
(GCs) of any galaxy so far, of which one nova and two SSSs (including the nova)
were found in M 31 GCs. To study the SSS state of CNe we carried out a
high-cadence X-ray monitoring of the M 31 central area with XMM-Newton and
Chandra. We analysed X-ray and optical data of a new transient X-ray source in
the M 31 GC Bol 126, discovered serendipitously in Swift observations. Our
optical data set was based on regular M 31 monitoring programmes from five
different small telescopes. Additionally, we made use of Pan-STARRS 1 data
obtained during the PAndromeda survey. Our observations reveal that the X-ray
source in Bol 126 is the third SSS in an M 31 GC and can be confirmed as the
second CN in the M 31 GC system. This nova is named M31N 2010-10f. Its
properties in the X-ray and optical regimes agree with a massive white dwarf
(M_WD >~ 1.3 M_sun) in the binary system. Incorporating the data on previously
found (suspected) novae in M 31 GCs we used our high-cadence X-ray monitoring
observations to estimate a tentative nova rate in the M 31 GC system of 0.05
/yr/GC. An optical estimate, based on the recent 10.5-year WeCAPP survey, gives
a lower nova rate, which is compatible with the X-ray rate on the 95%
confidence level. There is growing evidence that the nova rate in GCs is higher
than expected from primordial binary formation and under conditions as in the
field. Dynamical binary formation and/or additional accretion from the
intracluster medium are possible scenarios for an increased nova rate, but
observational confirmation for this enhancement has been absent, so far.
Regular X-ray monitoring observations of M 31 provide a promising strategy to
find these novae.Comment: 14 pages, 6 figures, 3 tables; accepted by Astronomy and Astrophysic
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