5,355 research outputs found
Semiclassical transport in nearly symmetric quantum dots II: symmetry-breaking due to asymmetric leads
In this work - the second of a pair of articles - we consider transport
through spatially symmetric quantum dots with leads whose widths or positions
do not obey the spatial symmetry. We use the semiclassical theory of transport
to find the symmetry-induced contributions to weak localization corrections and
universal conductance fluctuations for dots with left-right, up-down, inversion
and four-fold symmetries. We show that all these contributions are suppressed
by asymmetric leads, however they remain finite whenever leads intersect with
their images under the symmetry operation. For an up-down symmetric dot, this
means that the contributions can be finite even if one of the leads is
completely asymmetric. We find that the suppression of the contributions to
universal conductance fluctuations is the square of the suppression of
contributions to weak localization. Finally, we develop a random-matrix theory
model which enables us to numerically confirm these results.Comment: (18pages - 9figures) This is the second of a pair of articles (v3
typos corrected - including in equations
A History of Flips in Combinatorial Triangulations
Given two combinatorial triangulations, how many edge flips are necessary and
sufficient to convert one into the other? This question has occupied
researchers for over 75 years. We provide a comprehensive survey, including
full proofs, of the various attempts to answer it.Comment: Added a paragraph referencing earlier work in the vertex-labelled
setting that has implications for the unlabeled settin
3-D Models of Embedded High-Mass Stars: Effects of a Clumpy Circumstellar Medium
We use 3-D radiative transfer models to show the effects of clumpy
circumstellar material on the observed infrared colors of high mass stars
embedded in molecular clouds. We highlight differences between 3-D clumpy and
1-D smooth models which can affect the interpretation of data. We discuss
several important properties of the emergent spectral energy distribution
(SED): More near-infrared light (scattered and direct from the central source)
can escape than in smooth 1-D models. The near- and mid-infrared SED of the
same object can vary significantly with viewing angle, depending on the clump
geometry along the sightline. Even the wavelength-integrated flux can vary with
angle by more than a factor of two. Objects with the same average circumstellar
dust distribution can have very different near-and mid-IR SEDs depending on the
clump geometry and the proximity of the most massive clump to the central
source.
Although clumpiness can cause similar objects to have very different SEDs,
there are some observable trends. Near- and mid-infrared colors are sensitive
to the weighted average distance of clumps from the central source and to the
magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared
emission remains a robust measure of the total dust mass. We present simulated
SEDs, colors, and images for 2MASS and Spitzer filters. We compare to
observations of some UCHII regions and find that 3-D clumpy models fit better
than smooth models. In particular, clumpy models with fractal dimensions in the
range 2.3-2.8, smooth to clumpy ratios of <50%, and density distributions with
shallow average radial density profiles fit the SEDs best.Comment: accepted to ApJ; version with full-res figures:
http://www.astro.virginia.edu/~ri3e/clumpy3d.pd
Radio Astronomical Polarimetry and the Lorentz Group
In radio astronomy the polarimetric properties of radiation are often
modified during propagation and reception. Effects such as Faraday rotation,
receiver cross-talk, and differential amplification act to change the state of
polarized radiation. A general description of such transformations is useful
for the investigation of these effects and for the interpretation and
calibration of polarimetric observations. Such a description is provided by the
Lorentz group, which is intimately related to the transformation properties of
polarized radiation. In this paper the transformations that commonly arise in
radio astronomy are analyzed in the context of this group. This analysis is
then used to construct a model for the propagation and reception of radio
waves. The implications of this model for radio astronomical polarimetry are
discussed.Comment: 10 pages, accepted for publication in Astrophysical Journa
The behavior of standing waves near the end of an open pipe with low mean flow
The acoustic standing wave near the end of an open pipe is investigated using spectrally analyzed high-speed transmission electronic speckle pattern interferometry. It is shown that the standing wave extends beyond the open end of the pipe and the amplitude decays exponentially with distance from the end. Additionally, a pressure node is observed near the end of the pipe in a position that is not spatially periodic with the other nodes in the standing wave. A sinusoidal fit to the amplitude of the standing wave inside the pipe indicates that the end correction is well predicted by current theory
Electrical Control of Linear Dichroism in Black Phosphorus from the Visible to Mid-Infrared
The incorporation of electrically tunable materials into photonic structures
such as waveguides and metasurfaces enables dynamic control of light
propagation by an applied potential. While many materials have been shown to
exhibit electrically tunable permittivity and dispersion, including transparent
conducting oxides (TCOs) and III-V semiconductors and quantum wells, these
materials are all optically isotropic in the propagation plane. In this work,
we report the first known example of electrically tunable linear dichroism,
observed here in few-layer black phosphorus (BP), which is a promising
candidate for multi-functional, broadband, tunable photonic elements. We
measure active modulation of the linear dichroism from the mid-infrared to
visible frequency range, which is driven by anisotropic quantum-confined Stark
and Burstein-Moss effects, and field-induced forbidden-to-allowed optical
transitions. Moreover, we observe high BP absorption modulation strengths,
approaching unity for certain thicknesses and photon energies
Rapid Circumstellar Disk Evolution and an Accelerating Star Formation Rate in the Infrared Dark Cloud M17 SWex
We present a catalog of 840 X-ray sources and first results from a 100 ks
Chandra X-ray Observatory imaging study of the filamentary infrared dark cloud
G014.22500.506, which forms the central regions of a larger cloud complex
known as the M17 southwest extension (M17 SWex). In addition to the rich
population of protostars and young stellar objects with dusty circumstellar
disks revealed by Spitzer Space Telescope archival data, we discover a
population of X-ray-emitting, intermediate-mass pre--main-sequence stars (IMPS)
that lack infrared excess emission from circumstellar disks. We model the
infrared spectral energy distributions of this source population to measure its
mass function and place new constraints on the inner dust disk destruction
timescales for 2-8 stars. We also place a lower limit on the star
formation rate (SFR) and find that it is quite high ( yr), equivalent to several Orion Nebula Clusters in
G14.2250.506 alone, and likely accelerating. The cloud complex has not
produced a population of massive, O-type stars commensurate with its SFR. This
absence of very massive () stars suggests that either (1)
M17 SWex is an example of a distributed mode of star formation that will
produce a large OB association dominated by intermediate-mass stars but
relatively few massive clusters, or (2) the massive cores are still in the
process of accreting sufficient mass to form massive clusters hosting O stars.Comment: 29 pages, 9 figures, accepted to Ap
Geometric nature of the environment-induced Berry phase and geometric dephasing
We investigate the geometric phase or Berry phase (BP) acquired by a
spin-half which is both subject to a slowly varying magnetic field and
weakly-coupled to a dissipative environment (either quantum or classical). We
study how this phase is modified by the environment and find that the
modification is of a geometric nature. While the original BP (for an isolated
system) is the flux of a monopole-field through the loop traversed by the
magnetic field, the environment-induced modification of the BP is the flux of a
quadrupole-like field. We find that the environment-induced phase is complex,
and its imaginary part is a geometric contribution to dephasing. Its sign
depends on the direction of the loop. Unlike the BP, this geometric dephasing
is gauge invariant for open paths of the magnetic field.Comment: new version : more discussion and less technicalities (4pages 2fig
- …