6,144 research outputs found
Universality check of Abelian Monopoles
We study the Abelian projected SU(2) lattice gauge theory after gauge fixing
to the maximally Abelian gauge (MAG). In order to check the universality of the
Abelian dominance we employ the tadpole improved tree level (TI) action. We
show that the density of monopoles in the largest cluster (the IR component) is
finite in the continuum limit which is approximated already at relatively large
lattice spacing. The value itself is smaller than in the case of Wilson action.
We present results for the ratio of the Abelian to non-Abelian string tension
for both Wilson and TI actions for a number of lattice spacings in the range
0.06 fm < a < 0.35 fm. These results show that the ratio is between 0.9 and
0.95 for all considered values of lattice couplings and both actions. We
compare the properties of the monopole clusters in two gauges - in MAG and in
the Laplacian Abelian gauge (LAG). Whereas in MAG the infrared component of the
monopole density shows a good convergence to the continuum limit, we find that
in LAG it is even not clear whether a finite limit exists.Comment: 18 pages, 11 figure
Sequential Data-Adaptive Bandwidth Selection by Cross-Validation for Nonparametric Prediction
We consider the problem of bandwidth selection by cross-validation from a
sequential point of view in a nonparametric regression model. Having in mind
that in applications one often aims at estimation, prediction and change
detection simultaneously, we investigate that approach for sequential kernel
smoothers in order to base these tasks on a single statistic. We provide
uniform weak laws of large numbers and weak consistency results for the
cross-validated bandwidth. Extensions to weakly dependent error terms are
discussed as well. The errors may be {\alpha}-mixing or L2-near epoch
dependent, which guarantees that the uniform convergence of the cross
validation sum and the consistency of the cross-validated bandwidth hold true
for a large class of time series. The method is illustrated by analyzing
photovoltaic data.Comment: 26 page
Single-hole tunneling through a two-dimensional hole gas in intrinsic silicon
In this letter we report single-hole tunneling through a quantum dot in a
two-dimensional hole gas, situated in a narrow-channel field-effect transistor
in intrinsic silicon. Two layers of aluminum gate electrodes are defined on
Si/SiO using electron-beam lithography. Fabrication and subsequent
electrical characterization of different devices yield reproducible results,
such as typical MOSFET turn-on and pinch-off characteristics. Additionally,
linear transport measurements at 4 K result in regularly spaced Coulomb
oscillations, corresponding to single-hole tunneling through individual Coulomb
islands. These Coulomb peaks are visible over a broad range in gate voltage,
indicating very stable device operation. Energy spectroscopy measurements show
closed Coulomb diamonds with single-hole charging energies of 5--10 meV, and
lines of increased conductance as a result of resonant tunneling through
additional available hole states.Comment: 4 pages, 4 figures. This article has been submitted to Applied
Physics Letter
Variationnal study of ferromagnetism in the t1-t2 Hubbard chain
A one-dimensional Hubbard model with nearest and (negative) next-nearest
neighbour hopping is studied variationally. This allows to exclude saturated
ferromagnetism for . The variational boundary has a minimum
at a ``critical density'' and diverges for .Comment: 5 pages, LateX and 1 postscript figure. To appear in Physica
Spectroscopic parameters for silacyclopropynylidene, SiC, from extensive astronomical observations toward CW Leo (IRC +10216) with the Herschel satellite
A molecular line survey has been carried out toward the carbon-rich
asymptotic giant branch star CW Leo employing the HIFI instrument on board of
the Herschel satellite. Numerous features from 480 GHz to beyond 1100 GHz could
be assigned unambiguously to the fairly floppy SiC molecule. However,
predictions from laboratory data exhibited large deviations from the observed
frequencies even after some lower frequency data from this survey were
incorporated into a fit. Therefore, we present a combined fit of all available
laboratory data together with data from radio-astronomical observations.Comment: 7 pages, 1 figure, J. Mol. Spectrosc., appeared; CDMS links corrected
(version 2; current version: 3; may be updated later this year
The JCMT Spectral Legacy Survey: physical structure of the molecular envelope of the high-mass protostar AFGL2591
The understanding of the formation process of massive stars (>8 Msun) is
limited, due to theoretical complications and observational challenges.
We investigate the physical structure of the large-scale (~10^4-10^5 AU)
molecular envelope of the high-mass protostar AFGL2591 using spectral imaging
in the 330-373 GHz regime from the JCMT Spectral Legacy Survey. Out of ~160
spectral features, this paper uses the 35 that are spatially resolved.
The observed spatial distributions of a selection of six species are compared
with radiative transfer models based on a static spherically symmetric
structure, a dynamic spherical structure, and a static flattened structure. The
maps of CO and its isotopic variations exhibit elongated geometries on scales
of ~100", and smaller scale substructure is found in maps of N2H+, o-H2CO, CS,
SO2, CCH, and methanol lines. A velocity gradient is apparent in maps of all
molecular lines presented here, except SO, SO2, and H2CO. We find two emission
peaks in warm (Eup~200K) methanol separated by 12", indicative of a secondary
heating source in the envelope.
The spherical models are able to explain the distribution of emission for the
optically thin H13CO+ and C34S, but not for the optically thick HCN, HCO+, and
CS, nor for the optically thin C17O. The introduction of velocity structure
mitigates the optical depth effects, but does not fully explain the
observations, especially in the spectral dimension. A static flattened envelope
viewed at a small inclination angle does slightly better.
We conclude that a geometry of the envelope other than an isotropic static
sphere is needed to circumvent line optical depth effects. We propose that this
could be achieved in envelope models with an outflow cavity and/or
inhomogeneous structure at scales smaller than ~10^4 AU. The picture of
inhomogeneity is supported by observed substructure in at least six species.Comment: 17 pages; accepted for publication in A&
Twelve years of global observations of formaldehyde in the troposphere using GOME and SCIAMACHY sensors
This work presents global tropospheric formaldehyde columns retrieved from near-UV radiance measurements performed by the GOME instrument onboard ERS-2 since 1995, and by SCIAMACHY, in operation on ENVISAT since the end of 2002. A special effort has been made to ensure the coherence and quality of the CH<sub>2</sub>O dataset covering the period 1996–2007. Optimised DOAS settings are proposed in order to reduce the impact of two important sources of error in the derivation of slant columns, namely, the polarisation anomaly affecting the SCIAMACHY spectra around 350 nm, and a major absorption band of the O<sub>4</sub> collision complex centred near 360 nm. The air mass factors are determined from scattering weights generated using radiative transfer calculations taking into account the cloud fraction, the cloud height and the ground albedo. Vertical profile shapes of CH<sub>2</sub>O are provided by the global CTM IMAGES based on an up-to-date representation of emissions, atmospheric transport and photochemistry. A comprehensive error analysis is presented. This includes errors on the slant columns retrieval and errors on the air mass factors which are mainly due to uncertainties in the a priori profile and in the cloud properties. The major features of the retrieved formaldehyde column distribution are discussed and compared with previous CH<sub>2</sub>O datasets over the major emission regions
On Convergence of the Inexact Rayleigh Quotient Iteration with the Lanczos Method Used for Solving Linear Systems
For the Hermitian inexact Rayleigh quotient iteration (RQI), the author has
established new local general convergence results, independent of iterative
solvers for inner linear systems. The theory shows that the method locally
converges quadratically under a new condition, called the uniform positiveness
condition. In this paper we first consider the local convergence of the inexact
RQI with the unpreconditioned Lanczos method for the linear systems. Some
attractive properties are derived for the residuals, whose norms are
's, of the linear systems obtained by the Lanczos method. Based on
them and the new general convergence results, we make a refined analysis and
establish new local convergence results. It is proved that the inexact RQI with
Lanczos converges quadratically provided that with a
constant . The method is guaranteed to converge linearly provided
that is bounded by a small multiple of the reciprocal of the
residual norm of the current approximate eigenpair. The results are
fundamentally different from the existing convergence results that always
require , and they have a strong impact on effective
implementations of the method. We extend the new theory to the inexact RQI with
a tuned preconditioned Lanczos for the linear systems. Based on the new theory,
we can design practical criteria to control to achieve quadratic
convergence and implement the method more effectively than ever before.
Numerical experiments confirm our theory.Comment: 20 pages, 8 figures. arXiv admin note: text overlap with
arXiv:0906.223
A fabrication guide for planar silicon quantum dot heterostructures
We describe important considerations to create top-down fabricated planar
quantum dots in silicon, often not discussed in detail in literature. The
subtle interplay between intrinsic material properties, interfaces and
fabrication processes plays a crucial role in the formation of
electrostatically defined quantum dots. Processes such as oxidation, physical
vapor deposition and atomic-layer deposition must be tailored in order to
prevent unwanted side effects such as defects, disorder and dewetting. In two
directly related manuscripts written in parallel we use techniques described in
this work to create depletion-mode quantum dots in intrinsic silicon, and
low-disorder silicon quantum dots defined with palladium gates. While we
discuss three different planar gate structures, the general principles also
apply to 0D and 1D systems, such as self-assembled islands and nanowires.Comment: Accepted for publication in Nanotechnology. 31 pages, 12 figure
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