1,180 research outputs found
D-Branes and Fluxes in Supersymmetric Quantum Mechanics
Type 0A string theory in the (2,4k) superconformal minimal model backgrounds,
with background ZZ D-branes or R-R fluxes can be formulated non-perturbatively.
The branes and fluxes have a description as threshold bound states in an
associated one-dimensional quantum mechanics which has a supersymmetric
structure, familiar from studies of the generalized KdV system. The relevant
bound state wavefunctions in this problem have unusual asymptotics (they are
not normalizable in general, and break supersymmetry) which are consistent with
the underlying description in terms of open and closed string sectors. The
overall organization of the physics is very pleasing: The physics of the closed
strings in the background of branes or fluxes is captured by the generalized
KdV system and non-perturbative string equations obtained by reduction of that
system (the hierarchy of equations found by Dalley, Johnson, Morris and
Watterstam). Meanwhile, the bound states wavefunctions, which describe the
physics of the ZZ D-brane (or flux) background in interaction with probe FZZT
D-branes, are captured by the generalized mKdV system, and non-perturbative
string equations obtained by reduction of that system (the Painleve II hierachy
found by Periwal and Shevitz in this context).Comment: 41 pages, LaTe
Modelling the spatial distribution of DEM Error
Assessment of a DEM’s quality is usually undertaken by deriving a measure of DEM accuracy – how close the DEM’s elevation values are to the true elevation. Measures such as Root Mean Squared Error and standard deviation of the error are frequently used. These measures summarise elevation errors in a DEM as a single value. A more detailed description of DEM accuracy would allow better understanding of DEM quality and the consequent uncertainty associated with using DEMs in analytical applications. The research presented addresses the limitations of using a single root mean squared error (RMSE) value to represent the uncertainty associated with a DEM by developing a new technique for creating a spatially distributed model of DEM quality – an accuracy surface. The technique is based on the hypothesis that the distribution and scale of elevation error within a DEM are at least partly related to morphometric characteristics of the terrain. The technique involves generating a set of terrain parameters to characterise terrain morphometry and developing regression models to define the relationship between DEM error and morphometric character. The regression models form the basis for creating standard deviation surfaces to represent DEM accuracy. The hypothesis is shown to be true and reliable accuracy surfaces are successfully created. These accuracy surfaces provide more detailed information about DEM accuracy than a single global estimate of RMSE
String Theory and Water Waves
We uncover a remarkable role that an infinite hierarchy of non-linear
differential equations plays in organizing and connecting certain {hat c}<1
string theories non-perturbatively. We are able to embed the type 0A and 0B
(A,A) minimal string theories into this single framework. The string theories
arise as special limits of a rich system of equations underpinned by an
integrable system known as the dispersive water wave hierarchy. We observe that
there are several other string-like limits of the system, and conjecture that
some of them are type IIA and IIB (A,D) minimal string backgrounds. We explain
how these and several string-like special points arise and are connected. In
some cases, the framework endows the theories with a non-perturbative
definition for the first time. Notably, we discover that the Painleve IV
equation plays a key role in organizing the string theory physics, joining its
siblings, Painleve I and II, whose roles have previously been identified in
this minimal string context.Comment: 49 pages, 4 figure
Backlund Transformations, D-Branes, and Fluxes in Minimal Type 0 Strings
We study the Type 0A string theory in the (2,4k) superconformal minimal model
backgrounds, focusing on the fully non-perturbative string equations which
define the partition function of the model. The equations admit a parameter,
Gamma, which in the spacetime interpretation controls the number of background
D-branes, or R-R flux units, depending upon which weak coupling regime is
taken. We study the properties of the string equations (often focusing on the
(2,4) model in particular) and their physical solutions. The solutions are the
potential for an associated Schrodinger problem whose wavefunction is that of
an extended D-brane probe. We perform a numerical study of the spectrum of this
system for varying Gamma and establish that when Gamma is a positive integer
the equations' solutions have special properties consistent with the spacetime
interpretation. We also show that a natural solution-generating transformation
(that changes Gamma by an integer) is the Backlund transformation of the KdV
hierarchy specialized to (scale invariant) solitons at zero velocity. Our
results suggest that the localized D-branes of the minimal string theories are
directly related to the solitons of the KdV hierarchy. Further, we observe an
interesting transition when Gamma=-1.Comment: 17 pages, 3 figure
Assessing direct contributions of morphological awareness and prosodic sensitivity to children’s word reading and reading comprehension
We examined the independent contributions of prosodic sensitivity and morphological awareness to word reading, text reading accuracy, and reading comprehension. We did so in a longitudinal study of English-speaking children (N = 70). At 5 to 7 years of age, children completed the metalinguistic measures along with control measures of phonological awareness and vocabulary. Children completed the reading measures two years later. Morphological awareness, but not prosodic sensitivity made a significant independent contribution to word reading, text reading accuracy and reading comprehension. The effects of morphological awareness on reading comprehension remained after controls for word reading. These results suggest that morphological awareness needs to be considered seriously in models of reading development and that prosodic sensitivity might have primarily indirect relations to reading outcomes.
Keywords: Morphological Awareness; Prosody; Word Reading; Reading Comprehension
Sub-surface Oxygen and Surface Oxide Formation at Ag(111): A Density-functional Theory Investigation
To help provide insight into the remarkable catalytic behavior of the
oxygen/silver system for heterogeneous oxidation reactions, purely sub-surface
oxygen, and structures involving both on-surface and sub-surface oxygen, as
well as oxide-like structures at the Ag(111) surface have been studied for a
wide range of coverages and adsorption sites using density-functional theory.
Adsorption on the surface in fcc sites is energetically favorable for low
coverages, while for higher coverage a thin surface-oxide structure is
energetically favorable. This structure has been proposed to correspond to the
experimentally observed (4x4) phase. With increasing O concentrations, thicker
oxide-like structures resembling compressed Ag2O(111) surfaces are
energetically favored. Due to the relatively low thermal stability of these
structures, and the very low sticking probability of O2 at Ag(111), their
formation and observation may require the use of atomic oxygen (or ozone, O3)
and low temperatures. We also investigate diffusion of O into the sub-surface
region at low coverage (0.11 ML), and the effect of surface Ag vacancies in the
adsorption of atomic oxygen and ozone-like species. The present studies,
together with our earlier investigations of on-surface and
surface-substitutional adsorption, provide a comprehensive picture of the
behavior and chemical nature of the interaction of oxygen and Ag(111), as well
as of the initial stages of oxide formation.Comment: 17 pages including 14 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Interplay between spatially explicit sediment sourcing, hierarchical river-network structure, and in-channel bed material sediment transport and storage dynamics
Understanding how sediment moves along source to sink pathways through watersheds„from hillslopes to channels and in and out of floodplains„is a fundamental problem in geomorphology. We contribute to advancing this understanding by modeling the transport and in-channel storage dynamics of bed material sediment on a river network over a 600æyear time period. Specifically, we present spatiotemporal changes in bed sediment thickness along an entire river network to elucidate how river networks organize and process sediment supply. We apply our model to sand transport in the agricultural Greater Blue Earth River Basin in Minnesota. By casting the arrival of sediment to links of the network as a Poisson process, we derive analytically (under supply-limited conditions) the time-averaged probability distribution function of bed sediment thickness for each link of the river network for any spatial distribution of inputs. Under transport-limited conditions, the analytical assumptions of the Poisson arrival process are violated (due to in-channel storage dynamics) where we find large fluctuations and periodicity in the time series of bed sediment thickness. The time series of bed sediment thickness is the result of dynamics on a network in propagating, altering, and amalgamating sediment inputs in sometimes unexpected ways. One key insight gleaned from the model is that there can be a small fraction of reaches with relatively low-transport capacity within a nonequilibrium river network acting as ñbottlenecksî that control sediment to downstream reaches, whereby fluctuations in bed elevation can dissociate from signals in sediment supply. ©2017. American Geophysical Union. All Rights Reserved
Phonons from neutron powder diffraction
The spherically averaged structure function \soq obtained from pulsed
neutron powder diffraction contains both elastic and inelastic scattering via
an integral over energy. The Fourier transformation of \soq to real space, as
is done in the pair density function (PDF) analysis, regularizes the data, i.e.
it accentuates the diffuse scattering. We present a technique which enables the
extraction of off-center phonon information from powder diffraction experiments
by comparing the experimental PDF with theoretical calculations based on
standard interatomic potentials and the crystal symmetry. This procedure
(dynamics from powder diffraction(DPD)) has been successfully implemented for
two systems, a simple metal, fcc Ni, and an ionic crystal, CaF. Although
computationally intensive, this data analysis allows for a phonon based
modeling of the PDF, and additionally provides off-center phonon information
from powder neutron diffraction
Dispersion force for materials relevant for micro and nanodevices fabrication
The dispersion (van der Waals and Casimir) force between two semi-spaces are
calculated using the Lifshitz theory for different materials relevant for micro
and nanodevices fabrication, namely, gold, silicon, gallium arsenide, diamond
and two types of diamond-like carbon (DLC), silicon carbide, silicon nitride
and silicon dioxide. The calculations were performed using recent experimental
optical data available in the literature, usually ranging from the far infrared
up to the extreme ultraviolet bands of the electromagnetic spectrum. The
results are presented in the form of a correction factor to the Casimir force
predicted between perfect conductors, for the separation between the
semi-spaces varying from 1 nanometre up to 1 micrometre. The relative
importance of the contributions to the dispersion force of the optical
properties in different spectral ranges is analyzed. The role of the
temperature for semiconductors and insulators is also addressed. The results
are meant to be useful for the estimation of the impact of the Casimir and van
der Waals forces on the operational parameters of micro and nanodevices
Insights into the function of silver as an oxidation catalyst by ab initio, atomistic thermodynamics
To help understand the high activity of silver as an oxidation catalyst,
e.g., for the oxidation of ethylene to epoxide and the dehydrogenation of
methanol to formaldehyde, the interaction and stability of oxygen species at
the Ag(111) surface has been studied for a wide range of coverages. Through
calculation of the free energy, as obtained from density-functional theory and
taking into account the temperature and pressure via the oxygen chemical
potential, we obtain the phase diagram of O/Ag(111). Our results reveal that a
thin surface-oxide structure is most stable for the temperature and pressure
range of ethylene epoxidation and we propose it (and possibly other similar
structures) contains the species actuating the catalysis. For higher
temperatures, low coverages of chemisorbed oxygen are most stable, which could
also play a role in oxidation reactions. For temperatures greater than about
775 K there are no stable oxygen species, except for the possibility of O atoms
adsorbed at under-coordinated surface sites Our calculations rule out thicker
oxide-like structures, as well as bulk dissolved oxygen and molecular
ozone-like species, as playing a role in the oxidation reactions.Comment: 15 pages including 9 figures, Related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
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