478 research outputs found
Optical Hall conductivity in bulk and nanostructured graphene beyond the Dirac approximation
We present a perturbative method for calculating the optical Hall
conductivity in a tight-binding framework based on the Kubo formalism. The
method involves diagonalization only of the Hamiltonian in absence of the
magnetic field, and thus avoids the computational problems usually arising due
to the huge magnetic unit cells required to maintain translational invariance
in presence of a Peierls phase. A recipe for applying the method to numerical
calculations of the magneto-optical response is presented. We apply the
formalism to the case of ordinary and gapped graphene in a next-nearest
neighbour tight-binding model as well as graphene antidot lattices. In both
case, we find unique signatures in the Hall response, that are not captured in
continuum (Dirac) approximations. These include a non-zero optical Hall
conductivity even when the chemical potential is at the Dirac point energy.
Numerical results suggest that this effect should be measurable in experiments.Comment: 7 pages, 4 figures, accepted in Physical Review
Extracting Tree-structures in CT data by Tracking Multiple Statistically Ranked Hypotheses
In this work, we adapt a method based on multiple hypothesis tracking (MHT)
that has been shown to give state-of-the-art vessel segmentation results in
interactive settings, for the purpose of extracting trees. Regularly spaced
tubular templates are fit to image data forming local hypotheses. These local
hypotheses are used to construct the MHT tree, which is then traversed to make
segmentation decisions. However, some critical parameters in this method are
scale-dependent and have an adverse effect when tracking structures of varying
dimensions. We propose to use statistical ranking of local hypotheses in
constructing the MHT tree, which yields a probabilistic interpretation of
scores across scales and helps alleviate the scale-dependence of MHT
parameters. This enables our method to track trees starting from a single seed
point. Our method is evaluated on chest CT data to extract airway trees and
coronary arteries. In both cases, we show that our method performs
significantly better than the original MHT method.Comment: Accepted for publication at the International Journal of Medical
Physics and Practic
Optical properties of graphene antidot lattices
Undoped graphene is semi-metallic and thus not suitable for many electronic
and optoelectronic applications requiring gapped semiconductor materials.
However, a periodic array of holes (antidot lattice) renders graphene
semiconducting with a controllable band gap. Using atomistic modelling, we
demonstrate that this artificial nanomaterial is a dipole-allowed direct gap
semiconductor with a very pronounced optical absorption edge. Hence, optical
infrared spectroscopy should be an ideal probe of the electronic structure. To
address realistic experimental situations, we include effects due to disorder
and the presence of a substrate in the analysis.Comment: 11 pages, 9 figures, accepted for publication in Phys. Rev.
Graph Refinement based Airway Extraction using Mean-Field Networks and Graph Neural Networks
Graph refinement, or the task of obtaining subgraphs of interest from
over-complete graphs, can have many varied applications. In this work, we
extract trees or collection of sub-trees from image data by, first deriving a
graph-based representation of the volumetric data and then, posing the tree
extraction as a graph refinement task. We present two methods to perform graph
refinement. First, we use mean-field approximation (MFA) to approximate the
posterior density over the subgraphs from which the optimal subgraph of
interest can be estimated. Mean field networks (MFNs) are used for inference
based on the interpretation that iterations of MFA can be seen as feed-forward
operations in a neural network. This allows us to learn the model parameters
using gradient descent. Second, we present a supervised learning approach using
graph neural networks (GNNs) which can be seen as generalisations of MFNs.
Subgraphs are obtained by training a GNN-based graph refinement model to
directly predict edge probabilities. We discuss connections between the two
classes of methods and compare them for the task of extracting airways from 3D,
low-dose, chest CT data. We show that both the MFN and GNN models show
significant improvement when compared to one baseline method, that is similar
to a top performing method in the EXACT'09 Challenge, and a 3D U-Net based
airway segmentation model, in detecting more branches with fewer false
positives.Comment: Accepted for publication at Medical Image Analysis. 14 page
Field-Aligned and Ionospheric Currents by AMPERE and SuperMAG During HSS/SIR-Driven Storms
This study considers 28 geomagnetic storms with Dst nT driven by high-speed streams (HSSs) and associated stream interaction regions (SIRs) during 2010-2017. Their impact on ionospheric horizontal and field-aligned currents (FACs) have been investigated using superposed epoch analysis of SuperMAG and AMPERE data, respectively. The zero epoch () was set to the onset of the storm main phase. Storms begin in the SIR with enhanced solar wind density and compressed southward oriented magnetic field. The integrated FAC and equivalent currents maximise 40 and 58 min after , respectively, followed by a small peak in the middle of the main phase (+4h), and a slightly larger peak just before the Dst minimum (+5.3h). The currents are strongly driven by the solar wind, and the correlation between the Akasofu and integrated FAC is . The number of substorm onsets maximises near . The storms were also separated into two groups based on the solar wind dynamic pressure p_dyn in the vicinity of the SIR. High p_dyn storms reach solar wind velocity maxima earlier and have shorter lead times from the HSS arrival to storm onset compared with low p_dyn events. The high p_dyn events also have sudden storm commencements, stronger solar wind driving and ionospheric response at , and are primarily responsible for the first peak in the currents after . After days, the currents and number of substorm onsets become higher for low compared with high p_dyn events, which may be related to higher solar wind speed.publishedVersio
Extraction of Airways using Graph Neural Networks
We present extraction of tree structures, such as airways, from image data as
a graph refinement task. To this end, we propose a graph auto-encoder model
that uses an encoder based on graph neural networks (GNNs) to learn embeddings
from input node features and a decoder to predict connections between nodes.
Performance of the GNN model is compared with mean-field networks in their
ability to extract airways from 3D chest CT scans.Comment: Extended Abstract submitted to MIDL, 2018. 3 page
Effect of ICME-Driven Storms on Field-Aligned and Ionospheric Currents From AMPERE and SuperMAG
Funding Information: This work was supported by the Academy of Finland project 314664 and 314670. We thank the AMPERE team and the AMPERE Science Center for providing the Iridium derived data products ( https://ampere.jhuapl.edu/ ). For the ground magnetometer data and substorm onset list, we gratefully thank the SuperMAG collaboration and all organizations involved ( https://supermag.jhuapl.edu/info/ ). For the geomagnetic indices, solar wind and interplanetary magnetic field data, we gratefully thank NASA/GSFC's Space Physics Data Facility's OMNIWeb ( https://omniweb.gsfc.nasa.gov/ ). Funding Information: This work was supported by the Academy of Finland project 314664 and 314670. We thank the AMPERE team and the AMPERE Science Center for providing the Iridium derived data products (https://ampere.jhuapl.edu/). For the ground magnetometer data and substorm onset list, we gratefully thank the SuperMAG collaboration and all organizations involved (https://supermag.jhuapl.edu/info/). For the geomagnetic indices, solar wind and interplanetary magnetic field data, we gratefully thank NASA/GSFC's Space Physics Data Facility's OMNIWeb (https://omniweb.gsfc.nasa.gov/). Publisher Copyright: © 2022. The Authors.Peer reviewe
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