1,844 research outputs found
Trends in bulk electron-structural features of early transition-metal carbides
A detailed and systematic density-functional theory (DFT) study of a series
of early transition-metal carbides (TMC's) in the NaCl structure is presented.
The focus is on the trends in the electronic structure and nature of bonding,
which are essential for the understanding of the reactivity of TMC's. The
employed approach is based on a thorough complementary analysis of the electron
density differences, the density of states (DOS), the band structure, and the
real-space wave functions to gain insight into the bonding of this class of
materials and get a more detailed picture of it than previously achieved, as
the trend study allows for a systematic identification of the bond character
along the different bands. Our approach confirms the presence of both the
well-known TM--C and TM--TM bonds and, more importantly, it shows the existence
and significance of direct C--C bonds in all investigated TMC's, which are
frequently neglected but have been recently identified in some cases [Solid
State Commun. 121, 411 (2002); Phys. Rev. B 75, 235438 (2007)]. New information
on the spatial extent of the bonds, their \textit{k}-space location within the
band structure, and their importance for the bulk cohesion is provided. Trends
in covalency and ionicity are presented. The resulting electron-structural
trends are analyzed and discussed within a two-level model
Evaluation of Statistical Features for Medical Image Retrieval
In this paper we present a complete system allowing the classification of medical images in order to detect possible diseases present in them. The proposed method is developed in two distinct stages: calculation of descriptors and their classification. In the first stage we compute a vector of thirty-three statistical features: seven are related to statistics
of the first level order, fifteen to that of second level where thirteen are calculated by means of co-occurrence matrices and two with absolute gradient; the last thirteen finally are calculated using run-length matrices. In the second phase, using the descriptors already calculated, there is the actual image classification. Naive Bayes, RBF, Support VectorMa-
chine, K-Nearest Neighbor, Random Forest and Random Tree classifiers are used. The results obtained from the proposed system show that the analysis carried out both on textured and on medical images lead to have a high accuracy
Statistical Features for Image Retrieval: A Quantitative Comparison
In this paper we present a comparison between various statistical descriptors and analyze their goodness in
classifying textural images. The chosen statistical descriptors have been proposed by Tamura, Battiato and
Haralick. In this work we also test a combination of the three descriptors for texture analysis. The databases
used in our study are the well-known Brodatz’s album and DDSM(Heath et al., 1998). The computed features
are classified using the Naive Bayes, the RBF, the KNN, the Random Forest and Random Tree models. The
results obtained from this study show that we can achieve a high classification accuracy if the descriptors are
used all together
Atomic and molecular adsorption on transition-metal carbide (111) surfaces from density-functional theory: A trend study of surface electronic factors
This study explores atomic and molecular adsorption on a number of early
transition-metal carbides (TMC's) by means of density-functional theory
calculations. Trend studies are conducted with respect to both period and group
in the periodic table, choosing the substrates ScC, TiC, VC, ZrC, NbC,
delta-MoC, TaC, and WC and the adsorbates H, B, C, N, O, F, NH, NH2, and NH3.
Trends in adsorption strength are explained in terms of surface electronic
factors, by correlating the calculated adsorption energy values with the
calculated surface electronic structures. The results are rationalized with use
of a concerted-coupling model (CCM), which has previously been applied
succesfully to the description of adsorption on TiC(111) and TiN(111) surfaces
[Solid State Commun. 141, 48 (2007)]. First, the clean TMC(111) surfaces are
characterized by calculating surface energies, surface relaxations, Bader
charges, and surface-localized densities of states (DOS's). Detailed
comparisons between surface and bulk DOS's reveal the existence of
transition-metal localized SR's (TMSR's) in the pseudogap and of several
C-localized SR's (CSR's) in the upper valence band on all considered TMC(111)
surfaces. Then, atomic and molecular adsorption energies, geometries, and
charge transfers are presented. An analysis of the adsorbate-induced changes in
surface DOS's reveals a presence of both adsorbate--TMSR and adsorbate--CSR's
interactions, of varying strengths depending on the surface and the adsorbate.
These variations are correlated to the variations in adsorption energies. The
results are used to generalize the content and applications of the previously
proposed CCM to this larger class of substrates and adsorbates. Implications
for other classes of materials, for catalysis, and for other surface processes
are discussed
Shape matching by curve modelling and alignment
Automatic information retrieval in the eld of shape recognition has been widely covered by many
research elds. Various techniques have been developed using different approaches such as intensity-based, modelbased
and shape-based methods. Whichever is the way to represent the objects in images, a recognition method
should be robust in the presence of scale change, translation and rotation. In this paper we present a new recognition
method based on a curve alignment technique, for planar image contours. The method consists of various phases
including extracting outlines of images, detecting signicant points and aligning curves. The dominant points can
be manually or automatically detected. The matching phase uses the idea of calculating the overlapping indices
between shapes as similarity measures. To evaluate the effectiveness of the algorithm, two databases of 216 and
99 images have been used. A performance analysis and comparison is provided by precision-recall curves
Nature of Versatile Chemisorption on TiC(111) and TiN(111) Surfaces
Density-functional calculations on the polar TiX(111) (X = C, N) surfaces
show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR's) at
the Fermi level and X2p-derived SR's deep in the upper valence band and (ii)
for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies,
peaking at oxygen (9 eV). A concerted-coupling model, where adatom states
couple to both kinds of SR's in a concerted way, describes the adsorption. The
chemisorption versatility and the general nature of the model indicate
ramifications and predictive abilities in, e.g., growth and catalysis.Comment: 5 pages, 4 figures, submitted to Physical Review Letters (2006
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