162,266 research outputs found
Bound states in d-density-wave phases
We investigate the quasiparticle spectrum near surfaces in a two-dimensional
system with d-density-wave order within a mean-field theory. For Fermi surfaces
with perfect nesting for the ordering wave vector of the d-density-wave, a zero
energy bound state occurs at [110] surfaces, in close analogy with the known
effect in d-wave superconducting states or graphite. When the shape of the
Fermi surface is changed by doping, the bound state energy moves away from the
Fermi level. Furthermore, away from half-filling we find inhomogeneous phases
with domain walls of the d-density-wave order parameter. The domain walls also
support low energy bound states. These phenomena might provide an experimental
test for hidden d-density-wave order in the high-Tc cuprates.Comment: 6 pages, 5 figure
Collaborative Representation based Classification for Face Recognition
By coding a query sample as a sparse linear combination of all training
samples and then classifying it by evaluating which class leads to the minimal
coding residual, sparse representation based classification (SRC) leads to
interesting results for robust face recognition. It is widely believed that the
l1- norm sparsity constraint on coding coefficients plays a key role in the
success of SRC, while its use of all training samples to collaboratively
represent the query sample is rather ignored. In this paper we discuss how SRC
works, and show that the collaborative representation mechanism used in SRC is
much more crucial to its success of face classification. The SRC is a special
case of collaborative representation based classification (CRC), which has
various instantiations by applying different norms to the coding residual and
coding coefficient. More specifically, the l1 or l2 norm characterization of
coding residual is related to the robustness of CRC to outlier facial pixels,
while the l1 or l2 norm characterization of coding coefficient is related to
the degree of discrimination of facial features. Extensive experiments were
conducted to verify the face recognition accuracy and efficiency of CRC with
different instantiations.Comment: It is a substantial revision of a previous conference paper (L.
Zhang, M. Yang, et al. "Sparse Representation or Collaborative
Representation: Which Helps Face Recognition?" in ICCV 2011
UGC 4599: A Photometric Study of the Nearest Hoag-Type Ring Galaxy
We present a photometric study of UGC 4599, a low-luminosity galaxy
superficially resembling Hoag's Object in that on sky survey images it appears
to be a complete ring surrounding a roundish core. The nature of the outer ring
of Hoag-type galaxies is still debated and may be related either to slow
secular evolution or to environmental processes, such as galaxy-galaxy
interactions. we show that in UGC 4599 (a) the nearly round central body
follows well an r^1/4 light profile almost all the way to the centre, (b) the
isophotes are strongly twisted with a sharp 45 deg transition at a radius of
r~6 arcsec, (c) the blue ring seems to have reached near-equilibrium
configuration with the central body, (d) the ring is actually composed of a
one-and-a-half turn spiral feature, and (e) one side of the spiral shows
conspicuous star formation in the form of at least nine HII regions, revealed
by their H_alpha emission. Based on the photometric data, together with HI
information from the literature, we characterize UGC 4599 as an elliptical-like
object surrounded by a luminous ring and a massive, extremely extended HI disc.
Given its observed properties, we rule out UGC 4599 as representing a late
phase in barred early-type galaxies evolution. We discuss the origin of UGC
4599 and conclude that this galaxy could be the result of a major interaction
between two gas-rich spiral galaxies that took place at least 5 Gyr ago.
However, deep optical imaging and a detailed stellar population analysis are
required to determine whether the large gas reservoir could have been accreted
directly from the intergalactic medium onto a pre-existing elliptical galaxy in
the early Universe. A detailed kinematical study will shed light on the exact
nature of the central body and the ring of UGC 4599.Comment: 14 pages, 6 figures and 5 tables. Accepted for publication in MNRAS.
The abstract is abridged compared to the published versio
Strong correlations at topological insulator surfaces and the breakdown of the bulk-boundary correspondence
The criteria for strong correlations on surfaces of three-dimensional
topological insulators are discussed. Usually, the Coulomb repulsion at such
surfaces is too weak for driving a phase transition to a strongly correlated
regime. I discuss a mechanism and possibilities of its experimental
implementation by which the strength of the Coulomb interaction can be tuned
over a wide range. In the strongly interacting regime, the surface states are
gapped, even though the topological classification of the bulk band structure
predicts gapless surface states
VLT diffraction-limited imaging at 11 and 18 micron of the nearest active galactic nuclei
Mid-infrared imaging at resolutions of 300 mas of the central kpc region of
13 nearby, well-known active galaxies is presented. The bulk of the mid-IR
emission is concentrated on an unresolved central source within a size of less
than 5 to 130 pc, depending on the object distance. Further resolved emission
is detected in 70% of the sample in the form of circumnuclear star-forming
rings or diffuse nuclear extended emission. In the three cases with
circumnuclear star formation, the stellar contribution is at least as important
as that of the AGN. In those with extended nuclear emission -- a third of the
sample -- this emission represents a few per cent of the total measured;
however, this contribution may be underestimated because of the chopped nature
of these observations. This extended emission is generally collimated in a
preferential direction often coinciding with that of the extended ionized gas
or the jet.
In all cases, the nuclear fluxes measured at 11.8 and 18.7 micron represent a
minor contribution of the flux levels measured by large aperture IRAS data at
the nearest energy bands of 12 and 25 micron. This contribution ranges from 30%
to less than 10%. In only three cases do the AGN fluxes agree with IRAS to
within a factor of 2. In the AGNs with strong circumnuclear star formation,
this component can well account for most of the IRAS flux measured in these
objects. But in all other cases, either a low surface brightness component
extending over galactic scales or strong extra-nuclear IR sources -- e.g. HII
regions in spiral arms -- have to be the main source of the IRAS emission. In
either case, the contribution of these components dwarfs that of the AGN at
mid-IR wavelengths.Comment: 17 pages, 10 figures. Accepted for publication in MNRA
Pairing symmetry of the one-band Hubbard model in the paramagnetic weak-coupling limit: a numerical RPA study
We study the spin-fluctuation-mediated superconducting pairing gap in a
weak-coupling approach to the Hubbard model for a two dimensional square
lattice in the paramagnetic state. Performing a comprehensive theoretical study
of the phase diagram as a function of filling, we find that the superconducting
gap exhibits transitions from p-wave at very low electron fillings to
d_{x^2-y^2}-wave symmetry close to half filling in agreement with previous
reports. At intermediate filling levels, different gap symmetries appear as a
consequence of the changes in the Fermi surface topology and the associated
structure of the spin susceptibility. In particular, the vicinity of a van Hove
singularity in the electronic structure close to the Fermi level has important
consequences for the gap structure in favoring the otherwise sub-dominant
triplet solution over the singlet d-wave solution. By solving the full gap
equation, we find that the energetically favorable triplet solutions are chiral
and break time reversal symmetry. Finally, we also calculate the detailed
angular gap structure of the quasi-particle spectrum, and show how
spin-fluctuation-mediated pairing leads to significant deviations from the
first harmonics both in the singlet d_{x^2-y^2} gap as well as the chiral
triplet gap solution.Comment: 11 pages 11 figure
Classification via Incoherent Subspaces
This article presents a new classification framework that can extract
individual features per class. The scheme is based on a model of incoherent
subspaces, each one associated to one class, and a model on how the elements in
a class are represented in this subspace. After the theoretical analysis an
alternate projection algorithm to find such a collection is developed. The
classification performance and speed of the proposed method is tested on the AR
and YaleB databases and compared to that of Fisher's LDA and a recent approach
based on on minimisation. Finally connections of the presented scheme
to already existing work are discussed and possible ways of extensions are
pointed out.Comment: 22 pages, 2 figures, 4 table
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