7,325 research outputs found
Density of states near the Mott-Hubbard transition in the limit of large dimensions
The zero temperature Mott-Hubbard transition as a function of the Coulomb
repulsion U is investigated in the limit of large dimensions. The behavior of
the density of states near the transition at U=U_c is analyzed in all orders of
the skeleton expansion. It is shown that only two transition scenarios are
consistent with the skeleton expansion for U<U_c: (i) The Mott-Hubbard
transition is "discontinuous" in the sense that in the density of states finite
spectral weight is redistributed at U_c. (ii) The transition occurs via a point
at U=U_c where the system is neither a Fermi liquid nor an insulator.Comment: 4 pages, 1 figure; revised version accepted for publication in Phys.
Rev. Let
Mott-Hubbard transition in infinite dimensions
We calculate the zero-temperature gap and quasiparticle weight of the
half-filled Hubbard model with a random dispersion relation. After
extrapolation to the thermodynamic limit, we obtain reliable bounds on these
quantities for the Hubbard model in infinite dimensions. Our data indicate that
the Mott-Hubbard transition is continuous, i.e., that the quasiparticle weight
becomes zero at the same critical interaction strength at which the gap opens.Comment: 4 pages, RevTeX, 5 figures included with epsfig Final version for
PRL, includes L=14 dat
Deep Depth From Focus
Depth from focus (DFF) is one of the classical ill-posed inverse problems in
computer vision. Most approaches recover the depth at each pixel based on the
focal setting which exhibits maximal sharpness. Yet, it is not obvious how to
reliably estimate the sharpness level, particularly in low-textured areas. In
this paper, we propose `Deep Depth From Focus (DDFF)' as the first end-to-end
learning approach to this problem. One of the main challenges we face is the
hunger for data of deep neural networks. In order to obtain a significant
amount of focal stacks with corresponding groundtruth depth, we propose to
leverage a light-field camera with a co-calibrated RGB-D sensor. This allows us
to digitally create focal stacks of varying sizes. Compared to existing
benchmarks our dataset is 25 times larger, enabling the use of machine learning
for this inverse problem. We compare our results with state-of-the-art DFF
methods and we also analyze the effect of several key deep architectural
components. These experiments show that our proposed method `DDFFNet' achieves
state-of-the-art performance in all scenes, reducing depth error by more than
75% compared to the classical DFF methods.Comment: accepted to Asian Conference on Computer Vision (ACCV) 201
Rolling to the tachyon vacuum in string field theory
We argue that the rolling-tachyon solution in cubic OSFT proceeds at late
times to precisely the analytic tachyon-vacuum solution constructed by Schnabl.
In addition, we demonstrate the relationship between the rolling-tachyon
solution and the standard BCFT description by showing that there is a finite
gauge transformation which relates the two.Comment: 16 pages, 5 figures, References and comments adde
Local-Ansatz Approach with Momentum Dependent Variational Parameters to Correlated Electron Systems
A new wavefunction which improves the Gutzwiller-type local ansatz method has
been proposed to describe the correlated electron system. The ground-state
energy, double occupation number, momentum distribution function, and
quasiparticle weight have been calculated for the half-filled band Hubbard
model in infinite dimensions. It is shown that the new wavefunction improves
the local-ansatz approach (LA) proposed by Stollhoff and Fulde. Especially,
calculated momentum distribution functions show a reasonable momentum
dependence. The result qualitatively differs from those obtained by the LA and
the Gutzwiller wavefunction. Furthermore, the present approach combined with
the projection operator method CPA is shown to describe quantitatively the
excitation spectra in the insulator regime as well as the critical Coulomb
interactions for a gap formation in infinite dimensions.Comment: To be published in Phys. Soc. Jpn. 77 No.11 (2008
Tachyon condensation on brane sphalerons
We consider a sphaleron solution in field theory that provides a toy model
for unstable D-branes of string theory. We investigate the tachyon condensation
on a Dp-brane. The localized modes, including a tachyon, arise in the spectrum
of a sphaleron solution of a \phi^4 field theory on M^{p+1}\times S^1. We use
these modes to find a multiscalar tachyon potential living on the sphaleron
world-volume. A complete cancelation between brane tension and the minimum of
the tachyon potential is found as the size of the circle becomes small.Comment: To appear in JHEP, 13 pages, 2 eps figures, minor changes and
references adde
Zeeman Spectroscopy of the Star Algebra
We solve the problem of finding all eigenvalues and eigenvectors of the
Neumann matrix of the matter sector of open bosonic string field theory,
including the zero modes, and switching on a background B-field. We give the
discrete eigenvalues as roots of transcendental equations, and we give
analytical expressions for all the eigenvectors.Comment: (1, 25) pages, 2 Figure
Disk Partition Function and Oscillatory Rolling Tachyons
An exact cubic open string field theory rolling tachyon solution was recently
found by Kiermaier et. al. and Schnabl. This oscillatory solution has been
argued to be related by a field redefinition to the simple exponential rolling
tachyon deformation of boundary conformal theory. In the latter approach, the
disk partition function takes a simple form. Out of curiosity, we compute the
disk partition function for an oscillatory tachyon profile, and find that the
result is nevertheless almost the same.Comment: 17 pages, 2 figures. v4: discussion clarified, appendix added,
conclusions unchanged; version to appear in J.Phys.
Rolling down to D-brane and tachyon matter
We investigate the spatially inhomogeneous decay of an unstable D-brane and
construct an asymptotic solution which describes a codimension one D-brane and
the tachyon matter in boundary string field theory. In this solution, the
tachyon matter exists around the lower-dimensional D-brane.Comment: 8 pages, references added, solution corrected, revised and extended
version, v4:published versio
News discourses on distant suffering: A critical discourse analysis of the 2003 SARS outbreak
News carries a unique signifying power, a power to represent events in particular ways (Fairclough, 1995). Applying Critical Discourse Analysis and Chouliaraki's theory on the mediation of suffering (2006), this article explores the news representation of the 2003 global SARS outbreak. Following a case-based methodology, we investigate how two Belgian television stations have covered the international outbreak of SARS. By looking into the mediation of four selected discursive moments, underlying discourses of power, hierarchy and compassion were unraveled. The analysis further identified the key role of proximity in international news reporting and supports the claim that Western news media mainly reproduce a Euro-American centered world order. This article argues that news coverage of international crises such as SARS constructs and maintains the socio-cultural difference between 'us' and 'them' as well as articulating global power hierarchies and a division of the world in zones of poverty and prosperity, danger and safety
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