8,356 research outputs found
Spectrum of the Hermitian Wilson-Dirac Operator for a Uniform Magnetic Field in Two Dimensions
It is shown that the eigenvalue problem for the hermitian Wilson-Dirac
operator of for a uniform magnetic field in two dimensions can be reduced to
one-dimensional problem described by a relativistic analog of the Harper
equation. An explicit formula for the secular equations is given in term of a
set of polynomials. The spectrum exhibits a fractal structure in the infinite
volume limit. An exact result concerning the index theorem for the overlap
Dirac operator is obtained.Comment: 8 pages, latex, 3 eps figures, minor correction
Spin fluctuations probed by NMR in paramagnetic spinel LiVO: a self-consistent renormalization theory
Low frequency spin fluctuation dynamics in paramagnetic spinel LiVO,
a rare 3-electron heavy fermion system, is investigated. A parametrized
self-consistent renormalization (SCR) theory of the dominant AFM spin
fluctuations is developed and applied to describe temperature and pressure
dependences of the low- nuclear spin-lattice relaxation rate in this
material. The experimental data for available down to K are
well reproduced by the SCR theory, showing the development of AFM spin
fluctuations as the paramagnetic metal approaches a magnetic instability under
the applied pressure. The low- upturn of detected below 0.6 K under
the highest applied pressure of 4.74 GPa is explained as the nuclear spin
relaxation effect due to the spin freezing of magnetic defects unavoidably
present in the measured sample of LiVO.Comment: 11 pages, 2 figure
Charmless Final State Interaction in B-> pi pi decays
We estimate effects of the final state interactions in B -> pi pi decays
coming from rescattering of pi pi via exchange of rho, sigma, f_0 mesons. Then
we include the rho rho rescattering via exchange of pi, omega, a_1 mesons and
finally we consider contributions of the a_1 pi rescattering via exchange of
rho. The absorptive parts of amplitudes for these processes are determined. In
the case of pi^+ pi^- decay mode, due to model uncertainties, the calculated
contribution is |M_A| =< 1.7 x 10^-8 GeV. This produces a small relative strong
phase for the tree and color-suppressed B -> pi pi amplitudes consistent with
the result of a recent phenomenological analysis based on the BaBar and Belle
results for the B -> pi pi branching ratios and CP asymmetries.Comment: 10 pages, 2 figure
A Phase-Space Approach to Collisionless Stellar Systems Using a Particle Method
A particle method for reproducing the phase space of collisionless stellar
systems is described. The key idea originates in Liouville's theorem which
states that the distribution function (DF) at time t can be derived from
tracing necessary orbits back to t=0. To make this procedure feasible, a
self-consistent field (SCF) method for solving Poisson's equation is adopted to
compute the orbits of arbitrary stars. As an example, for the violent
relaxation of a uniform-density sphere, the phase-space evolution which the
current method generates is compared to that obtained with a phase-space method
for integrating the collisionless Boltzmann equation, on the assumption of
spherical symmetry. Then, excellent agreement is found between the two methods
if an optimal basis set for the SCF technique is chosen. Since this
reproduction method requires only the functional form of initial DFs but needs
no assumptions about symmetry of the system, the success in reproducing the
phase-space evolution implies that there would be no need of directly solving
the collisionless Boltzmann equation in order to access phase space even for
systems without any special symmetries. The effects of basis sets used in SCF
simulations on the reproduced phase space are also discussed.Comment: 16 pages w/4 embedded PS figures. Uses aaspp4.sty (AASLaTeX v4.0). To
be published in ApJ, Oct. 1, 1997. This preprint is also available at
http://www.sue.shiga-u.ac.jp/WWW/prof/hozumi/papers.htm
DRINet for medical image segmentation
Convolutional neural networks (CNNs) have revolutionized medical image analysis over the past few years. The UNet architecture is one of the most well-known CNN architectures for semantic segmentation and has achieved remarkable successes in many different medical image segmentation applications. The U-Net architecture consists of standard convolution layers, pooling layers, and upsampling layers. These convolution layers learn representative features of input images and construct segmentations based on the features. However, the features learned by standard convolution layers are not distinctive when the differences among different categories are subtle in terms of intensity, location, shape, and size. In this paper, we propose a novel CNN architecture, called Dense-Res-Inception Net (DRINet), which addresses this challenging problem. The proposed DRINet consists of three blocks, namely a convolutional block with dense connections, a deconvolutional block with residual Inception modules, and an unpooling block. Our proposed architecture outperforms the U-Net in three different challenging applications, namely multi-class segmentation of cerebrospinal fluid (CSF) on brain CT images, multi-organ segmentation on abdominal CT images, multi-class brain tumour segmentation on MR images
Comments on Closed Bianchi Models
We show several kinematical properties that are intrinsic to the Bianchi
models with compact spatial sections. Especially, with spacelike hypersurfaces
being closed, (A) no anisotropic expansion is allowed for Bianchi type V and
VII(A\not=0), and (B) type IV and VI(A\not=0,1) does not exist. In order to
show them, we put into geometric terms what is meant by spatial homogeneity and
employ a mathematical result on 3-manifolds. We make clear the relation between
the Bianchi type symmetry of space-time and spatial compactness, some part of
which seem to be unnoticed in the literature. Especially, it is shown under
what conditions class B Bianchi models do not possess compact spatial sections.
Finally we briefly describe how this study is useful in investigating global
dynamics in (3+1)-dimensional gravity.Comment: 14 pages with one table, KUCP-5
Nonlinear Supersymmetry, Brane-bulk Interactions and Super-Higgs without Gravity
We derive the coupling of a hypermultiplet of N=2 global supersymmetry to the
Dirac-Born-Infeld Maxwell theory with linear N=1 and a second nonlinear
supersymmetry. At the level of global supersymmetry, this construction
corresponds to the interaction with Maxwell brane fields of bulk
hypermultiplets, such as the universal dilaton of type IIB strings compactified
on a Calabi-Yau manifold. It displays in particular the active role of a
four-form field. Constrained N=1 and N=2 superfields and the formulation of the
hypermultiplet in its single-tensor version are used to derive the nonlinear
realization, allowing a fully off-shell description. Exact results with
explicit symmetries and supersymmetries are then obtained. The
electric-magnetic dual version of the theory is also derived and the gauge
structure of the interaction is exemplified with N=2 nonlinear QED of a charged
hypermultiplet. Its Higgs phase describes a novel super-Higgs mechanism without
gravity, where the goldstino is combined with half of the hypermultiplet into
an N=1 massive vector multiplet.Comment: 42 page
Hybridization Mechanism for Cohesion of Cd-based Quasicrystals
Cohesion mechanism of cubic approximant crystals of newly discovered binary
quasicrystals, CdM (M=Yb and Ca), are studied theoretically. It is found
that stabilization due to alloying is obtained if M is an element with
low-lying unoccupied states. This leads to conclusion that the cohesion of
the Cd-based compounds is due to the hybridization of the states of Yb and
Ca with a wide band. %unlike known stable quasicrystals without transition
elements %such as Al-Li-Cu and Zn-Mg-RE (RE:rare earth). Although a diameter of
the Fermi sphere coincides with the strong Bragg peaks for Cd-Yb and Cd-Ca, the
Hume-Rothery mechanism does not play a principal role in the stability because
neither distinct pseudogap nor stabilization due to alloying is obtained for
isostructural Cd-Mg. In addition to the electronic origin, matching of the
atomic size is very crucial for the quasicrystal formation of the Cd-based
compounds. It is suggested that the glue atoms, which do not participate in the
icosahedral cluster, play an important role in stabilization of the compound.Comment: 4 pages, 2 figure
LiV2O4: evidence for two-stage screening
LiV2O4, a frustrated mixed valent metal (d^1 d^2), is argued to undergo
two spin-screening processes. The first quenches the effective spin to produce
the spin 1/2 behavior seen below room temperature[1], while the second produces
the heavy fermi liquid character seen at low temperatures[2]. We present here a
preliminary discussion of a t-J model with strong Hund's coupling of the
strongly correlated d-electrons. Valence fluctuations of the Hubbard operators
(S = {1/2} 1) combined with the frustration of the underlying corner-shared
tetrahedral vanadium lattice are the essential components of our model.Comment: 5 pages, 1 figure, accepted for proceedings of SCES'2001 Physica B,
minor change
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