3,373 research outputs found
Global monopole, dark matter and scalar tensor theory
In this article, we discuss the space-time of a global monopole field as a
candidate for galactic dark matter in the context of scalar tensor theory.Comment: 8 pages, Accepted in Mod. Phys. Lett.
Self-forces in the Spacetime of Multiple Cosmic Strings
We calculate the electromagnetic self-force on a stationary linear
distribution of four-current in the spacetime of multiple cosmic strings. It is
shown that if the current is infinitely thin and stretched along a line which
is parallel to the strings the problem admits an explicit solution.Comment: This paper has been produced in Latex format and has 18 page
Nonplanar integrability at two loops
In this article we compute the action of the two loop dilatation operator on
restricted Schur polynomials that belong to the su(2) sector, in the displaced
corners approximation. In this non-planar large N limit, operators that
diagonalize the one loop dilatation operator are not corrected at two loops.
The resulting spectrum of anomalous dimensions is related to a set of decoupled
harmonic oscillators, indicating integrability in this sector of the theory at
two loops. The anomalous dimensions are a non-trivial function of the 't Hooft
coupling, with a spectrum that is continuous and starting at zero at large N,
but discrete at finite N.Comment: version to appear in JHE
Verification of Magnitude and Phase Responses in Fixed-Point Digital Filters
In the digital signal processing (DSP) area, one of the most important tasks
is digital filter design. Currently, this procedure is performed with the aid
of computational tools, which generally assume filter coefficients represented
with floating-point arithmetic. Nonetheless, during the implementation phase,
which is often done in digital signal processors or field programmable gate
arrays, the representation of the obtained coefficients can be carried out
through integer or fixed-point arithmetic, which often results in unexpected
behavior or even unstable filters. The present work addresses this issue and
proposes a verification methodology based on the digital-system verifier
(DSVerifier), with the goal of checking fixed-point digital filters w.r.t.
implementation aspects. In particular, DSVerifier checks whether the number of
bits used in coefficient representation will result in a filter with the same
features specified during the design phase. Experimental results show that
errors regarding frequency response and overflow are likely to be identified
with the proposed methodology, which thus improves overall system's
reliability
Equivalence of Fokker-Planck approach and non-linear -model for disordered wires in the unitary symmetry class
The exact solution of the Dorokhov-Mello-Pereyra-Kumar-equation for quasi
one-dimensional disordered conductors in the unitary symmetry class is employed
to calculate all -point correlation functions by a generalization of the
method of orthogonal polynomials. We obtain closed expressions for the first
two conductance moments which are valid for the whole range of length scales
from the metallic regime () to the insulating regime () and
for arbitrary channel number. In the limit (with )
our expressions agree exactly with those of the non-linear -model
derived from microscopic Hamiltonians.Comment: 9 pages, Revtex, one postscript figur
High-resolution abundance analysis of red giants in the metal-poor bulge globular cluster HP~1
The globular cluster HP~1 is projected at only 3.33 degrees from the Galactic
center. Together with its distance, this makes it one of the most central
globular clusters in the Milky Way. It has a blue horizontal branch (BHB) and a
metallicity of [Fe/H]~-1.0. This means that it probably is one of the oldest
objects in the Galaxy. Abundance ratios can reveal the nucleosynthesis pattern
of the first stars as well as the early chemical enrichment and early formation
of stellar populations. High-resolution spectra obtained for six stars were
analyzed to derive the abundances of the light elements C, N, O, Na, and Al,
the alpha-elements Mg, Si, Ca, and Ti, and the heavy elements Sr, Y , Zr, Ba,
La, and Eu.} High-resolution spectra of six red giants that are confirmed
members of the bulge globular cluster HP~1 were obtained with the 8m VLT
UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVES configuration.
The spectroscopic parameter derivation was based on the excitation and
ionization equilibrium of FeI and FeII. We confirm a mean metallicity of [Fe/H]
= -1.06~0.10, by adding the two stars that were previously analyzed in HP~1.
The alpha-elements O and Mg are enhanced by about +0.3<[O,Mg/Fe]<+0.5 dex, Si
is moderately enhanced with +0.15<[Si/Fe]<+0.35dex, while Ca and Ti show lower
values of -0.04<[Ca,Ti/Fe]<+0.28dex. The r-element Eu is also enhanced with
[Eu/Fe]~+0.4, which together with O and Mg is indicative of early enrichment by
type II supernovae. Na and Al are low, but it is unclear if Na-O are
anticorrelated. The heavy elements are moderately enhanced, with
-0.20<[La/Fe]<+0.43dex and 0.0<[Ba/Fe]<+0.75~dex, which is compatible with
r-process formation. The spread in Y, Zr, Ba, and La abundances, on the other
hand, appears to be compatible with the spinstar scenario or other additional
mechanisms such as the weak r-process.Comment: 15 pages, 8 figures In press in Astronomy & Astrophysics (2016
Path Integral Approach to the Scattering Theory of Quantum Transport
The scattering theory of quantum transport relates transport properties of
disordered mesoscopic conductors to their transfer matrix \bbox{T}. We
introduce a novel approach to the statistics of transport quantities which
expresses the probability distribution of \bbox{T} as a path integral. The
path integal is derived for a model of conductors with broken time reversal
invariance in arbitrary dimensions. It is applied to the
Dorokhov-Mello-Pereyra-Kumar (DMPK) equation which describes
quasi-one-dimensional wires. We use the equivalent channel model whose
probability distribution for the eigenvalues of \bbox{TT}^{\dagger} is
equivalent to the DMPK equation independent of the values of the forward
scattering mean free paths. We find that infinitely strong forward scattering
corresponds to diffusion on the coset space of the transfer matrix group. It is
shown that the saddle point of the path integral corresponds to ballistic
conductors with large conductances. We solve the saddle point equation and
recover random matrix theory from the saddle point approximation to the path
integral.Comment: REVTEX, 9 pages, no figure
Generalized Fokker-Planck Equation For Multichannel Disordered Quantum Conductors
The Dorokhov-Mello-Pereyra-Kumar (DMPK) equation, which describes the
distribution of transmission eigenvalues of multichannel disordered conductors,
has been enormously successful in describing a variety of detailed transport
properties of mesoscopic wires. However, it is limited to the regime of quasi
one dimension only. We derive a one parameter generalization of the DMPK
equation, which should broaden the scope of the equation beyond the limit of
quasi one dimension.Comment: 8 pages, abstract, introduction and summary rewritten for broader
readership. To be published in Phys. Rev. Let
The Effects of Phase Separation in the Cuprate Superconductors
Phase separation has been observed by several different experiments and it is
believed to be closely related with the physics of cuprates but its exactly
role is not yet well known. We propose that the onset of pseudogap phenomenon
or the upper pseudogap temperature has its origin in a spontaneous phase
separation transition at the temperature . In order to perform
quantitative calculations, we use a Cahn-Hilliard (CH) differential equation
originally proposed to the studies of alloys and on a spinodal decomposition
mechanism. Solving numerically the CH equation it is possible to follow the
time evolution of a coarse-grained order parameter which satisfies a
Ginzburg-Landau free-energy functional commonly used to model superconductors.
In this approach, we follow the process of charge segregation into two main
equilibrium hole density branches and the energy gap normally attributed to the
upper pseudogap arises as the free-energy potential barrier between these two
equilibrium densities below . This simulation provides quantitative
results %on the hole doping and temperature %dependence of the degree of the
charge inhomogeneity in agreement with %some experiments and the simulations
reproduce the observed stripe and granular pattern of segregation. Furthermore,
with a Bogoliubov-deGennes (BdG) local superconducting critical temperature
calculation for the lower pseudogap or the onset of local superconductivity, it
yields novel interpretation of several non-conventional measurements on
cuprates.Comment: Published versio
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