22,476 research outputs found
Random Network Models and Quantum Phase Transitions in Two Dimensions
An overview of the random network model invented by Chalker and Coddington,
and its generalizations, is provided. After a short introduction into the
physics of the Integer Quantum Hall Effect, which historically has been the
motivation for introducing the network model, the percolation model for
electrons in spatial dimension 2 in a strong perpendicular magnetic field and a
spatially correlated random potential is described. Based on this, the network
model is established, using the concepts of percolating probability amplitude
and tunneling. Its localization properties and its behavior at the critical
point are discussed including a short survey on the statistics of energy levels
and wave function amplitudes. Magneto-transport is reviewed with emphasis on
some new results on conductance distributions. Generalizations are performed by
establishing equivalent Hamiltonians. In particular, the significance of
mappings to the Dirac model and the two dimensional Ising model are discussed.
A description of renormalization group treatments is given. The classification
of two dimensional random systems according to their symmetries is outlined.
This provides access to the complete set of quantum phase transitions like the
thermal Hall transition and the spin quantum Hall transition in two dimension.
The supersymmetric effective field theory for the critical properties of
network models is formulated. The network model is extended to higher
dimensions including remarks on the chiral metal phase at the surface of a
multi-layer quantum Hall system.Comment: 176 pages, final version, references correcte
Unifying the Fixed Order Evolution of Fragmentation Functions with the Modified Leading Logarithm Approximation
An approach which unifies the Double Logarithmic Approximation at small x and
the leading order DGLAP evolution of fragmentation functions at large x is
presented. This approach reproduces exactly the Modified Leading Logarithm
Approximation, but is more complete due to the degrees of freedom given to the
quark sector and the inclusion of the fixed order terms. We find that data from
the largest x values to the peak region can be better fitted than with other
approaches
Factorization breaking in high-transverse-momentum charged-hadron production at the Tevatron?
We compare the transverse momentum (p_T) distribution of inclusive
light-charged-particle production measured by the CDF Collaboration at the
Fermilab Tevatron with the theoretical prediction evaluated at next-to-leading
order in quantum chromodynamics (QCD) using fragmentation functions recently
determined through a global data fit. While, in the lower p_T range, the data
agree with the prediction within the theoretical error or slightly undershoot
it, they significantly exceed it in the upper p_T range, by several orders of
magnitude at the largest values of p_T, where perturbation theory should be
most reliable. This disagreement is too large to be remedied by introducing
additional produced particles into the calculation, and potentially challenges
the validity of the factorization theorem on which the parton model of QCD
relies. Clearly, a breakdown of the factorization theorem, being a fundamental
property of QCD, would be extremely difficult to understand.Comment: 9 pages, 5 figures; discussion extended, references added; accepted
for publication in Physical Review Letter
Localized low-frequency Neumann modes in 2d-systems with rough boundaries
We compute the relative localization volumes of the vibrational eigenmodes in
two-dimensional systems with a regular body but irregular boundaries under
Dirichlet and under Neumann boundary conditions. We find that localized states
are rare under Dirichlet boundary conditions but very common in the Neumann
case. In order to explain this difference, we utilize the fact that under
Neumann conditions the integral of the amplitudes, carried out over the whole
system area is zero. We discuss, how this condition leads to many localized
states in the low-frequency regime and show by numerical simulations, how the
number of the localized states and their localization volumes vary with the
boundary roughness.Comment: 7 pages, 4 figure
Observables in the Decays of B to Two Vector Mesons
In general there are nine observables in the decay of a B meson to two vector
mesons defined in terms of polarization correlations of these mesons. Only six
of these can be detected via the subsequent decay angular distributions because
of parity conservation in those decays. The remaining three require the
measurement of the spin polarization of one of the decay products.Comment: 12 pages, no figur
Hysteresis in the de Haas-van Alphen Effect
A hysteresis loop is observed for the first time in the de Haas-van Alphen
(dHvA) effect of beryllium at low temperatures and quantizing magnetic field
applied parallel to the hexagonal axis of the single crystal. The irreversible
behavior of the magnetization occurs at the paramagnetic part of the dHvA
period in conditions of Condon domain formation arising by strong enough dHvA
amplitude. The resulting extremely nonlinear response to a very small
modulation field offers the possibility to find in a simple way the Condon
domain phase diagram. From a harmonic analysis, the shape and size of the
hysteresis loop is constructed.Comment: 4 pages, 5 figures, submitted to PR
On the variable-charged black holes embedded into de Sitter space: Hawking's radiation
In this paper we study the Hawking evaporation of masses of variable-charged
Reissner-Nordstrom and Kerr-Newman, black holes embedded into the de Sitter
universe by considering the charge to be function of radial coordinate of the
spherically symmetric metric.Comment: LaTex, p. 2
The Carbon content in the Galactic CygnusX/DR21 star forming region
Observations of Carbon bearing species are among the most important
diagnostic probes of ongoing star formation. CO is a surrogate for H and is
found in the vicinity of star formation sites. There, [CI] emission is thought
to outline the dense molecular cores and extend into the lower density regions,
where the impinging interstellar UV radiation field plays a critical role for
the dissociation and ionization processes. Emission of ionized carbon ([CII])
is found to be even more extended than [CI] and is linking up with the ionized
medium. These different tracers emphasize the importance of multi-wavelength
studies to draw a coherent picture of the processes driving and driven by high
mass star formation. Until now, large scale surveys were only done with low
resolution, such as the COBE full sky survey, or were biased to a few selected
bright sources (e.g. Yamamoto et al. 2001, Schneider et al. 2003). A broader
basis of unbiased, high-resolution observations of [CI], CO, and [CII] may play
a key role to probe the material processed by UV radiation.Comment: 4 pages, 4 figure, to appear in "Proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium", ed. S. Pfalzner, C. Kramer, C. Straubmeier,
and A. Heithausen (Springer Verlag
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