19,797 research outputs found
General Solutions for Tunneling of Scalar Fields with Quartic Potentials in de Sitter Space
The tunneling rates for scalar fields with quartic potentials in de Sitter
space in the limit of no gravitational back reaction are calculated numerically
and the results are fitted by analytic formulae.Comment: (Contours in Figure 1 corrected, two-dimensional fitting coefficient
corrected, references added.), 16 pages, KUNS 124
Upper bound for the conductivity of nanotube networks
Films composed of nanotube networks have their conductivities regulated by
the junction resistances formed between tubes. Conductivity values are enhanced
by lower junction resistances but should reach a maximum that is limited by the
network morphology. By considering ideal ballistic-like contacts between
nanotubes we use the Kubo formalism to calculate the upper bound for the
conductivity of such films and show how it depends on the nanotube
concentration as well as on their aspect ratio. Highest measured conductivities
reported so far are approaching this limiting value, suggesting that further
progress lies with nanowires other than nanotubes.Comment: 3 pages, 1 figure. Minor changes. Accepted for publication in Applied
Physics Letter
Integrated control of vector-borne diseases of livestock--pyrethroids: panacea or poison?
Tick- and tsetse-borne diseases cost Africa approximately US$4-5 billion per year in livestock production-associated losses. The use of pyrethroid-treated cattle to control ticks and tsetse promises to be an increasingly important tool to counter this loss. However, uncontrolled use of this technology might lead to environmental damage, acaricide resistance in tick populations and a possible exacerbation of tick-borne diseases. Recent research to identify, quantify and to develop strategies to avoid these effects are highlighted
Low-temperature thermopower study of YbRh2Si2
The heavy-fermion compound YbRh2Si2 exhibits an antiferromagnetic (AFM) phase
transition at an extremely low temperature of TN = 70 mK. Upon applying a tiny
magnetic field of Bc = 60 mT the AFM ordering is suppressed and the system is
driven toward a field-induced quantum critical point (QCP). Here, we present
low-temperature thermopower S(T) measurements of high-quality YbRh2Si2 single
crystals down to 30 mK. S(T) is found negative with comparably large values in
the paramagnetic state. In zero field no Landau-Fermi-liquid (LFL) like
behavior is observed within the magnetically ordered phase. However, a sign
change from negative to positive appears at lowest temperatures on the magnetic
side of the QCP. For higher fields B > Bc a linear extrapolation of S to zero
clearly evidences the recovery of LFL regime. The crossover temperature is
sharply determined and coincides perfectly with the one derived from
resistivity and specific heat investigations.Comment: LT25 conference proceedings in Journal of Physics: Conference Serie
Local Moments in an Interacting Environment
We discuss how local moment physics is modified by the presence of
interactions in the conduction sea. Interactions in the conduction sea are
shown to open up new symmetry channels for the exchange of spin with the
localized moment. We illustrate this conclusion in the strong-coupling limit by
carrying out a Schrieffer Wolff transformation for a local moment in an
interacting electron sea, and show that these corrections become very severe in
the approach to a Mott transition. As an example, we show how the Zhang Rice
reduction of a two-band model is modified by these new effects.Comment: Latex file with two postscript figures. Revised version, with more
fully detailed calculation
Scaling of excitations in dimerized and frustrated spin-1/2 chains
We study the finite-size behavior of the low-lying excitations of spin-1/2
Heisenberg chains with dimerization and next-to-nearest neighbors interaction,
J_2. The numerical analysis, performed using density-matrix renormalization
group, confirms previous exact diagonalization results, and shows that, for
different values of the dimerization parameter \delta, the elementary triplet
and singlet excitations present a clear scaling behavior in a wide range of
\ell=L/\xi (where L is the length of the chain and \xi is the correlation
length). At J_2=J_2c, where no logarithmic corrections are present, we compare
the numerical results with finite-size predictions for the sine-Gordon model
obtained using Luscher's theory. For small \delta we find a very good agreement
for \ell > 4 or 7 depending on the excitation considered.Comment: 4 pages, 4 eps figures, RevTeX 4 class, same version as in PR
Note on clock synchronization and Edwards transformations
Edwards transformations relating inertial frames with arbitrary clock
synchronization are reminded and put in more general setting. Their group
theoretical context is described.Comment: 11 pages, no figures; final version, to appear in Foundations of
Physics Letter
T-violation tests for relativity principles
We consider the implications of a violation of the equivalence principle or
of Lorentz invariance in the neutrino sector for the T-asymmetry in a
three-flavour framework. We find that additional mixing due to these
mechanisms, while obeying all present bounds, can lead to a substantial
enhancement, suppression, and/or sign change in for the preferred
energies and baselines of a neutrino factory. This in turn allows for the
possibility of improving existing constraints by several orders of magnitude.Comment: 2 pages; Talk given at the 4th NuFact '02 Workshop (Neutrino
Factories Based On Muon Storage Rings), 1-6 Jul 2002, London, England; To
appear in proceeding
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