25,952 research outputs found
Inversion mechanism for the transport current in type-II superconductors
The longitudinal transport problem (the current is applied parallel to some
bias magnetic field) in type-II superconductors is analyzed theoretically.
Based on analytical results for simplified configurations, and relying on
numerical studies for general scenarios, it is shown that a remarkable
inversion of the current flow in a surface layer may be predicted under a wide
set of experimental conditions. Strongly inhomogeneous current density
profiles, characterized by enhanced transport toward the center and reduced, or
even negative, values at the periphery of the conductor, are expected when the
physical mechanisms of flux depinning and consumption (via line cutting) are
recalled. A number of striking collateral effects, such as local and global
paramagnetic behavior, are predicted. Our geometrical description of the
macroscopic material laws allows a pictorial interpretation of the physical
phenomena underlying the transport backflow.Comment: 8 pages, 6 figures (Best quality pictures are available by author's
contact
One-dimensional relativistic dissipative system with constant force and its quantization
For a relativistic particle under a constant force and a linear velocity
dissipation force, a constant of motion is found. Problems are shown for
getting the Hamiltoninan of this system. Thus, the quantization of this system
is carried out through the constant of motion and using the quantization of the
velocity variable. The dissipative relativistic quantum bouncer is outlined
within this quantization approach.Comment: 11 pages, no figure
Comment on `Strong Vortex Liquid Correlation' from Multiterminal Measurements on Untwinned YBaCuO Single Crystals'
A.Rydh and \"O.Rapp [Phys. Rev. Lett. {\bf 86}, 1873 (2001).] claim that the
vortex liquid in untwinned YBaCuO crystals is correlated
above the melting transition, in striking contrast to previous work [D.L\'opez
{\it et al.}, Phys. Rev. Lett. {\bf 76}, 4034 (1996).]. In this Comment we
present new measurements using the same experimental technique on twinned and
untwinned YBaCuO crystals with similar overall
characteristics as those reported by Rydh and Rapp . The comparison of the
vortex correlation response in both cases indicates that the central conclusion
of their work is not correct. Our results reconfirm the work by L\'opez {\it et
al.} and points on the origin of the misinterpretation in the work of Rydh and
Rapp.Comment: comment on A.Rydh and \"O.Rapp, Phys. Rev. Lett. {\bf 86}, 1873
(2001). accepted in Phys. Rev. Let
Tensor mesons produced in tau lepton decays
Light tensor mesons (T = a_2, f_2 and K_2^*) can be produced in decays of tau
leptons. In this paper we compute the branching ratios of tau --> T pi nu
decays by assuming the dominance of intermediate virtual states to model the
form factors involved in the relevant hadronic matrix element. The exclusive
f_2(1270) pi^- decay mode turns out to have the largest branching ratio, of
O(10^-4) . Our results indicate that the contributions of tensor meson
intermediate states to the three-pseudoscalar channels of tau decays are rather
small.Comment: 10 pages, 1 figure. Version accepted for publication in PRD, some
typos are corrected and comments are added in section 4. Conclusions remain
unchange
Velocity quantization approach of the one-dimensional dissipative harmonic oscillator
Given a constant of motion for the one-dimensional harmonic oscillator with
linear dissipation in the velocity, the problem to get the Hamiltonian for this
system is pointed out, and the quantization up to second order in the
perturbation approach is used to determine the modification on the eigenvalues
when dissipation is taken into consideration. This quantization is realized
using the constant of motion instead of the Hamiltonian.Comment: 10 pages, 2 figure
Understanding co-operative R&D activity: evidence from four European countries
This paper investigates co-operative research activity by firms using data from
the 3rd Community Innovation Survey for four countries, France, Germany, Spain and the
UK. We build on the Cassiman and Veugelers (CV) (2002) study of Belgian manufacturing
firms, by incorporating information on the service sector, and considering the role of public
support in affecting firms’ decisions to co-operate. Our results support those in CV, in that
we find a positive relationship between the likelihood of undertaking co-operative R&D
and both incoming knowledge spillovers and the extent to which firms find strategic
methods important in appropriating the returns to innovative activity. We find that public
support is positively related to the probability of undertaking co-operative agreements
particularly with regard to the likelihood of co-operation with the research base. We find
some evidence, in particular for Spain, that firms carry out co-operative R&D to overcome
excessive perceived risks and financial constraints
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