2,265 research outputs found
A kinetic theory of diffusion in general relativity with cosmological scalar field
A new model to describe the dynamics of particles undergoing diffusion in
general relativity is proposed. The evolution of the particle system is
described by a Fokker-Planck equation without friction on the tangent bundle of
spacetime. It is shown that the energy-momentum tensor for this matter model is
not divergence-free, which makes it inconsistent to couple the Fokker-Planck
equation to the Einstein equations. This problem can be solved by postulating
the existence of additional matter fields in spacetime or by modifying the
Einstein equations. The case of a cosmological scalar field term added to the
left hand side of the Einstein equations is studied in some details. For the
simplest cosmological model, namely the flat Robertson-Walker spacetime, it is
shown that, depending on the initial value of the cosmological scalar field,
which can be identified with the present observed value of the cosmological
constant, either unlimited expansion or the formation of a singularity in
finite time will occur in the future. Future collapse into a singularity also
takes place for a suitable small but positive present value of the cosmological
constant, in contrast to the standard diffusion-free scenario.Comment: 17 pages, no figures. The present version corrects an erroneous
statement on the physical interpretation of the results made in the original
publicatio
Anisotropic properties of MgB2 by torque magnetometry
Anisotropic properties of superconducting MgB2 obtained by torque
magnetometry are compared to theoretical predictions, concentrating on two
issues. Firstly, the angular dependence of Hc2 is shown to deviate close to Tc
from the dependence assumed by anisotropic Ginzburg-Landau theory. Secondly,
from the evaluation of torque vs angle curves it is concluded that the
anisotropy of the penetration depth gamma_lambda has to be substantially higher
at low temperature than theoretical estimates, at least in fields higher than
0.2 T.Comment: 2 p.,2 Fig., submitted to Physica C (M2S-Rio proceedings); v2: 1 ref
adde
Neurasthenia in a longitudinal cohort study of young adults
This study examines the concept of neurasthenia in a longitudinal cohort of young adults selected from a community sample of the canton of Zurich, Switzerland. The major focus is on the validity of the case definition of neurasthenia. Close approximations of the proposed descriptive and research definitions of the ICD-10 are employed as well as the concept of ‘irritable weakness' as described in 1831 by Kraus (1926-1932). The prevalence of neurasthenia defined according to the ICD-10 criteria was: 1% across 10 years and 0·9% in 1988 for a duration criterion of ≥ 3 months; and 8·1% across 10 years and 12% in 1988 for a duration criterion of ≥ 1 month. The duration criterion of ≥ 3 months appeared to be excessively restrictive to represent individuals with neurasthenia in the community. Subjects with 1 month episodes of neurasthenia exhibited sufficient differences from controls and similarities to subjects with anxiety or depressive disorders to justify a 1 month duration criterion for neurasthenia in community samples. The clinical significance of neurasthenia was indicated by the magnitude of subjective distress, and occupational and social impairment reported by the majority of the cases. Prospective assessment of the longitudinal course of neurasthenia revealed that approximately 50% of the cases continued to exhibit this disorder at follow-up. Our findings suggest that neurasthenia is equally likely to represent an early manifestation of affective illness as it is a consequence in those neurasthenic subjects who exhibited comorbid affective disorders. The magnitude, chronicity, impairment, longitudinal stability and distinction from anxiety and depression associated with this condition in the general population, suggest that neurasthenia is an important diagnostic entity for which additional validation studies should be undertake
Anisotropy of the superconducting state properties and phase diagram of MgB2 by torque magnetometry on single crystals
The angular and temperature dependence of the upper critical field Hc2 in
MgB2 was determined from torque magnetometry measurements on single crystals.
The Hc2 anisotropy gamma_H was found to decrease with increasing temperature,
in disagreement with the anisotropic Ginzburg-Landau theory, which predicts
that the gamma_H is temperature independent. This behaviour can be explained by
the two band nature of superconductivity in MgB2. An analysis of measurements
of the reversible torque in the mixed state yields a field dependent effective
anisotropy gamma_eff, which can be at least partially explained by different
anisotropies of the penetration depth and the upper critical field. It is shown
that a peak effect in fields of about 0.85 Hc2 is a manifestation of an
order-disorder phase transition of vortex matter. The H-T phase diagram of MgB2
for H//c correlates with the intermediate strength of thermal fluctuations in
MgB2, as compared to those in high and low Tc superconductors.Comment: 12 pages, 8 figures, Physica C in print (invited paper for a special
issue on MgB2
- …