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

Dielectric properties of charge ordered LuFe2O4 revisited: The apparent influence of contacts

We show results of broadband dielectric measurements on the charge ordered, proposed to be mul- tiferroic material LuFe2O4. The temperature and frequency dependence of the complex permittivity as investigated for temperatures above and below the charge-oder transition near T_CO ~ 320 K and for frequencies up to 1 GHz can be well described by a standard equivalent-circuit model considering Maxwell-Wagner-type contacts and hopping induced AC-conductivity. No pronounced contribution of intrinsic dipolar polarization could be found and thus the ferroelectric character of the charge order in LuFe2O4 has to be questioned.Comment: 4 pages, 3 figure

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

Critical Currents and Order-Disorder Phase Transition in the Vortex States of YBa2Cu4O8 with Chemically Introduced Disorder

A series of YBa_{2-x}Sr_xCu_4O_8 single crystals was measured to study the influence of site disorder on the transition line Hss(T) between quasi-ordered vortex lattice and highly disordered vortex glass, as well as on the maximum critical current density within the glass phase, jcmax. When 32% of Ba is replaced by Sr, jcmax is an order of magnitude higher than in the unsubstituted compound. In contrast, the transition field Hss first drops by a factor of about five with a substitution of just 10% of Sr for Ba, and then remains approximately constant for higher Sr contents. Our results indicate that in very clean systems the order-disorder transition is affected very strongly by any crystallographic disorder, while above a certain threshold it is relatively robust with respect to additional disorder. In all substituted crystals Hss monotonically decreases with an increase of temperature.Comment: Text revised, some figures replaced, 11 pages, 10 figures, to appear in PR