Non-Langevin behaviour of the uncompensated magnetisation in nanoparticles of artificial ferritin

The magnetic behaviour of nanoparticles of antiferromagnetic ferritin has been investigated by 57Fe Mossbauer absorption spectroscopy and magnetisation measurements, in the temperature range 2.5K-250K and with magnetic fields up to 7T. Samples containing nanoparticles with an average number of Fe atoms ranging from 400 to 2500 were studied. The value of the anisotropy energy per unit volume was determined and found to be in the range 3-6 10**5 ergs/cm3, which is a value typical for ferric oxides. By comparing the results of the two experimental methods at large field, we show that, contratry to what is currently assumed, the uncompensated magnetisation of the feritin cores in the superparamagnetic regime does not follow a Langevin law. For magnetic fields below the spin-flop field, we propose an approximate law for the field and temperature variation of the uncompensated magnetisation which has so far never been applied in antiferromagnetic systems. This approach should more generally hold for randomly oriented antiferro- magnetic nanoparticles systems with weak uncompensated moments.Comment: 11 pages, 11 figure

Elliptic Flow from a Transversally Thermalized Fireball

The agreement of elliptic flow data at RHIC at central rapidity with the hydrodynamic model has led to the conclusion of very rapid thermalization. This conclusion is based on the intuitive argument that hydrodynamics, which assumes instantaneous local thermalization, produces the largest possible elliptic flow values and that the data seem to saturate this limit. We here investigate the question whether incompletely thermalized viscous systems may actually produce more elliptic flow than ideal hydrodynamics. Motivated by the extremely fast primordial longitudinal expansion of the reaction zone, we investigate a toy model which exhibits thermalization only in the transverse directions but undergoes collisionless free-streaming expansion in the longitudinal direction. For collisions at RHIC energies, elliptic flow results from the model are compared with those from hydrodynamics. With the final particle yield and \kt-distribution fixed, the transversally thermalized model is shown not to be able to produce the measured amount of elliptic flow. This investigation provides further support for very rapid local kinetic equilibration at RHIC. It also yields interesting novel results for the elliptic flow of massless particles such as direct photons.Comment: revtex4, 15 pages + 10 embedded EPS figure

Pion Interferometry for Hydrodynamical Expanding Source with a Finite Baryon Density

We calculate the two-pion correlation function for an expanding hadron source with a finite baryon density. The space-time evolution of the source is described by relativistic hydrodynamics and the Hanbury-Brown-Twiss (HBT) radius is extracted after effects of collective expansion and multiple scattering on the HBT interferometry have been taken into account, using quantum probability amplitudes in a path-integral formalism. We find that this radius is substantially smaller than the HBT radius extracted from the freeze-out configuration.Comment: 4 pages, 2 figure

Novel Bose-Einstein Interference in the Passage of a Fast Particle in a Dense Medium

When an energetic particle collides coherently with many medium particles at high energies, the Bose-Einstein symmetry with respect to the interchange of the exchanged virtual bosons leads to a destructive interference of the Feynman amplitudes in most regions of the phase space but a constructive interference in some other regions of the phase space. As a consequence, the recoiling medium particles have a tendency to come out collectively along the direction of the incident fast particle, each carrying a substantial fraction of the incident longitudinal momentum. Such an interference appearing as collective recoils of scatterers along the incident particle direction may have been observed in angular correlations of hadrons associated with a high-$p_T$ trigger in high-energy AuAu collisions at RHIC.Comment: 10 pages, 2 figures, invited talk presented at the 35th Symposium on Nuclear Physics, Cocoyoc, Mexico, January 3, 2012, to be published in IOP Conference Serie

Rare typhoon development near the equator

In Recent Advances in Atmospheric Sciences in Asia-Pacific. (Eds: K.N. Liou, M.D. Chou and H. H. Hsu), World Scientific Publication Company, 172-181.The formation of Typhoon Vamei on 27 December 2001 in the southern South China Sea was the first-observed tropical cyclogenesis within 1.5 degrees of the equator. This rare event was first detected by observations of typhoon strength winds from a US navy ship, and the existence of an eye structure was confirmed by satellite and radar imageries. This paper reviews these observations, and discusses the dynamic theory that may explain the process suggested by Chang et al. (2003) in which a strong cold surge event interacting with the Borneo vortex led to the equatorial development. As pointed out by Chang et al., the most intriguing question is not how Vamei could form so close to the equator, but is why such a formation was not observed before then. The formation of Typhoon Vamei on 27 December 2001 in the southern South China Sea was the first-observed tropical cyclogenesis within 1.5 degrees of the equator. This rare event was first detected by observations of typhoon strength winds from a US navy ship, and the existence of an eye structure was confirmed by satellite and radar imageries. This paper reviews these observations, and discusses the dynamic theory that may explain the process suggested by Chang et al. (2003) in which a strong cold surge event interacting with the Borneo vortex led to the equatorial development. As pointed out by Chang et al., the most intriguing question is not how Vamei could form so close to the equator, but is why such a formation was not observed before then

Today's problems with the evaluation methods of full lightning impulse parameters as described in IEC 60060-1

In this paper the present problems with the evaluation methods for lightning impulse parameters, as defined in IEC 60060-1, are described. Also the current practice of evaluation in many laboratories world-wide, that is obtained by a questionnaire, is presented. Some of the work performed up the present time and the initial conclusions are reported, then some recommendations are made for future work

Unification of bulk and interface electroresistive switching in oxide systems

We demonstrate that the physical mechanism behind electroresistive switching in oxide Schottky systems is electroformation, as in insulating oxides. Negative resistance shown by the hysteretic current-voltage curves proves that impact ionization is at the origin of the switching. Analyses of the capacitance-voltage and conductance-voltage curves through a simple model show that an atomic rearrangement is involved in the process. Switching in these systems is a bulk effect, not strictly confined at the interface but at the charge space region.Comment: 4 pages, 3 figures, accepted in PR

The Interference Term between the Spin and Orbital Contributions to M1 Transitions

We study the cross-correlation between the spin and orbital parts of magnetic dipole transitions M1 in both isoscalar and isovector channels. In particular, we closely examine certain cases where $\sum B(M1)$ is very close to $\sum B(M1)_{\sigma} + \sum B(M1)_l$, implying a cancellation of the summed interference terms. We gain some insight into this problem by considering special cases approaching the SU(3) limit, and by examining the behaviour of single-particle transitions at the beginning and towards the end of the s-d shell.Comment: 9 pages of latex file and no figure