9 research outputs found
Anomalous magnetic ordering in PrBa_2Cu_3O_{7-y} single crystals: Evidence for magnetic coupling between the Cu and Pr sublattices
In Al-free PrBa_2Cu_3O_{7-y} single crystals the kink in the temperature
dependence of magnetic susceptibility chi_{ab}(T), connected with Pr
antiferromagnetic ordering, disappears after field cooling (FC) in a field H ||
ab-plane. The kink in chi_c(T) remains unchanged after FC in H || c-axis. As a
possible explanation, freezing of the Cu magnetic moments, lying in the
ab-plane, caused by FC in H || ab, hinders their reorientation and, due to
coupling between the Pr and Cu(2) sublattices, ordering of the Pr^{3+} moments.
A field induced phase transition and a field dependence of the Pr^{3+} ordering
temperature have been found for both H || c and H || ab.Comment: 11 pages (LaTex with elsart.sty), 5 EPS figs. Accepted to Physica
Upper critical field and irreversibility line in superconducting MgB_2
The upper critical field Hc2(T) of sintered pellets of the recently
discovered MgB_2 superconductor was investigated by transport, ac
susceptibility and dc magnetization measurements in magnetic fields up to 16 T
covering a temperature range between Tc ~ 39 K and T = 3 K ~ 0.1Tc. The
temperature dependence of the upper critical field, Hc2(T), shows a positive
curvature near Tc similar to that found for the borocarbides YNi_2B_2C and
LuNi_2B_2C indicating that MgB_2 is in the clean limit. The irreversibility
line was consistently determined from dc magnetization measurements and from
the imaginary component of ac susceptibility. The irreversibility field was
found to increase up to 8.5 T at 10 K.Comment: 8 pages with 5 figures, submitted to Solid State Communication
Probing vacuum birefringence by phase-contrast Fourier imaging under fields of high-intensity lasers
In vacuum high-intensity lasers can cause photon-photon interaction via the
process of virtual vacuum polarization which may be measured by the phase
velocity shift of photons across intense fields. In the optical frequency
domain, the photon-photon interaction is polarization-mediated described by the
Euler-Heisenberg effective action. This theory predicts the vacuum
birefringence or polarization dependence of the phase velocity shift arising
from nonlinear properties in quantum electrodynamics (QED). We suggest a method
to measure the vacuum birefringence under intense optical laser fields based on
the absolute phase velocity shift by phase-contrast Fourier imaging. The method
may serve for observing effects even beyond the QED vacuum polarization.Comment: 14 pages, 9 figures. Accepted by Applied Physics
Thermopower and thermal conductivity of superconducting perovskite
The thermopower and thermal conductivity of superconducting perovskite
( 8 K) have been studied. The thermopower is negative
from room temperature to 10 K. Combining with the negative Hall coefficient
reported previously, the negative thermopower definetly indicates that the
carrier in is electron-type. The nonlinear temperature dependence of
thermopower below 150 K is explained by the electron-phonon interaction
renormalization effects. The thermal conductivity is of the order for
intermetallics, larger than that of borocarbides and smaller than . In
the normal state, the electronic contribution to the total thermal conductivity
is slightly larger than the lattice contribution. The transverse
magnetoresistance of is also measured. It is found that the classical
Kohler's rule is valid above 50 K. An electronic crossover occures at , resulting in the abnormal behavior of resistivity, thermopower, and
magnetoresistance below 50 K.Comment: Revised on 12 September 2001, Phys. Rev. B in pres
Local Ferromagnetism in Microporous Carbon with the Structural Regularity of Zeolite Y
Magnetization M(H,T) measurements have been performed on microporous carbon
(MC) with a three-dimensional nano-array structure corresponding to that of a
zeolite Y supercage. The obtained results unambiguously demonstrate the
occurrence of high-temperature ferromagnetism in MC, probably originating from
a topological disorder associated with curved graphene sheets. The results
provide evidence that the ferromagnetic behavior of MC is governed by isolated
clusters in a broad temperature range, and suggest the occurrence of
percolative-type transition with the temperature lowering. A comparative
analysis of the results obtained on MC and related materials is given.Comment: To be published in Physical Review B (2003
Tests of Classical and Quantum Electrodynamics with Intense Laser Fields
n this chapter classical and quantum electrodynamics in intense laser fields are discussed. We focus on the interaction of relativistic electrons with strong laser pulses. In particular, by analyzing the dynamics of this interaction, we show how the peak intensity of a strong laser pulse can be related to the spectrum of the radiation emitted by the electron during the interaction itself. The discussed method could be used to accurately measure high peak laser intensities exceeding 1020 W/cm2 up to about 1023 W/cm2 with theoretical envisaged accuracies of the order of 10 %. Furthermore, we investigate non-linear quantum effects originating from the interaction of an electron with its own electromagnetic field in the presence of an intense plane wave. These “radiative corrections” modify the electron wave-function in the plane wave. The self-interaction changes, amongst others, the dynamics of the electron’s spin in comparison with the prediction of the Dirac equation. We show that this effect can be measured, in principle, already at intensities of the order of 1022 W/cm