842 research outputs found
Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d)
Magnetic flux jumps in textured Bi2Sr2CaCu2O(8+d) have been studied by means
of magnetization measurements in the temperature range between 1.95 K and Tc,
in an external magnetic field up to 9 T. Flux jumps were found in the
temperature range 1.95 K - 6 K, with the external magnetic field parallel to
the c axis of the investigated sample. The effect of sample history on magnetic
flux jumping was studied and it was found to be well accounted for by the
available theoretical models. The magnetic field sweep rate strongly influences
the flux jumping and this effect was interpreted in terms of the influence of
both flux creep and the thermal environment of the sample. Strong flux creep
was found in the temperature and magnetic field range where flux jumps occur
suggesting a relationship between the two. The heat exchange conditions between
the sample and the experimental environment also influence the flux jumping
behavior. Both these effects stabilize the sample against flux instabilities,
and this stabilizing effect increases with decreasing magnetic field sweep
rate. Demagnetizing effects are also shown to have a significant influence on
flux jumping.Comment: 10 pages, 6 figures, RevTeX4, submitted to Phys. Rev.
Origin of the giant magnetic moments of Fe impurities on and in Cs films
To explore the origin of the observed giant magnetic moments ()
of Fe impurities on the surface and in the bulk of Cs films, we have performed
the relativistic LSDA + U calculations using the linearized muffin-tin orbital
(LMTO) band method. We have found that Fe impurities in Cs behave differently
from those in noble metals or in Pd. Whereas the induced spin polarization of
Cs atoms is negligible, the Fe ion itself is found to be the source of the
giant magnetic moment. The 3d electrons of Fe in Cs are localized as the 4f
electrons in rare-earth ions so that the orbital magnetic moment becomes as
large as the spin magnetic moment. The calculated total magnetic moment of , which comes mainly from Fe ion, is close to the experimentally
observed value.Comment: 4 pages including 3 figures and 1 table. Submitted to PR
Nano granular metallic Fe - oxygen deficient TiO composite films: A room temperature, highly carrier polarized magnetic semiconductor
Nano granular metallic iron (Fe) and titanium dioxide (TiO) were
co-deposited on (100) lanthanum aluminate (LaAlO) substrates in a low
oxygen chamber pressure using a pulsed laser ablation deposition (PLD)
technique. The co-deposition of Fe and TiO resulted in 10 nm
metallic Fe spherical grains suspended within a TiO matrix. The
films show ferromagnetic behavior with a saturation magnetization of 3100 Gauss
at room temperature. Our estimate of the saturation magnetization based on the
size and distribution of the Fe spheres agreed well with the measured value.
The film composite structure was characterized as p-type magnetic semiconductor
at 300 K with a carrier density of the order of . The
hole carriers were excited at the interface between the nano granular Fe and
TiO matrix similar to holes excited in the metal/n-type
semiconductor interface commonly observed in Metal-Oxide-Semiconductor (MOS)
devices. From the large anomalous Hall effect directly observed in these films
it follows that the holes at the interface were strongly spin polarized.
Structure and magneto transport properties suggested that these PLD films have
potential nano spintronics applications.Comment: 6 pages in Latex including 8 figure
Central peak position in magnetization loops of high- superconductors
Exact analytical results are obtained for the magnetization of a
superconducting thin strip with a general behavior J_c(B) of the critical
current density. We show that within the critical-state model the magnetization
as function of applied field, B_a, has an extremum located exactly at B_a=0.
This result is in excellent agreement with presented experimental data for a
YBCO thin film. After introducing granularity by patterning the film, the
central peak becomes shifted to positive fields on the descending field branch
of the loop. Our results show that a positive peak position is a definite
signature of granularity in superconductors.Comment: $ pages, 6 figure
Vortex State of TlBaCuO via Tl NMR at 2 Tesla
We report a Tl NMR study of vortex state for an aligned
polycrystalline sample of an overdoped high- superconductor
TlBaCuO (85 K) with magnetic field 2 T along
the c axis. We observed an imperfect vortex lattice, so-called Bragg glass at
=5 K, coexistence of vortex solid with liquid between 10 and 60 K, and
vortex melting between 65 and 85 K. No evidence for local antiferromagnetic
ordering at vortex cores was found for our sample.Comment: 4 pages with 5 figure
Critical currents, flux-creep activation energy and potential barriers for the vortex motion from the flux creep experiments
We present an experimental study of thermally activated flux creep in a
superconducting ring-shaped epitaxial YBCO film as well as a new way of
analyzing the experimental data. The measurements were made in a wide range of
temperatures between 10 and 83 K. The upper temperature limit was dictated by
our experimental technique and at low temperatures we were limited by a
crossover to quantum tunneling of vortices. It is shown that the experimental
data can very well be described by assuming a simple thermally activated
hopping of vortices or vortex bundles over potential barriers, whereby the
hopping flux objects remain the same for all currents and temperatures. The new
procedure of data analysis also allows to establish the current and temperature
dependencies of the flux-creep activation energy U, as well as the temperature
dependence of the critical current Ic, from the flux-creep rates measured at
different temperatures. The variation of the activation energy with current,
U(I/Ic), is then used to reconstruct the profile of the potential barriers in
real space.Comment: 12 pages, 13 Postscript figures, Submitted to Physical Review
Anomalous far infrared monochromatic transmission through a film of type-II superconductor in magnetic field
Anomalous far infrared monochromatic transmission through a lattice of
Abrikosov vortices in a type-II superconducting film is found and reported. The
transmitted frequency corresponds to the photonic mode localized by the defects
of the Abrokosov lattice. These defects are formed by extra vortices placed out
of the nodes of the ideal Abrokosov lattice. The extra vortices can be pinned
by crystal lattice defects of a superconductor. The corresponding frequency is
studied as a function of magnetic field and temperature in the framework of the
Dirac-type two-band model. While our approach is valid for all type-II
superconductors, the specific calculations have been performed for the
YBaCuO (YBCO). The control of the transmitted
frequency by varying magnetic field and/or temperature is analyzed. It is
suggested that found anomalously transmitted localized mode can be utilized in
the far infrared monochromatic filters.Comment: 9 pages, 2 figure
Functional Integration Approach to Hysteresis
A general formulation of scalar hysteresis is proposed. This formulation is
based on two steps. First, a generating function g(x) is associated with an
individual system, and a hysteresis evolution operator is defined by an
appropriate envelope construction applied to g(x), inspired by the overdamped
dynamics of systems evolving in multistable free energy landscapes. Second, the
average hysteresis response of an ensemble of such systems is expressed as a
functional integral over the space G of all admissible generating functions,
under the assumption that an appropriate measure m has been introduced in G.
The consequences of the formulation are analyzed in detail in the case where
the measure m is generated by a continuous, Markovian stochastic process. The
calculation of the hysteresis properties of the ensemble is reduced to the
solution of the level-crossing problem for the stochastic process. In
particular, it is shown that, when the process is translationally invariant
(homogeneous), the ensuing hysteresis properties can be exactly described by
the Preisach model of hysteresis, and the associated Preisach distribution is
expressed in closed analytic form in terms of the drift and diffusion
parameters of the Markovian process. Possible applications of the formulation
are suggested, concerning the interpretation of magnetic hysteresis due to
domain wall motion in quenched-in disorder, and the interpretation of critical
state models of superconducting hysteresis.Comment: 36 pages, 9 figures, to be published on Phys. Rev.
Research on the Geography of Agricultural Change: Redundant or Revitalized?
Future research directions for agricultural geography were the subject of debate in Area in the late 1980s. The subsequent application of political economy ideas undoubtedly revived interest in agricultural research. This paper argues that agricultural geography contains greater diversity than the dominant political economy discourse would suggest. It reviews ‘other’ areas of agricultural research on policy, post-productivism, people, culture and animals, presenting future suggestions for research. They should ensure that agricultural research continues revitalized rather than redundant into the next millennium
- …