17,520 research outputs found
Signatures of the self-modulation instability of relativistic proton bunches in the AWAKE experiment
We investigate numerically the detection of the self-modulation instability
in a virtual detector located downstream from the plasma in the context of
AWAKE. We show that the density structures, appearing in the temporally
resolving virtual detector, map the transverse beam phase space distribution at
the plasma exit. As a result, the proton bunch radius that appears to grow
along the bunch in the detector results from the divergence increase along the
bunch, related with the spatial growth of the self-modulated wakefields. In
addition, asymmetric bunch structures in the detector are a result of
asymmetries of the bunch divergence, and do not necessarily reflect asymmetric
beam density distributions in the plasma.Comment: Accepted for publication in NIM-A for the proceedings of the 3rd
European Advanced Accelerator Workshop. 5 pages, 2 figure
On the origin of unusual transport properties observed in densely packed polycrystalline CaAl_{2}
A possible origin of unusual temperature behavior of transport coefficients
observed in densely packed polycrystalline CaAl_{2} compound [M. Ausloos et
al., J. Appl. Phys. 96, 7338 (2004)] is discussed, including a power-like
dependence of resistivity with and N-like form of the
thermopower. All these features are found to be in good agreement with the
Shklovskii-Efros localization scenario assuming polaron-mediated hopping
processes controlled by the Debye energy
Ferromagnetic tendency at the surface of CE charge-ordered manganites
Most previous investigations have shown that the surface of a ferromagnetic
material may have antiferromagnetic tendencies. However, experimentally the
opposite effect has been recently observed: ferromagnetism appears in some
nano-sized manganites with a composition such that the antiferromagnetic
charge-ordered CE state is observed in the bulk. A possible origin is the
development of ferromagnetic correlations at the surface of these small
systems. To clarify these puzzling experimental observations, we have studied
the two-orbital double-exchange model near half-doping n=0.5, using open
boundary conditions to simulate the surface of either bulk or nano-sized
manganites. Considering the enhancement of surface charge density due to a
possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an
unexpected surface phase-separated state emerges when the model is studied
using Monte Carlo techniques on small clusters. This tendency suppresses the CE
charge ordering and produces a weak ferromagnetic signal that could explain the
experimental observations.Comment: 8 pages, 5 figure
Counterions at charge-modulated substrates
We consider counterions in the presence of a single planar surface with a
spatially inhomogeneous charge distribution using Monte-Carlo simulations and
strong-coupling theory. For high surface charges, multivalent counterions, or
pronounced substrate charge modulation the counterions are laterally correlated
with the surface charges and their density profile deviates strongly from the
limit of a smeared-out substrate charge distribution, in particular exhibiting
a much increased laterally averaged density at the surface.Comment: 7 page
Structural and insulator-to-metal phase transition at 50 GPa in GdMnO3
We present a study of the effect of very high pressure on the orthorhombic
perovskite GdMnO3 by Raman spectroscopy and synchrotron x-ray diffraction up to
53.2 GPa. The experimental results yield a structural and insulator-to-metal
phase transition close to 50 GPa, from an orthorhombic to a metrically cubic
structure. The phase transition is of first order with a pressure hysteresis of
about 6 GPa. The observed behavior under very high pressure might well be a
general feature in rare-earth manganites.Comment: 4 pages, 3 figures and 2 table
Dynamical reentrance and geometry imposed quantization effects in Nb-AlOx-Nb Josephson junction arrays
In this paper, we report on different phenomena related to the magnetic
properties of artificially prepared highly ordered (periodic) two-dimensional
Josephson junction arrays (2D-JJA) of both shunted and unshunted Nb-AlOx-Nb
tunnel junctions. By employing mutual-inductance measurements and using a
high-sensitive bridge, we have thoroughly investigated (both experimentally and
theoretically) the temperature and magnetic field dependence of complex AC
susceptibility of 2D-JJA. We also demonstrate the use of the scanning SQUID
microscope for imaging the local flux distribution within our unshunted arrays
Irreversibility line and low-field grain-boundary pinning in electron-doped superconducting thin films
AC magnetic susceptibilities of electron-doped Pr_{1.85}Ce_{0.15}CuO_4 (PCCO)
and Sm_{1.85}Ce_{0.15}CuO_4 (SCCO) granular thin films have been measured as a
function of temperature and magnetic-field strength. Depending on the level of
homogeneity of our films, two different types of the irreversibility line (IL)
defined as the intergrain-loss peak temperature in the imaginary part of
susceptibility have been found. The obtained results are described via the
critical-state model taking into account the low-field grain-boundary pinning.
The extracted pinning-force densities in more granular SCCO films turn out to
be four times larger than their counterparts in less granular PCCO films
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