787 research outputs found
Successive transition from superconducting to antiferromagnetic phase in (Ca_6(Al, Ti)_4O_y)Fe_2As_2 studied via ^{75}As and ^{27}Al NMR
An unusual successive phase transition from superconducting (SC) to
antiferromagnetic (AF) phases was discovered via ^{75}As and ^{27}Al nuclear
magnetic resonance (NMR) in (Fe_2As_2)(Ca_6(Al, Ti)_4O_y) with four (Al, Ti)O
layers intercalated between FeAs planes. Although the spatially-uniform AF
ordering is clearly visible from ^{27}Al spectra, the ordered moments are very
small and the low-frequency fluctuation is much suppressed, contrary to
existing pnictides with localized magnetic elements. Furthermore, the
temperature (T) dependence of the fluctuation at both nuclei is very similar
throughout the entire temperature range. These facts suggest that some
hybridization between Ti and Fe orbitals induces a uniform electronic state
within FeAs and (Al, Ti)O layers accompanied by the SC and AF transitions. The
iron-based pnictide with Ti-doped blocking layers is the first high-T_c
compound having metallic blocking layers
Structural Features of Layered Iron Pnictide Oxides (Fe2As2)(Sr4M2O6)
Structural features of newly found perovskite-based iron pnictide oxide
system have been systematically studied. Compared to REFePnO system,
perovskite-based system tend to have lower Pn-Fe-Pn angle and higher pnictogen
height owing to low electronegativity of alkaline earth metal and small
repulsive force between pnictogen and oxygen atoms. As-Fe-As angles of
(Fe2As2)(Sr4Cr2O6), (Fe2As2)(Sr4V2O6) and (Fe2Pn2)(Sr4MgTiO6) are close to
ideal tetrahedron and those pnictogen heights of about 1.40 A are close to
NdFeAsO with optimized carrier concentration. These structural features of this
system may leads to realization of high Tc superconductivity.Comment: 3pages, 2figures, 1table, proceedings of M2S 200
Quench and Normal Zone Propagation Characteristics of RHQT-Processed Nb3Al Wires under Cryocooler-Cooling Conditions
Lyapunov exponent and natural invariant density determination of chaotic maps: An iterative maximum entropy ansatz
We apply the maximum entropy principle to construct the natural invariant
density and Lyapunov exponent of one-dimensional chaotic maps. Using a novel
function reconstruction technique that is based on the solution of Hausdorff
moment problem via maximizing Shannon entropy, we estimate the invariant
density and the Lyapunov exponent of nonlinear maps in one-dimension from a
knowledge of finite number of moments. The accuracy and the stability of the
algorithm are illustrated by comparing our results to a number of nonlinear
maps for which the exact analytical results are available. Furthermore, we also
consider a very complex example for which no exact analytical result for
invariant density is available. A comparison of our results to those available
in the literature is also discussed.Comment: 16 pages including 6 figure
Self-organization in systems of self-propelled particles
We investigate a discrete model consisting of self-propelled particles that
obey simple interaction rules. We show that this model can self-organize and
exhibit coherent localized solutions in one- and in two-dimensions.In
one-dimension, the self-organized solution is a localized flock of finite
extent in which the density abruptly drops to zero at the edges.In
two-dimensions, we focus on the vortex solution in which the particles rotate
around a common center and show that this solution can be obtained from random
initial conditions, even in the absence of a confining boundary. Furthermore,
we develop a continuum version of our discrete model and demonstrate that the
agreement between the discrete and the continuum model is excellent.Comment: 4 pages, 5 figure
Role of magnetic dopants in the phase diagram of Sm1111 pnictides: The Mn case
The deliberate insertion of magnetic Mn dopants in the Fe sites of the
optimally-doped SmFeAsO0.88-F0.12 iron-based superconductor can modify in a
controlled way its electronic properties. The resulting phase diagram was
investigated across a wide range of manganese contents (x) by means of
muon-spin spectroscopy (muSR), both in zero- and in transverse fields,
respectively, to probe the magnetic and the superconducting order. The pure
superconducting phase (at x < 0.03) is replaced by a crossover region at
intermediate Mn values (0.03 =< x < 0.08), where superconductivity coexists
with static magnetic order. After completely suppressing superconductivity for
x = 0.08, a further increase in Mn content reinforces the natural tendency
towards antiferromagnetic correlations among the magnetic Mn ions. The sharp
drop of Tc and the induced magnetic order in the presence of magnetic
disorder/dopants, such as Mn, are both consistent with a recent theoretical
model of unconventional superconductors [M. Gastiasoro et al., ArXiv
1606.09495], which includes correlation-enhanced RKKY-couplings between the
impurity moments.Comment: 10 pages, 13 figures. Accepted on Physical Review
Abrupt Change of Josephson Plasma Frequency at the Phase Boundary of the Bragg Glass in Bi_2Sr_2CaCu_2O_{8+\delta}
We report the first detailed and quantitative study of the Josephson coupling
energy in the vortex liquid, Bragg glass and vortex glass phases of
Bi_2Sr_2CaCu_2O_{8+\delta} by the Josephson plasma resonance. The measurements
revealed distinct features in the T- and H-dependencies of the plasma frequency
for each of these three vortex phases. When going across either
the Bragg-to-vortex glass or the Bragg-to-liquid transition line,
shows a dramatic change. We provide a quantitative discussion on the properties
of these phase transitions, including the first order nature of the
Bragg-to-vortex glass transition.Comment: 5pages, 4figure
The Northern Eurasia Earth Science Partnership: An Example of Science Applied to Societal Needs
Northern Eurasia, the largest landmass in the northern extratropics, accounts for ~20% of the global land area. However, little is known about how the biogeochemical cycles, energy and water cycles, and human activities specific to this carbon-rich, cold region interact with global climate. A major concern is that changes in the distribution of land-based life, as well as its interactions with the environment, may lead to a self-reinforcing cycle of accelerated regional and global warming. With this as its motivation, the Northern Eurasian Earth Science Partnership Initiative (NEESPI) was formed in 2004 to better understand and quantify feedbacks between northern Eurasian and global climates. The first group of NEESPI projects has mostly focused on assembling regional databases, organizing improved environmental monitoring of the region, and studying individual environmental processes. That was a starting point to addressing emerging challenges in the region related to rapidly and simultaneously changing climate, environmental, and societal systems. More recently, the NEESPI research focus has been moving toward integrative studies, including the development of modeling capabilities to project the future state of climate, environment, and societies in the NEESPI domain. This effort will require a high level of integration of observation programs, process studies, and modeling across disciplines
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