2,622 research outputs found
Measurement and simulation of anisotropic magnetoresistance in single GaAs/MnAs core/shell nanowires
We report four probe measurements of the low field magnetoresistance in
single core/shell GaAs/MnAs nanowires synthesized by molecular beam epitaxy,
demonstrating clear signatures of anisotropic magnetoresistance that track the
field-dependent magnetization. A comparison with micromagnetic simulations
reveals that the principal characteristics of the magnetoresistance data can be
unambiguously attributed to the nanowire segments with a zinc blende GaAs core.
The direct correlation between magnetoresistance, magnetization and crystal
structure provides a powerful means of characterizing individual hybrid
ferromagnet/semiconductor nanostructures.Comment: Submitted to Applied Physics Letters; some typos corrected and a
defective figure replace
Uncovering the Hidden Order in URu2Si2 by Impurity Doping
We report the use of impurities to probe the hidden order parameter of the
strongly correlated metal URu_2Si_2 below the transition temperature T_0 ~ 17.5
K. The nature of this order parameter has eluded researchers for more than two
decades, but is accompanied by the development of a partial gap in the single
particle density of states that can be detected through measurements of the
electronic specific heat and nuclear spin-lattice relaxation rate. We find that
impurities in the hidden order phase give rise to local patches of
antiferromagnetism. An analysis of the coupling between the antiferromagnetism
and the hidden order reveals that the former is not a competing order parameter
but rather a parasitic effect of the latter.Comment: 4 pages, 4 figure
Complex Landau Ginzburg Theory of the Hidden Order in URu_2Si_2
We develop a Landau Ginzburg theory of the hidden order phase and the local
moment antiferromagnetic phase of URu_2Si_2. We unify the two broken symmetries
in a common complex order parameter and derive many experimentally relevant
consequences such as the topology of the phase diagram in magnetic field and
pressure. The theory accounts for the appearance of a moment under application
of stress and the thermal expansion anomaly across the phase transitions. It
identifies the low energy mode which is seen in the hidden order phase near the
conmensurate wavector (0,0, 1) as the pseudo-Goldstone mode of the approximate
U(1) symmetry.Comment: 4 pages, 3 figure
Long range order and two-fluid behavior in heavy electron materials
The heavy electron Kondo liquid is an emergent state of condensed matter that
displays universal behavior independent of material details. Properties of the
heavy electron liquid are best probed by NMR Knight shift measurements, which
provide a direct measure of the behavior of the heavy electron liquid that
emerges below the Kondo lattice coherence temperature as the lattice of local
moments hybridizes with the background conduction electrons. Because the
transfer of spectral weight between the localized and itinerant electronic
degrees of freedom is gradual, the Kondo liquid typically coexists with the
local moment component until the material orders at low temperatures. The
two-fluid formula captures this behavior in a broad range of materials in the
paramagnetic state. In order to investigate two-fluid behavior and the onset
and physical origin of different long range ordered ground states in heavy
electron materials, we have extended Knight shift measurements to
URuSi, CeIrIn and CeRhIn. In CeRhIn we find that the
antiferromagnetic order is preceded by a relocalization of the Kondo liquid,
providing independent evidence for a local moment origin of antiferromagnetism.
In URuSi the hidden order is shown to emerge directly from the Kondo
liquid and so is not associated with local moment physics. Our results imply
that the nature of the ground state is strongly coupled with the hybridization
in the Kondo lattice in agreement with phase diagram proposed by Yang and
Pines.Comment: 9 pages, 13 figure
On two problems in graph Ramsey theory
We study two classical problems in graph Ramsey theory, that of determining
the Ramsey number of bounded-degree graphs and that of estimating the induced
Ramsey number for a graph with a given number of vertices.
The Ramsey number r(H) of a graph H is the least positive integer N such that
every two-coloring of the edges of the complete graph contains a
monochromatic copy of H. A famous result of Chv\'atal, R\"{o}dl, Szemer\'edi
and Trotter states that there exists a constant c(\Delta) such that r(H) \leq
c(\Delta) n for every graph H with n vertices and maximum degree \Delta. The
important open question is to determine the constant c(\Delta). The best
results, both due to Graham, R\"{o}dl and Ruci\'nski, state that there are
constants c and c' such that 2^{c' \Delta} \leq c(\Delta) \leq 2^{c \Delta
\log^2 \Delta}. We improve this upper bound, showing that there is a constant c
for which c(\Delta) \leq 2^{c \Delta \log \Delta}.
The induced Ramsey number r_{ind}(H) of a graph H is the least positive
integer N for which there exists a graph G on N vertices such that every
two-coloring of the edges of G contains an induced monochromatic copy of H.
Erd\H{o}s conjectured the existence of a constant c such that, for any graph H
on n vertices, r_{ind}(H) \leq 2^{c n}. We move a step closer to proving this
conjecture, showing that r_{ind} (H) \leq 2^{c n \log n}. This improves upon an
earlier result of Kohayakawa, Pr\"{o}mel and R\"{o}dl by a factor of \log n in
the exponent.Comment: 18 page
Towards improved socio-economic assessments of ocean acidification’s impacts
Ocean acidification is increasingly recognized as a component of global change that could have a wide range of impacts on marine organisms, the ecosystems they live in, and the goods and services they provide humankind. Assessment of these potential socio-economic impacts requires integrated efforts between biologists, chemists, oceanographers, economists and social scientists. But because ocean acidification is a new research area, significant knowledge gaps are preventing economists from estimating its welfare impacts. For instance, economic data on the impact of ocean acidification on significant markets such as fisheries, aquaculture and tourism are very limited (if not non-existent), and non-market valuation studies on this topic are not yet available. Our paper summarizes the current understanding of future OA impacts and sets out what further information is required for economists to assess socio-economic impacts of ocean acidification. Our aim is to provide clear directions for multidisciplinary collaborative research
Microwave Devices Employing Magnetic Waves
Contains reports on three research projects.Joint Services Electronics Program (Contract DAAB07-76-C-1400)National Science Foundation (Grant ENG76-18359
On discretization in time in simulations of particulate flows
We propose a time discretization scheme for a class of ordinary differential
equations arising in simulations of fluid/particle flows. The scheme is
intended to work robustly in the lubrication regime when the distance between
two particles immersed in the fluid or between a particle and the wall tends to
zero. The idea consists in introducing a small threshold for the particle-wall
distance below which the real trajectory of the particle is replaced by an
approximated one where the distance is kept equal to the threshold value. The
error of this approximation is estimated both theoretically and by numerical
experiments. Our time marching scheme can be easily incorporated into a full
simulation method where the velocity of the fluid is obtained by a numerical
solution to Stokes or Navier-Stokes equations. We also provide a derivation of
the asymptotic expansion for the lubrication force (used in our numerical
experiments) acting on a disk immersed in a Newtonian fluid and approaching the
wall. The method of this derivation is new and can be easily adapted to other
cases
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