98,788 research outputs found
NiTi shape-memory transformations: minimum-energy pathways between austenite, martensites, and kinetically-limited intermediate states
NiTi is the most used shape-memory alloy, nonetheless, a lack of
understanding remains regarding the associated structures and transitions,
including their barriers. Using a generalized solid-state nudge elastic band
(GSSNEB) method implemented via density-functional theory, we detail the
structural transformations in NiTi relevant to shape memory: those between
body-centered orthorhombic (BCO) groundstate and a newly identified stable
austenite ("glassy" B2-like) structure, including energy barriers (hysteresis)
and intermediate structures (observed as a kinetically limited R-phase), and
between martensite variants (BCO orientations). All results are in good
agreement with available experiment. We contrast the austenite results to those
from the often-assumed, but unstable B2. These high- and low-temperature
structures and structural transformations provide much needed atomic-scale
detail for transitions responsible for NiTi shape-memory effects.Comment: 4 pages, 4 figure
Multi-Agent Complex Systems and Many-Body Physics
Multi-agent complex systems comprising populations of decision-making
particles, have many potential applications across the biological,
informational and social sciences. We show that the time-averaged dynamics in
such systems bear a striking resemblance to conventional many-body physics. For
the specific example of the Minority Game, this analogy enables us to obtain
analytic expressions which are in excellent agreement with numerical
simulations.Comment: Accepted for publication in Europhysics Letter
Low-energy, planar magnetic defects in BaFe2As2: nanotwins, twins, antiphase and domain boundaries
In BaFe2As2, structural and magnetic planar defects begin to proliferate
below the structural phase transition, affecting descriptions of magnetism and
superconductivity. We study using density-functional theory the stability and
magnetic properties of competing antiphase and domain boundaries, twins and
isolated twins (twin nuclei) - spin excitations proposed and/or observed.
These nanoscale defects have very low surface energy (-~Jm),
with twins favorable to the mesoscale. Defects exhibit smaller moments confined
near their boundaries -- making a uniform-moment picture inappropriate for
long-range magnetic order in real samples. {\it{Nano}}twins explain features in
measured pair distribution functions, so should be considered when analyzing
scattering data. All these defects can be weakly mobile and/or have
fluctuations that lower assessed "ordered" moments from longer spatial and/or
time averaging, and should be considered directly.Comment: 6 pages, 6 figures, 1 tabl
Anomalous magneto-structural behavior of MnBi explained: a path towards an improved permanent magnet
Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice
constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation
and magnetic susceptibility maximum near 90 K, and, importantly for
high-temperature magnetic applications, an increasing coercivity (unique to
MnBi) above 180 K. We calculate the total energy and magneto-anisotropy energy
(MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above
anomalies. We predict that coercivity and MAE increase due to increasing a,
suggesting means to improve MnBi permanent magnets.Comment: 4 pages, 5 figure
Predictability of large future changes in a competitive evolving population
The dynamical evolution of many economic, sociological, biological and
physical systems tends to be dominated by a relatively small number of
unexpected, large changes (`extreme events'). We study the large, internal
changes produced in a generic multi-agent population competing for a limited
resource, and find that the level of predictability actually increases prior to
a large change. These large changes hence arise as a predictable consequence of
information encoded in the system's global state.Comment: 10 pages, 3 figure
Testing equality of variances in the analysis of repeated measurements
The problem of comparing the precisions of two instruments using repeated measurements can be cast as an extension of the Pitman-Morgan problem of testing equality of variances of a bivariate normal distribution. Hawkins (1981) decomposes the hypothesis of equal variances in this model into two subhypotheses for which simple tests exist. For the overall hypothesis he proposes to combine the tests of the subhypotheses using Fisher's method and empirically compares the component tests and their combination with the likelihood ratio test. In this paper an attempt is made to resolve some discrepancies and puzzling conclusions in Hawkins's study and to propose simple modifications.\ud
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The new tests are compared to the tests discussed by Hawkins and to each other both in terms of the finite sample power (estimated by Monte Carlo simulation) and theoretically in terms of asymptotic relative efficiencies
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