1,732 research outputs found
Influence of strain and oxygen vacancies on the magnetoelectric properties of multiferroic bismuth ferrite
The dependencies on strain and oxygen vacancies of the ferroelectric
polarization and the weak ferromagnetic magnetization in the multiferroic
material bismuth ferrite, BiFeO_3, are investigated using first principles
density functional theory calculations. The electric polarization is found to
be rather independent of strain, in striking contrast to most conventional
perovskite ferroelectrics. It is also not significantly affected by oxygen
vacancies, or by the combined presence of strain and oxygen vacancies. The
magnetization is also unaffected by strain, however the incorporation of oxygen
vacancies can alter the magnetization slightly, and also leads to the formation
of Fe^{2+}. These results are discussed in light of recent experiments on
epitaxial films of BiFeO_3 which reported a strong thickness dependence of both
magnetization and polarization.Comment: 9 pages, 3 figure
Origin of ferroelectricity in the multiferroic barium fluorides BaMF4
We present a first principles study of the series of multiferroic barium
fluorides with the composition BaMF4, where M is Mn, Fe, Co, or Ni. We discuss
trends in the structural, electronic, and magnetic properties, and we show that
the ferroelectricity in these systems results from the "freezing in" of a
single unstable polar phonon mode. In contrast to the case of the standard
perovskite ferroelectrics, this structural distortion is not accompanied by
charge transfer between cations and anions. Thus, the ferroelectric instability
in the multiferroic barium fluorides arises solely due to size effects and the
special geometrical constraints of the underlying crystal structure.Comment: 8 pages, 6 figures, 3 table
Effects of Lateral Ligament Sectioning on the Stability of the Ankle and Subtalar Joint
Patients with subtalar joint instability are often diagnosed with ankle instability. Only after a prolonged period of time in which a patient does not improve after treatment for ankle instability is subtalar joint instability considered. To develop a clinically relevant method to diagnose subtalar joint instability, the kinematics of the simulated unstable subtalar joint were examined. A 6 degree-of-freedom positioning and loading device was developed. Plantarflexion/dorsiflexion, inversion/eversion, and internal/external rotation were applied individually or as coupled motions along with an anterior/posterior drawer. Kinematic data were collected from sensors attached to the calcaneus, talus, and tibia by keeping all the ligaments intact, and by serially sectioning anterior talofibular ligament (ATFL), calcaneofibular ligament (CFL), cervical ligament, and talocalceneal interosseous ligament. Kinematic results were reported using Euler angles. The ATFL and CFL contributed talocrural instability, similar to previous studies. The interosseous ligament was the greatest contributor to subtalar joint stability. The hindfoot motion (calcaneus relative to tibia) showed significant increases in motion when the ankle and/or subtalar joint was made to be unstable. Therefore, it is difficult to diagnose subtalar joint instability on physical examination alone. (C) 2011 Orthopaedic Research Society
First principles study of the multiferroics BiFeO, BiFeCrO, and BiCrO: Structure, polarization, and magnetic ordering temperature
We present results of an {\it ab initio} density functional theory study of
three bismuth-based multiferroics, BiFeO, BiFeCrO, and
BiCrO. We disuss differences in the crystal and electronic structure of
the three systems, and we show that the application of the LDA+ method is
essential to obtain realistic structural parameters for BiFeCrO. We
calculate the magnetic nearest neighbor coupling constants for all three
systems and show how Anderson's theory of superexchange can be applied to
explain the signs and relative magnitudes of these coupling constants. From the
coupling constants we then obtain a mean-field approximation for the magnetic
ordering temperatures. Guided by our comparison of these three systems, we
discuss the possibilities for designing a multiferroic material with large
magnetization above room temperature.Comment: 8 Pages, 4 Figure
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Utilisation of prophylactic mastectomy in 10 European centers
ABSTRACT:
Increasingly women at high risk of
breast cancer are opting for prophylactic surgery to
reduce their risks. Data from 10 European centres that
offer a risk counselling and screening service to
women at risk show different approaches to the option
of preventive surgery, although most centres adhere to
a protocol including at least two risk counselling
sessions and a psychological assessment. Thus far the
combined centres have data on 174 women who have
undergone prophylactic mastectomy with in excess of
400 women years of follow up. Operations were
carried out on women with lifetime risks of 25–80%,
with an average annual expected incidence rate of 1%
per women. No breast cancers have occurred in this
cohort. Long term follow up on an extended group of
women will be necessary to truly address the risk of
subsequent breast cancer and the psychological
sequelae
Wavepacket reconstruction via local dynamics in a parabolic lattice
We study the dynamics of a wavepacket in a potential formed by the sum of a
periodic lattice and of a parabolic potential. The dynamics of the wavepacket
is essentially a superposition of ``local Bloch oscillations'', whose frequency
is proportional to the local slope of the parabolic potential. We show that the
amplitude and the phase of the Fourier transform of a signal characterizing
this dynamics contains information about the amplitude and the phase of the
wavepacket at a given lattice site. Hence, {\em complete} reconstruction of the
the wavepacket in the real space can be performed from the study of the
dynamics of the system.Comment: 4 pages, 3 figures, RevTex
Spectral method for the time-dependent Gross-Pitaevskii equation with a harmonic trap
We study the numerical resolution of the time-dependent Gross-Pitaevskii
equation, a non-linear Schroedinger equation used to simulate the dynamics of
Bose-Einstein condensates. Considering condensates trapped in harmonic
potentials, we present an efficient algorithm by making use of a spectral
Galerkin method, using a basis set of harmonic oscillator functions, and the
Gauss-Hermite quadrature. We apply this algorithm to the simulation of
condensate breathing and scissors modes.Comment: 23 pages, 5 figure
Science and Ideology in Economic, Political, and Social Thought
This paper has two sources: One is my own research in three broad areas: business cycles, economic measurement and social choice. In all of these fields I attempted to apply the basic precepts of the scientific method as it is understood in the natural sciences. I found that my effort at using natural science methods in economics was met with little understanding and often considerable hostility. I found economics to be driven less by common sense and empirical evidence, then by various ideologies that exhibited either a political or a methodological bias, or both. This brings me to the second source: Several books have appeared recently that describe in historical terms the ideological forces that have shaped either the direct areas in which I worked, or a broader background. These books taught me that the ideological forces in the social sciences are even stronger than I imagined on the basis of my own experiences.
The scientific method is the antipode to ideology. I feel that the scientific work that I have done on specific, long standing and fundamental problems in economics and political science have given me additional insights into the destructive role of ideology beyond the history of thought orientation of the works I will be discussing
Measurements in two bases are sufficient for certifying high-dimensional entanglement
High-dimensional encoding of quantum information provides a promising method
of transcending current limitations in quantum communication. One of the
central challenges in the pursuit of such an approach is the certification of
high-dimensional entanglement. In particular, it is desirable to do so without
resorting to inefficient full state tomography. Here, we show how carefully
constructed measurements in two bases (one of which is not orthonormal) can be
used to faithfully and efficiently certify bipartite high-dimensional states
and their entanglement for any physical platform. To showcase the practicality
of this approach under realistic conditions, we put it to the test for photons
entangled in their orbital angular momentum. In our experimental setup, we are
able to verify 9-dimensional entanglement for a pair of photons on a
11-dimensional subspace each, at present the highest amount certified without
any assumptions on the state.Comment: 11+14 pages, 2+7 figure
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