1,664 research outputs found
Calculations of Magnetic Exchange Interactions in Mott--Hubbard Systems
An efficient method to compute magnetic exchange interactions in systems with
strong correlations is introduced. It is based on a magnetic force theorem
which evaluates linear response due to rotations of magnetic moments and uses a
novel spectral density functional framework combining our exact diagonalization
based dynamical mean field and local density functional theories. Applications
to spin waves and magnetic transition temperatures of 3d metal mono--oxides as
well as high--T_{c} superconductors are in good agreement with experiment
Consistency of the Bootstrap Procedure in Individual Bioequivalence
Recently, new concepts have been proposed for assessing bioequivalence of two drug formulations, namely the so-called population and individual bioequivalence. Using moment-based and probability-based measures for evaluating the proposed bioequivalence concepts, criteria have been formulated to decide whether two formulations should be regarded as bioequivalent or not. This decision has of course to be based on an adequate statistical method where the Food and Drug Administration (FDA) guidance (1997) recommends the use of a bootstrap percentile interval. In this paper, we discuss theoretical properties such as consistency and accuracy of the recommended bootstrap intervals. We focus our investigations on the concept of individual bioequivalence and here especially on the scaled versions of the moment-based as well as the probability-based measures as recommended by the FDA. As estimates for the former, we consider those obtained from an according analysis of variance and restricted maximum likelihood estimators under mixed effect models, where an unbiased estimator of the latter can be derived from the corresponding relative frequencies
Currents, Torques, and Polarization Factors in Magnetic Tunnel Junctions
Application of Bardeen's tunneling theory to magnetic tunnel junctions having
a general degree of atomic disorder reveals the close relationship between
magneto-conduction and voltage-driven pseudo-torque, as well as the thickness
dependence of tunnel-polarization factors. Among the results: 1) The torque
generally varies as sin theta at constant applied voltage. 2) Whenever
polarization factors are well defined, the voltage-driven torque on each moment
is uniquely proportional to the polarization factor of the other magnet. 3) At
finite applied voltage, this relation predicts significant voltage-asymmetry in
the torque. For one sign of voltage the torque remains substantial even when
the magnetoconductance is greatly diminished. 4) A broadly defined junction
model, called ideal middle, allows for atomic disorder within the magnets and
F/I interface regions. In this model, the spin dependence of a state-weighting
factor proportional to the sum over general state index of evaluated within the
(e.g. vacuum) barrier generalizes the local state density in previous theories
of the tunnel-polarization factor. 5) For small applied voltage,
tunnel-polarization factors remain legitimate up to first order in the inverse
thickness of the ideal middle. An algebraic formula describes the first-order
corrections to polarization factors in terms of newly defined lateral
auto-correllation scales.Comment: This version no. 3 is thoroughly revised for clarity. Just a few
notations and equations are changed, and references completed. No change in
results. 17 pages including 4 figure
Differential Changes in Expression of Stress- and Metabolic-related Neuropeptides in the Rat Hypothalamus during Morphine Dependence and Withdrawal
Chronic morphine treatment and naloxone precipitated morphine withdrawal activates stress-related brain circuit and results in significant changes in food intake, body weight gain and energy metabolism. The present study aimed to reveal hypothalamic mechanisms underlying these effects. Adult male rats were made dependent on morphine by subcutaneous implantation of constant release drug pellets. Pair feeding revealed significantly smaller weight loss of morphine treated rats compared to placebo implanted animals whose food consumption was limited to that eaten by morphine implanted pairs. These results suggest reduced energy expenditure of morphine-treated animals. Chronic morphine exposure or pair feeding did not significantly affect hypothalamic expression of selected stress- and metabolic related neuropeptides - corticotropin-releasing hormone (CRH), urocortin 2 (UCN2) and proopiomelanocortin (POMC) compared to placebo implanted and pair fed animals.
Naloxone precipitated morphine withdrawal resulted in a dramatic weight loss starting as early as 15-30 min after naloxone injection and increased adrenocorticotrophic hormone, prolactin and corticosterone plasma levels in morphine dependent rats. Using real-time quantitative PCR to monitor the time course of relative expression of neuropeptide mRNAs in the hypothalamus we found elevated CRH and UCN2 mRNA and dramatically reduced POMC expression. Neuropeptide Y (NPY) and arginine vasopressin (AVP) mRNA levels were transiently increased during opiate withdrawal. These data highlight that morphine withdrawal differentially affects expression of stress- and metabolic-related neuropeptides in the rat hypothalamus, while relative mRNA levels of these neuropeptides remain unchanged either in rats chronically treated with morphine or in their pair-fed controls
Impurity and boundary effects in one and two-dimensional inhomogeneous Heisenberg antiferromagnets
We calculate the ground-state energy of one and two-dimensional spatially
inhomogeneous antiferromagnetic Heisenberg models for spins 1/2, 1, 3/2 and 2.
Our calculations become possible as a consequence of the recent formulation of
density-functional theory for Heisenberg models. The method is similar to
spin-density-functional theory, but employs a local-density-type approximation
designed specifically for the Heisenberg model, allowing us to explore
parameter regimes that are hard to access by traditional methods, and to
consider complications that are important specifically for nanomagnetic
devices, such as the effects of impurities, finite-size, and boundary geometry,
in chains, ladders, and higher-dimensional systems.Comment: 4 pages, 4 figures, accepted by Phys. Rev.
Wannier-function approach to spin excitations in solids
We present a computational scheme to study spin excitations in magnetic
materials from first principles. The central quantity is the transverse spin
susceptibility, from which the complete excitation spectrum, including
single-particle spin-flip Stoner excitations and collective spin-wave modes,
can be obtained. The susceptibility is derived from many-body perturbation
theory and includes dynamic correlation through a summation over ladder
diagrams that describe the coupling of electrons and holes with opposite spins.
In contrast to earlier studies, we do not use a model potential with adjustable
parameters for the electron-hole interaction but employ the random-phase
approximation. To reduce the numerical cost for the calculation of the
four-point scattering matrix we perform a projection onto maximally localized
Wannier functions, which allows us to truncate the matrix efficiently by
exploiting the short spatial range of electronic correlation in the partially
filled d or f orbitals. Our implementation is based on the FLAPW method.
Starting from a ground-state calculation within the LSDA, we first analyze the
matrix elements of the screened Coulomb potential in the Wannier basis for the
3d transition-metal series. In particular, we discuss the differences between a
constrained nonmagnetic and a proper spin-polarized treatment for the
ferromagnets Fe, Co, and Ni. The spectrum of single-particle and collective
spin excitations in fcc Ni is then studied in detail. The calculated spin-wave
dispersion is in good overall agreement with experimental data and contains
both an acoustic and an optical branch for intermediate wave vectors along the
[100] direction. In addition, we find evidence for a similar double-peak
structure in the spectral function along the [111] direction.Comment: 16 pages, 11 figures, 5 table
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