20,317 research outputs found
General Green's function formalism for transport calculations with spd-Hamiltonians and giant magnetoresistance in Co and Ni based magnetic multilayers
A novel, general Green's function technique for elastic spin-dependent
transport calculations is presented, which (i) scales linearly with system size
and (ii) allows straightforward application to general tight-binding
Hamiltonians (spd in the present work). The method is applied to studies of
conductance and giant magnetoresistance (GMR) of magnetic multilayers in CPP
(current perpendicular to planes) geometry in the limit of large coherence
length. The magnetic materials considered are Co and Ni, with various
non-magnetic materials from the 3d, 4d, and 5d transition metal series.
Realistic tight-binding models for them have been constructed with the use of
density functional calculations. We have identified three qualitatively
different cases which depend on whether or not the bands (densities of states)
of a non-magnetic metal (i) form an almost perfect match with one of spin
sub-bands of the magnetic metal (as in Cu/Co spin valves); (ii) have almost
pure sp character at the Fermi level (e.g. Ag); (iii) have almost pure d
character at the Fermi energy (e.g. Pd, Pt). The key parameters which give rise
to a large GMR ratio turn out to be (i) a strong spin polarization of the
magnetic metal, (ii) a large energy offset between the conduction band of the
non-magnetic metal and one of spin sub-bands of the magnetic metal, and (iii)
strong interband scattering in one of spin sub-bands of a magnetic metal. The
present results show that GMR oscillates with variation of the thickness of
either non-magnetic or magnetic layers, as observed experimentally.Comment: 22 pages, 9 figure
Preliminary flight evaluation of an engine performance optimization algorithm
A performance seeking control (PSC) algorithm has undergone initial flight test evaluation in subsonic operation of a PW 1128 engined F-15. This algorithm is designed to optimize the quasi-steady performance of an engine for three primary modes: (1) minimum fuel consumption; (2) minimum fan turbine inlet temperature (FTIT); and (3) maximum thrust. The flight test results have verified a thrust specific fuel consumption reduction of 1 pct., up to 100 R decreases in FTIT, and increases of as much as 12 pct. in maximum thrust. PSC technology promises to be of value in next generation tactical and transport aircraft
Suppression of Giant Magnetoresistance by a superconducting contact
We predict that current perpendicular to the plane (CPP) giant
magnetoresistance (GMR) in a phase-coherent magnetic multilayer is suppressed
when one of the contacts is superconducting. This is a consequence of a
superconductivity-induced magneto-resistive (SMR) effect, whereby the
conductance of the ferromagnetically aligned state is drastically reduced by
superconductivity. To demonstrate this effect, we compute the GMR ratio of
clean (Cu/Co)_nCu and (Cu/Co)_nPb multilayers, described by an ab-initio spd
tight binding Hamiltonian. By analyzing a simpler model with two orbitals per
site, we also show that the suppression survives in the presence of elastic
scattering by impurities.Comment: 5 pages, 4 figures. Submitted to PR
Negative 4-Probe Conductances of Mesoscopic Superconducting Wires
We analyze the longitudinal 4-probe conductance of mesoscopic normal and
superconducting wires and predict that in the superconducting case, large
negative values can arise for both the weakly disordered and localized regimes.
This contrasts sharply with the behaviour of the longitudinal 4-probe
conductance of normal wires, which in the localized limit is always
exponentially small and positive.Comment: Latex, 3 figures available on request to [email protected]
(Simon Robinson
Long-term Effects of Famine on Life Expectancy: A Re-analysis of the Great Finnish Famine of 1866-1868
Famines are extreme cases of environmental stress, and have been used by a series of studies to explore the long-term consequences of the fetal or childhood environment. Results are inconsistent and do not support negative long-term effects on mortality. The authors test the hypothesis that selection during famine changes the frailty distributions of cohorts and may hide negative long-term effects. They use death counts from age 60+ from the Human Mortality Data Base for the birth cohorts 1850-1854, 1855-1859, 1860-1865, 1866-1868, 1869-1874, 1875-1879, 1880-1884 and 1885-1889 to explore the effect of being born during the Great Finnish Famine 1866-1868. Swedish cohorts without famine exposure are analysed as a control group. Cohorts born in Finland during the Great Finnish Famine are highly heterogeneous in their distribution of deaths after age 60. By contrast, cohorts born in the years immediately after the famine are particularly homogeneous. Accounting for these differences results into a lower remaining life expectancy at age 60 for cohorts born during the famine. Statistically, long-term effects of famine on mortality become only visible when changes in the frailty distribution of cohorts are explicitly considered.old-age mortality, selection, debilitation, early life circumstances
Deconstructing graviphoton from mass-deformed ABJM
Mass-deformed ABJM theory has a maximally supersymmetric fuzzy two-sphere
vacuum solution where the scalar fields are proportional to the TGRVV matrices.
We construct these matrices using Schwinger oscillators. This shows that the
ABJM gauge group that corresponds to the fuzzy two-sphere geometry is
. We deconstruct the graviphoton term in the D4 brane
theory. The normalization of this term is fixed by topological reasons. This
gives us the correct normalization of the deconstructed U(1) gauge field and
fixes the Yang -Mills coupling constant to the value which corresponds to M5
brane compactified on \mb{R}^ {1,2} \times S^3/{\mb{Z}_k}. The graviphoton
term also enable us to show that the zero mode contributions to the partition
functions for the D4 and the M5 brane agree.Comment: 26 page
(2,0) theory on circle fibrations
We consider (2,0) theory on a manifold M_6 that is a fibration of a spatial
S^1 over some five-dimensional base manifold M_5. Initially, we study the free
(2,0) tensor multiplet which can be described in terms of classical equations
of motion in six dimensions. Given a metric on M_6 the low energy effective
theory obtained through dimensional reduction on the circle is a Maxwell theory
on M_5. The parameters describing the local geometry of the fibration are
interpreted respectively as the metric on M_5, a non-dynamical U(1) gauge field
and the coupling strength of the resulting low energy Maxwell theory. We derive
the general form of the action of the Maxwell theory by integrating the reduced
equations of motion, and consider the symmetries of this theory originating
from the superconformal symmetry in six dimensions. Subsequently, we consider a
non-abelian generalization of the Maxwell theory on M_5. Completing the theory
with Yukawa and phi^4 terms, and suitably modifying the supersymmetry
transformations, we obtain a supersymmetric Yang-Mills theory which includes
terms related to the geometry of the fibration.Comment: 24 pages, v2 References added, typos correcte
Sub-gap conductance in ferromagnetic-superconducting mesoscopic structures
We study the sub-gap conductance of a ferromagnetic mesoscopic region
attached to a ferromagnetic and a superconducting electrode by means of tunnel
junctions. In the absence of the exchange field, the ratio of the two tunnel junction resistances determines the behaviour of
the sub-gap conductance which possesses a zero-bias peak for and for
a peak at finite voltage. We show that the inclusion of the exchange
field leads to a peak splitting for , while it shifts the zero-bias
anomaly to finite voltages for .Comment: 5 pages revte
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