4,004 research outputs found
Spontaneous Magnetization in the Disorder dominated Phase of the Twodimensional Random Bond Ising Model
The selfconsistent approach to the 2D Ising Model with quenched random bonds
is extended to the full lattice theory of four real fermions. The additional
degrees of freedom, neglected in the renormalization-group theory, lead to a
new phase between the ferromagnetic and the paramagnetic phase. The disorder
averaged spin-spin correlation function decays exponentially with distance. The
corresponding correlation length is , where denotes the order
parameter of the new phase introduced by Ziegler.Comment: 18 pages,plainTEX,TKM-74-9
Conductivity of a quasiperiodic system in two and three dimensions
A generalization of the Aubry-Andre model in two and three dimensions is
introduced which allows for quasiperiodic hopping terms in addition to the
quasiperiodic site potentials. This corresponds to an array of interstitial
impurities within the periodic host crystal. The resulting model is exactly
solvable and I compute the density of states and the ac-conductivity. There is
no mobility edge as in completely disordered systems but the regular
ac-conductivity and the strongly reduced Drude weight indicate a precursor of
the Anderson transition as the Fermi energy goes from the center to the band
edges.Comment: 4 pages,6 figures, references adde
On the Spectrum of the XXZ-chain at roots of unity
In a recent paper (cond-mat/0009279), Fabricius and McCoy studied the
spectrum of the spin 1/2 XXZ-model at Delta = (q+q^{-1})/2 and q^{2N}=1 for
integer N >1. They found a certain pattern of degeneracies and linked it to the
sl(2)-loop symmetry present in the commensurable spin sector (N divides S^z).
We show that the degeneracies are due to zero-energy, transparent excitations,
the cyclic bound states. These exist both in commensurable and incommensurable
sectors, indicating a symmetry, of which sl(2)-loop is a partial manifestation.
Our approach treats both sectors on even footing and yields an analytical
expression for the degeneracies in the case N = 3.Comment: 27 page
Large inverse tunneling magnetoresistance in CoCrFeAl/MgO/CoFe magnetic tunnel junctions
Magnetic tunnel junctions with the layer sequence
CoCrFeAl/MgO/CoFe were fabricated by magnetron sputtering
at room temperature (RT). The samples exhibit a large inverse tunneling
magnetoresistance (TMR) effect of up to -66% at RT. The largest value of -84%
at 20 K reflects a rather weak influence of temperature. The dependence on the
voltage drop shows an unusual behavior with two almost symmetric peaks at
mV with large inverse TMR ratios and small positive values around zero
bias
Structural and magneto-transport characterization of Co_2Cr_xFe_(1-x)Al Heusler alloy films
We investigate the structure and magneto-transport properties of thin films
of the Co_2Cr_xFe_(1-x)Al full-Heusler compound, which is predicted to be a
half-metal by first-principles theoretical calculations. Thin films are
deposited by magnetron sputtering at room temperature on various substrates in
order to tune the growth from polycrystalline on thermally oxidized Si
substrates to highly textured and even epitaxial on MgO(001) substrates,
respectively. Our Heusler films are magnetically very soft and ferromagnetic
with Curie temperatures up to 630 K. The total magnetic moment is reduced
compared to the theoretical bulk value, but still comparable to values reported
for films grown at elevated temperature. Polycrystalline Heusler films combined
with MgO barriers are incorporated into magnetic tunnel junctions and yield 37%
magnetoresistance at room temperature
Run-time Spatial Mapping of Streaming Applications to Heterogeneous Multi-Processor Systems
In this paper, we define the problem of spatial mapping. We present reasons why performing spatial mappings at run-time is both necessary and desirable. We propose what is—to our knowledge—the first attempt at a formal description of spatial mappings for the embedded real-time streaming application domain. Thereby, we introduce criteria for a qualitative comparison of these spatial mappings. As an illustration of how our formalization relates to practice, we relate our own spatial mapping algorithm to the formal model
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