5,598 research outputs found
Ab-initio study of disorder effects on the electronic and magnetic structures of SrFeMoO
We have investigated the electronic structure of ordered and disordered
SrFeMoO using {\it ab-initio} band structure methods. The effect of
disorder was simulated within super-cell calculations to realize several
configurations with mis-site disorders. It is found that such disorder effects
destroy the half-metallic ferro-magnetic state of the ordered compound. It also
leads to a substantial reduction of the magnetic moments at the Fe sites in the
disordered configurations. Most interestingly, it is found for the disordered
configurations, that the magnetic coupling within the Fe sub-lattice as well as
that within the Mo sub-lattice always remain ferro-magnetic, while the two
sub-lattices couple anti-ferromagnetically, in close analogy to the magnetic
structure of the ordered compound, but in contrast to recent suggestions.Comment: 7 pages, 3 figure
Critical behavior of diluted magnetic semiconductors: the apparent violation and the eventual restoration of the Harris criterion for all regimes of disorder
Using large-scale Monte Carlo calculations, we consider strongly disordered
Heisenberg models on a cubic lattice with missing sites (as in diluted magnetic
semiconductors such as Ga_{1-x}Mn_{x}As). For disorder ranging from weak to
strong levels of dilution, we identify Curie temperatures and calculate the
critical exponents nu, gamma, eta, and beta finding, per the Harris criterion,
good agreement with critical indices for the pure Heisenberg model where there
is no disorder component. Moreover, we find that thermodynamic quantities (e.g.
the second moment of the magnetization per spin) self average at the
ferromagnetic transition temperature with relative fluctuations tending to zero
with increasing system size. We directly calculate effective critical exponents
for T > T_{c}, yielding values which may differ significantly from the critical
indices for the pure system, especially in the presence of strong disorder.
Ultimately, the difference is only apparent, and eventually disappears when T
is very close to T_{c}.Comment: 11 pages, 9 figure
Magnetic Percolation and the Phase Diagram of the Disordered RKKY model
We consider ferromagnetism in spatially randomly located magnetic moments, as
in a diluted magnetic semiconductor, coupled via the carrier-mediated indirect
exchange RKKY interaction. We obtain via Monte Carlo the magnetic phase diagram
as a function of the impurity moment density and the relative carrier
concentration . As evidenced by the diverging correlation length
and magnetic susceptibility, the boundary between ferromagnetic (FM) and
non-ferromagnetic (NF) phases constitutes a line of zero temperature critical
points which can be viewed as a magnetic percolation transition. In the dilute
limit, we find that bulk ferromagnetism vanishes for . We also
incorporate the local antiferromagnetic direct superexchange interaction
between nearest neighbor impurities, and examine the impact of a damping factor
in the RKKY range function.Comment: 5 pages, 3 figures; figure formatting modified, typos fixe
Effects of large induced superconducting gap on semiconductor Majorana nanowires
With the recent achievement of extremely high-quality epitaxial interfaces
between InAs nanowires and superconducting Al shells with strong
superconductor-semiconductor tunnel coupling, a new regime of proximity-induced
superconductivity in semiconductors can be explored where the induced gap may
be similar in value to the bulk Al gap (large gap) with negligible subgap
conductance (hard gap). We propose several experimentally relevant consequences
of this large-gap strong-coupling regime for tunneling experiments, and we
comment on the prospects of this regime for topological superconductivity. In
particular, we show that the advantages of having a strong spin-orbit coupling
and a large spin g-factor in the semiconductor nanowire may both be compromised
in this strongly coupled limit, and somewhat weaker interface tunneling may be
necessary for achieving optimal proximity superconductivity in the
semiconductor nanowire. We derive a minimal, generic theory for the
strong-coupling hard-gap regime obtaining good qualitative agreement with the
experiment and pointing out future directions for further progress toward
Majorana nanowires in hybrid semiconductor-superconductor structures.Comment: 8 pages, 5 figures; published versio
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