746 research outputs found
Optimal Recombination in Genetic Algorithms
This paper surveys results on complexity of the optimal recombination problem
(ORP), which consists in finding the best possible offspring as a result of a
recombination operator in a genetic algorithm, given two parent solutions. We
consider efficient reductions of the ORPs, allowing to establish polynomial
solvability or NP-hardness of the ORPs, as well as direct proofs of hardness
results
Competing rhombohedral and monoclinic crystal structures in Mn compounds: an {\em ab-initio} study
Based on the relativistic spin-polarized density functional theory
calculations we investigate the crystal structure, electronic and magnetic
properties of a family MnPn2Ch4 compounds, where pnictogen metal atoms (Pn) are
Sb and Bi; chalcogens (Ch) are Se, Te. We show that in the series the compounds
of this family with heavier elements prefer to adopt rhombohedral crystal
structure composed of weakly bonded septuple monoatomic layers while those with
lighter elements tend to be in the monoclinic structure. Irrespective of the
crystal structure all compounds of the MnPn2Ch4 series demonstrate a weak
energy gain (of a few meV per formula unit or even smaller than meV) for
antiferromagnetic (AFM) coupling for magnetic moments on Mn atoms with respect
to their ferromagnetic (FM) state. For rhombohedral structures the interlayer
AFM coupling is preferable while in monoclinic phases intralayer AFM
configuration with ferromagnetic ordering along the Mn chain and
antiferromagnetic ordering between the chains has a minimum energy. Over the
series the monoclinic compounds are characterized by substantially wider
bandgap than compounds with rhombohedral structure
Spin Polarization and Transport of Surface States in the Topological Insulators Bi2Se3 and Bi2Te3 from First Principles
We investigate the band dispersion and the spin texture of topologically
protected surface states in the bulk topological insulators Bi2Se3 and Bi2Te3
by first-principles methods. Strong spin-orbit entanglement in these materials
reduces the spin-polarization of the surface states to ~50% in both cases; this
reduction is absent in simple models but of important implications to
essentially any spintronic application. We propose a way of controlling the
magnitude of spin polarization associated with a charge current in thin films
of topological insulators by means of an external electric field. The proposed
dual-gate device configuration provides new possibilities for electrical
control of spin.Comment: 4+ pages, 3 figure
Optimal recombination in genetic algorithms for combinatorial optimization problems: Part II
This paper surveys results on complexity of the optimal recombination problem
(ORP), which consists in finding the best possible offspring as a result of a
recombination operator in a genetic algorithm, given two parent solutions. In
Part II, we consider the computational complexity of ORPs arising in genetic
algorithms for problems on permutations: the Travelling Salesman Problem, the
Shortest Hamilton Path Problem and the Makespan Minimization on Single
Machine and some other related problems. The analysis indicates that the
corresponding ORPs are NP-hard, but solvable by faster algorithms, compared
to the problems they are derived from
Spin-helical Dirac states in graphene induced by polar-substrate surfaces with giant spin-orbit interaction: a new platform for spintronics
Spintronics, or spin electronics, is aimed at efficient control and
manipulation of spin degrees of freedom in electron systems. To comply with
demands of nowaday spintronics, the studies of electron systems hosting giant
spin-orbit-split electron states have become one of the most important
directions providing us with a basis for desirable spintronics devices. In
construction of such devices, it is also tempting to involve graphene, which
has attracted great attention because of its unique and remarkable electronic
properties and was recognized as a viable replacement for silicon in
electronics. In this case, a challenging goal is to make graphene Dirac states
spin-polarized. Here, we report on absolutely new promising pathway to create
spin-polarized Dirac states based on coupling of graphene and polar-substrate
surface states with giant Rashba-type spin-splitting. We demonstrate how the
spin-helical Dirac states are formed in graphene deposited on the surface of
BiTeCl. This coupling induces spin separation of the originally spin-degenerate
graphene states and results in fully helical in-plane spin polarization of the
Dirac electrons.Comment: 5 pages, 3 figure
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