1,351 research outputs found
Antiferromagnetic order in (Ga,Mn)N nanocrystals: A density functional theory study
We investigate the electronic and magnetic properties of (Ga,Mn)N
nanocrystals using the density functional theory. We study both wurtzite and
zinc-blende structures doped with one or two substitutional Mn impurities. For
a single Mn dopant placed close to surface, the behavior of the empty
Mn-induced state, hereafter referred to as "Mn hole", is different from bulk
(Ga,Mn)N. The energy level corresponding to this off-center Mn hole lies within
the nanocrystal gap near the conduction edge. For two Mn dopants, the most
stable magnetic configuration is antiferromagnetic, and this was unexpected
since (Ga,Mn)N bulk shows ferromagnetism in the ground state. The surprising
antiferromagnetic alignment of two Mn spins is ascribed also to the holes
linked to the Mn impurities located close to surface. Unlike Mn holes in
(Ga,Mn)N bulk, these Mn holes in confined (Ga,Mn)N nanostructures do not
contribute to the ferromagnetic alignment of the two Mn spins
Optical spin control in nanocrystalline magnetic nanoswitches
We investigate the optical properties of (Cd,Mn)Te quantum dots (QDs) by
looking at the excitons as a function of the Mn impurities positions and their
magnetic alignments. When doped with two Mn impurities, the Mn spins, aligned
initially antiparallel in the ground state, have lower energy in the parallel
configuration for the optically active spin-up exciton. Hence, the
photoexcitation of the QD ground state with antiparallel Mn spins induces one
of them to flip and they align parallel. This suggests that (Cd,Mn)Te QDs are
suitable for spin-based operations handled by light
First-principles calculations of the magnetic properties of (Cd,Mn)Te nanocrystals
We investigate the electronic and magnetic properties of Mn-doped CdTe
nanocrystals (NCs) with 2 nm in diameter which can be experimentally
synthesized with Mn atoms inside. Using the density-functional theory, we
consider two doping cases: NCs containing one or two Mn impurities. Although
the Mn d peaks carry five up electrons in the dot, the local magnetic moment on
the Mn site is 4.65 mu_B. It is smaller than 5 mu_B because of the sp-d
hybridization between the localized 3d electrons of the Mn atoms and the s- and
p-type valence states of the host compound. The sp-d hybridization induces
small magnetic moments on the Mnnearest- neighbor Te sites, antiparallel to the
Mn moment affecting the p-type valence states of the undoped dot, as usual for
a kinetic-mediated exchange magnetic coupling. Furthermore, we calculate the
parameters standing for the sp-d exchange interactions. Conduction N0\alpha and
valence N0\beta are close to the experimental bulk values when the Mn
impurities occupy bulklike NCs' central positions, and they tend to zero close
to the surface. This behavior is further explained by an analysis of
valence-band-edge states showing that symmetry breaking splits the states and
in consequence reduces the exchange. For two Mn atoms in several positions, the
valence edge states show a further departure from an interpretation based in a
perturbative treatment. We also calculate the d-d exchange interactions |Jdd|
between Mn spins. The largest |Jdd| value is also for Mn atoms on bulklike
central sites; in comparison with the experimental d-d exchange constant in
bulk Cd0.95Mn0.05Te, it is four times smaller
A geometrical analysis of the field equations in field theory
In this review paper we give a geometrical formulation of the field equations
in the Lagrangian and Hamiltonian formalisms of classical field theories (of
first order) in terms of multivector fields. This formulation enables us to
discuss the existence and non-uniqueness of solutions, as well as their
integrability.Comment: 14 pages. LaTeX file. This is a review paper based on previous works
by the same author
Multivector Field Formulation of Hamiltonian Field Theories: Equations and Symmetries
We state the intrinsic form of the Hamiltonian equations of first-order
Classical Field theories in three equivalent geometrical ways: using
multivector fields, jet fields and connections. Thus, these equations are given
in a form similar to that in which the Hamiltonian equations of mechanics are
usually given. Then, using multivector fields, we study several aspects of
these equations, such as the existence and non-uniqueness of solutions, and the
integrability problem. In particular, these problems are analyzed for the case
of Hamiltonian systems defined in a submanifold of the multimomentum bundle.
Furthermore, the existence of first integrals of these Hamiltonian equations is
considered, and the relation between {\sl Cartan-Noether symmetries} and {\sl
general symmetries} of the system is discussed. Noether's theorem is also
stated in this context, both the ``classical'' version and its generalization
to include higher-order Cartan-Noether symmetries. Finally, the equivalence
between the Lagrangian and Hamiltonian formalisms is also discussed.Comment: Some minor mistakes are corrected. Bibliography is updated. To be
published in J. Phys. A: Mathematical and Genera
On the Multimomentum Bundles and the Legendre Maps in Field Theories
We study the geometrical background of the Hamiltonian formalism of
first-order Classical Field Theories. In particular, different proposals of
multimomentum bundles existing in the usual literature (including their
canonical structures) are analyzed and compared. The corresponding Legendre
maps are introduced. As a consequence, the definition of regular and
almost-regular Lagrangian systems is reviewed and extended from different but
equivalent ways.Comment: LaTeX file, 19 pages. Replaced with the published version. Minor
mistakes are correcte
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