Mogus Mochena

Ground-state properties and the molecular theory of the Curie temperature in the coherent potential approximation of diluted magnetic semiconductors

Using a spin-1/2 description of valence holes and Kondo coupling between local spins and carriers, GaAs-based III–V diluted magnetic semiconductors (DMS) are studied in the coherent potential approximation (CPA). Our calculated relation between ground-state energy and impurity magnetization shows that ferromagnetism is always favourable at low temperatures. For very weak Kondo coupling, the density of states (DOS) of the host semiconductor is not modified much. Impurity bands can be generated at the bottom of the host band only when Kondo coupling is strong enough. Using Weiss molecular theory, we predict a linear relation of Curie temperature with respect to Kondo coupling and doping concentration x if the hole density is proportional to x.

THEORY OF FERROMAGNETISM IN (A, Mn) B SEMICONDUCTORS

Abstract A brief review of theory of ferromagnetism of diluted magnetic semiconductors of the form (A, Mn) B based on the double-exchange model is first given. A systematic investigation of the phenomena extending the current theory is outlined. We begin with an investigation of regions of instability of the non-magnetic towards the ferromagnetic state of a system of Mn atoms doped in an AB-type semiconductor. A self-consistent many-body theory of the ferromagnetic state is then developed, going beyond the mean-field approaches by including fluctuations of the Mn spins and the itinerant hole gas. A functional theory suitable for computation of system properties such as the Curie temperature as a function of the hole and the Mn concentrations, the spin current, etc., is formulated.

Kinetics of monolayer and bilayer nanoparticle film formation during electrophoretic deposition

Abstract The kinetics of electrophoretic deposition are studied for the growth of monolayers and bilayers of ∼10 nm iron oxide nanoparticles. The net deposited nanoparticle film is measured as a function of time for five voltages through analysis of scanning electron microcopy images of the films. The collected data suggest that films deposited at different voltages have different kinetic behaviours. Additionally, we observe monolayer and bilayer growth separately, from which we assert that the initiation of the bilayer may be the source of variation in kinetic behaviour.

A low-temperature derivation of spin–spin exchange in Kondo lattice model

Abstract Using Hubbard–Stratonovich transformation and drone-fermion representations for spin- 1 2 and for spin- 3 2 , which is presented for the first time, we make a path-integral formulation of the Kondo lattice model. In the case of weak coupling and low temperature, the functional integral over conduction fermions can be approximated to the quadratic order and this gives the well-known RKKY interaction. In the case of strong coupling, the same quadratic approximation leads to an effective local spin–spin interaction linear in hopping energy t .

Broken-symmetry-adapted Green function theory of condensed matter systems: Towards a vector spin-density-functional theory

The group theory framework developed by Fukutome for a systematic analysis of the various broken symmetry types of Hartree-Fock solutions exhibiting spin structures is here extended to the general many body context using spinor-Green function formalism for describing magnetic systems. Consequences of this theory are discussed for examining the magnetism of itinerant electrons in nanometric systems of current interest as well as bulk systems where a vector spin-density form is required, by specializing our work to spin-density-functional formalism. We also formulate the linear response theory for such a system and compare and contrast them with the recent results obtained for localized electron systems. The various phenomenological treatments of itinerant magnetic systems are here unified in this group-theoretical description.

Optimal packing size of non-ligated CdSe nanoclusters for microstructure synthesis

Structural and electrostatic properties of nanoclusters of CdSe of diameter 1–2 nm are studied with first principle calculations to determine the optimal size for synthesizing microstructures. Based on robustness of the core structure, i.e., the retention of tetrahedral geometry, hexagonal ring structure, and overall w u¨rtzite structure to surface relaxations, we conclude that nanoclusters of ∼2 nm diameter are the best candidates to form a dense microstructure with minimal interstitial space. Se-terminated surfaces retain a zigzag structure as Se atoms are pulled out and Cd atoms are pulled in due to relaxation, therefore, are best suited for inter-nanocluster formations.

Analysis of the Zeeman effect on the energy spectrum in graphenes

An analysis of the Zeeman effect with a strong external magnetic field on the energy spectrum in graphene is presented. In general, the Hamiltonian of graphene in applied electric and magnetic fields is not of SO(1, 2) invariance even in the nearest-neighbor approximation because of the Zeeman coupling. But an approximate SO(1, 2) invariance can be obtained when the applied magnetic field is very strong. This approximate invariance can be used to relate the energy structure of graphene in the presence of both electric and magnetic fields to that when there is only magnetic field. Therefore, it can help explain the recently found quantum Hall conductance (4q2/h)L for L = 0.1.

Spin-wave softening and Hund's coupling in ferromagnetic manganites

Using a one-orbital model of hole-doped manganites, we show with the help of the Holstein-Primakov transformation that finite Hunds coupling is responsible for the spin-wave softening in the ferromagnetic B-phase manganites. We obtain an analytical result for the spin-wave spectrum for [Formula: see text]. In the limit of infinite Hunds coupling, the spectrum is the conventional nearest-neighbour Heisenberg ferromagnetic spin-wave. The o (t/J(H))-order correction is negative and thus accounts for the softening near the zone boundary.

Manganese nanoclusters and nanowires on GaAs surfaces

We have computed the ground-state local magnetic moments of manganese and neighboring arsenic for various cluster configurations on the (001) surface of a GaAs bulk crystal using a cluster of 512 atoms. For manganese we obtained a substantial local magnetic moment of (3.66′0.01) μ B for all cases considered. The induced magnetic moment of arsenic is less than that of manganese by two orders of magnitude, and falls off drastically beyond the nearest-neighbor distance. A small amount of charge is transferred from the manganese atom to the arsenic atom. The possibility of a spin-polarized wire channel on the arsenic layer below the surface is suggested.