Akira Masago

Luminescent centers in GaN codoped with Eu and Mg: Calculation on density functional theory

We investigate GaN codoped with Eu and Mg from calculations on the density functional theory. As a result, we obtain the total energies that are almost independent of Mg positions relative to Eu positions. The result suggests the coexistence of the luminescent centers that are Eu–Mg complexes and typical Eu atoms at least. Eu ions are trivalent in both centers; hence, the magnetization in the ground state 7F0 is zero. Considering the low excited states via the Boltzmann distribution, we can expect the finite magnetization on each Eu atom, but the Curie temperature is unlikely to be room temperature.

Efficient luminescent center by codoping (Eu,Mg,O) into GaN

From a theoretical standpoint, we propose that the addition of oxygen enhances the luminescence for codoping europium and magnesium in gallium nitride. From the additional oxygen, donor levels are created to an energy range between the europium f*-level and the conduction band, whereas magnesium acceptor levels are between the europium f-level and the valence band. Because the donor and acceptor levels form exciton paths from the host bands to Eu luminescent levels, the luminescence must be efficiently promoted.

First-principles investigations of defect and phase stabilities in thermoelectric (GeTe)x(AgSbTe2)1−x

We focus on the defect and phase stabilities in the pseudo binary alloy (GeTe)x(AgSbTe2)1−x (TAGS; tellurium antimony germanium silver). TAGS is expected to be a high effective thermoelectric material because its thermal conductivity shows anomalous behavior around the concentration of x = 0.8. The origin of the anomalous thermal conductivity and the stable structure in TAGS have not been well understood. To clarify the stable structure, we calculate the formation energies of the point and complex defects. It is found that the chain structure of Ag–Te–Sb has a lower formation energy in GeTe, and the system becomes more stable by assembling the Ag–Te–Sb chain structures. Moreover, the calculated mixing energy also shows that the TAGS system tends to undergo phase separation. In such structures, the grain boundary plays an important role in inducing large phonon scattering, leading to the thermal conductivity reduction.

Theoretical simulation of Kelvin probe force microscopy for Si surfaces by taking account of chemical forces.

A new method of theoretical simulation for Kelvin probe force microscopy (KPFM) imaging on semiconductor or metal samples is proposed. The method is based on a partitioned real space (PR) density functional based tight binding (DFTB) calculation of the electronic states to determine the multi-pole electro-static force, which is augmented with the chemical force obtained by a perturbation treatment of the orbital hybridization. With the PR-DFTB method, the change of the total energy is calculated together with the induced charge distribution in the tip and the sample by their approach under an applied bias voltage, and the KPFM images, namely the patterns of local contact potential difference (LCPD) distribution, are obtained with the minimum condition of the interaction force. However, since the interaction force is due to electro-static multi-poles, the spatial resolution of the KPFM images obtained by PR-DFTB is limited to the nano-scale range and an atom-scale resolution cannot be attained. By introducing an additional chemical force, i.e., the force due to the orbital hybridization, we succeeded in reproducing atom-scale resolution of KPFM images. Case studies are performed for clean and impurity embedded Si surfaces with Si tip models.

Simulation of Noncontact Atomic Force Microscopy of Hydrogen- and Methyl-Terminated Si(001) Surfaces

Simulations of noncontact atomic force microscopy (ncAFM) are presented for Si(111), Si(001), and hydrogen-terminated and methylterminated Si(001) surfaces. The force between tip and surface consists of the long-range van der Waals force and the short-range chemical force. The long-range van der Waals force is estimated by a continuum model using only the Si–Si Hamaker constant in a vacuum, whether or not the surface is terminated by H atoms or methyl radicals. The short-range chemical force is predicted by the densityfunctional based tight-binding method because its computational cost is much lower than ab initio calculations using density functional theory. The experimental range of the frequency shifts and the ncAFM images of Si(111), Si(001), and H-terminated and methyl-terminated Si(001) surfaces were reproduced excellently. In the images, the methyl radical adsorbed is observed as a cavity-like region despite of its protrusion. # 2009 The Japan Society of Applied Physics

Inherent instability by antibonding coupling in AgSbTe2

In the present paper, an inherent instability in the ternary chalcogenide compound AgSbTe2 is described from the electronic structure viewpoint. Our calculations, which are based on the cluster expansion method, suggest nine stable crystal structures involving the most stable structure with symmetry. The effective pair interactions calculated by the generalized perturbation method point out that the stability of these structures originates from the number of linear arrangements of the Ag–Te–Sb atomic bonds. Moreover, it is found that AgSbTe2 has a special electronic structure, where the dominant components of the top of the valence band are the Te-5p antibonding states. Such an antibonding contribution leads to an inherent instability, such that the system spontaneously forms various mutation phases caused by charge-compensated defect complexes. We propose that these mutation phases play an important role in the thermal conductivity and thermoelectric efficiency in AgSbTe2.

Computational nano-materials design for circularly polarized luminescence in (Eu,Mg,O)-codoped GaN

We describe the production of circularly polarized luminescence from a luminescent center, which is generated by the spontaneous spinodal nano-decomposition of Eu, Mg, and O impurities doped into GaN. In this material, Zeners p–f exchange interaction creates an electronic structure suitable for the production of circularly polarized luminescence. Moreover, the 7P2 ground state causes finite magnetic moments due to the reduction of the local symmetry around the Eu atom. These factors provide us with an expectation of not only circularly polarized luminescence but also potential control of the lifetime, color, and intensity, using the self-organized magnetic nano-structures.

The Pressure Dependence of Solid Boron

We study the stability of icosahedron-based solid boron called α- and β-boron by first principles calculations. It is found that α-boron is the most stable phase at zero temperature. The actual structure of β-boron has disorders in the atomic arrangement. This affects slightly and decreases the total energy of β-boron, but does not change the above conclusion. The effect of this disorder, however, gives stability of β-boron at finite temperatures through the entropy term. On applying pressure, the stability of α-boron is increased. Under hydrostatic pressures, the deformation of icosahedra of β-boron exhibits differently from both of α-boron and boron carbide.

Group theoretical classification of broken symmetry states of the two-fold degenerate Hubbard model on a triangular lattice

By a group theoretical procedure we derive the possible spontaneously broken-symmetry states for the two-fold degenerate Hubbard model on a two-dimensional triangular lattice. For ordering wave vectors corresponding to the points Γ and K in the first BZ we find 22 states which include 16 collinear and six non-collinear states. The collinear states include the usual SDW and CDW states which appear also in the single-band Hubbard model. The non-collinear states include exotic ordering states of orbitals and spins as well as the triangular arrangement of spins. cience B.V. All rights reserved.

Group Theoretical Classification of Doubly Degenerate Orbital Systems on a Triangular Lattice

On the basis of a group theoretical theory we have classified the mean field solutions of an e g degenerate Hubbard model on a two dimensional triangular lattice. For the crystal structure we have ...