The relative stability between wurtzite and zinc blende structures in group III–V semiconductor nanowires is systematically investigated based on an empirical potential, which incorporates electrostatic energy due to valence-bond and ionic charges. The energy differences between wurtzite and zinc blende structures of 12 compound nanowires with diameter of 1–22 nm show that the wurtzite nanowires are stabilized for small diameter. This structural trend is found to be due to the contribution of two- and three-coordinated atoms on the nanowire facets to the system energy. We also find that the critical diameters, where the nanowires turn out be bistable forming both wurtzite and zinc blende structures, exist at the diameter of 12–32 nm depending on the ionicity of semiconductors. The bistability implies the synthesis of nanowires exhibiting polytypes, and supports the experimental results in GaP, GaAs, InP, and InAs nanowires.

Controlling and designing quantum magnetic properties by an external electric field is a key challenge in modern magnetic physics. Here, from first principles, the effects of an external electric field on the magnetocrystalline anisotropy (MCA) in ferromagnetic transition-metal monolayers are demonstrated which show that the MCA in an Fe(001) monolayer [but not in Co(001) and Ni(001) monolayers] can be controlled by the electric field through a change in band structure, in which small components of the p orbitals near the Fermi level, which are coupled to the d states by the electric field, play a key role. This prediction obtained opens a way to control the MCA by the electric field and invites experiments.

The enantioselective Mannich-type reaction of an enolate or an enolate anion equivalent with aldimines constitutes a useful method for the preparation of chiral b-amino carbonyl compounds, which are the precursors of biologically important compounds such as b-lactams and b-amino acids. The development of chiral catalysts for the asymmetric Mannichtype reaction has attracted the attention of synthetic organic chemists.[1] Although stoichiometric amounts of chiral acid were employed initially, [2] a number of enantioselective catalysts such as chiral Lewis acid catalysts [3] and chiral base catalysts [4] have been developed lately. In addition to metal-based chiral catalysts, [5] the use of small organic molecules as catalysts to promote asymmetric reactions has emerged as a new frontier in reaction methodology.[6] Accordingly, l-proline derivatives [7] and peptide derivatives [8] have been developed as catalysts for the Mannich-type reactions. We previously reported that Mannich-type reactions [9] and the aza-Diels-Alder reaction [10] proceed smoothly in the presence of a catalytic amount of a strong Brønsted acid. We thus postulated that the use of a chiral Brønsted acid, in which the proton is surrounded by bulky substituents, may lead to effective asymmetric induction. We report herein an enantioselective Mannich-type reaction of silyl enolates with aldimines catalyzed by a chiral metal-free Brønsted acid. [11,12] First, treatment of aldimine 1 a (Scheme 1, R 1 = Ph) and ketene silyl acetal 2 (3.0 equiv) with 0.3 equivalents of the chiral phosphate 4 a [13,14,15] (which is readily prepared from (R)-BINOL; Scheme 2) in toluene at À78 8C led to a smooth Mannich-type reaction to give 3 a (R 1 = Ph). However, no enantioselectivity was observed (Table 1, entry 1), as deterScheme 1. Mannich-type reaction of aldimines 1 and ketene silyl acetals 2 to form b-aminoesters 3.

The electric-field-induced switching of magnetocrystalline anisotropy ͑MCA͒ between in-plane and out-ofplane orientations is investigated by first-principles calculations for the prototypical Fe on MgO͑001͒ system. Our results predict that an ideal abrupt Fe/MgO interface gives rise to a large out-of-plane MCA due to weak Fe-O hybridization at the interface, but the MCA switching by an applied electric field is found to be difficult to achieve. Instead, the existence of an interfacial FeO layer plays a key role in demonstrating the MCA switching that accompanies an electric-field-induced displacement of Fe atoms on the interfacial FeO layer.Controlling magnetic properties by means of applied electric field ͑E field͒ is a key challenge in materials physics. Materials being studied include magnetoelectric multiferroics 1-4 and magnetic semiconductors. 5-7 Even in itinerant thin films, magnetic properties such as the magnetocrystalline anisotropy ͑MCA͒ are modified by application of a voltage; 8 there is general consensus that modification of the surface d-electron charge/spin density induced by the E field contributes to the effect. [9][10][11][12] Magnetic metal-oxide junctions are seen as an important avenue toward ultrahigh density and nonvolatile electronics 13,14 needed for information processing and storage technologies. Recent experiments have reported that the perpendicular MCA energy is significantly reduced by an applied voltage in Au/Fe/MgO junctions, 15 i.e., at ultrathin Fe/MgO interfaces. Moreover, the direct switching from out of plane to in plane has been achieved in Au/ Fe 0.8 Co 0.2 / MgO. 16 Such E-field-driven MCA switching offers a pathway to control magnetism at the nanoscale with ultralow-energy power consumption compared to traditional switching using magnetic fields. Despite its importance, an understanding of the underlying physics and mechanism of the E-field-driven MCA modification in metal-oxide interfaces is still lacking due to the structural and electronic complexity of the interfaces, thus, hindering the search for other promising metal-oxide interface candidates.Here, we present a mechanism for the E-field-driven MCA modification at the Fe/MgO interface, based on fullpotential linearized augmented plane-wave ͑FLAPW͒ calculations. 17,18 These calculations predict that an ideal abrupt Fe/MgO interface gives rise to a large out-of-plane MCA due to weak Fe-O hybridization at the interface, and that the MCA modification is caused by changes in the d-band structure at the Fermi level ͑E F ͒ when an E field is introduced. However, the MCA switching by an applied E field is found to be difficult to achieve. Instead, the existence of an interfacial FeO x layer between the metallic Fe layer and the MgO substrate plays a key role in facilitating the MCA switching, which accompanies an E-field-induced displacement of the Fe atoms in the interfacial FeO layer.Calculations were performed using the FLAPW method which treats a single slab geometry 17,18 that allows a natural way to include an exte...

The stability of Mg-incorporated GaN surfaces with semipolar ( 101 1 1 1) orientation is investigated by performing first-principles pseudopotential calculations. Several Mg-incorporated surfaces, in which a single Mg atom is substituted for the topmost Ga atom, can be formed when the surfaces include step edges in the [0 1 100] direction. This implies that on the stepped surfaces Mg atoms can be easily incorporated into electrically active substitutional lattice sites, leading to high hole concentrations. The calculated results provide a possible explanation for experimentally observed high hole concentrations in Mg-doped semipolar ( 101 1 1 1) GaN on vicinal (100) MgAl 2 O 4 substrates miscut in the h011i direction.

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We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such as partial pressure and temperature, on the surface stability and growth kinetics. First, we examine the feasibility of this approach by comparing calculated surface phase diagrams of GaAs(001) with experimental results. In addition, the Ga diffusion length on GaAs(001) during molecular beam epitaxy is discussed. Next, this approach is systematically applied to the reconstruction, adsorption and incorporation on various nitride semiconductor surfaces. The calculated results for nitride semiconductor surface reconstructions with polar, nonpolar, and semipolar orientations suggest that adlayer reconstructions generally appear on the polar and the semipolar surfaces. However, the stable ideal surface without adsorption is found on the nonpolar surfaces because the ideal surface satisfies the electron counting rule. Finally, the stability of hydrogen and the incorporation mechanisms of Mg and C during metalorganic vapor phase epitaxy are discussed.