Tatsuki Oda

Finite electric field effects in the large perpendicular magnetic anisotropy surface Pt/Fe/Pt(001): a first-principles study.

We investigate crystalline magnetic anisotropy in the electric field (EF) for the FePt surface which has a large perpendicular anisotropy, by means of the first-principles approach. Anisotropy is reduced linearly with respect to the inward EF, associated with the induced spin density around the Fe layer. Although the magnetic anisotropy energy (MAE) density reveals large variation around the atoms, the intrinsic contribution to the MAE is found to mainly come from the Fe layer. The surface without the capping Pt layer also shows similar linear dependence.

Inverted micelle formation of cell-penetrating peptide studied by coarse-grained simulation: Importance of attractive force between cell-penetrating peptides and lipid head group

Arginine-rich peptide and Antennapedia are cell-penetrating peptides (CPPs) which have the ability to permeate plasma membrane. Deformation of the plasma membrane with CPPs is the key to understand permeation mechanism. We investigate the dynamics of CPP and the lipid bilayer membrane by coarse-grained simulation. We found that the peptide makes inverted micelle in the lipid bilayer membrane, when the attractive potential between the peptide and lipid heads is strong. The inverted micelle is formed to minimize potential energy of the peptide. For vesicle membrane, the peptide moves from the outer vesicle to the inner vesicle through the membrane. The translocation of the peptide suggests inverted micelle model as a possible mechanism of CPPs.

Valley spin polarization by using the extraordinary Rashba effect on silicon

The addition of the valley degree of freedom to a two-dimensional spin-polarized electronic system provides the opportunity to multiply the functionality of next-generation devices. So far, however, such devices have not been realized due to the difficulty to polarize the valleys, which is an indispensable step to activate this degree of freedom. Here we show the formation of 100% spin-polarized valleys by a simple and easy way using the Rashba effect on a system with C3 symmetry. This polarization, which is much higher than those in ordinary Rashba systems, results in the valleys acting as filters that can suppress the backscattering of spin-charge. The present system is formed on a silicon substrate, and therefore opens a new avenue towards the realization of silicon spintronic devices with high efficiency.

Magic Numbers of Graphene Multivacancies

The stabilities and atomic geometries of graphene multivacancies are studied using a first-principles calculation. We find that the atomic relaxation allows the system to have pentagons in vacancies and that this pentagon formation stabilizes the defects considerably. It is concluded that the magic numbers of the multivacancies are 2, 4, and 6. This result differs from that predicted using the conventional dangling bond counting model, which does not include any effect of lattice relaxation. An extended model, which is additionally parametrized by the number of pentagons, is constructed.

A comparative ab initio study on electric-field dependence of magnetic anisotropy in MgO/Fe/Pt and MgO/Fe/Au films

We have investigated the electric field (EF) effect on the magnetic anisotropy energy (MAE) in thin films MgO/Fe/M(001) (M = Au and Pt) by means of first-principles density-functional calculations. The EF dependence of the MAE is enhanced significantly in the film with the Pt substrate compared to that with the Au substrate. This enhancement is attributed to the EF-induced hybridization between Fe 3d- and Pt 5d-orbitals. This implies that the Pt layer stacked on the magnetic layer strengthens the sensitivity of devices for bias-voltage-induced magnetic control.

Atomic Geometry and Stability of Mono-, Di-, and Trivacancies in Graphene

Stability and atomic geometry of mono-, di-, and trivacancies in graphene sheets are studied by using first-principles calculations. We find that the atomic relaxation substantially contributes to the stability of the vacancies. The monovacancy is found to have a nonplanar structure, i.e., its symmetry is C1h, while the ideal monovacancy has D3h symmetry. The divacancy is found to have a 5-8-5 membered ring structure. The trivacancy is also found to have two five membered rings. The energetics of these vacancies are not explained by the conventional dangling-bond counting model, which does not include lattice relaxation. Our calculations show that the divacancy is very stable and is thus expected to be detected under some experimental conditions.

Electric-field effects on magnetic anisotropy in Pd/Fe/Pd(0 0 1) surface

Electric-field (EF) effects have been studied on magnetic anisotropy in the metallic surfaces Pt/Fe/Pt(0 0 1) and Pd/Fe/Pd(0 0 1) by means of the first-principles electronic structure calculation which employs the generalized gradient approximation. The variation of anisotropy energy with respect to the EF is found to be opposite to each other. The modulus rate of the variation is larger by a few factors in the Pt substrate than in the Pd one. These results agree qualitatively well with the available experimental data. The electronic structures are presented and the origins in EF effects are discussed along a line of the second perturbative fashion.

Superconducting H5S2 phase in sulfur-hydrogen system under high-pressure

Recently, hydrogen sulfide was experimentally found to show the high superconducting critical temperature (Tc) under high-pressure. The superconducting Tc shows 30–70 K in pressure range of 100–170 GPa (low-Tc phase) and increases to 203 K, which sets a record for the highest Tc in all materials, for the samples annealed by heating it to room temperature at pressures above 150 GPa (high-Tc phase). Here we present a solid H5S2 phase predicted as the low-Tc phase by the application of the genetic algorithm technique for crystal structure searching and first-principles calculations to sulfur-hydrogen system under high-pressure. The H5S2 phase is thermodynamically stabilized at 110 GPa, in which asymmetric hydrogen bonds are formed between H2S and H3S molecules. Calculated Tc values show 50–70 K in pressure range of 100–150 GPa within the harmonic approximation, which can reproduce the experimentally observed low-Tc phase. These findings give a new aspect of the excellent superconductivity in compressed sulfur-hydrogen system.

Effect of atomic monolayer insertions on electric-field-induced rotation of magnetic easy axis

We have investigated the electric field (EF) effect on the magnetic anisotropy energy (MAE) in the thin films MgO/M/Fe/Au(001) and MgO/Fe/M(001) (M = Pd, Pt, and Au) by means of first-principles density-functional calculations. We find that the MAE varies linearly with the EF and investigate the change in slope of the MAE as a function of the EF as the buffer layer is changed. We find that a single monatomic buffer layer may be useful for devices that use EF-modified MAE. We simulate the critical EF for easy-axis rotation and discuss interface effects of Mg/Fe and Fe/Au on MAE.

Structure and Magnetism of Anion Iron Oxide Clusters

We have studied structural and magnetic properties in the anion iron oxide clusters, (m=1−6) and (m=1−5), by means of first-principles molecular dynamics based on the density functional theory. The additional electron on the ground state neutral cluster was found to affect the geometry and magnetism of clusters. The vertical detachment energy (VDE) was estimated at the ground state geometry of anion clusters. A comparison with the experimental data indicates good agreement of tendency in the number of oxygen atoms.