Spin textures and Larmor precession of holes modulated by the spin-orbit coupling confined in the two-dimensional Si x Ge1-x mixed-alloy quantum well system.

We theoretically study the spin textures of holes confined in the two-dimensional (2D) [Formula: see text] quantum well (QW) system. We particularly focus on the spin-orbit interaction (SOI) caused by the bulk-inversion-asymmetry (BIA) and explore the effective magnetic field (EMF) generated by the combination of the SOI couplings. For the study of the semiconductor mixed-alloy (MA) system, we propose the extended [Formula: see text] perturbation approach including possible perturbation terms crossing with the SOI couplings up to the second order ones. We then study the distribution and orientation of the EMF and investigate how the EMF changes the spin textures of the heavy-mass holes (HHs), light-mass holes (LHs) and separated holes (SHs), respectively. Finally, we study the Larmor spin precession of these holes having the characteristic spin textures driven by the SOI couplings.

Ground-state molecular and electronic structures of group-IV nanoribbons, nanorings and nanotubes.

Based on ab initio molecular orbital (MO) theory and first-principles band calculations, we systematically study the ground-state molecular and electronic structures of group-IV nanoribbons (NRBs), nanorings (NRGs) and nanotubes (NTBs) by substituting the honeycomb skeletal atoms with C, Si or Ge atoms. We then explore the energetics in the ground-state singlet-triplet (ST) crossover, particularly focusing on the configuration hybridization by electron correlation.

Molecular dynamical approach to the conformational transition in peptide nanorings and nanotubes.

We study the conformational transition in d,l-peptide nanorings (PNRs) and nanotubes (PNTs) computationally based on the total energy calculation. Ab initio energy calculation has been carried out to investigate the static states of PNRs, whereas the molecular dynamics (MD) calculation has been employed to examine PNRs' dynamical states. We, then, discuss the time-dependent (TD) feature via the transition process from E-type to B-type and vice versa. The conformational transition occurs easily from E-type equatorial (Eeq) to B-type axial (Bax) but is unreversible for the opposite direction because of a larger activation energy. The TD tracing of the two dihedral angles in the individual amino acid residues reveals that the conformational change propagates along the peptide skeleton ring nearly at the sound velocity. We further expand our study to the tubular forms and reveal that the PNT has an ability to produce the two kinds of homogeneous tubes, being composed of E rings (E-tube) and of B rings (B-tube), and also that these two PNRs should be mixed to produce a binary alloyed PNT.

Conformational transitions of cyclic D,L-peptides.

Conformational transitions of cyclic D,L-hexapeptides have been studied by first-principles calculations. Geometry optimizations for 20 types of homoresidue cyclic D,L-hexapeptide revealed that the cyclic peptides have two types of energetically stable backbone (extended (E) and bound (B) types); and for each type, the amino acid side chains have two orientations (equatorial and axial). Among the four types of isomer [E-type equatorial (E(eq)), B-type equatorial (B(eq)), E-type axial (E(ax)), and B-type axial (B(ax))], B(ax) is the energetically most preferred by most of the 20 encoded amino acid residues, whereas E(ax) is the least preferred. A search for transition states indicated that six types of conformational transition are possible between the isomers of the cyclic peptide, i.e., the backbone-backbone conversions (E(eq)-B(eq) and E(ax)-B(ax) transitions), the side chain-side chain conversions (E(eq)-E(ax) and B(eq)-B(ax) transitions), and the simultaneous conversions of the backbone and the side-chain orientation (E(eq)-B(ax) and E(ax)-B(eq) transitions). All the six transitions proceed with the breaking of the high molecular symmetry (S(6)) and go through the triangular (C(3)) intermediate structure with either equatorial or axial side-chain orientation.

Theoretical study of the time-dependent phenomenon of photon-assisted tunneling through a charged quantum dot.

We have studied numerically the time-dependent photon-assisted tunneling (TD-PAT) process for electrons confined in quantum dots (QDs) by employing the finite difference method under the scheme of the TD-density functional theory (DFT). We have found the quasi-dark state (quasi-DS), where the injected electron remains in the QD. By varying the barrier thickness, we have calculated the TD profile of the electron density in a QD, and found the optimal geometry of the lozenge QD. We have also discussed how the charged QD modulates the PAT process.

Chelation of transition metal ions by peptide nanoring.

We have computationally studied the energetics and electronic structures of a chelate system where the guest cation is a transition metal (TM) and the host ligand is a peptide nanoring (PNR). The trapping of a TM cation by a cyclic peptide skeleton is primarily caused by the electrostatic interaction. The exchange interaction plays a secondary role in determining the relative stability in accordance with the spin multiplicity. An interesting feature of this chelate system is that a TM cation can also be trapped by the side-chain aromatic groups of the PNR via pi-d hybridization. However, the spin multiplicity of the system changes the trapped form. When the chelate system has spin singlet multiplicity, a Fe(2+) cation, for example, is not trapped by the single-phenyl group but is preferentially sandwiched by the two phenyl groups. In contrast, a Fe(2+) cation can be trapped by single as well as by double-phenyl groups when the chelate system has higher spin multiplicity, such as triplet and quintet. These two different trapping forms are caused by the difference in the number of valence electrons of TM cations. For this chelate system, the newly occupied molecular orbital (MO) has an interbenzene antibonding character. Therefore, an electron occupying this MO state favors the mutual separation of two benzene molecules. Because the electron occupation of this MO varies in accordance with the spin multiplicity, one can predict the preference for the single-phenyl-group trapping process rather than the double-phenyl-group process systematically as well as consistently.

Variety of the molecular conformation in Peptide nanorings and nanotubes.

Possible molecular conformations in peptide nanorings and nanotubes were theoretically investigated by a mathematical conformation analysis as well as ab initio Hartree-Fock calculations. The mathematical analysis predicts not only the conventional nanorings having an extended-type (E-type) backbone (trans zigzag) but also the novel ones having bound-type (B-type) backbones with a smaller internal diameter. Ab initio calculations for the amino acid substitution reveal that all 20 encoded residues can form both types of the above nanorings as a local minimum. However, the energetically stable type is determined in accordance with the kind of the replaced side chains. Moreover, the present work theoretically reveals that both types of nanorings stack to form nanotubes through inter-ring hydrogen bonds, i.e., larger E-type nanotubes and smaller B-type nanotubes. Electronically, the HOMO and LUMO states of the nanoring and nanotube backbones are formed by the in-plane pi state. The replacement by the appropriate residues is furthermore predicted to intrude additional levels in the energy gap and to form the frontier states localized at the side chains.