Electric-field effects on photoluminescence properties in a GaAs/AlAs marginal type-I superlattice

We have investigated the electric-field effects on the cw and time-resolved photoluminescence (PL) properties in a marginal type-I GaAs/AlAs superlattice (SL) whose lowest X state (X₁) is situated in the lowest Γ(Γ₁) miniband. In the low bias voltage regime, the PL spectra reveal the transition between type-I and type-II radiative recombination processes caused by Wannier–Stark localization. In contrast, in the high bias voltage regime, the decay time of the time-resolved PL is prolonged. This is because of delayed carrier transport caused by Γ–X transfer. From these results, it was found that marginal type-I SLs present various interesting phenomena that originate from the competitive carrier transport among the Γ miniband, the localized Γ Stark–ladder states, and the X₁ state.     

High-resolution optical storage by use of minute spheres

We have developed a new optical storage system that uses minute spheres arranged upon a surface-relief grating. By using dye-doped minute spheres as recording bits we can limit a sensitive region within a sphere's diameter. Using a reflection-type confocal optical system, we read out shape signals from minute spheres at high resolution. The shape signals from minute spheres are utilized as clock signals in recording and readout.     

Field induced magnetic moment and anisotropy of HoAG and TbAG

A theoretical and experimental study of the field induced magnetic moment and magnetic anisotropy of holmium aluminum garnet and terbium aluminium garnet is presented. The torque curves due to the induced field dependent anisotropy obtained at 4.2 K in applied fields up to 7 T are compared with the theoretical calculation. The results indicate that the two systems show remarkably similar magnetic behaviours, and the calculation based upon a simplified effective Hamiltonian with the crystal field parameters determined from spectroscopic data accounts for the experimental results with satisfactory accuracy.     

Highly Enantioselective Bromocyclization of Allylic Amides with a P/P=O Double‐Site Lewis Base Catalyst

The enantioselective bromocyclization of allylic amides catalyzed by phosphorus‐containing Lewis bases was examined in detail. A series of control experiments and NMR studies showed that a partially oxidized bis‐phosphine generated in situ serves as the actual enantioselective catalyst. The reaction mechanism involves distinct roles of two Lewis basic sites, P and P=O, with P⁺Br serving as a fine‐tuning element for substrate fixation in the chiral environment, and P⁺OBr as the Br⁺ transfer agent to the olefin. Catalyst loading could be reduced to as little as 1 mol %, and the reaction affords enantioenriched oxazolines with up to >99.5 % ee.     

ON GENERAL TYPE OF DIGITAL OPTIMAL PREVIEW SERVO SYSTEM

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Spin waves in the (π,0) magnetically ordered iron chalcogenide Fe1.05Te

We use neutron scattering to show that spin waves in the iron chalcogenide Fe(1.05)Te display novel dispersion clearly different from both the first principles density functional calculations and recent observations in the related iron pnictide CaFe(2)As(2). By fitting to a Heisenberg Hamiltonian, we find that although the nearest-neighbor exchange couplings in the two systems are quite different, their next-nearest-neighbor (NNN) couplings are similar. This suggests that superconductivity in the pnictides and chalcogenides share a common magnetic origin that is intimately associated with the NNN magnetic coupling between the irons.     

Asymmetric Dearomative Fluorination of 2‐Naphthols with a Dicarboxylate Phase‐Transfer Catalyst

A linked dicarboxylate phase‐transfer catalyst enables smooth asymmetric dearomative fluorination of 2‐naphthols with Selectfluor under mild conditions to give the corresponding 1‐fluoronaphthalenone derivatives in a highly enantioselective manner. This reaction, which is compatible with a range of functional groups, is the first example of catalytic asymmetric fluorination of 2‐naphthols, and is expected to be useful in the synthesis of bioactive molecules.     

Crystal structures of Ln4Ni3O8 (Ln = La, Nd) triple layer T'-type nickelates

Single-phase Ln4Ni3O8 (Ln = La, Nd) nickelates were synthesized and their crystal structures were determined by Rietveld refinement of powder neutron diffraction data. The crystal structures of these mixed-valent Ni1+/Ni2+ phases belong to the T'-type and are built by intergrowth of LnO2 fluorite layers with triple NiO2 infinite-layer structural blocks. The major driving force of transformation of the LnO rock-salt block of the parent Ln4Ni3O10-delta Ruddlesden-Popper phases to the fluorite arrangement in the reduced Ln4Ni3O8 phases is attributed to internal structural stress. This transformation allows longer Ni-O bonds in Ln4Ni3O8 without overstretching of the Ln-O bonds, especially in the equatorial plane. The observed displacement of Ni atoms from the outer NiO2 planes toward the Ni atom of the central NiO2 plane in Ln4Ni3O8 is ascribed to large electrostatic repulsion from the fluorite part of the structure. X-ray absorption spectra near the K-edge of Ni suggest that the charge density on the nickel ion is similar for all members of the T'-type Lnn+1NinO2n+2 homologous series, which correlates with nearly constant Ni-O bond lengths observed in all the reduced nickelates. This suggests that the formal changes in the valence state of Ni affect the covalency of the Ni-O bond.     

Real-space calculations for electron transport properties of nanostructures

Recent developments in the fabrication and investigation of conductors of atomic dimensions have stimulated a large number of experimental and theoretical studies on these nanoscale devices. In this paper, we introduce examples presenting the efficiencies and advantages of a first-principles transport calculation scheme based on the real-space finite-difference (RSFD) formalism and the overbridging boundary-matching (OBM) method. The RSFD method does not suffer from the artificial periodicity problems that arise in methods using plane-wave basis sets or the linear dependence problems that occur in methods using atomic basis sets. Moreover, the algorithm of the RSFD method is suitable for massively parallel computers and, thus, the combination of the RSFD and OBM methods enables us to execute first-principles transport calculations using large models. To demonstrate the advantages of this method, several applications of the transport calculations in various systems ranging from jellium nanowires to the tip and surface system of scanning tunneling microscopy are presented.