Nobuhisa Fujima

First‐order Raman scattering in MgO microcrystals

Raman scattering is observed in MgO microcrystals with three different particle sizes (105, 103, and 102 A). The second‐order Raman lines in the bulk crystals of MgO disappear in all the microcrystals, while new peaks emerge at 280, 446, and 1088 cm−1 except for specimen prepared by grinding a large single crystal. Raman lines with lower frequencies, which are enhanced with decrease of the particle size, are discussed as due to the first‐order Raman scattering forbidden in the bulk crystals. The assignment is confirmed in terms of photon density of states in microcrystals. Another line with the highest frequency is coincident with a previously reported Raman line whose origin has been attributed to a surface phonon mode.

Chemical Bonding in Mn Clusters, MnN and Mn+N (N=2–7)

Electronic states of the neutral and cationic Mn N clusters for N =2–7 are calculated by the spin-polarized density functional method. For neutral clusters, the equilibrium interatomic distances are 20% or more larger than that of the bulk crystal, and the characteristics of the electronic structure remain the same as those of an isolated atom. Atoms consisting of the neutral clusters are bonded non-metallically. For smaller cationic Mn N + clusters ( N =2–4), similar situations exist although the 3 d -4 s mixing increases. For larger cationic Mn N + clusters ( N =5–7), the equilibrium interatomic distances are less than 90% of the value of the bulk crystal. The 3 d levels broaden in energy and constitute band-like structure, and the 4 s metallic bonding occurs. This change from non-metallic to metallic bonding in the cationic clusters seems to correspond to the magic number 5 which is observed in the cationic clusters.

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.

Geometrical magnetic structures of transition-metal clusters

Abstract We calculate electronic states of the transition-metal clusters, Ni 19 , Ni 55 , Fe 15 , Fe 35 Cr 15 , and Cr 35 by using the local spin density functional method. We discuss geometrical magnetic structures of the ferromagnetic and antiferromagnetic clusters. For the fcc Ni and bcc Fe clusters, the minimum moment appears on the second layer from the surface of the clusters. The local moments are nearly equal to or larger than the value in the bulk crystal except for the second layer. For the bcc Cr clusters, the local moments strongly depend on the atomic site, and decrease with increasing the coordination number of the atomic site. The moment of the Cr clusters also strongly depends on the interatomic distance.

Electronic States of Si(001) Stepped Surface

Electronic states of steps on the Si(001) surface are calculated by the DV-Xα method. The calculated steps are the unreconstructed S b , reconstructed S b , unreconstructed S a , buckled S a , and dimer-chain S a steps. The dangling bonds near the steps interact with each other. The dangling-bond levels appear in the middle of the band gap. The charge distribution of dangling-bond levels of the dimer on the upper terrace of the unreconstructed S a step is slightly asymmetric, although the dimer is symmetric. The charge distribution of the buckled S a step where the dimer is tilted by 10 degrees is buckled, but its shape does not reproduce the STM image. A new model of the S a step is proposed. The atomic structure of step edge of the higher terrace is a zig-zag chain of dimers. The charge distribution of the dimer-chain S a step well reproduces the large buckling of the STM image.

Shell Structure of Electronic State of Icosahedral Al and Cu Clusters

Electronic states of icosahedral (Ih) clusters Al 13 , Al 12 , Cu 13 , and Cu 12 were calculated by the DV-Xα method which took into account of many-electron effects. The valence 3 d orbitals of Ih-Cu 13 cluster are localized within atom and isolated from the valence 4 s and 4 p orbitals extended over the whole cluster. The characteristic feature of the states of s and p valence electrons of Ih-Al 3 and Cu 13 clusters well corresponds to that of one-electron picture i.e. the shell model of the 3-dimensional isotropic harmonic oscillator potential. Electronic states of valence levels of Ih-Al 12 and Ih-Cu 12 clusters which have no central atom fairly well correspond to those of a combined potential of the harmonic oscillator and a 3-dimensional Gaussian potential barrier at the center of the cluster.

Magnetic Anomaly and Shell Structure of Electronic States of Nickel Microclusters

Magnetism and electronic structure of Ni N clusters are calculated by the spin-polarized DV-Xα-LCAO method for N =4, 6, 8, 13, 14 and 19. The valence 3 d electrons are localized within the atoms of the clusters and separated from the 4 s valence electrons which extend over the whole cluster. The 4 s electronic states well correspond to those of the shell model. Since the highest occupied molecular orbital is within the 3 d band, there exist 3 d holes. Physically, the number is 2 for 4< N <8, 8 for 8≦ N ≦18, and 18 for N =19 and 20. This results in the stepwise change of magnitude of the total spin as a function of N . This 3 d magnetic anomaly is closely related to the 4 s shell structure and expected to appear in a series of transition-metal clusters. This anomaly is different from the so-called Kubo effect of fine particles of simple metals.

Non-Collinear Magnetic Moments of Five-Atom Transition-Metal Clusters

Non-collinear magnetic moments are calculated for trigonal bipyramid clusters, V 5 , Cr 5 , Mn 5 , Fe 5 , Co 5 , and Ni 5 by a simple method employing the locality of the local spin-density functional approximation and the discrete variational scheme. We discuss the magnetic properties of the transition-metal clusters with the bond lengths of 80%–100% of the crystalline interatomic distance. Parallel magnetic moments appear for the Ni, Co, and Fe clusters with almost all the bond lengths. Tilting magnetic moments also appear for the Co and Fe clusters in a certain bond length. Non-collinear magnetic arrangements appear for the Mn and Cr clusters, which change to antiparallel arrangements with decreasing bond length. Only an antiparallel alignment appears for the V cluster.

Magnetic shells of Co clusters

Magnetic properties of Co N clusters for N =13-147 are discussed by calculating the electronic states of the clusters with the local spin-density functional method. Two kinds of oscillatory behavior are observed in the magnetism of the Co clusters. First, the magnetic moment per atom oscillates with increasing the cluster size. This oscillatory size dependence is simulated by an electronic shell model, where the discreteness of the energy levels derived from 4 s /4 p -electrons originates the oscillation. Second, the local magnetic moment oscillates from the surface to the center of the clusters, that is, a large magnetic moment at the surface atomic shell decreases at the second shell, increases at the third shell and again decreases at the fourth shell from the surface. The local density of states of 3 d electrons changes shell by shell, parallel to the oscillation of the local magnetic moment.

Local structures and structural phase change in Ni-Zr-Nb glassy alloys composed of Ni5Zr5Nb3 icosahedral clusters

We construct a local structural model for Ni-Zr-Nb glassy alloys, Ni48Zr40Nb24, which contains eight icosahedra of Ni5Zr5Nb3 as the structural units. We fully optimize the 112-atom structures by first principles calculation, and find two characteristic phases in the structures depending on the alignment of the icosahedra: an amorphous phase where the icosahedral structure well remains and a periodic phase corresponding to crystallization where the icosahedra change to fcc-like cuboctahedra. Nb atom-clustering may play a key role in anti-crystallization.