Yoshiyuki Kawazoe

Magnetic properties of nanoscale Fe clusters in Cu

Abstract The magnetic and electronic properties of fcc Fe clusters embedded in a Cu matrix are studied by the first-principles spin-polarized calculations within the local density functional formalism. A single Fe atom in a Cu host is found to have a magnetic moment of 3.05 μ B . Small Fe n clusters ( n ≤ 13) in Cu exhibit high-moment ferromagnetic coupling, while larger clusters ( n ≥ 13) in Cu show low-moment antiferromagnetic coupling. The magnetic properties of embedded fcc Fe clusters in Cu have some features in common with nanoscale fcc Fe/Cu(100) films.

An ab initio approach to phonon spectrum of trans-polyacetylene

By an ab initio calculation based on the local density approximation of densityfunctional theory, the force constants up to the third nearest-neighbour carbon-hydrogen unit are determined to obtain the phonon spectrum of trans-polyacetylene for the first time. The calculated phonon frequencies at the 0 point coincide well with those of the observed infrared and Raman spectra of trans-polyacetylene. The motion of hydrogen atoms is shown to be important in determining the phonon spectrum of polyacetylene due to the contribution of the bending between the CH and CC bonds. Trans-polyacetylene [1], as a prototype of conducting polymers, has been studied extensively due to its promising electric, magnetic and optical properties [2]. In particular, Su, Schrieffer and Heeger (SSH) [3] proposed a soliton model to explain these properties of conducting polymers. The resulting physics turned out to be quite rich and the model has been very successful in interpreting the experimental phenomena. However, in the SSH model some approximations have been made to describe trans-(CH)x , the main one being that only a translation of the CH group as a whole in the chain direction is considered among the six degrees of freedom for the group. To study the physical properties quantitatively, especially the vibrational spectra in detail, one has to consider all these coordinates. Vibrational spectra of trans-(CH)x have been studied in many ways. Within the discrete one-dimensional SSH model, all vibrational modes around the soliton excitation [4] as well as in the dimerized lattice [5] are obtained. Three infrared localized modes are found to fit the observed three infrared absorption peaks in doped polyacetylene in total. Furthermore, by use of a two-dimensional model, including the contribution of the bending between the CC bonds, a few more localized modes are found [6], depending on the adjustable model parameters, which are not completely clear for trans-(CH)x . Based onab initio second-order Møller–Plesset perturbation calculations of trans-oligoenes, Hirataet al [7] extrapolated the vibrational force field of trans-(CH)x using scale factors. Although their result seems close to the experimental data, it is still believed that further parameter-free calculations are needed for the phonon spectra. Ab initio methods based on density-functional theory (DFT) are now common and there are well established tools for studying structual and vibrational properties of materials. The plane-wave pseuodopotential method and local-density approximation (LDA) to DFT have provided a simple framework and the accuracy and predictive power of this method have been demonstrated convincingly in a large variety of systems. The calculation of reliable phonon spectra in semiconductors is well within reach of DFT. In this letter, we report the result of anab initio calculation on the dimerized lattice of trans-polyacetylene based on the 0953-8984/97/240351+04

A Recording Process Simulation For Double-layered Magneto-optical Disk

19.50c

Electron interaction and soliton energy in conjugated polymer

Magnetic Multi-Valued (MMV) recording process is studied by numerical simulation including thermomagnetic effect. A medium, which consists of PtCo capping layer coupled to TbFeCo recording layer, switches the signal of Kerr rotation angle responding to the external field in four levels. The time evolution of generated pattern of recording mark is calculated by extending the previous magneto-optical (MO) recording simulation for single layer system.

Simulated annealing of small silicon clusters by tight-binding

The effect of e-e interaction on the creation energy of soliton in polymer has been extensively studied by using the Hubbard model. However, the polymer possesses wide bandwidth, which is even larger than the on-site repulsion U, then it cant be guaranteed that the Hubbard model is always suitable, since this model originates from the systems with narrow band. Actually the Hubbard model only contains the diagonal part of e-e interaction, for wide band system, the off-diagonal part of e-e interaction may have substantial impact. This paper goes beyond the Hubbard model by using a screened Coulomb interaction, which includes both diagonal and off-diagonal parts. Our calculation shows that the effect of e-e interaction on the creation energy of soliton is distinctly different from the result based on the Hubbard model, the physical reason cause this difference is analysed.

Macroscopic magnetization tunneling and coherence in antiferromagnetic particles

Using a recently developed Adaptive Simulated Annealing algorithm, we have performed total energy minimizations for small silicon clusters of sizes from 10 to 36. The total energy and forces are calculated with the generalized tight-binding parametrization by Kwon et al. The final ground state structures are reached after about 100,000 to 200,000 molecular dynamics time steps. A few stable clusters with relatively large HOMO-LUMO gap, which is a signature of their stability can be identified. For the stable, high cohesive energy clusters, we also calculate the vibrational spectra, and predict the modes that can be detected under IR and Raman spectroscopy.

Ferromagnetic Instanton Interference in Magnetic Fields — Detailed Calculation of Tunneling-Rate

Abstract We systematically investigate the macroscopic quantum effects of antiferromagnetic particles for three different forms of the magnetic anisotropy and the external magnetic field employing the method of two sublattices. The tunneling rates are obtained in each case with the help of the instanton method. The effect of the topological term is also studied.

EFFECTS OF QUANTUM SIZE AND POTENTIAL SHAPE ON THE SPECTRA OF AN ELECTRON AND A DONOR IN QUANTUM DOTS

Macroscopic quantum coherence and tunneling of small ferromagnetic particles in the presence of a magnetic field are investigated by using the instanton method. Both the Wentzel–Kramers–Brillouin exponent and the van Vleck fluctuation determinant are obtained for each case. The topological quenching at H=0 for half-integral spin particles is evident and such effect can be destroyed by a large applied magnetic field. The oscillating relations of the tunneling rate with the weak applied magnetic field are clearly shown and the quenching of tunneling at some definite values of the field is predicted. The experiment condition for observing the topological quenching is suggested.