Keiichiro Nasu

Extended Peierls-Hubbard Model for One-Dimensional N-Sites N-Electrons System. I. Phase Diagram by Mean Field Theory

The phase diagram of a one-dimensional many-electron system with site-diagonal electron-phonon(e-p) coupling and short-ranged inter-site electron-electron(e-e) repulsion as well as intra-site one is studied within the mean field theory for electrons and in the adiabatic limit for phonons. The theory covers the whole region of four parameters characterizing the system: the transfer energy of electron T , the e-p coupling energy S , the intra- and inter-site e-e repulsive enegies U and V , and the phase diagram is given in a tetrahedral coordinate space spanned by T , U , S and V . It is proved that the “ T - U - S - V tetrahedron” is divided into SDW and CDW regions by the interface U = S +2 V , that is, “ S ” competes with “ U ” but cooperates with “ V ”. The “ S ↔ U ” and “ S ↔ T ” competitions appeared in the energy gap of CDW can explain the experimental results of halogen-bridged mixed-valence metal complexs.

Tunneling Process from Free State to Self-Trapped State of Exciton

The tunneling of the Wannier exciton from its metastable free state to the stable self-trapped state is described in the context of the multi-phonon nonradiative transition. The escape path through the adiabatic potential barrier is determined by a variational method: the tunneling is assumed to occur along a trial interaction mode, with its degree of localization being the variational parameter determined so as to maximize the rate of the tunneling. The rate is obtained as a function of the exciton band width, coupling constants and the Wannier radius, and the tunneling is found to be more probable than the radiative annihilation in a wide range of these parameters. The rate is also found to depend exponentially on the band width and the Wannier radius, in agreement with the experimental results in alkali iodides and rare gas solids.

Extended Peierls-Hubbard Model for One-Dimensional N-Sites N-Electrons System. II. Effect of Fluctuation, Optical and Magnetic Excitations

A one-dimensional many-electron system with site-diagonal electron-phonon coupling, intra- and inter-site electron-electron (e-e) repulsions is studied within the RPA for electrons and in the adiabatic limit for phonons, so as to clarify effects of fluctuation on the phase diagram of the ground state within the mean field theory (MFT), and also on various excitations therefrom. Energies of bound electron-hole (e-h) pairs in the MFT energy-gap such as magnon and charge-transfer (CT) excitons are calculated together with the ground-state energy-gain due to their virtual excitations. The resultant new phase diagram is such that the SDW phase erodes the CDW region given by the MFT, because of the magnon excitation. It is shown that the inter-site e-e repulsion always gives a stable singlet CT exciton that dominates the optical property of the CDW.

Extended Peierls-Hubbard Model for One-Dimensional N-Sites N-Electrons System. III. Lattice Relaxation after Optical Excitation in CDW

The self-trapping of exciton in the CDW phase of one-dimensional many-electron system with short-ranged electron-phonon(e-p) and electron-electron(e-e) interactions is studied by a variational method for exciton and in the adiabatic limit for phonons, so as to clarify its lattice relaxation after the light excitation. The adiabatic potential surface describing the relaxation process is calculated in a wide region of four parameters characterizing the system: the transfer energy of electron T , the e-p coupling energy S , intra- and inter-site e-e repulsive energies U and V . It is shown that in the strong coupling case S ≫2 T , the exciton relaxes to metastable ground states such as an island of SDW phase, and only in the inter-mediate case S ∼2 T it relaxes to a luminescent state localized in the middle of the energy-gap. These results can well explain the luminescence of Wolfframs red salt.

Theory for Spectral Shape of Optical Absorption in Halogen-Bridged Mixed-Valent Metal Complexes

Spectral shapes of optical absorptions in the charge density wave states of halogen-bridged mixed-valent metal complexes are theoretically determined for the first time, using a one-dimensional extended Peierls-Hubbard model. Two important effects beyond the mean field theory are taken into account; the electron-hole (e-h) correlation on the metal atoms, and the classical fluctuation of the halogen sublattice coordinates. It is shown that the exciton strongly affects the spectral shape of the charge transfer (CT) band. When the e-h binding is weak, the CT band is the mixture of the exciton absorption and the interband ones. When it is strong, on the other hand, the exciton dominates over all the CT band and, consequently, its higher energy side becomes very close to a Lorentzian. From the comparison with experimental data, it is concluded that platinum complexes correspond to strong binding cases. The intragap absorptions associated with nonlinear excitations such as solitons and polarons are also studied...

Charge Transfer Instability with Structural Change. II. Mean Field Theory for One-Dimensional N-Sites N-Electrons System

Stable and metastable phases of a one-dimensional many-electron system with short range electron-electron (e-e) repulsion and site-diagonal electron-phonon (e-p) interaction are studied within the mean field theory and in the adiabatic limit. The total energy of the system is calculated as a function of the three parameters: the transfer energy of the electron between neighbouring two sites T , the on-site e-e repulsive energy U and the e-p coupling energy S . The resultant phase diagram is such that the “ T - U - S triangle ” is bisected into the SDW and the CDW regions by the border-line with U = S . The metastable SDW (CDW) phase exists in addition to the stable CDW (SDW) phase in a narrow region around the bisector. The border-lines for the metastable phases and the bisector between the two stable phases flow together into the T - vertex of the triangle, indicating the intriguing instability of one-dimensional metal against U and S .

Dynamical study on polaron formation in a metal/polymer/metal structure

By considering a metal/polymer/metal structure within a tight-binding one-dimensional model, we have investigated the polaron formation in the presence of an electric field. When a sufficient voltage bias is applied to one of the metal electrodes, an electron is injected into the polymer chain, and then a self-trapped polaron is formed at a few hundreds of femtoseconds while it moves slowly under a weak electric field (not larger than

Theory for photoinduced ionic-neutral structural phase transition in quasi one-dimensional organic molecular crystal TTF-CA

1.0\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{V}/\mathrm{c}\mathrm{m}).

Theory for exciton effects on light absorption spectra of f.c.c. type C60 crystal

At an electric field between

Tunneling and relaxation from the free state to the self-localized state of the exciton: Nondiabatic multi-step theory

1.0\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{V}/\mathrm{c}\mathrm{m}