Makoto Kuwabara

Interchain hopping of solitons and polarons in polyacetylene

Interchain hopping of solitons and polarons in polyacetylene is studied by numerical simulations of their motion under the electric field. The used model is the Su-Schrieffer-Heeger model supplemented with the intrachain electron-electron interaction and dopant potential. We find that charged solitons can hop to the opposite chain by forming a pair (bipolaron). For the case of polarons also hopping in a pair is more favorable than single polaron hopping. Interchain hopping of a polaron pair is more effective than that of a soliton pair.

Coexistence of Charge Order and Spin–Peierls Lattice Distortion in One-Dimensional Organic Conductors

The electronic properties of quarter-filled organic materials showing spin–Peierls transition are investigated theoretically. By studying the one-dimensional extended Peierls–Hubbard model analytically as well as numerically, we find that there is a competition between two different spin–Peierls states due to the tetramized lattice distortion in the strongly correlated regime. One is accompanied by lattice dimerization which can be interpreted as a spontaneous Mott insulator, while the other shows the existence of charge order of Wigner crystal-type. Results of numerical density matrix renormalization group computations on sufficiently large system sizes show that the latter is stabilized in the ground state when both the on-site and the inter-site Coulomb interactions are large.

MOTION OF CHARGED SOLITON IN POLYACETYLENE DUE TO ELECTRIC FIELD. II, BEHAVIOR OF WIDTH

By numerical simulations treating the motion of a charged soliton in polyacetylene, we analyze the width of a moving soliton. The motion of the soliton is induced by applying an electric field as in the previous work. The soliton width is estimated by two methods; one is from the excess charge density profile and the other from the distortion in the lattice displacement. Both results indicate that the width decreases as the soliton velocity increases. Quantitatively, however, the amount of decrease is different in the two cases; the maximum change of the width is about 10% in the former case whereas it is 20% in the latter. Furthermore, the width obtained from the lattice distortion is found to oscillate as a function of time, consistently with the excitation of the amplitude mode around a soliton.

Electronic and Lattice Dynamics in the Photoinduced Ionic-to-Neutral Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model

Real-time dynamics of charge density and lattice displacements is studied during photoinduced ionic-to-neutral phase transitions by using a one-dimensional extended Peierls–Hubbard model with alternating potentials for the one-dimensional mixed-stack charge–transfer complex, TTF-CA. The time-dependent Schrodinger equation and the classical equation of motion are solved for the electronic and lattice parts, respectively. We show how neutral domains grow in the ionic background. As the photoexcitation becomes intense, more neutral domains are created. Above threshold intensity, the neutral phase is finally achieved. After the photoexcitation, ionic domains with wrong polarization also appear. They quickly reduce the averaged staggered lattice displacement, compared with the averaged ionicity. As the degree of initial lattice disorder increases, more solitons appear between these ionic domains with different polarizations, which obstruct the growth of neutral domains and slow down the transition.

Polaron versus Bipolaron in Conducting Polymers: a Density Matrix Renormalization Group Study.

Competition between polaron and bipolaron in conjugated polymers with nondegenerate ground state is systematically studied in the extended Hubbard-Peierls model with the symmetry-breaking Brazovskii-Kirova term, using the density matrix renormalization group method combined with lattice optimization in the adiabatic approximation. We demonstrate that the relative stability of a bipolaron over two separated polarons sensitively depends on both on-site Hubbard U and nearest-neighbor repulsion V . When U is much larger than V , the bipolaron state is more stabilized compared with mean field calculations.Competition between polaron and bipolaron in conjugated polymers with nondegenerate ground state is systematically studied in the extended Hubbard-Peierls model with the symmetry-breaking Brazovskii-Kirova term, using the density matrix renormalization group method combined with lattice optimization in the adiabatic approximation. We demonstrate that the relative stability of a bipolaron over two separated polarons sensitively depends on both on-site Hubbard U and nearest-neighbor repulsion V . When U is much larger than V , the bipolaron state is more stabilized compared with mean field calculations.

Motion of Charged Soliton in Polyacetylene Due to Electric Field.III. : Energetics

In numerical simulations on a moving charged soliton in polyacetylene, the behavior of the total, electric, lattice potential and lattice Kinetic energies are analyzed. In order to induce physically natural motion, the soliton is accelerated by a uniform electronic field as in the previous papers of this series. During the acceleration the electronic energy shows an almost monotone decrease while the lattice potential energy increases in an almost monotone way. These two energies have also an oscillating component which is related to the previously reported oscillation of the soliton width. On the contrary the total and the lattice kinetic energies show no oscillation. The relation between the total energy e tot and the soliton velocity υ is well fitted to a functional form e tot (υ)=e tot (0)-( M s υ m 2 /2) ln (1-υ 2 /υ m 2 ) where υ m is the saturation velocity of about three times the sound velocity and M s the soliton effective mass of four to five times the bare electron mass.

Dynamics of an Acoustic Polaron in One-Dimensional Electron-Lattice System

The dynamical behavior of an acoustic polaron in typical non-degenerate conjugated polymer, polydiacetylene, is numerically studied by using Su-Schrieffer-Heegers model for the one dimensional electron-lattice system. It is confirmed that the velocity of a polaron accelerated by a constant electric field shows a saturation to a velocity close to the sound velocity of the system, and that the width of a moving polaron decreases as a monotonic function of the velocity tending to zero at the saturation velocity. The effective mass of a polaron is estimated to be about one hundred times as heavy as the bare electron mass. Furthermore the linear mode analysis in the presence of a polaron is carried out, leading to the conclusion that there is only one localized mode, i.e. the translational mode. This is confirmed also from the phase shift of extended modes. There is no localized mode corresponding to the amplitude mode in the case of the soliton in polyacetylene. Nevertheless the width of a moving polaron shows small oscillations in time. This is found to be related to the lowest odd symmetry extended mode and to be due to the finite size effect.

Motion of Charged Soliton in Polyacetylene Due to Electric Field. IV. : Damping of Soliton Velocity

We study velocity damping of a moving soliton in polyacetylene by using numerical simulations within Su, Schrieffer and Heegers model. The system is assumed to be at absolute zero of temperature and involves no impurity. In order to get an initial velocity the soliton is accelerated by applying an external electric field. The relaxation time of the soliton velocity has a minimum value when the initial velocity is around the sound velocity of this system. This is a result of the interaction between the moving soliton and the acoustic phonons emitted by the soliton itself. We find that the damping of the soliton velocity is caused by the energy transfer from the moving soliton to the acoustic phonons. The transfer is enhanced when the soliton is surrounded by the acoustic phonons.

Charge ordering and lattice modulation in quasi-one-dimensional halogen-bridged binuclear metal complexes

Abstract Ground state phase diagrams of the MMX chains are studied in a one-dimensional dimerized 3/4-filled model by exactly diagonalizing 12-site clusters. Experimentally observed phases are reproduced by changing relative strengths of an electron-lattice coupling, the inter-dimer transfer integral, and an elastic constant whose variation is roughly estimated from the interatomic spacing, the species of the halogen ions and the presence/absence of counter ions.

Numerical Studies of Ground State Phase Diagrams for the MMX Chains

Abstract We study ground state phase diagrams for the MMX chains, using a one-dimensional dimerized 3/4-filled extended Hubbard-Peierls model with site diagonal and off-diagonal electron-lattice interactions. The ground states are obtained mainly in the Hartree-Fock approximation, and their accuracy is checked by the exact diagonalization of small clusters. We find a new phase in addition to frequently considered phases and compare our results with experimental results.