An electron transport is studied in the system that consists of a scanning tunneling microscopy, single-molecule magnet metal. Because of quantum tunneling of magnetization in a single-molecule magnet, linear response conductance exhibits stepwise behavior with increasing longitudinal field, and each step is maximized at a certain value of field sweeping speed. The conductance at each step oscillates as a function of the additional transverse magnetic field along the hard axis. A rigorous theory is presented that combines the exchange model with the Landau-Zener model.

Electronic Transport in Single-Molecule Magnets on Metallic Surfaces

The stability of charge carriers in conjugated polymers is investigated in terms of a nonadiabatic evolution method by using an extended version of the Su–Schrieffer–Heeger (SSH) model that includes the effects of an external electric field and temperature. On the basis of this physical picture, different patterns of applied electric field and temperature dependence of polaron and bipolaron kinematics as well as the transitions between different regimes are found. Phase transitions from subsonic to supersonic velocities are also discussed in terms of the system conditions. We were able to describe at which thermal regime each quasi-particle loses its stability and also to determine under which circumstances do the electric field and temperature rise or dampen its motion. The results indicate that thermal effects on polaron and bipolaron stability may provide guidance for improving the charge carrier conduction in organic optoelectronic devices.

Effects of temperature and electric field induced phase transitions on the dynamics of polarons and bipolarons

The intrachain recombination dynamics between oppositely charged polarons is theoretically investigated through the use of a version of the Su-Schrieffer-Heeger (SSH) model modified to include an external electric field, an extended Hubbard model, Coulomb interactions, and temperature effects in the framework of a nonadiabatic evolution method. Our results indicate notable characteristics concerning the polaron recombination: (1) it is found that there exists a critical temperature regime, below which an exciton is formed directly and (2) a pristine lattice is the resulting product of the recombination process, if the temperature is higher than the critical value. Additionally, it is found that the critical electric field regime plays the role of drastically modifying the system dynamics. These facts suggest that thermal effects in the intrachain recombination of polarons are crucial for the understanding of electroluminescence in optoelectronic devices, such as Polymer Light Emitting Diodes.

Critical temperature and products of intrachain polaron recombination in conjugated polymers

Combining the one-dimensional tight-binding Su–Schrieffer–Heeger (SSH) model and the extended Hubbard model (EHM), the scattering of two oppositely charged bipolarons and a bipolaron–polaron pair is investigated under the influence of impurity effects using a nonadiabatic evolution method. These novel results for bipolarons show that the oppositely charged quasi-particles scatter into a mixed state composed of bipolarons and excitons. The excitation yield depends sensitively on the strength of the applied electric field. In the presence of an impurity, the critical electric field regime for formation of a state composed by bipolarons and excitons is increased. Additionally, we were able to obtain critical values of electric fields that played the role of drastically modifying the system dynamics. These facts suggest that the scattering between bipolarons and a bipolaron–polaron pair in the presence of impurities is crucial for the understanding of electroluminescence in optoelectronics devices, such as po...

Dynamical study of impurity effects on bipolaron-bipolaron and bipolaron-polaron scattering in conjugated polymers.

Substantial progress of the time-dependent density matrix renormalisation group (t-DMRG) method in the recent 15 years is reviewed in this paper. By integrating the time evolution with the sweep pr...

The time-dependent density matrix renormalisation group method

By employing an adaptive time-dependent density-matrix-renormalization-group method, we investigate the dynamics of a charged bipolaron in the presence of both electron-phonon and electron-electron interactions. We use a Su–Schrieffer–Heeger model modified to include electron-electron interactions via a Hubbard Hamiltonian, a Brazovskii–Kirova symmetry-breaking term, and an external electric field. Our results show that the velocity of the bipolaron increases first and then decreases with the increasing of the on-site Coulomb interaction, U. Furthermore, the dependence of the bipolaron velocity, bipolaron effective mass, and bipolaron stability on the lattice structures is discussed.

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Correlation effects on the dynamics of bipolarons in nondegenerate conjugated polymers.

By employing an adaptive time-dependent density-matrix-renormalization-group method, the spin-flip process of polarons is investigated in a polymer chain with magnetic impurities. Being driven by an external electric field, a polaron carrying both spin 1/2 and charge e moves at a constant speed in the polymer chain. When the polaron passes through a specific site, which couples to a magnetic impurity via spin-exchange interaction, a spin-flip process is observed if its spin is antiparallel to the impurity spin. Our results show that the spin-flip probability is enhanced by the on-site Coulomb interaction and increases with increase in the spin-exchange integral. Additionally, some possible applications of the spin-state swap between the polaron and the impurity are discussed. © 2009 American Institute of Physics. doi:10.1063/1.3247189

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Xu-Ming Zhang

Papers

Correlation effects on the dynamics of bipolarons in nondegenerate conjugated polymers.

Spin-flip processes of polarons by magnetic impurities in conjugated polymers.

Erratum: “Spin-flip processes of polarons by magnetic impurities in conjugated polymers” [J. Chem. Phys. 131, 154901 (2009)]