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Europhysics Letters (EPL)

Correlation effects on the scattering of soliton pairs in conjugated polymers

Hui Chao Zhao, Yuguang Chen, Yonghong Yan, Zhong An, Chang-Qin Wu

Published Dec 1, 2012• 2 citations
Publisher

Abstract

\n\nBy employing an adaptive time-dependent density matrix renormalization group (t-DMRG) method, we investigate the electron-electron correlation effects on the inelastic-scattering processes of oppositely charged-soliton anti-soliton pairs in conjugated polymers under the influence of an external electric field. The simulations are performed based on the Su-Schrieffer-Heeger (SSH) model modified to include electron-electron interactions and an external electric field. The percent yield of product is numerically calculated by a projection method. Our results show that the on-site Coulomb interaction is of fundamental importance and favors the recombination between the pairs of soliton. Additionally, the effects of external electric field and nearest-neighbor electron-electron interaction on the collision processes are also discussed.\n

DOI
10.1209/0295-5075/100/57005

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•Jan 1, 2017•0 citations
\n\nBy employing an adaptive time-dependent density matrix renormalization group (t-DMRG) method, we investigate the electron-electron correlation effects on the inelastic-scattering processes of oppositely charged-soliton anti-soliton pairs in conjugated polymers under the influence of an external electric field. The simulations are performed based on the Su-Schrieffer-Heeger (SSH) model modified to include electron-electron interactions and an external electric field. The percent yield of product is numerically calculated by a projection method. Our results show that the on-site Coulomb interaction is of fundamental importance and favors the recombination between the pairs of soliton. Additionally, the effects of external electric field and nearest-neighbor electron-electron interaction on the collision processes are also discussed.\n
The motion of polarons, which serve as charge carriers in conjugated polymers, is of fundamental importance for understanding transport properties of organic optoelectronic devices. We investigate the dynamics of a charged polaron in the presence of both electron-phonon and electron-electron interactions under the influence of an external electric field, which is modeled by the one-dimensional tight-binding Su-Schrieffer-Heeger (SSH) model supplemented with a Hubbard on-site repulsion term. For this many-body dynamical evolution problem, we develop an adaptive time-dependent density matrix renormalization group (ttt-DMRG) method in combination with a Newtonian equation of motion for atomic displacements. Our results show that the velocity of the polaron is suppressed by the on-site Coulomb interaction UUU. The polaron moves with a supersonic velocity, about four times the sound velocity at the small UUU limit, and approaches the sound velocity at the large UUU limit. Furthermore, the dependence of the polaron velocity and the polaron effective mass on the lattice structures are discussed.
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.
In this paper, we address the role of electron-electron interactions on the velocities of spin and charge transport in one-dimensional systems typified by conjugated polymers. We employ the Hubbard model to model electron-electron interactions. The recently developed technique of time dependent Density Matrix Renormalization Group (tdDMRG) is used to follow the spin and charge evolution in an initial wavepacket described by a hole doped in the ground state of the neutral system. We find that the charge and spin velocities are different in the presence of correlations and are in accordance with results from earlier studies; the charge and spin move together in the noninteracting picture while interaction slows down only the spin velocity. We also note that dimerization of the chain only weakly affects these velocities.
We present a simulation method to compute the time-dependent properties of a system of fermions coupled to classical degrees of freedom. The method is employed to calculate the static and dynamic properties of the Su-Schrieffer-Heeger model for polyacetylene chains. Our results for the static quantities for chains, containing up to 256 electrons on 256 sites, are in excellent agreement with previous work and confirm that solitons are only present for very small doping. Our simulation data for the optical absorption as a function of dopant concentration are in good agreement with experiment.
We apply the Hubbard-Peierls Hamiltonian to study the low-lying excited states of several model conjugated polymers. The calculations employ a numerical renormalization-group method for chains of intermediate length, and a more approximate truncated configuration-interaction scheme to extrapolate the results to longer systems. In the range of on-site Coulomb repulsion strengths of physical interest, we find that a correlated even-parity singlet excited state competes with the odd-parity singlet excitation expected in the U=0 limit of the model, in agreement with previous studies on the Pariser-Parr-Pople model. The lowest electronic excitations in the model are identified as triplet excitations. Self-consistently-relaxed lattice configurations surrounding these electronic excitations are studied. In addition to the stable photoexcitations predicted in the U=0 limit of the theory, we find that the interelectronic repulsive potential leads to a rich spectrum of competing neutral structures, which are studied and characterized. Coulomb correlations provide a mechanism for production of long-lived spin-1/2 neutral excitations for degenerate ground-state systems, and spin-1 neutral excitations for nondegenerate ground-state systems.
The present thesis deals with a theoretical study of electronic structures in -conjugated molecular materials with focus on their application in organic elec-tronics. We also discuss a modified and efficient symmetrized DMRG algorithm for studying excited states in these systems. In recent times, organic conjugated systems have emerged as potential candidates in a wide range of fascinating fields by virtue of their tunable electronic properties, easy processability and low cost. Tunability in the electronic and optical properties primarily are centered on the or-dering and nature of the low-lying excited states. Probing these important excited states also demands development of efficient and adaptable techniques. Chapter 1 provides a basic overview of conjugated organic polymers which have been utilized over decades in diverse fields as in organic light emitting diodes (OLED), organic solar cells (OSC) and non-linear optical (NLO) devices. These systems also contribute significantly to theoretical understanding as they pro vide important insights of one and quasi-one dimensional systems. In this chapter, we have given basic description of the electronic processes in OLED and OSC along with a brief theoretical description of -conjugated organic systems. Chapter 2 gives an account of the numerical techniques which are necessary for the study of low-dimensional strongly correlated systems like -conjugated sys-tems. For this purpose, effective low-energy model Hamiltonians viz. Huckel,¨ Hubbard and Pariser-Parr-Pople Hamiltonians are discussed. Exact diagonalization technique within the diagrammatic valence bond (DVB) basis and density matrix renormalization group (DMRG) technique are discussed in details. We have also given brief accounts of the methods employed to study real-time dynamics. A short description of different computational techniques for the study of NLO properties in -conjugated systems is also provided. Engineering the position of the lowest triplet state (T1) relative to the first excited singlet state (S1) is of great importance in improving the efficiencies of organic light emitting diodes and organic photovoltaic cells. In chapter 3, we have carried out model exact calculations of substituted polyene chains to understand the fac-tors that affect the energy gap between S1 and T1. The factors studied are backbone dimerization, different donor-acceptor substitutions and twisted backbone geome-try. The largest system studied is an eighteen carbon polyene which spans a Hilbert space of about 991 million in the triplet subspace. We show that for reverse inter-system crossing (RISC) process, the best choice involves substituting all carbon sites on one half of the polyene with donors and the other half with acceptors. Singlet fission (SF) is a potential pathway for significant enhancement of efficiency in OSC. In chapter 4, we study singlet fission in a pair of polyene molecules in two different stacking arrangements employing exact many-body wave packet dy-namics. In the non-interacting model, SF…