Neutral Solitons Research Articles

Pattern Formation and Aggregation in Ensembles of Solitons in Quasi One-Dimensional Electronic Systems

Broken symmetries of quasi one-dimensional electronic systems give rise to microscopic solitons taking roles of carriers of the charge or spin. The double degeneracy gives rise to solitons as kinks of the scalar order parameter A; the continuous degeneracy for the complex order parameter Aexp(iθ) gives rise to phase vortices, amplitudes solitons, and their combinations. These degrees of freedom can be controlled or accessed independently via either the spin polarization or the charge doping. The long-range ordering in dimensions above one imposes super-long-range confinement forces upon the solitons, leading to a sequence of phase transitions in their ensembles. The higher-temperature T transition enforces the confinement of solitons into topologically bound complexes: pairs of kinks or the amplitude solitons dressed by exotic half-integer vortices. At a second lower T transition, the solitons aggregate into rods of bi-kinks or into walls of amplitude solitons terminated by rings of half-integer vortices. With lowering T, the walls multiply, passing sequentially across the sample. Here, we summarize results of a numerical modeling for different symmetries, for charged and neutral soliton, in two and three dimensions. The efficient Monte Carlo algorithm, preserving the number of solitons, was employed which substantially facilitates the calculations, allowing to extend them to the three-dimensional case and to include the long-range Coulomb interactions.

Generating Pure Spin Current in Heterojunction Organic Solar Cells.

We propose a mechanism for generating pure spin current in heterojunction organic solar cells, with the donor and acceptor both being degenerate ground-state polymers; thus, solitons can be formed. This mechanism contains the following steps: (i) the donor is photoexcited to create the electron-hole (e-h) pairs; (ii) the excited electrons are transferred to the acceptor; (iii) the net charges in the donor and acceptor are evolved into the localized charged solitons; (iv) the intermolecule bias is applied to drive the transferred electrons back to donor, and concomitantly, charged solitons are converted to neutral solitons. Here, the on-site Coulomb interaction plays an important role in ensuring the neutral solitons' spins in the donor and acceptor are oppositely polarized. Because spins are separated between the donor and acceptor without any charge separations, pure spin current can be formed. Our mechanism opens a new avenue for exploring potential organic spintronic devices.

Fate of soliton matter upon symmetry-breaking ferroelectric order

In a one-dimensional (1D) system with degenerate ground states, their domain boundaries, dubbed solitons, emerge as topological excitations often carrying unconventional charges and spins; however, the soliton excitations are only vital in the non-ordered 1D regime. Then a question arises; how do the solitons conform to a 3D ordered state? Here, using a quasi-1D organic ferroelectric, TTF-CA, with degenerate polar dimers, we pursue the fate of a spin-soliton charge-soliton composite matter in a 1D polar-dimer liquid upon its transition to a 3D ferroelectric order by resistivity, NMR and NQR measurements. We demonstrate that the soliton matter undergoes neutral spin-spin soliton pairing and spin-charge soliton pairing to form polarons, coping with the 3D order. The former contributes to the magnetism through triplet excitations whereas the latter carries electrical current. Our results reveal the whole picture of a soliton matter that condenses into the 3D ordered state.

Molecular dynamic simulation of photogenerated soliton pairs in trans-polyacetylene

Abstract The formation and evolution of photogenerated soliton pairs in trans-polyacetylene is simulated by using a nonadiabatic dynamical method. Electron–lattice interaction as well as electron correlation effects are both included by employing the Su–Schrieffer–Heeger+Pariser–Parr–Pople model. We adopt the multiconfigurational time-dependent Hartree–Fock formulas which allows for the distinguishing between charged and neutral soliton pairs. It is found that charged soliton pairs are created in the initial stage of the evolution after photoexcitation. Subsequently, the formed charged soliton pairs transform into neutral ones during the dynamical evolutions. The conversion time from charged to neutral state depends on the strength of Coulomb interactions. However, neutral soliton pairs are suppressed in the non-interacting limit, which is in good agreement with the previous estimates based on the Su–Schrieffer–Heeger model. We hope these results can provide further insight into some of the basic issues of interest in photogenerated processes in conducting polymers.

Gross-Pitaevskii-Poisson model for an ultracold plasma: Density waves and solitons

We introduce 1D and 2D models of a degenerate bosonic gas composed of ions with positive and negative charges (cations and anions). The system may exist in the mean-field condensate state, enabling the competition of the Coulomb coupling, contact repulsion, and kinetic energy of the particles, provided that their effective mass is reduced by means of a lattice potential. The model combines the Gross-Pitaevskii (GP) equations for the two-component wave function of the cations and anions, coupled to the Poisson equation for the electrostatic potential mediating the Coulomb coupling. The contact interaction in the GP system can be derived, in the Thomas-Fermi approximation, from a system of three GP equations, which includes the wave function of heavy neutral atoms. In the system with fully repulsive contact interactions, we construct stable spatially periodic patterns (density waves, DWs). The transition to DWs is identified by analysis of the modulational instability of a uniformly mixed neutral state. The DW pattern, which represents the system's ground state (GS), is predicted by a variational approximation. In 2D, a stable pattern is produced too, with a quasi-1D shape. The 1D system with contact self-attraction in each component produces bright solitons of three types: neutral ones, with fully mixed components; dipoles, with the components separated by the inter-species contact repulsion; and quadrupoles, with a layer of one component sandwiched between side lobes formed by the other. The transition from the neutral solitons to dipoles is accurately modeled analytically. A chart of the GSs of the different types (neutral solitons, dipoles, or quadrupoles) is produced. Different soliton species do not coexist as stable states. Collisions between traveling solitons are elastic for dipole-dipole pairs, while dipole-antidipole ones merge into stable quadrupoles via multiple collisions.

Charge Carrier Scattering in Polymers: A New Neutral Coupled Soliton Channel

The dynamical scattering of two oppositely charged bipolarons in non-degenerate organic semiconducting lattices is numerically investigated in the framework of a one-dimensional tight-biding–Hubbard model that includes lattice relaxation. Our findings show that it is possible for the bipolaron pair to merge into a state composed of a confined soliton-antisoliton pair, which is characterized by the appearance of states within less than 0.1 eV from the Fermi level. This compound is in a narrow analogy to a meson confining a quark-antiquark pair. Interestingly, solitons are quasi-particles theoretically predicted to arise only in polymer lattices with degenerate ground state: in the general case of non-degenerate ground state polymers, isolated solitons are not allowed.

Phase transitions in ensembles of solitons induced by an optical pumping or a strong electric field

The latest trend in studies of modern electronically and/or optically active materials is to provoke phase transformations induced by high electric fields or by short (femtosecond) powerful optical pulses. The systems of choice are cooperative electronic states whose broken symmetries give rise to topological defects. For typical quasi-one-dimensional architectures, those are the microscopic solitons taking from electrons the major roles as carriers of charge or spin. Because of the long-range ordering, the solitons experience unusual super-long-range forces leading to a sequence of phase transitions in their ensembles: the higher-temperature transition of the confinement and the lower one of aggregation into macroscopic walls. Here we present results of an extensive numerical modeling for ensembles of both neutral and charged solitons in both two- and three-dimensional systems. We suggest a specific Monte Carlo algorithm preserving the number of solitons, which substantially facilitates the calculations, allows to extend them to the three-dimensional case and to include the important long-range Coulomb interactions. The results confirm the first confinement transition, except for a very strong Coulomb repulsion, and demonstrate a pattern formation at the second transition of aggregation.

Spin-polarized electron transport through magnetic poly-BIPO molecule: the role of soliton–antisoliton separation

Spin-polarized transport through a one-dimensional metal/poly-BIPO/metal model junction with the soliton–antisoliton separation is investigated. Nonlinear spin and charge densities are considered in magnetic poly-BIPO molecule, as a neutral soliton and charged antisoliton with different separations. The calculations are performed based on Su–Schrieffer–Heeger Hamiltonian which is extended with Heisenberg and Hubbard Hamiltonians to include the spin and electron–electron interactions. The spin-dependent transport properties are obtained within the framework of the Landauer–Buttiker formalism based on Green’s function theory. This study demonstrates the reduction in current and spin polarization as the separation between soliton and antisoliton centers is increased. We have found that when the soliton–antisoliton separation is less than 14 sites, the spin polarization is almost 100 % plato, over the voltage ranges more than 0.3 V. Also the energy differences between the soliton–antisoliton mid-gap states for up- and down-spin electrons and the Fermi energy of the system are reduced. However, for the soliton–antisoliton separation lengths more than 14 sites, these quantities tend to constant values with enhancement of the distance between the excitation centers.

Influence of Soliton Spin Density on the Spin Filtering Properties of Magnetic Poly-BIPO Molecule

Spin-polarized transport through the magnetic polyacetylene (poly-BIPO) in the presence of a soliton spin density distribution is numerically investigated in the metal/poly-BIPO/metal model junction. The study was performed using the modified Su–Schrieffer–Heeger Hamiltonian with the extended Hubbard model for the electron–electron interaction in the case of finite electron correlation. The spin transport properties were calculated within the framework of Green’s function theory and Landauer–Buuttiker formalism. Our results indicate that the presence of neutral solitons gives rise to the extension of spin density distribution which induces the extended soliton states in the middle of an energy gap of the molecule such that the band gap width of the molecule reduced It is also shown that the spin transport depends on the relative direction of the spin of soliton and the spin of radicals, and also, the maximum polarization happens when the spin direction of the radicals and soliton is in the opposite directions.

Exploring the competition between localization and delocalization of the neutral soliton defect in polyenyl chains with the orbital optimized second order opposite spin method

Theory and implementation of the analytical nuclear gradient is presented for orbital optimized scaled opposite-spin perturbation theory (O2). Evaluation of the O2 analytical gradient scales with the 4th power of molecular size, like the O2 energy. Since the O2 method permits optimization of the orbitals in the presence of wavefunction-based electron correlation, it is suitable for problems where correlation effects determine the competition between localization and delocalization of an odd electron, or hole. One such problem is the description of a neutral soliton defect on an all-trans polyacetylene chain with an odd number of carbon atoms. We show that the results of the O2 method compare well to benchmark values for small polyenyl radicals. O2 is also efficient enough to be applied to longer chains where benchmark coupled cluster methods are unfeasible. For C(41)H(43), unrestricted orbital O2 calculations yield a soliton length of about 9 carbon atoms, while other unrestricted orbital methods such as Hartree-Fock, and the B3LYP and ωB97X-D density functionals, delocalize the soliton defect over the entire chain. The O2 result is about half the width inferred experimentally.

A new perspective on optoelectric conversion in conjugated polymers**Project supported by the Special Funds of the National Natural Foundation of China (Grant No. 11047148), and the Jining University Research Program, China (Grant No. 2010QNKJ04).

Photoexcitation of a neutral soliton will create a polaron and a charged soliton. According to a tight-binding model and a nonadiabatic method, we investigate the dynamical process of these two photogenerated charge carriers in an external electric field. It is found that the polaron and the soliton can pass through each other, which excludes the possibility of carrier recombination that usually occurs in existing organic solar cells. The results indicate a more efficient way to realize the optoelectric conversion by photoexciting polymer materials with soliton defects. On the other hand, it is found that solitons take on greater stability than polarons during collision.

A comparative ab initio study of neutral and charged kink solitons on conjugated carbon chains

The ground state of odd-N polyynic oligomers CNH2 features kink solitons in carbon-carbon bond-length alternation (BLA) patterns. We perform a systematic first-principles computational study of neutral and singly charged kinks in long oligomers addressing relationships between BLA patterns, electron energy gaps, and accompanying distributions of spin and charge densities, both in vacuum and in the screening solvent environment. A quantitative comparison is made of the results derived with four different ab initio methods: from pure density-functional theory to pure Hartree–Fock (HF) and including two popular hybrid density functionals, B3LYP and BHandHLYP. A clear correlation is demonstrated between the derived spatial extent of kinks and the amount of HF exchange used in the functionals. For charged kinks, we find a substantial difference in the behavior of charge and spin densities.

Decay Kinetics of Photoinduced Solitons on a One-Dimensional Lattice with Traps

The survival probability of solitons diffusing on a one-dimensional lattice with traps is studied numerically. Solitons are assumed to collapse once they collide with one another or reach traps. It turns out that, as the density of traps increases, the decay profile tends to change into an exponential-like form from an extremely nonexponential one specific to the geminate recombination on a trap-free lattice. The randomness of the distance between adjacent traps, as well as the disorder of the intersite energy barriers for the hopping motion of solitons, functions to reduce the deviation of the decay profile from that in the case of the trap-free lattice. Our calculation well reproduces the temperature-dependent decay profiles of the photoinduced neutral solitons that have been observed recently in the MX -chain compound {[Pt(en) 2 ][Pt(en) 2 Cl 2 ]} 3 (CuCl 4 ) 4 ·12H 2 O. It is suggested that traps are located at every 7 to 8 segments of the lattice and that the height of energy barriers is irregular to...

Multichanneled relaxation of halogen‐bridged diplatinum chain complexes photoexcited

AbstractEmploying a time‐dependent Hartree‐Fock method, we simulate photoexcitation and the following nonlinear lattice relaxation of quasi‐one‐dimensional binuclear platinum complexes with bridging halogens. High‐energy excitations above the electron‐hole continuum tend to relax into polarons, while excitons pumped within the optical gap can be stabilized into solitons.Both neutral and charged solitons appear as stable photoproducts, which is not the case with conventional platinum‐halide chains. When a pair of kink and antikink is born out of a self‐trapped exciton and recombine to disappear, excess spin and charge densities alternate and oscillate on them. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoproduction of spin and charge carriers in halogen-bridged binuclear platinum chaincomplexes

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds istheoretically investigated within a one-dimensional extended Peierls–Hubbard model. Wefirst illuminate the whole relaxation scenario in terms of variational wavefunctions and thenvisualize each relaxation channel numerically integrating the Schrödinger equation.High-energy excitations above the electron–hole continuum tend to relax into polarons,while excitons pumped within the optical gap, unless luminescent, turn into solitonic statesnonradiatively. Neutral and charged solitons coexist as stable photoproducts,which has never been observed in conventional platinum-halide chains, and theyare highly resonant on the occasion of their birth and geminate recombination.

Dynamics of photoexcitations with interchain coupling in conjugated polymers

AbstractIn this article, we study the photoexcitation dynamics of two coupled trans‐polyacetylene chains. The approach used was an extended version of the Su, Scherieffer, and Heeger model for a pair of interacting chains and impurity. An electron excitation from one fully dimerized chain to another was simulated. The chain length influence were analyzed and shown to be of importance when the spectrum is concerned. It was also observed that the electron excitation generates a pair of neutral solitons on the second chain. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008

Re-excited States in Conjugated Polymers

We investigate the re-exciting process of a soliton, a polaron and abipolaron in conjugated polymers. For a soliton case, the processesS0+hν⇒S±+P∓ and S±+hν⇒S0+P± will take place, where S0, S± and P±express a neutral soliton, charged solitons and charged polarons,respectively. For a polaron case, the processes P±+hν⇒P∓+BP±2 will occur, where BP±2 expresses a charged bipolaron with two electronic units. For abipolaron case, the processes BP±2+hν⇒P±+P± and BP±2+hν⇒BP±2+E0 are obtained.

Obliquely propagating large amplitude solitary waves in charge neutral plasmas

Abstract. This paper deals in a consistent way with the implications, for the existence of large amplitude stationary structures in general plasmas, of assuming strict charge neutrality between electrons and ions. With the limit of pair plasmas in mind, electron inertia is retained. Combining in a fluid dynamic treatment the conservation of mass, momentum and energy with strict charge neutrality has indicated that nonlinear solitary waves (as e.g. oscillitons) cannot exist in electron-ion plasmas, at no angle of propagation with respect to the static magnetic field. Specifically for oblique propagation, the proof has turned out to be more involved than for parallel or perpendicular modes. The only exception is pair plasmas that are able to support large charge neutral solitons, owing to the high degree of symmetry naturally inherent in such plasmas. The nonexistence, in particular, of oscillitons is attributed to the breakdown of the plasma approximation in dealing with Poisson's law, rather than to relativistic effects. It is hoped that future space observations will allow to discriminate between oscillitons and large wave packets, by focusing on the time variability (or not) of the phase, since the amplitude or envelope graphs look very similar.

Theoretical Investigation of Static Characterization on Nonlinear Elementary Excitations in trans-Polyacetylene

Semiempirical quantum chemical studies on neutral and positively charged H(CH)nH homologues have been performed for systems with n up to 101, where different kinds of nonlinear excitation are found with increasing chain length. The Pariser-Parr-Pople (PPP) model has been employed and solved with the density matrix renormalization group (DMRG) method. The geometrical and electronic distortions induced by defects are obtained and compared with previous theoretical work, indicating that an adequate account of the electron correlation is essential for describing such systems. The structural distortion of a charged soliton (half-width calculated as L = 13) is shown to be more extended than that of a neutral soliton (L = 6); the geometrical distortion is even more extended in a polaron. In bipolarons, our calculations show that the coupling of the soliton-antisoliton pair might be longer ranged than expected. The phase transition from a bipolaron to a separated soliton-antisoliton pair occurs when n is close to 100.

Nonlinear lattice relaxation of photoexcited diplatinum-halide chain compounds

In order to reveal the relaxation mechanism of photogenerated charge-transfer excitations in quasi-one-dimensional halogen-bridged diplatinum complexes, we calculate the low-lying adiabatic potential energy surfaces of a one-dimensional extended Peierls-Hubbard model. High-energy excitations above the electron-hole continuum may relax into polarons, while excitons pumped within the optical gap are self-localized and then either decay by luminescence or divide into solitons. Neutral solitons, charged solitons, and polarons may be simultaneously photogenerated in a diplatinum-halide chain, which has never been observed in any conventional platinum-halide chain. Optical conductivity is also simulated along the decay paths for experimental verification.