arXiv subjects

A. R. Bishop

Publications and source records attributed to A. R. Bishop.

198 records · Page 11Linked to original sources

arXiv1997

Excitons in two coupled conjugated polymer chains

We have studied the exciton states in two coupled conjugated polymer chains which are modeled individually by the Su-Schrieffer-Heeger Hamiltonian and coupled by an interchain electron-transfer term. Both the intra- and inter-chain long range Coulomb interactions are taken into account. The properties of the lowest symmetric and antisymmetric exciton states are extensively discussed for both the parallel and anti-parallel ordering between these two chains. It is found that, for these two kinds of ordering, the features of excitons are quite different. Possible implications for the experiment of luminescent polymers are also addressed.

cond-mat.soft↗

arXiv1997

Excitons in quasi-one dimensional organics: Strong correlation approximation

An exciton theory for quasi-one dimensional organic materials is developed in the framework of the Su-Schrieffer-Heeger Hamiltonian augmented by short range extended Hubbard interactions. Within a strong electron-electron correlation approximation, the exciton properties are extensively studied. Using scattering theory, we analytically obtain the exciton energy and wavefunction and derive a criterion for the existence of a $B_u$ exciton. We also systematically investigate the effect of impurities on the coherent motion of an exciton. The coherence is measured by a suitably defined electron-hole correlation function. It is shown that, for impurities with an on-site potential, a crossover behavior will occur if the impurity strength is comparable to the bandwidth of the exciton, corresponding to exciton localization. For a charged impurity with a spatially extended potential, in addition to localization the exciton will dissociate into an uncorrelated electron-hole pair when the impurity is sufficiently strong to overcome the Coulomb interaction which binds the electron-hole pair. Interchain coupling effects are also discussed by considering two polymer chains coupled through nearest-neighbor interchain hopping $t_{\perp}$ and interchain Coulomb interaction $V_{\perp}$. Within the $t$ matrix scattering formalism, for every center-of-mass momentum, we find two poles determined only by $V_{\perp}$, which correspond to the interchain excitons. Finally, the exciton state is used to study the charge transfer from a polymer chain to an adjacent dopant molecule.

cond-mat.soft↗

arXiv1997

Persistent Current in the Ferromagnetic Kondo Lattice Model

In this paper, we study the zero temperature persistent current in a ferromagnetic Kondo lattice model in the strong coupling limit. In this model, there are spontaneous spin textures at some values of the external magnetic flux. These spin textures contribute a geometric flux, which can induce an additional spontaneous persistent current. Since this spin texture changes with the external magnetic flux, we find that there is an anomalous persistent current in some region of magnetic flux: near Phi/Phi_0=0 for an even number of electrons and Phi/Phi_0=1/2 for an odd number of electrons.

cond-mat.str-el↗

arXiv1997

Interchain Coupling Effects and Solitons in CuGeO_3

The effects of interchain coupling on solitons and soliton lattice structures in CuGeO3 are explored. It is shown that interchain coupling substantially increases the soliton width and changes the soliton lattice structures in the incommensurate phase. It is proposed that the experimentally observed large soliton width in CuGeO3 is mainly due to interchain coupling effects.

cond-mat.str-el↗

arXiv1997

First-Order Melting of a Moving Vortex Lattice: Effects of Disorder

We study the melting of a moving vortex lattice through numerical simulations with the current driven 3D XY model with disorder. We find that there is a first-order phase transition even for large disorder when the corresponding equilibrium transition is continuous. The low temperature phase is an anisotropic moving glass.

cond-mat.supr-con↗

arXiv1996

On the stability of polaronic superlattices in strongly coupled electron-phonon systems

We investigate the interplay of electron-phonon (EP) coupling and strong electronic correlations in the frame of the two-dimensional (2D) Holstein t-J model (HtJM), focusing on polaronic ordering phenomena for the quarter-filled band case. The use of direct Lanczos diagonalization on finite lattices allows us to include the effects of quantum phonon fluctuations in the calculation of spin/charge structure factors and hole-phonon correlation functions. In the adiabatic strong coupling regime we found evidence for ``self-localization'' of polaronic carriers in a $(π,π)$ charge-modulated structure, a type of superlattice solidification reminiscent of those observed in the nickel perovskites $La_{2-x}Sr_{x}NiO_{4+y}$.

cond-mat.str-el↗

arXiv1996

Charge Localization in Disordered Colossal-Magnetoresistance Manganites

The metallic or insulating nature of the paramagnetic phase of the colossal-magnetoresistance manganites is investigated via a double exchange Hamiltonian with diagonal disorder. Mobility edge trajectory is determined with the transfer matrix method. Density of states calculations indicate that random hopping alone is not sufficient to induce Anderson localization at the Fermi level with 20-30% doping. We argue that the metal-insulator transtion is likely due to the formation of localized polarons from nonuniform extended states as the effective band width is reduced by random hoppings and electron-electron interactions.

cond-mat.str-el↗

arXiv1996

Nonequilibrium transport and population inversion in double quantum dot systems

We present a microscopic theory for both equilibrium and nonequilibrium transport properties of coupled double quantum dots (DQD). A general formula for current tunneling through the DQD is derived by the nonequilibrium Green's function method. Using a Hartree-Fock approach, effects of multi-level coupling and nonequilibrium electron distributions in resonant tunneling are considered. We find that the peak in the resonant tunneling current through two symmetric dots will split only when the inter-dot coupling is stronger than dot-lead coupling. We predict that population inversion can be achieved in one dot in the nonequilibrium regime.

cond-mat.mes-hall↗

arXiv1996

Low Temperature Magnetic Properties of the Double Exchange Model

We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener to explain ferromagnetism has unexpected properties when there is more than one itinerant electron. We find that, in general, the many-body ground state of the DE model is {\it not} globally FM ordered (except for special filled-shell cases). Also, the low energy excitations of this model are distinct from spin wave excitations in usual Heisenberg ferromagnets, which will result in unusual dynamic magnetic properties.

cond-mat↗

arXiv1996

Quantum Breathers in a Nonlinear Lattice

We study nonlinear phonon excitations in a one-dimensional quantum nonlinear lattice model using numerical exact diagonalization. We find that multi-phonon bound states exist as eigenstates which are natural counterparts of breather solutions of classical nonlinear systems. In a translationally invariant system, these quantum breather states form particle-like bands and are characterized by a finite correlation length. The dynamic structure factor has significant intensity for the breather states, with a corresponding quenching of the neighboring bands of multi-phonon extended states.

cond-mat↗

arXiv1995

Double Degeneracy and Jahn-Teller Effects in CMR Perovskites

Jahn-Teller (JT) electron-phonon coupling effects in the colossal magnetoresistance perovskite compounds $La_{1-x}A_xMnO_3$ are investigated. Electron-electron correlations between two degenerate Mn $e_g$ orbitals are studied in the Gutzwiller approximation. The static JT distortion and antiadiabatic polaron effects are studied in a modified Lang-Firsov approximation. We find that (i) the electron or hole character of the charge carrier depends on the static JT distortion, and (ii) due to the two-component nature of the JT coupling, fluctuations in the JT distortion direction contribute to the charge transport in similar fashion as the local spins.

cond-mat↗

arXiv1995

Frustrated Spin-Peierls Chains

We study the phase diagram, ground state properties, and excitation gaps of a frustrated spin-Peierls chain, i.e. a spin-Peierls chain with both nearest neighbor exchange $J_1$ and next nearest neighbor exchange $J_2$. The phase diagram is calculated using a bosonization Renormalization-Group (RG) method. We propose a (bosonic) bond-operator mean field approximation to calculate the ground state properties and excitation gaps.

cond-mat↗

arXiv1995

Superconductivity Fluctuations in a One-dimensional Two Band Electron-Phonon Model with Strong Repulsive Interactions

We study a one-dimensional, two band model with short range electron-electron repulsions (onsite $U$ and nearest-neighbour $V$ terms) and electron-phonon coupling. We show that there is a region of $U$,$V$ and band filling in which singlet superconductivity fluctuations are dominant. This region is absent without electron-phonon interactions and includes large values of $U$,$V$.

cond-mat↗

arXiv1993

Localized States in Discrete Nonlinear Schrödinger Equations

A new 1-D discrete nonlinear Schrödinger (NLS) Hamiltonian is introduced which includes the integrable Ablowitz-Ladik system as a limit. The symmetry properties of the system are studied. The relationship between intrinsic localized states and the soliton of the Ablowitz-Ladik NLS is discussed. It is pointed out that a staggered localized state can be viewed as a particle of a {\em negative} effective mass. It is shown that staggered localized states can exist in the discrete dark NLS. The motion of localized states and Peierls-Nabarro pinning are studied.

patt-sol↗

arXiv1992

Length-scale competition in the damped sine-Gordon chain

It is shown that there are two different regimes for the damped sine-Gordon chain driven by the spatio-temporal periodic force $Γsin(ωt - k_{n} x)$ with a flat initial condition. For $Γ_{c}(n)$ to a translating {\em 2-breather} excitation from a state locked to the driver. For $ω< k_{n}$, the excitations of the system are the locked states with the phase velocity $ω/k_{n}$ in all the region of $Γ$ studied. In the first regime, the frequency of the breathers is controlled by $ω$, and the velocity of the breathers, controlled by $k_{n}$, is shown to be the group velocity determined from the linear dispersion relation for the sine-Gordon equation. A linear stability analysis reveals that, in addition to two competing length-scales, namely, the width of the breathers and the spatial period of the driving, there is one more length-scale which plays an important role in controlling the dynamics of the system at small driving. In the second regime the length-scale $k_{n}$ controls the excitation. The above picture is further corroborated by numerical nonlinear spectral analysis. An energy balance estimate is also presented and shown to predict the critical value of $Γ$ in good agreement with the numerics.

cond-mat↗

arXiv1992

Spin-Peierls ground states and frustration in a multi-band Peierls-Hubbard model

We discuss the consequences of including both electron-phonon and electron-electron couplings in multi-band models, focusing on numerical studies of a one-dimensional two-band model in the intermediate regime for both coupling strengths. Spin-Peierls as well as long-period, frustrated ground states are identified, reminisent of those found in antiferromagnetic next-nearest neighbor (ANNNI) models. We speculate on experimentally observable signatures of this rich phase diagram. 1992 PACS: 75.30.Fv, 71.45.Lr, 71.38+i, 64.70.Rh

cond-mat↗

arXiv1992

Electron-Phonon Driven Spin Frustration in Multi-Band Hubbard Models: MX Chains and Oxide Superconductors

We discuss the consequences of both electron-phonon and electron-electron couplings in 1D and 2D multi-band (Peierls-Hubbard) models. After briefly discussing various analytic limits, we focus on (Hartree-Fock and exact) numerical studies in the intermediate regime for both couplings, where unusual spin-Peierls as well as long-period, frustrated ground states are found. Doping into such phases or near the phase boundaries can lead to further interesting phenomena such as separation of spin and charge, a dopant-induced phase transition of the global (parent) phase, or real-space (``bipolaronic'') pairing. We discuss possible experimentally observable consequences of this rich phase diagram for halogen-bridged, transition metal, linear chain complexes (MX chains) in 1D and the oxide superconductors in 2D.

cond-mat↗

arXiv1992

Photoinduced charge separation in Q1D heterojunction materials: Evidence for electron-hole pair separation in mixed-halide $MX$ solids

Resonance Raman experiments on doped and photoexcited single crystals of mixed-halide $MX$ complexes ($M$=Pt; $X$=Cl,Br) clearly indicate charge separation: electron polarons preferentially locate on PtBr segments while hole polarons are trapped within PtCl segments. This polaron selectivity, potentially very useful for device applications, is demonstrated theoretically using a discrete, 3/4-filled, two-band, tight-binding, extended Peierls-Hubbard model. Strong hybridization of the PtCl and PtBr electronic bands is the driving force for separation.

cond-mat↗