Showing papers on "Bipolaron published in 1997"

EPR and charge-transport studies of polyaniline

Abstract: The study of dc and microwave (140 GHz) electrical conductivities using multifrequency electron-spin resonance in undoped and HCl-doped polyaniline is reported. The accidental quasi-three-dimensional (3D) charge hopping between the pinned and mobile small polarons dominates the bulk conductivity of the emeraldine base form of polyaniline. The increase in mobility and the number of excitations upon light doping of the polymer leads to the isoenergetic interpolaron charge hopping between the polaron and bipolaron states. 1D variable-range hopping of a charge between conducting islands, which correlates with a superslow torsional dynamics of the polymer chains, dominates bulk conductivity of heavily doped polyaniline at low temperatures. Intrinsic microconductivity is determined by the interaction of the charge with the lattice phonons at high temperatures. Following Epstein and MacDiarmid we propose that emeraldine salt of polyaniline represents a 1D disordered conducting compound consisting of metal-like islands of well coupled chains with 3D delocalized charge carriers.

Combined ab initio and density functional study on polaron to bipolaron transitions in oligophenyls and oligothiophenes

Abstract: Ab initio self-consistent-field (SCF), two-configuration SCF (TCSCF), and density functional theory (DFT) calculations on the charge–transfer complexes of doubly Li/Cl-doped oligothiophenes and oligo(p-phenyls) and on respective charged systems without counterions have been carried out in order to study polaron to bipolaron transitions. Oligomer chains up to octamers and the ring structures cyclo-dodecathiophene and cyclo-dodeca(p-phenyl) have been investigated. Special attention is paid to the open-shell biradical character of two isolated polaronic defects. It is found that the TCSCF and the spin-unrestricted DFT methods can be successfully applied. A bipolaron structure is obtained when the doping atoms are located on neighboring rings and when there is one undoped ring separating the two doped ones. If there are two or more undoped rings in between a two-polaron configuration (biradical) is found. The bipolaron system is calculated to be more stable than the two-polaron case when counterions are taken...

Mobile intersite bipolarons in the discrete holstein-hubbard model

Abstract: We have explored the properties of a two-fermion system interacting with the phonon field in the framework of the one-dimensional discrete Holstein-Hubbard model The variational method employed here introduces the correlations between the phonons and the electron wave functions, therefore the composite trial state is not factorized As a consequence our approach gives reliable results in the whole space of the parameters We will direct our attention to a new class of solutions found in the intermediate range of values of the nonadiabaticity parameter \ensuremath{\gamma} These solutions are characterized by anomalous (non-Gaussian) fluctuations of the position of the oscillators An intersite bipolaron with a relatively small effective mass is stable in a wide region of the parameters due to both exchange and nonadiabaticity effects

Polaron and bipolaron formation in a cubic perovskite lattice

Abstract: The Rice-Sneddon model for BaBiO${}_{3}$ offers a starting point for discussing polarons and bipolarons in a three-dimensional oxide crystal. We use exact diagonalization methods on finite samples to study the stability and properties of polarons and bipolarons. Because polarons, when they form, turn out to be very well localized, we are able to converge accurately our calculations for two-electron bipolaron wave functions, accounting for the Coulomb interaction without approximation. Some of our results are compared with and interpreted by reference to the variational method of Landau and Pekar. We calculate both electronic and vibrational excitations of the small polaron solutions, finding a single vibrational state localized with the full symmetry of the polaron, which has its energy significantly decreased at the onset of polaron formation. Both on-site (Hubbard) and long-range Coulomb repulsion are included in the bipolaron calculation, but due to the high degree of localization, the long-range part has only a small influence. For a reasonable on-site repulsion $U$ equal to two times the band width $W$, bipolaron formation is significantly suppressed; there is a large window of electron-phonon coupling where the polaron is stable but the bipolaron decays into two polarons.

Origin of pair-tunneling states in semiconductor quantum dots

Abstract: We analyze the role optical and acoustic phonons and physical confinement play in stabilizing bound bipolarons in a GaAs quantum dot. We find that while acoustic phonons play a significant role in polaron formation, they play virtually no role in the stability of a bipolaron. In the weak-coupling limit, we find that the bipolaron is stable only when a local potential is present to physically confine the two electrons. Our results are then applied to the problem of pair electron tunneling @Phys. Rev. Lett. 68, 3088 ~1992!# in GaAs quantum dots. We find that although GaAs is a weakly polar semiconductor, optical phonons and the physical confinement of the quantum dot can conspire to create a barrier to single-electron tunneling. Such a barrier to single-electron tunneling signifies the formation of 2U pairing centers. The physical potential in which the two electrons are bound can be composed of a Si impurity and a parabolic well that originates from the image potential created by the d dopants in the backing layer of the dot. We find that pair binding is unfavorable at small separations between the two wells where the Coulomb repulsion obviates pair binding. A minimal separation of ’800 A is found for pair binding to occur. Hence, we argue that when the confining radius of the dot is smaller than ’800 A, pair tunneling states should not be observed. In addition, we find that the pair state is unstable at moderate magnetic field strengths (’ 2T !, as is seen experimentally. @S0163-1829~97!06508-9#

Molecular quantum chemical calculations of the formation of interfaces between Al, Ca and Mg with poly(phenylene vinylene) oligomers

Abstract: Molecular quantum chemical calculations were performed both at the ab initio and at the semi-empirical level to model the molecular conformations and electronic structure of oligomers of poly(phenylene vinylene) during the early stages of interface formation with Al, Ca and Mg. We found that the divalent metals, Mg and Ca, disrupt the conformation of the oligomers less than Al does. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) move into the energy gap both for Ca- and for Mg-doped systems, resulting in gap-state formation. This is consistent with the polaron/bipolaron picture. The electron density plots indicate that the de-localization of electrons is reduced more significantly by Al than it is by Ca and Mg. Our simulation results have been confirmed experimentally via XPS and NEXAFS.

The electronic structure of liquid and amorphous Se: chain models

Abstract: We present a numerical study of the electronic structure of simple models of liquid and amorphous selenium. The geometry is based upon random walks; the electronic structure is described by a tight-binding Hamiltonian. We investigate the influence of the bond- and dihedral-angle distribution, the range and nature of the tight-binding hopping matrix element and the sign and magnitude of the electron - electron interaction upon the density of states. Localization properties and charge profiles have been computed. To reproduce the experimentally observed optical gap, it is essential to fix the dihedral angles and to introduce an intraorbital electron - electron attraction of modest strength . In contrast to local descriptions of bond breaking in disordered Se, we observe the formation of two negatively charged chain ends, compensated by the creation of a bipolaron within the chain. Only bipolaron states are able to occupy an impurity band. A simple mechanism for Fermi level pinning is discussed. We give an outlook on the electronic properties of related three-dimensional models.

The design of Optical Limiters Based on the Formation of Bipolaron-Like Dications

Abstract: During the past decade, there has been considerable effort in the design of new materials with greatly enhanced nonlinear absorption characteristics. In recent years, it has been shown that incorporation of bipolaronic charges in the pi-electron sequence in conjugated polymers or copolymers leads to enhanced third-order nonlinear optical properties. In this current study a design strategy for the synthesis of optical power limiting copolymers is presented whose properties are based on the photogeneration of stable bipolaron-like dications.

Polaronic excitations in the doped polyacene

Abstract: The polyacene as two coupled chains of transpolyacetylene has been studied based on an extended SSHHubbard model. The dimerized displacement u0 is found to be similar to the case of trans-polyacetylene, and equals to 0.04 A. The energy-band gap is 0.38 eV, in agreement with other authors. In particular, we have considered some cases where polyacene is doped with one and two donor electrons. In the case of doping with one electron, a polaron spreading over two chains has been found; in the case of doping with two electrons having different spins, a stable bipolaron has been obtained, which is different from the one of transpolyacetylene; in the case of doping with two electrons having the same spins, two stable polarons which spread over two chains have been found. The bound electronic states corresponding to these cases are obtained.

Stability of bipolarons in the presence of a magnetic field

Abstract: The stability of large Frohlich bipolarons in the presence of a static magnetic field is investigated with the path integral formalism. We find that the application of a magnetic field (characterized by the cyclotron frequence ω c) favors bipolaron formation: (i) the critical electronphonon coupling parameter α c (above which the bipolaron is stable) decreases with increasing ω c and (ii) the critical Coulomb repulsion strength U c (below which the bipolaron is stable) increases with increasing ω c. The binding energy and the corresponding variational parameters are calculated as a function of α, U and ω c. Analytical results are obtained in various limiting cases. In the limit of strong electron-phonon coupling (α ≫ 1) we obtain for ω c ≫ 1 that E estim ⋍ E estim(ω c = 0) + c(u)ω c/α 4 with c(u) an explicitly calculated constant, dependent on the ratio u = U/α where U is the strength of the Coulomb repulsion. This relation applies both in 2D and in 3D, but with a different expression for c(u). For ω c ≫ α 2≫ 1 we find in 3D E estim ⋍ ω c - α 2 A(u) ln2(ω c/α 2), (also with an explicit analytical expression for A(u)) whereas in 2D E estim 2D ⋍ ω c - α√ω cπ(u-2-√2)/2. The validity region of the Feynman-Jensen inequality for the present problem, bipolarons in a magnetic field, remains to be examined.

Bipolarons in the intermediate coupling method

Abstract: The properties of polarons and bipolarons are studied by the intermediate coupling method, which takes electron-lattice coupling into account. A Coulomb correlation in the electron motion is included. When the Frohlich coupling constant is reduced, at α*=5.7 a bipolaron decays suddenly from a self-trapped state into two delocalized polarons. A phase diagram is constructed for the region where the stable bipolaron exists. These results are compared with those obtained by integration along trajectories.

Small Bipolaron Conductivity in the Holstein–Hubbard Model

Abstract: We examine a narrow-band Holstein–Hubbard model where electrons are coupled to a local lattice distortion and form small polarons. When the on-site Coulomb interactions are overscreened by strong electron–phonon coupling, the polarons attract each other through the effective potential, bind into pairs, and form small bipolarons. The optical conductivity of small bipolaron is calculated as a function of frequency, electron density, and temperature, within a limited temperature and parameter range.

Interchain Pair Hopping of Solitons and Polarons Via Dopants in Polyacetylene

Abstract: Interchain hopping of solitons and polarons in polyacetylene is studied by numerical simulation of their motion under an electric field. Use is made of the Su-Schrieffer-Heeger model supplemented with intrachain electron-electron interactions and dopant potentials. We find that charged solitons can hop to the opposite chain by forming bound pairs (bipolarons). 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 efficient than that of a soliton pair.

Large polarons, bipolarons and Boson-Fermion model of superconductivity

Abstract: Large polaron and bipolaron formation is reviewed with particular emphasis on the screening effects which become important at relatively high carrier density (plasma frequency, ωp, larger than longitudinal optical frequency, ω1). At high density polarons are mobile particles even at very high electron-phonon coupling constant (a»l) and bipolaron binding energy tends to vanish giving evidence of the existence of metastable bipolaron states. The coexistence of metastable bipolarons (bosons) and polarons (fermions) supports the so-called boson-fermion model of superconductivity. This model can be generalized in order to take into account the dependence on the carrier density given in the polaron-bipolaron framework. The results are in good agreement with the density dependence of the critical temperature exhibited by many high-Tc superconductors.

Bipolaron kinetics of high-Tc cuprates

Abstract: Based on the generic Hamiltonian including the electron-phonon interaction and the direct Coulomb repulsion the ground state of cuprates is shown to be a charged 2e Bose liquid of small bipolarons if the electron-phonon coupling constant λ is about or larger than 0.5. Hole bipolaron band structure is derived for perovskite lattices. A microscopic theory of the normal state in-plane and c-axis transport of copper oxides is developed. The fundamental relationship between the anisotropy of resistivity and the spin susceptibility is derived. The temperature and doping dependence of the in-plane and out-of-plane resistivity as well as the spin susceptibility are found to be in remarkable agreement with the experimental data in underdoped, optimally and overdoped La 2 - x Sr x CuO 4 for the entire temperature regime from Tc up to 600 K. The normal state gap is explained and its doping and temperature dependence is clarified. Unusual logarithmic temperature dependence of resistivity at very low temperatures is quantitatively described as well. A new phase diagram of doped cuprates is proposed.