Search results for "Bipolaron"
showing 7 items of 7 documents
Spectroscopic and theoretical study of the molecular and electronic structures of a terthiophene-based quinodimethane.
2004
The UV/Vis, infrared absorption, and Raman scattering spectra of 3',4'-dibutyl-5,5"-bis(dicyanomethylene)-5,5"-dihydro-2,2':5',2"-terthiophene have been analyzed with the aid of density functional theory calculations. The compound exhibits a quinoid structure in its ground electronic state and presents an intramolecular charge transfer from the terthiophene moiety to the C(CN)2 groups. The molecular system therefore consists of an electron-deficient terthiophene backbone end-capped with electron-rich C(CN)2 groups. The molecule is characterized by a strong absorption in the red, due to the HOMO-->LUMO pi-pi* electronic transition of the terthiophene backbone that shifts hypsochromically on …
Quantum chemical modelling of electron polarons and excitons in ABO3perovskites
2000
Quantum chemical calculations using the intermediate neglect of the differential overlap (INDO) method, combined with the large unit cell periodic model argue for an existence of the self-trapped electrons in KNbO3 and KTaO3 perovskite crystals. An electron in the ground state occupies predominantly t2g orbital of a Nb4+ ion. Its orbital degeneracy is lifted by a combination of the breathing and Jahn-Teller modes where four nearest equatorial O atoms are displaced outwards and two oxygens shift inwards along the z axis. Triplet exciton is shown to be in a good approximation of a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron) which is very likely responsible for the `gr…
Computer Modeling of Luminescence in ABO3 Perovskites
2001
ABSTRACTWe suggest theoretical interpretation to a long-debated discussion on a nature of the intrinsic “green” luminescence observed in many ABO3 perovskites. For this purpose we performed quantum chemical calculations using the Intermediate Neglect of the Differential Overlap combined with the Large Unit Cell periodic model. Triplet exciton which is very likely responsible for the “green” luminescence is shown to be in a good approximation a pair of nearest Jahn-Teller electron and hole polarons (a bipolaron).
Controlling the thermoelectric properties of polymers: application to PEDOT and polypyrrole
2015
Poly(3,4-ethylenedioxythiophene) (PEDOT) and polypyrrole (PPy) films have been prepared by an electrochemical method in a three electrode cell. The films have been obtained at different oxidation levels regarded as bipolaron, polaron and neutral states by varying the voltage, as is usually done in conjugated heterocyclic polymers. The voltage (-0.2V1.0 V) has been applied versus a Ag/AgCl reference electrode, producing a variation of one order of magnitude in the electrical conductivity and the Seebeck coefficient of the films. In the voltage range explored, the electrical conductivity increases from 80 to 766 S cm(-1) in PEDOT and from 15 to 160 S cm(-1) in PPy, while the Seebeck coefficie…
Superlight small bipolarons from realistic long-range Coulomb and Fröhlich interactions
2011
We report analytical and numerical results on the two-particle states of the polaronic t-Jp model derived recently with realistic Coulomb and electron-phonon (Frohlich) interactions in doped polar insulators. Eigenstates and eigenvalues are calculated for two different geometries. Our results show that the ground state is a bipolaronic singlet, made up of two polarons. The bipolaron size increases with increasing ratio of the polaron hopping integral t to the exchange interaction Jp but remains small compared to the system size in the whole range 0<t/Jp<1. Furthermore, the model exhibits a phase transition to a superconducting state with a critical temperature well in excess of 100K. In the…
Possibility of unconventional superconductivity of SrTiO3−δ
2003
Abstract SrTiO3−δ can show metallic behavior and superconductivity at T It is demonstrated that by variation of the annealing temperature in vacuum the transport properties of SrTiO3−δ can be tuned continuously from semiconducting to metallic. We present measurements of the upper critical field Bc2(T) which show near Tc a positive curvature. This unusual temperature dependence is consistent with a model of weakly interacting charged bosons which condense in the superconducting state (local pairing). However, measurements of current–voltage curves reveal only small critical currents of our samples. This observation is discussed in the framework of doping inhomogeneities.
Many-body Green's function theory for electron-phonon interactions: ground state properties of the Holstein dimer
2015
We study ground-state properties of a two-site, two-electron Holstein model describing two molecules coupled indirectly via electron-phonon interaction by using both exact diagonalization and self-consistent diagrammatic many-body perturbation theory. The Hartree and self-consistent Born approximations used in the present work are studied at different levels of self-consistency. The governing equations are shown to exhibit multiple solutions when the electron-phonon interaction is sufficiently strong whereas at smaller interactions only a single solution is found. The additional solutions at larger electron-phonon couplings correspond to symmetry-broken states with inhomogeneous electron de…