Dissociation Of Bipolarons Research Articles

Unified percolation model for bipolaron-assisted organic magnetoresistance in the unipolar transport regime

The fact that in organic semiconductors the Hubbard energy is usually positive appears to be at variance with a bipolaron model to explain magnetoresistance (MR) in those systems. Employing percolation theory, we demonstrate that a moderately positive $U$ is indeed compatible with the bipolaron concept for MR in unipolar current flow, provided that the system is energetically disordered, and the density of states (DOS) distribution is partially filled, so that the Fermi level overlaps with tail states of the DOS. By exploring a broad parameter space, we show that MR becomes maximal around $U=0$ and even diminishes at large negative values of $U$ because of spin independent bipolaron dissociation. Trapping effects and reduced dimension enhance MR.

Interaction of bipolaron with the H2O/O2 redox couple causes current hysteresis in organic thin-film transistors

Hysteresis in the current-voltage characteristics is one of the major obstacles to the implementation of organic thin-film transistors in large-area integrated circuits. The hysteresis has been correlated either extrinsically to various charge-trapping/transfer mechanisms arising from gate dielectrics or surrounding ambience or intrinsically to the polaron-bipolaron reaction in low-mobility conjugated polymer thin-film transistors. However, a comprehensive understanding essential for developing viable solutions to eliminate hysteresis is yet to be established. By embedding carbon nanotubes in the polymer-based conduction channel of various lengths, here we show that the bipolaron formation/recombination combined with the H2O/O2 electrochemical reaction is responsible for the hysteresis in organic thin-film transistors. The bipolaron-induced hysteresis is a thermally activated process with an apparent activation energy of 0.29 eV for the bipolaron dissociation. This finding leads to a hysteresis model that is generally valid for thin-film transistors with both band transport and hopping conduction in semiconducting thin films.

Bipolarons and polarons in the Holstein-Hubbard model: analogies and differences

The single bipolaron problem is examined in the context of the 1D Holstein-Hubbard model, emphasizing analogies and differences with respect to the complementary single polaron physics. The bipolaron band structure below the phonon threshold is revealed, showing a complex relationship between numerous excited bands as the adiabatic limit is approached. Light bipolarons with significant binding energy, the stability of large bipolarons, the small to large bipolaron crossover as a function of the Hubbard repulsion, as well as the bipolaron dissociation, are investigated in detail, disentangling adiabatic, nonadiabatic and lattice coarsening effects. It is emphasized that condensation of bipolarons occurs in the dilute limit only at very low temperatures.

BIPOLARON DYNAMICS IN NON-DEGENERATE POLYMERS

Bipolaron dynamics in non-degenerate polymers are discussed using the nonadiabatic dynamic method. First, charge injection process from metal electrode to a nondegenerate polymer in a metal/polymer/metal structure has been investigated. We demonstrate that the dynamical formation of a bipolaron sensitively depends on the work function of metal electrode. We also study the bipolaron dissociation process. It is found that the electric field that can dissociate the bipolaron is up to 106 V/cm, which is consistent with experiments.

Comparative Study of Composition Dependences of Photorefractive and Related Effects in LiNbO3 and LiTaO3 Crystals

The results of a comparative study of composition dependences of the light-induced refractive index change, photorefractive sensitivity, polaron luminescence, and intrinsic optical absorption in nominally pure LiNbO 3 and LiTaO 3 crystals are reported. Our investigation demonstrates that the particular light-induced defects traced by the photoluminescence are secondary photorefractive centers itself. The experimental results obtained are well described by light-induced dissociation of bipolarons into small polarons that are unstable at room temperature.

Massive spectral weight transfer and colossal magneto-optical effect in doped manganites

We calculate the value of the Fröhlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction (∼1 eV) is much stronger than any relevant magnetic interaction. This suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. As these materials are cooled below the Curie temperature, the colossal magnetoresistance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by the exchange interaction with localized Mn spins during the transition. The corresponding current carrier density collapse in doped manganites leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition, in agreement with available experimental observations.

Colossal magneto-optical conductivity in doped manganites

We show that the current carrier density collapse in doped manganites, which results from bipolaron formation in the paramagnetic phase, leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition. As with the colossal magnetoresistance itself, the corresponding magneto-optical effect is explained by the dissociation of bipolarons into polarons owing to the exchange interaction with the localized Mn spins in the ferromagnetic phase. The effect is positive at low frequencies and negative in the high-frequency region. The present results agree with available experimental observations.

Light-induced absorption changes in reduced lithium niobate

Reduced LiNbO3 crystals change their optical absorption under illumination with blue or green pump light. We measure the dependence of this effect on pump intensity I at different probe wavelengths. Steady-state absorption changes and build-up velocities are found to be proportional to the square root of I. Rates of relaxation towards equilibrium in the dark increase exponentially with temperature. Our experimental results can be well described by light-induced dissociation of bipolarons into small polarons unstable at room temperature.

Direct measurements of bipolaron-band development in doped polypyrrole with inverse photoemission.

Inverse-photoemission spectroscopy has been used to investigate dopant-dependent electronic properties of polypyrrole (PPy) films doped with ${\mathrm{BF}}_{4}^{\mathrm{\ensuremath{-}}}$ (tetrafluoroborate) or (PSS${)}^{\mathrm{\ensuremath{-}}}$ [poly(4-styrenesulfonate)]. The results show that the bonding and antibonding bipolaron bands appear at \ensuremath{\sim}1.0 and \ensuremath{\sim}2.6 eV above the valence-band maximum for heavily doped PPy films (\ensuremath{\sim}1 dopant per 3 pyrrole units). The bandwidth of each of these bipolaron bands is \ensuremath{\sim}0.4 eV and the band gap for heavily doped films is \ensuremath{\sim}3.6 eV. The bipolaron bands move toward the band edges and the gap shrinks with decreasing dopant concentration. Annealing of heavily doped films at 300 \ifmmode^\circ\else\textdegree\fi{}C leads to dopant loss from the surface. This effect is less pronounced for the larger polymeric anions such as (PSS${)}^{\mathrm{\ensuremath{-}}}$ than for the smaller counterions such as ${\mathrm{BF}}_{4}^{\mathrm{\ensuremath{-}}}$, suggesting that the former is more stable or has a greater thermal activation energy for bipolaron dissociation into single polarons.