Bipolaron

About: Bipolaron is a research topic. Over the lifetime, 1335 publications have been published within this topic receiving 29154 citations. The topic is also known as: bipolarons.

Dynamical evolution of polaron to bipolaron in conjugated polymers

Abstract: We investigate the insertion of holes in conjugated polymers bearing polarons. We use a Su-Schrieffer-Heeger model modified to include electron-electron interactions via an extended Hubbard Hamiltonian, a Brazovskii-Kirova symmetry breaking term, and an external electric field. We study the dynamics performing numerical calculations within the time-dependent unrestricted Hartree-Fock approximation. We find that adding a hole in a conducting polymer bearing a single positively charged polaron leads to the direct transition of polaron to bipolaron state. The transition produced is a single-polaron to biplaron transition whose excitation spectrum explains the experimental data. The competing mechanism of two polarons merging to form a bipolaron is not observed. We also find that depending on how fast the hole is inserted, a structure that contains a bipolaron coupled to a breather is created. The bipolaron-breather pair can be decoupled under the action of an external electric field. We determine the value of the critical electric field to untrap the bipolaron from the breather.

Polaron and bipolaronlike states in n-doped bithiophene.

Abstract: Bithiophene is studied with varying Cs dosing as a model for the n-doping of thiophene-based conducting polymers. The evolution of the electronic structure with increasing Cs concentration is followed both indirectly, by electron-energy-loss spectroscopy (EELS), and directly by ultraviolet-photoemission spectroscopy (UPS). In both spectroscopies the behavior of the states formed in the gap indicates two doping regimes. The EELS results parallel the optical-absorption results for the p-doping of longer thiophene oligomers and polythiophene, and suggest a polaron (anion) to bipolaronlike (dianion) transformation. The UPS results are consistent with this interpretation, showing first in the low-doping regime the appearance of two states in the band gap which move closer together in the higher-doping regime. This constitutes direct observation of a transition from polaron to bipolaron states, albeit in a model system. Consideration of the photoionization cross sections indicates that the deeper (bi)polaron level has C 2p character while the shallow level, filled by donation from the Cs, has significant S 3p character.

Theory of colossal magnetoresistance in doped manganites

Abstract: The exchange interaction of polaronic carriers with localized spins leads to a ferromagnetic/paramagnetic transition in doped charge-transfer insulators with strong electron-phonon coupling. The relative strength of the exchange and electron-phonon interactions determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of local bound-pair (bipolaron) formation in the paramagnetic phase, is extremely sensitive to an external magnetic field. Below the critical temperature of the transition, , the binding of the polarons into immobile pairs competes with the ferromagnetic exchange between polarons and the localized spins on Mn ions, which tends to align the polaron moments and, therefore, breaks up those pairs. The number of carriers abruptly increases below leading to a sudden drop in resistivity. We show that the carrier-density collapse explains the colossal magnetoresistance of doped manganites close to the transition. Below , transport occurs by polaronic tunnelling, whereas at high temperatures the transport is by hopping processes. The transition is accompanied by a spike in the specific heat, as experimentally observed. The gap feature in tunnelling spectroscopy is related to the bipolaron binding energy, which depends on the ion mass. This dependence explains the giant isotope effect of the magnetization and resistivity upon substitution of for . It is shown also that the localization of polaronic carriers by disorder cannot explain the observed huge sensitivity of the transport properties to the magnetic field in doped manganites.

Polarons in C60.

Abstract: A simple Su-Schrieffer-Heeger-type model of ${\mathrm{C}}_{60}$ and doped ${\mathrm{C}}_{60}$ is presented The model is solved numerically in the adiabatic approximation In the half-filled case the cluster dimerizes, in agreement with experiment and more complicated calculations When an electron is added to half-filling, a polaron is present in the ground state This polaron possesses an unusual geometry; it is a loop circling the ${\mathrm{C}}_{60}$ cluster For two added electrons a bipolaron forms with a similar geometry It is suggested that this bipolaron may be observable in optical experiments