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.

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.

Computational modelling of defects and charge trapping in amorphous and crystalline metal oxides

Abstract: Thin films of metal oxides, like Al₂O₃ and LaAlO₃, play a crucial role in emerging nanoelectronic devices. Using density functional theory (DFT) and other computational methods, the properties of defects and intrinsic polaron trapping have been calculated in LaAlO₃ and amorphous Al₂O₃. The spectroscopic properties of neutral (Vₒ⁰) and charged (Vₒ⁺) oxygen vacancies in cubic and rhombohedral LaAlO₃ have been investigated using Time Dependent DFT and the embedded cluster method. The peaks of the optical absorption spectra are predicted at 3.5 and 4.2 eV for Vₒ⁰ and 3.6 eV for Vₒ⁺ in rhombohedral LaAlO₃. The calculated electron paramagnetic resonance (EPR) parameters of Vₒ⁺ accurately predict the width (3 mT) and position of its EPR spectrum. Amorphous Al₂O₃ is then investigated, which has applications in non-volatile memory and a-IGZO (amorphous indium-gallium-zinc oxide) thin film transistors. Amorphous Al₂O₃ structures were generated using a molecular dynamics melt-quench approach and found to be in good agreement with experiment. DFT calculations, using a tuned hybrid functional, determined that the a-Al₂O₃ band gap decreases to 5.5 eV, compared to 8.6 eV in α-Al₂O₃, because of the reduction in Al coordination number in the amorphous phase. This causes a shift in the electrostatic potential that lowers the conduction band minimum, adding support to experimental measurements of band offsets. Then intrinsic polaron and bipolaron trapping in a-Al₂O₃ is modelled. The average trapping energy of hole polarons in a-Al₂O₃ was calculated to be 1.26 eV, much higher than the 0.38 eV calculated for α-Al₂O₃. Electrons were found not to trap in both crystalline and amorphous Al₂O₃. To explain the negative charging of Al₂O₃ films the properties of oxygen, hydrogen and aluminium defects were calculated. A mechanism is proposed to explain experimental trap spectroscopy measurements, whereby negatively charge defects are compensated by positively charged defects that have unoccupied states in the band gap. These predictions will facilitate experimental identification of defect states in LaAlO₃ and Al₂O₃ and their effect on nanodevices.

Photoinduced splitting of bipolarons in polymers

Abstract: In conducting polymers, a polaron carries charge with spin and a bipolaron carries charge without spin. Hence, the polaron is a spin carrier but the bipolaron is not. Through a dynamical simulation, our study of photoinduced phenomena shows that, by absorbing a photon, a bipolaron is split into two polarons. This photoinduced splitting converts a charge carrier (bipolaron) into two spin carriers (polarons). (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)