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.

Unified theory of colossal magnetoresistance in manganites and high temperature superconductivity in cuprates

Abstract: The bipolaron theory of superconductivity provides a parameter-free fit of the superconducting critical temperature and the upper critical field of cuprates. It describes their non-Fermi-liquid normal state, including the in-plane and out-of-plane resistivity, Hall effect, magnetic susceptibility and tunnelling and photoemission spectra. The theory is based on the experimental fact that the electron-phonon interaction in doped oxides is the largest one. We show that the same interaction together with the p − d exchange interaction explains the ferromagnetism and colossal magnetoresistance (CMR) of doped manganites as a result of the pairing of polaronic carriers in the paramagnetic phase and a pair-breaking effect in the ferromagnetic phase. The theory is largely independent of the type of the electron-phonon interaction and the size of bipolarons and, in addition to perovskite manganites, applies to pyrochlore manganites where the Jahn-Teller phonons and double-exchange mechanism are absent. It accounts for the dc and ac colossal magnetoresistivity, for low mobility, for the sudden spectral weight transfer with temperature in the optical conductivity and photoemission, for a strongly suppressed Drude weight in the ferromagnetic phase, and for the giant isotope effect. The theory suggests that by replacing the magnetic ions of Mn by nonmagnetic Cu one can turn a doped charge-transfer magnetic insulator into a high-temperature superconductor owing to the Bose-Einstein condensation of bipolarons as observed in cuprates.

Translational-invariant bipolarons and superconductivity

Abstract: A translation-invariant (TI) bipolaron theory of superconductivity based, like Bardeen-Cooper-Schrieffer theory, on Frohlich Hamiltonian is presented. Here the role of Cooper pairs belongs to TI bipolarons which are pairs of spatially delocalized electrons whose correlation length of a coupled state is small. The presence of Fermi surface leads to stabilization of such states in its vicinity and a possibility of their Bose-Einstein condensation (BEC). The theory provides a natural explanation of the existence of a pseudogap phase preceding the superconductivity and enables one to estimate the temperature of a transition $T^*$ from a normal state to a pseudogap one. It is shown that the temperature of BEC of TI bipolarons determines the temperature of a superconducting transition $T_c$ which depends not on the bipolaron effective mass but on the ordinary mass of a band electron. This removes restrictions on the upper limit of $T_c$ for a strong electron-phonon interaction. A natural explanation is provided for the angular dependence of the superconducting gap which is determined by the angular dependence of the phonon spectrum. It is demonstrated that a lot of experiments on thermodynamic and transport characteristics, Josephson tunneling and angle-resolved photoemission spectroscopy (ARPES) of high-temperature superconductors does not contradict the concept of a TI bipolaron mechanism of superconductivity in these materials. Possible ways of enhancing $T_c$ and producing new room-temperature superconductors are discussed on the basis of the theory suggested.

Microscopic theory of organic magnetoresistance based on kinetic equations for quantum spin correlations

Abstract: The correlation kinetic equation approach is developed that allows describing spin correlations in a material with hopping transport. The quantum nature of spin is taken into account. The approach is applied to the problem of the bipolaron mechanism of organic magnetoresistance (OMAR) in the limit of large Hubbard energy and small applied electric field. The spin relaxation that is important to magnetoresistance is considered to be due to hyperfine interaction with atomic nuclei. It is shown that the lineshape of magnetoresistance depends on short-range transport properties. Different model systems with identical hyperfine interaction but different statistics of electron hops lead to different lineshapes of magnetoresistance including the two empirical laws $H^2/(H^2 + H_0^2)$ and $H^2/(|H| + H_0)^2$ that are commonly used to fit experimental results.

Vibrational Spectroscopic Study of Iodine-Doped Poly(isothianaphthene)

Abstract: Far-infrared, mid-infrared, and Raman spectroscopy were used to characterize iodine-doped poly(isothianaphthene) (PITN) films and powders. The far-infrared and mid-infrared results show changes from absorption mode to reflective mode as the doping level increases, consistent with the iodine-doped PITN becoming more metallic and more conductive at higher doping levels. The far-IR and Raman (514.5-nm laser excitation) results show that I3- is dominant in iodine-doped PITN. The Raman spectral changes observed using 1064-nm excitation are different from those measured using 514.5-nm excitation. The spectra recorded with 514.5-nm excitation show features due to the undoped parts of the polymer, and these indicate that the effective conjugated chain length decreases with increased doping. The Raman spectra obtained by using 1064-nm excitation show features due to polaron and bipolaron states in the doped polymer.