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Abstract: The BCS theory has been extended by us to the strong-coupling regime wherecarriers are small lattice polarons and bipolarons. Here I review themulti-polaron strong-coupling theory of superconductivity. Attractive electroncorrelations, prerequisite to any superconductivity, are caused by an almostunretarded electron-phonon interaction sufficient to overcome the directCoulomb repulsion in this regime. Low energy physics is that of small polaronsand bipolarons, which are itinerant quasiparticles existing in the Bloch statesat temperatures below the characteristic phonon frequency. I identify thelong-range Froehlich electron-phonon interaction with c-axis polarised opticalphonons as the most essential for pairing in superconducting cuprates. A numberof key observations have been predicted or explained with polarons andbipolarons including unusual isotope effects and upper critical fields, normalstate pseudogaps, kinetic properties and diamagnetism, and giant proximityeffects. These and some other observations provide strong evidence for a novelstate of electronic matter in layered cuprates, which is a charged Bose-liquidof small mobile bipolarons.



Author: A. S. Alexandrov

Source: https://arxiv.org/







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