Abstract
It is shown that the translation-invariant bipolaron theory of superconductivity can explain the dependence of the isotope coefficient in high-temperature superconductors on the critical temperature of a superconducting transition: in the case of strong electron–phonon interaction, the isotope coefficient is low when doping is optimal and high when it is weak. It is demonstrated that in the case of London penetration depth, the absolute value of the isotope coefficient behaves in the opposite way. A conclusion of the great role of non-adiabaticity in the case of weak doping is made. The criteria for d-wave phonon input into the isotope effect is established.
Highlights
Understanding the high-temperature superconductivity (HTSC) in cuprate superconductors and other HTSC materials is at the heart of current research in condensed matter physics
A significant number of new experimental facts has led researchers to return to the electron–phonon interaction (EPI) as a dominant mechanism for explaining the HTSC effect
In the limit of weak doping, the isotope coefficient for London penetration depth β as distinct from the isotope coefficient α for Tc will be very small
Summary
Understanding the high-temperature superconductivity (HTSC) in cuprate superconductors and other HTSC materials is at the heart of current research in condensed matter physics. A lack of this effect in optimally doped high-temperature superconductors was the reason for discarding the phonon mechanism in HTSC and, as a consequence, the Bardeen–Cooper–Schrieffer theory (BCS) [1]. Generalization of the BCS to the case of strong EPI (Eliashberg theory) has been unable to explain many important phenomena attending HTSC, such as the pseudogap state. To overcome these difficulties, the author has developed a translation-invariant (TI) bipolaron theory of HTSC where the role of Cooper pairs belongs to TI bipolarons [3,4,5,6]. The aim of this paper is to explain isotope effects observed in HTSC on the basis of the TI bipolaron theory
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