Isotope effect in the translation-invariant bipolaron theory of high-temperature superconductivity

TL;DR

This paper demonstrates that the translation-invariant bipolaron theory explains how isotope effects vary with doping and transition temperature in high-temperature superconductors, emphasizing the importance of non-adiabaticity and phonon interactions.

Contribution

It introduces a novel explanation for isotope coefficient behavior in high-Tc superconductors based on bipolaron theory and establishes criteria for d-wave phonon involvement.

Findings

Isotope coefficient is low at optimal doping and high at weak doping.

Absolute isotope coefficient behaves oppositely for London penetration depth.

Non-adiabatic effects are crucial in weak doping regimes.

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.