Large isotope effect on $T_c$ in cuprates despite of a small electron-phonon coupling

TL;DR

This study shows that the isotope effect on the superconducting transition temperature in cuprates is large at low doping levels despite a small electron-phonon coupling, due to spectral weight depletion caused by the pseudogap phase.

Contribution

It demonstrates that the large isotope effect at low doping is a universal feature arising from spectral weight depletion, independent of specific density of states features.

Findings

Isotope coefficient $eta$ increases significantly at low doping.

Small electron-phonon coupling constant $ ightarrow$ negligible effect on pseudogap temperature.

Large isotope effect is due to spectral weight depletion by the pseudogap.

Abstract

We calculate the isotope coefficients $\alpha$ and $\alpha^\ast$ for the superconducting critical temperature $T_c$ and the pseudogap temperature $T^\ast$ in a mean-field treatment of the t-J model including phonons. The pseudogap phase is identified with the $d$-charge-density wave ($d$-CDW) phase in this model. Using the small electron-phonon coupling constant $\lambda_d \sim 0.02$ obtained previously in LDA calculations in YBa$_2$Cu$_3$O$_7$, $\alpha^{\ast}$ is negative but negligible small whereas $\alpha$ increases from about 0.03 at optimal doping to values around 1 at small dopings in agreement with the general trend observed in many cuprates. Using a simple phase fluctuation model where the $d$-CDW has only short-range correlations it is shown that the large increase of $\alpha$ at low dopings is rather universal and does not depend on the existence of sharp peaks in the density of states in the pseudogap state or on specific values of the phonon cutoff. It rather is caused by the large depletion of spectral weight at low frequencies by the $d$-CDW and thus should also occur in other realizations of the pseudogap.