Oxygen isotope effect of the plane-copper NQR frequency inYBa2Cu4O8

Abstract

We report high-precision measurements of the temperature dependence of the plane-${}^{63}\mathrm{Cu}$ NQR line frequency ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ and the linewidth in normal and superconducting ${}^{16}\mathrm{O}$ and ${}^{18}\mathrm{O}$-exchanged ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}.$ Whereas ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ passes ${T}_{c}$ very smoothly without a discontinuity either in value or slope, the linewidth increases in the normal conducting phase down to ${T}_{c}$ and starts to decrease sharply in the superconducting phase to finally resume its high-temperature value of the normal phase. There is a well discernible oxygen isotope effect on the ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ temperature dependencies. The temperature dependence of ${\ensuremath{\nu}}_{Q}(\mathrm{Cu}2)$ is described by an empirical expression consisting of two parts, one related to the thermal expansion of the lattice and the other due to charge redistribution during the formation of new electronic structures in the ${\mathrm{CuO}}_{2}$ planes. From the fit to the experimental data we determine for the conjectured formation of new electronic structures an energy scale $\ensuremath{\Delta}{(}^{16}\mathrm{O})=188.0(1.6) \mathrm{K}$ and $\ensuremath{\Delta}{(}^{18}\mathrm{O})=180.0(1.6) \mathrm{K}.$ This results in a partial oxygen isotope effect coefficient ${\ensuremath{\alpha}}_{{\ensuremath{\nu}}_{Q}}=0.42(11)$ which is larger than both the spin pseudogap coefficient ${\ensuremath{\alpha}}_{\mathrm{PG}}=0.061(8)$ and the ${T}_{c}$ coefficient ${\ensuremath{\alpha}}_{{T}_{c}}=0.056(12)$ [F. Raffa, T. Ohno, M. Mali, J. Roos, D. Brinkmann, K. Conder, and M. Eremin, Phys. Rev. Lett. $81,$ 5912 (1998)].