Contrasting oxygen and copper isotope effects inYBa2Cu4O8superconducting and normal states

Abstract

We have used Raman, susceptibility, and ${}^{89}\mathrm{Y}$ NMR measurements on the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ superconductor synthesized using pure ${}^{63}\mathrm{Cu}$ or ${}^{65}\mathrm{Cu}$ to investigate the oxygen and copper isotope effects in ${T}_{c}$ and the normal-state pseudogap. We find no copper or oxygen isotope effects in the normal-state ${}^{89}\mathrm{Y}$ Knight shift for temperatures in excess of 150 K. There is a positive copper isotope effect in ${T}_{c}$ and, for temperatures less than 150 K, a negative copper isotope effect in the normal-state ${}^{89}\mathrm{Y}$ Knight shift. This differs from the oxygen isotope effect which is positive for both ${T}_{c}$ and the ${}^{89}\mathrm{Y}$ Knight shift below 150 K. These results indicate that the normal-state pseudogap may not originate from precursor superconducting pairing.