Contrasting oxygen and copper isotope effects in YBa2Cu4O8 superconductingand normal states

We have used Raman, susceptibility, and Y-89 NMR measurements on the YBa2Cu 4O8 superconductor synthesized using pure Cu-63 or Cu-65 to investigate the oxygen and copper isotope effects in T-c and the normal-state pseudogap. W e find no copper or oxygen isotope effects in the normal-state Y-89 Knight shift for temperatures in excess of 150 K. There is a positive copper isoto pe effect in T-c and, for temperatures less than 150 K, a negative copper i sotope effect in the normal-state Y-89 Knight shift. This differs from the oxygen isotope effect which is positive for both T-c and the Y-89 Knight sh ift below 150 K. These results indicate that the normal-state pseudogap may not originate from precursor superconducting pairing.