Doping-dependent isotope effects in HTS cuprates

Abstract

The doping-dependence of the oxygen isotope effect in over-doped high Tc superconducting cuprates (HTSCs) is examined in Ca doped Y-123 and (Pb, Y) doped Bi-2212. Oxygen isotope exchange is performed by repeatedly annealing samples in 16O and 18O environments. Raman spectra are collected for all samples to identify the site selective degree of isotopic substitution. Isotope shifts in Tc are determined by magnetometry and susceptibility measurements. Thermo-electric power measurements are made on all samples to directly determine hole concentration, p, and to identify possible differences in Tc due to small differences in hole concentration in each sample pair. Small positive oxygen isotope effects. αo(p) ~ 0.10 ± 0.05, are observed for over-doped Ca0.16 Y-123 Ca0.2 Y-123 and Pb0.2 Bi-2212 in the range 0.19 < p < 0.22. Two consistent measurements are presented for each Y-123 sample pair - one for as-prepared samples, the other following a co-anneal to attempt to equalise hole concentrations by equalising oxygen content. When compared with previous studies, these results clearly indicate that αo(p) does not become negative beyond optimal doping, as some theories predict, but are inconclusive regarding a possible upturn in αo(p) in the over-doped region. Raman spectra for 16O and 18O exchanged Pb0.2 Bi-2212 showing impurity peaks and highly site selective oxygen mobility at 400° C are discussed. No definite barium isotope effect is observed in optimally and under-doped Y-123 and naturally doped Y-124. This suggests that barium motion plays no part at all in the superconducting pairing mechanism. A copper effect of αcu = 0.10 ± 0.04 is resolved in naturally doped Y-124 which is consistent with observed trends. A compound copper-oxygen effect is examined in 16O63Cu and 18O65Cu Y-124 and shows the odd result of an isotope shift intermediate between the individual copper and oxygen effects. Results are discussed in terms of the Loram formulation of the pseudo-gap in which the normal state pseudo-gap and superconducting gaps compete. In this picture isotope effects in Tc can result from isotope effects in either the superconducting gap or the pseudo-gap. Various theoretical predictions of αo(p) for over-doping are reviewed and re-examined in the light of the present results.