A Precise Measurement of the Oxygen Isotope Effect on the Neel Temperature in Cuprates

A limiting factor in the ability to interpret isotope effect measurements in cuprates is the absence of sufficiently accurate data for the whole phase diagram; there is precise data for T-c, but not for the antiferromagnetic transition temperature T-N. Extreme sensitivity of T-N to small changes in the amount of oxygen in the sample is the major problem. This problem is solved here by using the novel compound (Ca0.1La0.9)(Ba1.65La0.35)Cu3Oy for which there is a region where T-N is independent of oxygen doping. Meticulous measurements of T-N for samples with O-16 and O-18 find the absence of an oxygen isotope effect on T-N with unprecedented accuracy. A possible interpretation of our finding and existing data is that isotope substitution affects the normal state charge carrier density.