| This thesis examines isotope effects in four different materials. Two of the investigations deal with High Temperature Superconductors (HTSCs), specifically the copper isotope effect (63Cu/65Cu) on Tc in oxygen deficient YBa2Cu3O7-delta and the oxygen isotope effect (16O/18O) on Tc in Bi2Sr2CaCu2O8+y as a function of oxygen content, y. The importance of CU-O planes and antiferromagnefic fluctuations in HTSCs leads to a search for isotope effects (both copper and oxygen) on the magnetic ordering temperatures of CuO. The fourth investigation concerns boron and carbon isotope effects on Tc in Ni and Pd based borocarbide superconductors.;A brief introduction outlines some of the common properties of HTSCs and a sketch of the conventional theory concerning isotope effects in superconductors is given. Practical aspects of isotope effect experiments are discussed. The importance of sample preparation and characterization are emphasized. Background information is given for each material followed by a description of the experimental process and a summary of results.;The investigation on the borocarbides shows clear evidence of a boron isotope effect in both YNi2B2C (alphaB = 0.26 +/- 0.03) and YPd2B2C (alphaB = 0.32 +/- 0.04). Results for a carbon isotope effect are suggestive for YNi2B2C (alphaC = 0.07 +/- 0.06), but essentially at the limit of experimental accuracy.;Isotopically enriched samples of 63CuO,65CuO, Cu16O and Cu18O showed no difference in either Neel temperature within experimental uncertainty.;Results for alphaCu(delta) in YBa2Cu3 O7-delta for 0.06 ≤ delta ≲ 0.60 show that it is zero at optimal doping (delta = 0.06) and negative (Tc(65Cu) > Tc(63Cu)) for all other values of delta. It is a nonmonotonic function of delta, showing large negative values away from the 60 K plateau and smaller values in the plateau (0.35 < delta < 0.45).;Oxygen content (y) in Bi2Sr2CaCu2O 8+y was determined by iodometric titration. A wide variation was encountered in the data, but for several samples with identical y values, alpha o is small (<0.1) and positive, in both the over (y ≈ 0.24) and under (y ≈ 0.10) doped regions. |
