Disorder and Doping in the Oxygenated Electron-doped Superconductor PCCO

Abstract

This thesis is composed of two parts: the first part deals with the high temperature electron-doped superconductor Pr_(2-x)Ce_(x)CuO_(4-delta); the second part deals with the diluted magnetic semiconductor Ti_(1-x)Co_(x)O_(2-delta).

It is not clear why oxygen reduction and cerium doping are necessary to obtain superconductivity in the electron-doped Pr_(2-x)Ce_(x)CuO_(4-delta). I investigated the effects of oxygenation in this material using resistivity and Hall measurements. For various oxygen contents, I was able to determine that there is a separable doping and a disorder contribution to the superconducting transition temperature. I was able to quantitatively separate out these two effects and show that these two effects are opposite with regards to changes in T_(c) for overdoped thin films. The disorder component is roughly twice as large as the doping component. This analysis is also shown to be self consistent in demonstrating that the doping component of oxygen variation follows the trends of Cerium doping.

For the diluted magnetic semiconductor Ti_(1-x)Co_(x)O_(2-delta), I investigated the intrinsic nature of the ferromagnetism observed in thin films. Hall effect measurements were used as the technique because ferromagnetic materials exhibit an anomalous Hall effect, which is due to an interaction between the charge carriers and the magnetic moments. I found that low carrier concentration anatase phase films did not exhibit an anomalous Hall effect, whereas high carrier concentration rutile phase films do. The presence of the anomalous Hall effect at this point cannot be attributed to an intrinsic ferromagnetism as cobalt clusters are observed in these films.