Electronic Transport Measurements in the Electron-Doped High-Temperature Superconductor Pr2-xCexCuO4-d

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Bach, Paul Leonard
Greene, Richard L
This thesis is composed of four major parts centered around the electron-doped superconductor Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub>: angular magnetoresistance studies of antiferromagnetism, doping effects of oxygenation, Tc enhancement by the creation of superlattices, and comparison of high-temperature resistivity with the pnictides. The first part focuses on transport measurements of the magnetism in Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub> and La<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub>. It was found that the thermal evolution of the angular dependence of magnetoresistance in Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub> can be used to determine the Neel temperature in that material. This angular magnetoresistance technique was also applied to La<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub>; evidence for antiferromagnetism in this system was observed as well. This section additionally develops angular magnetoresistance as a useful probe in other cuprate projects here described. The second part investigates over-oxygenated and irradiated Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub>, in under- and optimal dopings. Resistivity, Hall effect, and angular magnetoresistance measurements show oxygen both doping and disordering the system, in agreement with over-doped films. The evolution of both the Hall effect and angular magnetoresistance shows that over-oxygenation results in significant changes in the number of charge carriers, regardless of the increase in scattering incurred. Additionally, this study indicates that annealing primarily removes apical oxygen, rather than other proposed behaviors. The third part studies multi-layer films of Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub> and La<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub>. These superlattices exhibit a significant Tc enhancement over component layers' Tc s. Interface effects are excluded as a source of this Tc increase based upon critical current measurments. The Tc enhancement is found to be due to charge redistribution. Based on Hall and angular magnetoresistance measurements, the result of this redistribution is slightly net-under-doped films. The fourth part uses Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub> and Nd<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub> as comparisons to in- vestigate the high-temperature resistivity behavior in the SrFe<sub>2</sub>As<sub>2</sub> system. Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub> and Nd<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub> are shown to be similar to other cuprate systems in that they strongly violate the Mott-Ioffe-Regel limit, an indication of strong correlations. The SrFe<sub>2</sub>As<sub>2</sub> system was found to saturate near the Mott-Ioffe-Regel resistivity, ruling out strong, cuprate-like correlations in the pnictide superconductors. Finally, some aborted attempts at synthesis of thin films of superconducting Pr<sub>2</sub>CuO<sub>4</sub> and iron-based superconductors are discussed. Pr<sub>2</sub>CuO<sub>4</sub> is suggested to be an important system for understanding oxygenation in Pr<sub>2-x</sub>Ce<sub>x</sub>CuO<sub>4&plusmn;&delta;</sub>.