1. Doping and Disorder in the Oxygenated, Electron-doped High-temperature Superconductor Pr 2-x Ce x CuO 4±  The building blocks of the high-temperature superconductors are two-dimensional copper-oxygen planes separated by charge reservoirs. In a hole-doped compound, La 2-x Sr x CuO 4, superconductivity is achieved by substituting Sr for La, or through changing the oxygen content in the undoped (x=0) compound. However in the electron-doped cuprate Pr 2- x Ce x CuO 4± , neither substitution nor oxygen reduction alone result in superconductivity. The need for oxygen reduction is still not clear. For example, it is not clear where the oxygen are removed during reduction. Additionally, it is not clear whether the effects of oxygen reduction on the transport properties (i.e., resistivity, Hall effect, T c, etc.) are due to disorder or charge carrier doping. In our research, we have separated out the effects on the superconducting transition temperature T c due to carrier doping and disorder when the oxygen content is changed in Pr 2-x Ce x CuO 4± . In the figure, the black (square) data show how T c changes in optimally prepared thin films with various cerium contents. The blue (down triangle) data show how T c decreases in cerium-doped films (x=0.17) as oxygen increases. The red data show the oxygenated x=0.17 films after the effects from disorder are subtracted. This figure shows that a change in the oxygen content has a doping effect (red data) on T c similar to that of cerium-doping (black data), as well as a dominant disorder effect. We expect that this new understanding of doping and disorder will help us understand the cause of high-temperature superconductivity. J. S. Higgins et al., Phys. Rev. B 73, 104510 (2006) R. L. Greene, University of Maryland, DMR-0352735 Change in T c versus the change in the Hall coefficient (  R H ). The Hall coefficient is used as a measure of the carrier density. The black data (squares) show how T c changes in optimally prepared cerium-doped thin films. The blue data (down triangles) shows how T c changes in x=0.17 cerium-doped films as the oxygen content increases (to the left of zero). The red data (up triangles) show a positive doping contribution to T c, which follows the trends of cerium doping. Disorder dominates, however, as can be seen in the blue data.

2. Education One undergraduate ( Matt Barr), one graduate student (Joshua Higgins) and one postdoc (Yoram Dagan) contributed to this work. Matt Barr has been accepted at Harvard for graduate studies in Physics and will begin his studies in the Fall of 2006. Joshua Higgins received his Ph.D. in 2006 and is currently looking for a post- doc position. Yoram Dagan is now an Assistant Professor at Tel Aviv University. Societal Impact An understanding of the mechanism causing high temperature superconductivity may enable the development of new materials that are superconducting above room temperature. This would have a large impact on electronic devices and electricity generation and distribution. R. L. Greene, University of Maryland, DMR-0352735 Doping and Disorder in the Oxygenated, Electron-doped High-temperature Superconductor Pr 2-x Ce x CuO 4± 