A study of unconventional transport in electron-doped oxide superconductors

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Non-Technical Abstract:Superconductivity, the complete absence of electrical resistance, is an amazing low temperature property of some elements and compounds. This quantum property of solids was discovered in 1911 and was believed to be fully understood in 1957 by the Bardeen, Cooper, Schrieffer (BCS) theory. In 1987 some copper oxide compounds (cuprates) were discovered with unexpectedly high superconducting transition temperatures (up to 140 Kelvin). This high-temperature superconductivity cannot be explained by the conventional BCS mechanism and it is not presently understood. A complete understanding of high-temperature superconductivity is one of the major unsolved problems of condensed matter physics. The solution to this problem could lead to the discovery of room temperature (300 Kelvin) superconductors, a development with very significant practical applications. This project focuses on experimental transport studies on one class of cuprate compounds where the superconductivity can be completely suppressed by the application of a small magnetic field. This enables the normal metallic state to be studied over a full range of temperature from 300 K to well below 1K. An understanding of the normal metal state is believed to be crucial for a full understanding of the origin of the superconductivity. The transport experiments are planned on specially prepared thin films with variation of the magnetic field, the temperature and electron doping, all of which change the normal state properties in ways that are expected to give deep insight into the nature of the superconductivity. This project supports the education of PhD and undergraduate students at the University of Maryland---an urban university with a diverse population---in advanced vacuum deposition and electronic characterization techniques. These techniques have proven to be excellent training for productive scientific careers in academic and technology settings.Technical Abstract:Understanding the mechanism of superconductivity and the non-Fermi liquid normal state in strongly correlated copper oxides (cuprates) is one of the most significant unsolved problems of condensed matter physics. Much progress has been made, but there are still many puzzles to be solved. In the last few years some dramatic new experimental results have been found that give new insights into the physics of the hole-doped cuprates. Since hole- and electron-doped cuprates should obey the same fundamental physics, the goal of this project is to gain a more detailed understanding of the normal state and superconducting properties of the electron-doped (n-doped) cuprate superconductors. This new information should allow the mechanism of the unconventional superconductivity in all cuprates to be determined. A comprehensive set of experiments helps understand the nature of the normal state when superconductivity is suppressed by a magnetic field. The n-doped cuprates are particularly advantageous for this because the low temperature normal state (0 T Tc) can be reached with a modest dc magnetic field (H 10 T). An understanding of the nature of the normal state of the cuprates is believed to be crucial for understanding the cause of high-Tc superconductivity in both n- and p-type cuprates. The experimental techniques to be used are: resistivity, Hall Effect, magnetoresistance, Nernst effect, thermopower, and tunneling. Some transport experiments are performed at very high magnetic field at the NSF supported high magnetic field labs in Tallahassee and Los Alamos. A study of the superconducting state in the over doped region of the electron-doped phase diagram is also to be done, since anomalous behavior recently found there is suggestive of fundamentally new physics. With outside collaborators, penetration depth, high-field Nernst and thermal diffusivity experiments are attempted. This project incorporates the training of high-school students, undergraduate science majors, graduate students, and postdoctoral scientists in various experimental aspects of condensed matter physics research. As in the past, the principal investigator encourages women and underrepresented groups to participate in this project. The external collaborations in this project provide a unique vehicle for students to experience research in different laboratory environments, i.e., university, industry, and government. An ongoing participation in the Graduate Resources Advancing Diversity (GRADMAP) program at the University of Maryland involves undergraduates in research exposure programs designed to attract a broader audience to graduate studies. Efforts to encourage a broader education in science via existing Physics department outreach programs with predominately minority public-school students near the University of Maryland are continued. These include, Physics is Phun, Physics Discovery Days, and the Summer School Girl's Program.