Investigation of the strange metal normal state of electron-doped oxide superconductors

University Of Maryland, College Park, College Park MD

Investigators

Abstract

Non-technical abstract Superconductivity, the complete absence of electrical resistance, is an amazing low temperature property of some elements and compounds. This 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 performs experimental studies on one class of cuprate compounds where the superconductivity can be completely eliminated by the application of a small magnetic field. This enables the non-superconducting, or normal, metallic state to be studied over a wide range of temperature from 600 K to well below 1K. The normal state of the cuprates has quite different physical properties from those of well-known metals, such as copper or lead, and the cuprates have been called strange metals. An understanding of this strange metal state is believed to be crucial for an understanding of the origin of the superconductivity. In this project a variety of transport and other experiments are performed on specially prepared thin films. Variation of the magnetic field, the temperature and electron doping can change the normal state properties in ways that are expected to give deep insight into the nature of the strange metal state and 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 electrical 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 (strange metal) normal state in strongly correlated oxides is one of the most significant unsolved problems of condensed matter physics. Recently some dramatic experimental results have been reported by the PI (and others) that give new insights into the physics of the copper oxides (cuprates) and that also report the discovery of superconductivity in a related nickelate system, Sr doped NdNiO2. This project follows up on these breakthroughs to gain a more detailed understanding of the strange normal state of the electron-doped cuprates and the doped nickelates. This project provides a comprehensive set of experiments to study the nature of the normal state when superconductivity is suppressed, either by magnetic field, disorder or critical current. The electron-doped cuprates are particularly advantageous for this research 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-temperature superconductivity in the cuprates. The experimental techniques used are resistivity, Hall Effect, magnetoresistance, Nernst effect, thermopower, specific heat and strain (pressure). Some transport experiments are done at very high magnetic field at the NSF supported National High Magnetic Field Laboratory (NHMFL) in Tallahassee and Los Alamos. The research includes an in-depth study of the itinerant ferromagnetism discovered by the PI in 2019 in the highly doped region of the n-type cuprates The combination of materials science expertise and physics measurement expertise of the PI is essential for making progress in this very important area of correlated physics materials research. 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 will encourage 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 will involve 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 will be continued. These include, Physics is Phun, Physics Discovery Days, and the Summer School Girl’s Program. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.