International Research Fellowship Program: Microscopic Role of Magnetism in High Temperature Superconductivity

Kavich Jerald J, Chicago IL

Investigators

Abstract

0853415 Kavich This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). The International Research Fellowship Program enables U.S. scientists and engineers to conduct nine to twenty-four months of research abroad. The program's awards provide opportunities for joint research, and the use of unique or complementary facilities, expertise and experimental conditions abroad. This award will support a twenty-four-month research fellowship by Dr. Jerald J. Kavich to work with Dr. Pietro Gambardella at the Institut Catala de Nanotechnologia in Spain. This project examines the local electronic and magnetic structure of substitutional magnetic impurities in high-temperature superconducting materials. Magnetism plays a non-trivial, but elusive role in unconventional superconductivity. Previous impurity studies have concentrated on the macroscopic properties of the superconductors, leaving the impurities themselves an unexplored area and permitting their electronic structure and symmetries to be oversimplified in theoretical models. To address these issues, focus is being directed toward three key aspects related to a realistic description of the impurity, including determination of the electronic ground state configuration, spatial dependence of orbital occupancies, and anisotropy of the magnetic order in all major phases exhibited by high-temperature superconducting materials, i.e. the superconducting phase, the pseudo-gap phase, and the normal state. The investigation of these advanced materials is being accomplished using variable polarization soft x-ray spectroscopy, requiring state-of-the-art, ultra-bright synchrotron facilities. Temperature and field dependent studies are being conducted with both circularly and linearly polarized x-rays to map the magnitude of the local impurity moment and to determine anisotropies in the orbital occupancies and magnetism. In combination with experimental methods, advanced calculations using multi-configurational multiplet techniques, are being carried out to obtain a quantitative analysis of experimental data allowing for correlations between micro- and macroscopic properties of high-temperature superconductors and the impurity. The spectroscopic study of Mn, Fe, Co, and Ni substitutional impurities in materials such as Bi2Sr2Ca1Cu2O8 is expected to provide fundamental insight into their valence state and related local moment, settling a long standing debate about the degree of hybridization of the impurity states, and screening or coupling of their moment with the environment. Also, for the first time, the behavior of the spin and orbital impurity magnetization will be separately monitored across the superconducting transition temperature of a doped superconductor, illuminating magnetic effects on electron paring processes. Finally, by measuring the sample in different fields, we plan to determine the impurity-selective magnetization curves. These are normally masked below the transition temperature in superconductors by the large diamagnetic signal measured in standard magnetometry experiments. This proposal promises not only to advance superconductor physics, but also to increase the fundamental knowledge of isolated magnetic atoms in a strongly correlated environment. The development and incorporation of superconducting materials into new and existing technologies is expected to have a profound impact on global environmental, industrial, and economic factors. The benefits are extensive, ranging from the improved reliability of the lossless transportation and storage of electrical energy, to aspirations of quantum computing with superconducting qubits. The fundamental research in this proposal is consistent with core aspects of long-term scientific research goals in the U.S.