In rare earth and actinide materials, hybridization between localized f-electron and conduction-electron states yields novel phenomena such as heavy fermion behavior, non-Fermi liquid behavior, Kondo insulating behavior and superconductivity. The objectives of this research program are to characterize unconventional electronic ordered phases found in f-electron materials, determine the conditions under which they are formed, identify underlying microscopic mechanisms, and test relevant theories. These goals are sought experimentally through the measurement of the transport, thermodynamic, and magnetic properties of single crystals and thin films of novel rare earth and actinide compounds at low temperatures (down to the mK range), high magnetic fields, and high pressures. This investigation features three main thrusts: non-Fermi liquid behavior, unconventional superconductivity and other ground states, and the search for new correlated electron materials. Materials currently at the core of these studies include the CeMIn5 compounds, the Pr-based filled skutterudites, and U-based intermetallics such as URu2Si2. The study of strongly correlated electron phenomena in this laboratory serves to train the next generation of condensed matter and materials physicists who will contribute to academic, industrial, and governmental arenas throughout their careers. In addition to improving basic understanding of correlated electron behavior, these studies may also directly lead to technological advances such as thermoelectric refrigeration and magnetic information storage.

Nontechnical Abstract

The objective of this research program is to experimentally characterize the class of unusual strong interactions between electrons referred to as "correlated electron" behavior. Correlated electrons lead to a variety of interesting effects, such as superconductivity and unconventional magnetism. These phenomena are often observed in "f-electron" materials, those composed of rare earth or actinide elements, which include cerium and uranium. This investigation encompasses the synthesis of these f-electron materials and the subsequent experimental measurement of their electrical and magnetic properties. Via this approach, the different types of behavior attributed to correlated electron effects will be analyzed with the goal of better describing these effects and ultimately understanding the various conditions under which they occur. This investigation further involves a search for new materials exhibiting correlated electron behavior. This program serves to train the next generation of condensed matter and materials physicists who will contribute to academic, industrial, and governmental arenas throughout their careers. In addition to forming a deeper understanding of correlated electronic behavior, the knowledge developed from these studies may also lead to new advanced sensors, devices, and technologies for use in the energy, information, environmental, medical, and defense sectors.

Project Report

M. Brian Maple June 18, 2012 Many of the fundamental properties of metals are governed by electrons and their interactions. For example, a metal's ability to conduct an electrical current or whether it can be a permanent magnet is partially determined by how strongly electrons interact with each other. Some properties are particularly difficult to predict theoretically so one effective way to study these electronic phenomena is to make new materials and tune them by putting them in magnetic fields, squeezing them by applying pressure, and adding small concentrations of other elements. Some of the most interesting physical phenomena include superconductivity, a phenomenon wherein a material has zero electrical resistance and expels small magnetic fields when cooled below a material specific critical temperature, and heavy fermion behavior, in which electrons behave as if they were much more massive than typical electrons by factors of several hundred. Many of the materials that have been responsible for recent advances in physics and technology are multinary (consisting of three or more elements) rare earth and actinide compounds in which the localized (attached to a specific ion) d-and f-electrons are admixed with conduction electrons (free to move within the metal). Examples of such materials that were investigated in this research program include a unique correlated electron state in Ce1-xYbxCoIn5, enhancement of the mysterious "hidden order" phase boundary in the URu2-xFexSi2 system, development of a small magnetic signature indicative of an unconventional type of superconductivity in the systems Pr(Os1-xRux)4Sb12 and Pr1-yLayOs4Sb12, anisotropy of superconductivity and magnetism in LnFePO (Ln = La, Pr, and Nd) single crystals, multiple magnetic states in Yb2Co12P7, and unusual heavy fermion behavior in the ferromagnetic samarium compounds Sm2Fe12P7 and SmOs4Sb12. Our studies of the physics of strongly correlated electron systems impact science and engineering research in many beneficial ways. The physical properties of f-electron materials are measured as functions of temperature, magnetic field, applied pressure, and chemical substitution, yielding important clues that shed light on the underlying mechanisms of phenomena such as superconductivity. Unraveling what these various materials share in common and what aspects of their behavior are system specific will lead to basic scientific insights with important practical relevance to developing functional materials for technological applications. General studies of magnetism and superconductivity will likely continue to benefit substantially from this research, and increased understanding of these phenomena may help light the way toward technological advances such as more powerful permanent magnetic materials, denser magnetic storage media, high efficiency thermoelectric materials, and superconductors with yet higher values of the superconducting critical temperature and critical current density (the maximum amount of current per unit cross-sectional area a wire can carry and still remain superconducting).

National Science Foundation (NSF)
Division of Materials Research (DMR)
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Daniele Finotello
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University of California San Diego
La Jolla
United States
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