Cerium-ruthenium based compounds: A source of materials for strongly correlated electron physics

The phenomena of strongly correlated electrons have been at the forefront of contemporary research in condensed matter physics for more than four decades. Over the years this study field has remained resourceful in providing a wealth of new physics and has compelled new thinking in many aspects of our understanding of the behavior of electrons in metals. Underpinning the vigor with which the field has kept on reinventing itself has been an ever-broadening materials class that convey the fascinating physics of correlated electrons in one guise or another. One particular group of compounds stem from the combination of the chemical elements cerium and ruthenium. The element cerium has been one of the most profitable starting points from which to synthesize compounds that show interesting and often anomalous magnetic, structural and correlated electron behavior. Among the key ingredients for characterizing these compounds is the nearest-neighbor Ce−Ru separation in the crystal lattice. Several examples are found in which this distance is notably shorter than the sum of the Ce and Ru covalent bonding radii, and this leads to hybridization of the normally well-localized 4f−electron orbital of Ce, with degenerate conduction electron bands. In this way an unstable or non-integer valence is produced on the Ce ion, which is responsible for many correlated electron features and a variety of magnetic phenomena that range from long-range order, through metamagnetism and phenomena at the edge of magnetism, to quantum criticality when order is suppressed to the absolute zero of temperature. Here we review results of a selected number of cerium-ruthenium compounds as examples with extraordinary physical properties. We highlight the commonalities of strongly correlated electron behavior in these compounds, in an effort to guide and expand the materials base for future studies.