magnetic, and structural degrees of freedom. The presence of oxygen is particularly critical to the creation of exotic properties—the orbital energies of the oxygen ion are well matched to transition metal orbital energies, yielding the added complexity of a balance between covalent and ionic bonding that differs from compound to compound. Finally, the relatively straightforward methods generally used in the synthesis of oxides and their chemical stability make them widely available to the condensed-matter and solid-state chemistry communities, resulting in vigorous worldwide research on many different compound systems. Examples of challenges that drive this field are the quest for room-temperature superconductivity and the production of a room-temperature ferromagnetic insulator. There is also a vigorous effort devoted to realizing qualitatively new magnetic phases such as two-dimensional critical spin liquids.
Intermetallics are materials made of combinations of two or more metallic elements. When one of the elements is a magnetic rare-earth or actinide metal (e.g., cerium and uranium), such crystalline systems embody the concentrated limit of the Kondo effect, in which the conduction electron forms a pair with the magnetic ion in a lattice periodic fashion. In this many-body limit, novel “heavy” electron states emerge. These materials constitute the frontier of exploration into the possibility of conducting states of matter that are qualitatively different from the conventional Landau Fermi-liquid, which describes metals such as copper and aluminum. Intermetallic compounds are widely used, though for different reasons, in today’s mechanical and electrical systems.