Revival of the Kondo effect

33 WHY would anyone still want to study a physical phenomenon that was discovered in the 1930s, explained in the 1960s and has been the subject of numerous reviews since the 1970s? Although the Kondo effect is a well known and widely studied phenomenon in condensedmatter physics, it continues to capture the imagination of experimentalists and theorists alike. The effect arises from the interactions between a single magnetic atom, such as cobalt, and the many electrons in an otherwise non-magnetic metal. Such an impurity typically has an intrinsic angular momentum or “spin” that interacts with the electrons. As a result, the mathematical description of the system is a difficult many-body problem. However, the Kondo problem is well defined, making it an attractive testing ground for the new numerical and analytical tools that have been developed to attack other challenging many-body problems. Interest in the Kondo effect has therefore persisted because it provides clues to understanding the electronic properties of a wide variety of materials where the interactions between electrons are particularly strong, for instance in heavy-fermion materials and hightemperature superconductors. Physicists’ fascination with the phenomenon has continued since it was first explained by Japanese theorist Jun Kondo in 1964. In fact, interest in the Kondo effect has recently peaked thanks to new experimental techniques from the rapidly developing field of nanotechnology, which have given us unprecedented control over Kondo systems.