Scaling the Kondo lattice

The two types of magnetic order that appear in metals have very different origins: 'itinerant-electron' magnetism occurs where a spin-density wave instability in a sea of itinerant electrons causes magnetic order, and 'local-moment' magnetism occurs where magnetic order comes from an instability in a liquid of local atomic magnetic moments. A class of strongly correlated electron materials exist, the heavy-electron intermetallic compounds such as alloys containing Ce or Yb, that bridges these two extremes, with itinerant-electron magnetism reigning at low temperature and local-moment magnetism at high temperature. It has been a long-standing goal to quantitatively describe the transition between the two and to discover what determines its temperature scale. Yang et al. now report a simple semiquantitative solution to the latter problem that provides a new framework for interpreting the physics of heavy electron materials. This offers the prospect to quantitatively determine the physical origin of some of the remarkable properties, such as their superconducting behaviour.