Colloquium on May 25, 2010

Gertrud Zwicknagl
Institut für Mathematische Physik, TU Braunschweig

The f-electron's "double life"

When atoms form a solid, the fate of their outer-shell electrons is determined qualitatively by a trade-off between kinetic and potential energies that leads to the formation of dispersive bands. This simple picture, combined with Pauli's exclusion principle, forms the basis for understanding why solids are metallic, semiconducting or insulating and why solids crystallize in particular structures. Our present understanding of the 4f- and 5f -materials, on the other hand, appears incomplete. These materials exhibit highly complex and anomalous behaviour which arises from the very strong repulsive Coulomb interactions among the electrons in the partially filled inner shells. The rich spectrum of physical states and exotic behaviours pose the grandest challenging questions for today's condensed matter physics. The present talk focuses on the “dual nature” of f-electrons which is a consequence of the extremely strong correlations. Compelling experiments on cerium compounds show that the single 4f-electron of cerium should be considered as localized at room temperature. With decreasing temperature, however, it may ‘dissolve’ into a highly correlated state. While the low-energy excitations are itinerant heavy quasi-particles, the high-energy excitations still reflect the localized nature of the 4f state. Similarly, low-temperature experiments on correlated uranium compounds find that electrons in uranium's 5f-orbital simultaneously participate in magnetic ordering of localized electrons and in forming a correlated band of itinerant electrons out of which unconventional superconductivity emerges.