Ordering Phenomena in Transition Metal Oxides

Quantum criticality in heavy fermion compounds   

Matter at the absolute zero in temperature may reach a highly exotic state: Where two distinctly different ground states are separated by a second order phase transition the system is far from being frozen; it is undecided in which state to be and therefore undergoes strong collective quantum fluctuations. Quantum criticality describes these fluctuations and their extension to finite temperature. Quantum critical behavior has been reported in systems as distinct as heavy fermion compounds, high-temperature superconductors, metamagnets, multilayer ³He films, or single-molecule quantum dots. Heavy fermion compounds have in the past few years emerged as prototypical systems. The talk deals with the recent discovery of a new energy scale in one such system, YbRh₂Si₂, that vanishes at the quantum critical point and is in addition to the second-order phase transition scale that governs the behavior of conventional quantum critical points. Completely new theoretical approaches are called for to describe this situation. To put this result on a broader experimental basis attempts to identify other compounds with similar behavior are on the way and one such candidate, Ce₃Pd₂0Si₆, shall be discussed.