Quantum critical behavior in Kondo systems

This article briefly reviews three topics related to the quantum critical b ehavior of certain heavy-fermion systems. First, we summarize an extended d ynamical mean-field theory for the Kondo lattice, which treats on an equal footing the quantum fluctuations associated with the Kondo and RKKY couplin gs. The resulting dynamical mean-field equations describe a Kondo impurity model with an additional coupling to vector bosons. Two types of quantum ph ase transition appear to be possible within this approach the first a conve ntional spin-density-wave transition, the second driven by local physics. F or the second type of transition to be realized, the effective impurity mod el must have a quantum critical point exhibiting an anomalous local spin su sceptibility. In the second part of the paper, such a critical point is sho wn to occur in two variants of the Kondo impurity problem. Finally, we prop ose an operational test for the existence of quantum critical behavior driv en by local physics. Neutron scattering results suggest that CeCu6-Au-x(x) passes this test.