Percolation Physics in doped and stochiometric Quantum Critical Systems

We argue that spontaneous fragmentation of the magnetic lattice in strongly correlated electron systems close to the quantum critical point accounts for most of the observed non-Fermi liquid behavior. Upon cooling, magnetic fragmentation of the Kondo lattice is caused by a distribution of Kondo temperatures, which in turn originate from small variations in interionic separations (0.05-0.1 A). This temperature-dependent fragmentation transforms the magnetic lattice to a percolation system resulting in the creation of isolated clusters which dominate the low-temperature response of quantum critical systems. We argue the validity of this new scenario for the physics near a QCP using literature data on systems where interionic distances have been modified by means of chemical doping (e.g., UCu₄Au) as well as stoichiometric systems where zero-point phonons are responsible for such variations (e.g., CeRu₂Si₂). The percolation physics describing QCP-systems represents a new universality class (see Fayfar et al.). This new class appears to violate the Harris criterion, thereby providing a natural explanation for the lack of universality of the critical exponents observed in QCP-systems.