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330 Handbook on the Physics and Chemistry of Rare Earths parameter creates special difficulties. After all, disorder can fundamentally disturb the basic properties of the material to be tuned. This is particularly true for strongly correlated electron systems. Examples of such are crystal and band-structure changes, the formation of inhomogeneous and nonrandom clusters, unwelcome magnetic behavior, a metal-to-insulator transition, etc. Hence disorder tuning is a delicate procedure and must be carefully controlled via metallurgical and chemical analyses. Although disorder can create NFL behavior, the latter can be taken incorrectly as an indication of a QCP. The prime example here is the Y1xUxPd3 pseudobinary alloy, an original NFL without a well-defined ground-state or a QPT (Seaman et al., 1991). Fur- thermore, magnetic disorder can give rise to a low-temperature SG phase (Mydosh, 2015). This can easily mask the putative QPT by smearing out the QCP into a region of glassy dynamics. Such effects then pose the question of a percolation transition (Stauffer and Aharony, 1994) caused by lattice or magnetic disorder creating a QPT. Percolation occurs intrinsically from massive disorder and random- ness. At the critical percolation concentration, a phase transition occurs at zero temperature, does this percolation transition represent a QPT and is the critical concentration a QCP? At the present stage of experimental investigation, the percolation behavior does not clearly correspond to a QPT, eg, the case of Pd1xNix (Kalvius et al., 2014). And such a result illustrates the stringent challenge of doping disorder as a tuning parameter for a QPT. The theoretical situation, on the other hand, is at present confused, even for “clean” transitions tuned by magnetic field or pressure. Sure, there exists the HM theory of itinerant electrons that provides satisfactory answers for a large group of materials, such as CeNi2Ge2. However, many com- pounds display clear characteristics that deviate from HM theory, for which only phenomenological theories can currently provide insights. Microscopi- cally, a fundamental unanswered question is the Kondo lattice problem, as well as the theory of quantum critical fluctuations interacting where the electrons cannot be integrated out. Answers to those purely theoretical ques- tions are needed before one can even start to understand specific materials. Luckily, this makes the field of quantum criticality in heavy fermion mate- rials one of the most active and entertaining areas of condensed matter physics. ACKNOWLEDGMENTS We gratefully acknowledge discussions with Elihu Abrahams, Piers Coleman, Philipp Gegenwart, Hilbert von L€ohneysen, and Rina Takashima. This work has been supported through an NWO Rubicon fellowship (L.R.).PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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