Role of the tuning parameter at magnetic quantum phase transitions

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Strongly correlated electron systems can be driven via various routes to a magnetic quantumphase transition (QPT) occurring for temperature T → 0. The most extensively studied routeto quantum criticality in f-electron systems is the competition between RKKY interaction andKondo effect that can be affected by different tuning parameters, such as hydrostaticpressure, chemical doping, or external magnetic field. Here the system CeCu6-xAux isprototypical and thus has been investigated thoroughly. These investigations have led to thediscovery of a new type of quantum criticality not explicable in terms of the “conventional”Hertz-Millis-Moriya approach. More recently, we showed that the tuning parameter plays akey role in determining the scaling functions of the critical fluctuations that can be determined by inelastic neutron scattering. These findings have prompted new important questions to be addressed in this project. Do the different scaling functions signal a complex phase diagram for T → 0 ? Are there “subdominant” effects that determine quantum criticality? For instance, the strength of the Kondo effect can be affected by crystalline electric field excitations and/or magnetic anisotropies. This issue will be studied with the isostructural antiferromagnets CeAu2Ge2 and CeAg2Ge2 that are promising candidates for exploring this influence. Another parameter determining the formation of a magnetic vs. non-magnetic ground state is geometrical frustration, which tends to suppress magnetic order, either globally or locally. An example of the latter is the hexagonal antiferromagnet CePdAl. Finally, the role of disorder will be investigated by comparing the low temperature properties of CeCu6-xAux with those of Ce1-y LayCu6 and Ce1-y LayCu5Au.