Role of the tuning parameter at magnetic quantum phase transitions

强相关电子系统可以通过各种途径驱动到温度T→0时发生的磁量子相变（QPT）。在f电子系统中，最广泛研究的量子临界路线是RKKY相互作用和近藤效应之间的竞争，这种竞争可能受到不同调谐参数的影响，例如静水压力，化学掺杂或外部磁场。在这里，CeCu6-xAux系统是典型的，因此已经进行了彻底的研究。这些研究导致了一种新型量子临界的发现，这种量子临界不能用“传统的”赫兹-米利斯-莫里亚方法来解释。最近，我们证明了调谐参数在确定临界波动的标度函数中起着关键作用，而临界波动可以由非弹性中子散射确定。这些发现引发了本项目需要解决的新的重要问题。不同的比例函数是否表示T→0的复杂相图？是否存在决定量子临界性的“亚显性”效应？例如，近藤效应的强度可以受到晶体电场激励和/或磁各向异性的影响。这一问题将用同结构反铁磁体CeAu2Ge2和CeAg2Ge2进行研究，它们是探索这种影响的有希望的候选者。另一个决定磁性基态与非磁性基态形成的参数是几何挫折，它倾向于抑制磁性秩序，无论是全局还是局部。后者的一个例子是六方反铁磁体CePdAl。最后，通过比较CeCu6-xAux与Ce1-y LayCu6和Ce1-y LayCu5Au的低温性能，探讨无序对CeCu6-xAux的影响。

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.