Role of the tuning parameter at magnetic quantum phase transitions

强关联电子系统可以通过各种途径驱动到温度T→为0时发生的磁量子相变。F电子系统中最广泛研究的量子临界性途径是RKKY相互作用和Kondo效应之间的竞争，而RKKY相互作用和Kondo效应可以受到不同的调谐参数的影响，如流体静压、化学掺杂或外部磁场。在这里，CeCu6-xAux系统是典型的，因此已经被深入研究。这些研究导致了一种新类型的量子临界性的发现，这种临界性不是“传统的”赫兹-米利斯-莫里亚方法所能解释的。最近，我们证明了调谐参数在确定可由非弹性中子散射确定的临界涨落的标度函数中起着关键作用。这些发现促使这个项目提出了新的重要问题。不同的标度函数是否表示T→0的复杂相图？是否存在决定量子临界性的“次优”效应？例如，近藤效应的强度可能受到晶态电场激发和/或磁各向异性的影响。这个问题将用等结构反铁磁体CeAu2Ge2和CeAg2Ge2来研究，它们是探索这种影响的有前途的候选者。决定磁性基态与非磁性基态形成的另一个参数是几何受挫，它倾向于全局或局部地抑制磁秩序。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-yLayCu6和Ce1-yLayCu5Au的低温性质，研究了无序的作用。

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.