Ferromagnetic quantum criticality and superconductivity in Yb-based heavy-fermion systems

铁磁量子临界性和非常规超导现象的发生是强相关电子系统研究的中心课题。对于具有三价Yb的重费米子系统，这是一个实验挑战，因为显示这些现象的基于Yb的化合物的数量非常有限。这与现有的大量反铁磁近道晶格系统和在ce基重费米子材料中已确定的超导性形成鲜明对比。造成这种不平衡的一个重要原因是由于Yb的高蒸气压，这使得这些材料的晶体生长非常困难。因此，在两位作者的参与下，最近在YbNi4P2中发现了铁磁量子临界性，并在YbRh2Si2中出现了超导性，这是实验研究yb基重费米子金属中这些现象的重要一步。本研究计划的主要目标是单晶生长和接近磁量子临界点的yb基重费米子化合物的低温表征。我们将重点讨论铁磁量子临界以及量子临界点附近超导现象的发生。为此，我们将详细研究(i) YbNi4P2和相关取代系列，（ii） Yb(RhCo)2Si2和（iii） YbRh2Si2单晶。将采用几种晶体生长技术来生长具有低缺陷和可重复物理性质的非常纯净的单晶，这些单晶将通过最先进的低温物理测量来表征。

Ferromagnetic quantum criticality and the occurrence of unconventional superconductivity are central research topics among strongly correlated electron systems. For heavy-fermion systems with trivalent Yb, this is an experimental challenge, because the number of Yb-based compounds showing these types of phenomena is very limited. This is in stark contrast to the huge number of existing antiferromagnetic Kondo-lattice systems and the established occurrence of superconductivity in Ce-based heavy-fermion materials. One important reason for this imbalance is due to the high-vapor pressure of Yb, which makes the crystal growth of these materials very difficult. The recent discoveries of ferromagnetic quantum criticality in YbNi4P2 and the emergence of superconductivity in YbRh2Si2, under the participation of both proposers, was therefore an essential step forward to experimentally investigate these phenomena in Yb-based heavy-fermion metals.The main objectives of this research proposal are the single crystal growth and the low-temperature characterization of Yb-based heavy-fermion compounds close to a magnetic quantum critical point. We will focus on ferromagnetic quantum criticality as well as the occurrence of superconductivity in the vicinity of the quantum critical point. For this purpose, we will investigate in detail (i) YbNi4P2 and related substitution series, (ii) Yb(RhCo)2Si2 and (iii) YbRh2Si2 single crystals. Several crystal growth techniques will be employed to grow very pure single crystals with low amount of defects and reproducible physical properties, which will be characterized by state-of-the art low-temperature physical measurements.