Single Crystal Synthesis And Physical Properties Of The Kondo Lattice Compound YbCo₆Ge₆

Quantum phase transition has attracted enormous research interests, while heavy fermion (HF) system has been proved to be an ideal touch stone to study it. In HF system, the strength of the microscopic interactions can be tuned efficiently by utilizing various experimental methods, e.g. pressure, doping and magnetic or electrical field. Under proper tuning conditions, novel quantum phenomenon like quantum critical behavior can emerge.Although considerable examples of quantum criticalities have been discovered in Ce- and U- based compounds, Yb-based materials rarely display quantum criticality Moreover, the quantum criticalities in few certain examples are quite unconventional, as in the case of YbRh2Si2.In YbRh2Si2,an antiferromagnetic (AFM) quantum critical point(QCP) can by induced by applying a small magnetic field, where the Kondo effect is destructed simultaneously, and its physical properties cannot be understood by the theory of the conventional SDW-type QCP model. Therefore, searching for new Yb-based materials which displays quantum criticality might provide exclusively important insights to an universal description of quantum phase transition.In this thesis, we present the synthesis of YbCo6Ge6 compound and the study of its physical properties. The contents of this thesis will be divided into three chapters:Chapter.1 briefly introduces the history of HF compounds and the basic knowledge of Kondo effect, RKKY interaction, and the competition between them. We also discussed the exotic phenomenon in the vicinity of QCP.Chapter.2 describes the experimental techniques used in this thesis, including the experimental methods for sample synthesis and physical properties measurements.Chapter.3 presents the physical properties of YbCo6Ge6 and its doped compounds. Single crystals of YbCo6Ge6 and LuCo6Ge6 have been grown successfully for the first time with excessive Sn as a flux. Structural identification followed by other physical properties experiments, such as magnetic, thermal, and transport properties measurements have been performed and studied. displays broad and large enhancement of specific heat coefficient developed below 10K down to 1K, followed by broad peak-like feature below 1K with the peak position of 0.55K. This enhancement in specific heat coefficient with similar divergent behavior in magnetic susceptibility in the same temperature range resembled those observed in YbRh2Si2 and YbCo2Ge4 compounds which are Yb-based heavy fermion systems near their quantum critical point. The temperature dependence of the resistivity data measured along the crystallographic c-axis, however, shows good metallic behavior with presence of Kondo effect, which often appears in Kondo lattice systems in their localized regime. Under the applied magnetic field, the peak-like feature in specific heat data and the corresponding weak kink in resistivity data below 1K appears to be suppressed gradually into lower temperature, and becomes undetectable around the magnetic field B=1.5 T, suggesting possible antiferromagnetic characteristics of this low temperature feature. We have also studied the system with doping of Rh or Ni in Co-site with nominal concentration of 10% to further understand the nature of the low temperature state. No significant changes in physical properties have been observed upon doping of Rh or Ni at the current stage. Further study is required to understand the ground state properties of this system.