Investigation of CeRu4Sn6 under high pressure

Abstract

Heavy fermion systems are a wide class of intermetallic compounds where the interaction of the conduction electrons with a sublattice of localized magnetic moments via the Kondo effect leads to the formation of strongly renormalized bands of heavy quasiparticles. These systems are characterized by two competing energy scales (the Kondo and RKKY interaction), making them highly responsive to external stimuli like pressure or magnetic field. In the presence of nontrivial electronic topology the landscape of novel quantum phases is expected to be further enriched. A candidate material to explore this regime is the Kondo semimetal CeRu4Sn6. Recent theoretical work predicted that it hosts Weyl points in its renormalized bandstructure near the Fermi level, which would make it a Weyl-Kondo semimetal. Together with evidence from inelastic neutron scattering that CeRu4Sn6 is quantum critical without tuning this raises the interesting questions whether a magnetic phase may be located nearby, and whether a Weyl-Kondo semimetal phase may be an emergent phase stabilized by quantum fluctuations. This thesis addresses these questions by investigating the specific heat (AC calorimetry) and magnetotransport properties of CeRu4Sn6 single crystals under multiple extreme conditions of high pressures (up to 10.5 kbar in a hydrostatic piston-cylinder cell), low temperatures (down to 50 mK), and high magnetic fields (up to 15 T). In zero field, applying pressure appears to tune CeRu4Sn6 away from quantum criticality as seen by a slight suppression of the specific heat (Sommerfeld) coefficient. At the same time, the electrical resistivity is gradually enhanced with pressure. However, neither of the two shows signs of a pressure induced phase transition. Application of magnetic fields has more drastic effects. The specific heat coefficient is strongly suppressed and, under combined finite pressure and field, an anomaly suggestive of a phase transition appears. The nature of the associated phase and its precise location need to be clarified in future experiments. As to Weyl-Kondo semimetal signatures, both the specific heat and the zero-field Hall response provide tentative evidence, but again additional measurements are necessary pin this assignment down.