Weyl-Kondo semimetal candidate Ce3Bi4Pd3 and its nonmagnetic counterpart La3Bi4Pd3

Abstract

Heavy fermion systems have fascinated researchers worldwide since their discovery in the 1970s. Despite the amount of exciting materials found in these systems like unconventional superconductors (e.g. CeCoIn5 [1] and CeCu2Si2 [2]) or Kondo insulators (e.g. YbB12 [3], Ce3Bi4Pt3 [4] and SmB6 [5]), many current investigations show unanticipated results like spin-triplet superconductivity in UTe2 [6] which indicates that there is much more to be discovered in this field. In parallel, Weyl fermions, predicted by Hermann Weyl for high energy physics already in 1929, were proposed for solid state materials [7, 8] and first detected in TaAs in 2015 [9]. A material that might combine the physics of both fields is Ce3Bi4Pd3, identified as a Weyl-Kondo semimetal candidate [10, 11], where the Kondo effect arising from the single 4f electron of the rare earth element cerium pins the Weyl nodes near the Fermi level. Recently, a spontaneous Hall effect was discovered in Ce3Bi4Pt3 [12] that provides strong evidence for this assignment. Still many questions remain unanswered: What is the impact of high magnetic fields on the band structure? As the Weyl nodes only appear in certain points in the band structure, is there any anisotropy seen in the transport properties? This thesis seeks to address these questions by further investigating transport properties of Ce3Bi4Pd3 at low temperatures. The annihilation of the Weyl nodes with increasing magnetic field [13] is confirmed on a new lead-flux grown batch of Ce3Bi4Pd3 and a phase transition to a heavy fermion metal at high magnetic fields is found. An anisotropy of the spontaneous Hall effect is observed by changing the angle of the excitation current on a single sample. We confirm different Berry curvature driven Hall signals [12], which were recently also found in WTe2 [14] and are a characteristic of a Weyl semimetal [15], for an excitation current flow along different crystal directions. For a full characterisation of Ce3Bi4Pd3, we study the nonmagnetic reference material by substituting cerium with the non-magnetic element lanthanum to form the metallic compound La3Bi4Pd3. In this material we find a transition to the superconducting phase at T=1.9K by transport measurements. The findings of this study provide important insight into a better understanding of the phenomena happening in the Weyl-Kondo semimetal candidate Ce3Bi4Pd3.