Magnetic-field quantum phase driven transitions in CeBiPt and CeCu$_{6-x}$Au$_{x}$

The half-Heussler compounds CeBiPt and LaBiPt are semimetals with very low charge-carrier concentrations as evidenced by Shubnikov-de Haas (SdH) and Hall-effect measurements. Elastic neutron-scattering results reveal a simple antiferromagnetic structure in CeBiPt below $T_{N}$ = 1.15 K. The band structure of CeBiPt sensitively depends on temperature, magnetic field, and stoichiometry. Above a certain, sample-dependent, threshold field ($B \quad >$ 25 T) the SdH signal disappears and the Hall coefficient reduces significantly. These effects are absent in the non-4$f$ compound LaBiPt. Electronic-band-structure calculations can well explain the observed behavior by a 4$f$-polarization-induced Fermi-surface modification. CeCu$_{6-x}$Au$_{x}$ orders for $x \quad >$ 0.1 with an incommensurate antiferromagnetic structure. Here we compare the magnetic fluctuation spectrum obtained from inelastic neutron scattering for a field-driven quantum phase transition at $x$ = 0.2 with that for zero-field transition at the critical concentration $x_{c}$ = 0.1. \newline \newline Work performed in collaboration with J. Wosnitza, G. Goll, A. D. Bianchi, B. Bergk, N. Kozlova, I. Ophale, S. Elgazzar, M. Richter, O. Stockert, T. Yoshino, T. Takabatake and M. Enderle.