Probing quantum criticality in ferromagnetic CeRh6Ge4

CeRh_6Ge_4 is unusual in that its ferromagnetic transition can be suppressed continuously to zero temperature, i.e., to a ferromagnetic quantum-critical point (QCP), through the application of modest hydrostatic pressure. This discovery has raised the possibility that the ferromagnetic QCP may be of the Kondo-breakdown type characterized by a jump in Fermi volume, to which thermopower S measurements should be sensitive. Though S/T changes both sign and magnitude around the critical pressure P_c≈0.8 GPa, these changes are not abrupt but extend over a pressure interval from within the ferromagnetic state up to P_c. Together with temperature and pressure variations in electrical resistivity and previously reported heat capacity, thermopower results point to the near coincidence of two sequential effects near P_c, delocalization of 4f degrees-of-freedom through orbital-selective hybridization followed by quantum criticality of itinerant ferromagnetism.