Probing quantum criticality in ferromagnetic CeRh6Ge4

CeRh6Ge4 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 Pc≈0.8 GPa, these changes are not abrupt but extend over a pressure interval from within the ferromagnetic state up to Pc. 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 Pc, delocalization of 4f degrees of freedom through orbital-selective hybridization followed by quantum criticality of itinerant ferromagnetism.