Quantum critical behavior in Ce(Fe_0.76Ru_0.24)₂Ge₂

Systems with embedded magnetic ions that exhibit a competition between magnetic order and disorder down to absolute zero can display unusual low-temperature behaviors of the resistivity, susceptibility, and specific heat. Moreover, the dynamic response of such a system can display hyperscaling behavior in which the relaxation back to equilibrium when an amount of energy E is given to the system at temperature T only depends on the ratio E/T. Ce(Fe_0.76Ru_0.24)₂Ge₂ is a system that displays these behaviors. We show that these complex behaviors are rooted in a fragmentation of the magnetic lattice upon cooling caused by a distribution of local Kondo screening temperatures, and that the hyperscaling behavior can be attributed to the flipping of the total magnetic moment of magnetic clusters that spontaneously form and order upon cooling. We present our arguments based on the review of two-decades worth of neutron scattering and transport data on this system, augmented with new polarized and unpolarized neutron scattering experiments.