T/B scaling without quasiparticle mass divergence: YbCo2Ge4

Abstract

YbCo2Ge4 is a clean paramagnetic Kondo lattice which displays non-Fermi-liquid behavior. We report a detailed investigation of the specific heat, magnetic Grüneisen parameter (Γmag), and temperature derivative of the magnetization (M) on a high-quality single crystal at temperatures down to 0.1 K and magnetic fields up to 7 T. Γmag and dM/dT display a divergence upon cooling and obey T/B scaling. Similar behavior has previously been found in several other Yb-based Kondo lattices and related to a zero-field quantum critical point without fine tuning of pressure or composition. However, in the approach of B→0 the electronic heat capacity coefficient of YbCo2Ge4 saturates at low T, excluding ferromagnetic quantum criticality. This indicates that T/B scaling is insufficient to prove a zero-field quantum critical point.