Development of the valence fluctuation in the nearly divalent compound YbCu₂Ge₂under high pressure

Pressure driven valence fluctuations of a paramagnetic compound YbCu ₂Ge₂, having the almost divalent configuration, has been investigated via the electrical resistivity (ρ) measurement. Temperature dependence of the resistivity ρ (T) shows typical metallic behavior expected for the non-magnetic metals at low pressure. With increasing pressure, the ρ is strongly enhanced, and a broad shoulderlike anomaly appears in the ρ (T) curve, as observed in the valence fluctuating system. The effective mass deduced from the ρ (T) is strongly enhanced, indicating that YbCu ₂Ge₂ changes to the heavy fermion system. The observation of the strong enhancement of the residual resistivity above 10 GPa, where the mass also starts to increase drastically, indicating the development of the valence fluctuation.