Kondo effect in C_eX_c (X_c = S, S_e, T_e) studied by electrical resistivity measurements under high pressure

We have measured the electrical resistivity of cerium monochalcogenides, CeS, CeSe, and CeTe, under high pressures of up to 8 GPa. The pressure dependences of the antiferromagnetic ordering temperature _N, crystal field splitting, and the ln T anomaly of the Kondo effect have been studied to cover the entire region from the magnetic ordering regime at low pressure to the Fermi liquid regime at high pressure. _N initially increases with increasing pressure, and starts to decrease at high pressure as expected from Doniach's diagram. Simultaneously, the ln T behavior in the resistivity is enhanced, indicating the enhancement of the Kondo effect by pressure. It is also characteristic of C_eX_c that the crystal field splitting rapidly decreases at a common rate of -12.2K=GPa. This leads to the increase in the degeneracy of the f state and the further enhancement of the Kondo effect. It is shown that the pressure-dependent degeneracy of the f state is a key factor for understanding the pressure dependence of T_N,, the Kondo effect, magnetoresistance, and the peak structure in the temperature dependence of resistivity.