Experimental evidence for the formation of a singlet ground state at low temperatures in the dense Kondo system CeAl3

Heat-capacity measurements have been performed on Ce${\mathrm{Al}}_3$ and La${\mathrm{Al}}_3$ between 1.8 and 300 K. The difference in the heat capacities of these two compounds ($\Delta C_p$) has been regarded as the additional contribution due to the local moments on ${\mathrm{Ce}}^{3+}$ ($J=\frac{5}{2}$) in Ce${\mathrm{Al}}_3$. The $\Delta C_p-\mathrm{vs}-T$ curve displays two pronounced anomalies peaking near 5 and 25 K. The total entropy under this curve is nearly $R$ ln6 which is in excess of the maximum possible crystal-field entropy by $R$ ln2. Of the two anomalies, the one at 25 K has been interpreted as being caused by crystal-field effects alone since it accounts for nearly $R$ ln3 of the measured entropy. Neutron-diffraction measurements, performed at 3.5, 4.6, and 300 K, show that the lower-temperature heat-capacity anomaly is not caused by magnetic ordering in Ce${\mathrm{Al}}_3$. The results have therefore been interpreted on the basis of the combined effects of crystal-field splitting and the Kondo effect on the ${\mathrm{Ce}}^{3+}$ ions. Analysis of the measured entropy leads to the conclusion that the low-temperature ground state in this Kondo system is a spin- and orbit-compensated singlet, in accord with recent theoretical predictions.