Thermodynamic properties of Kondo insulators

A family of heavy-fermion materials that exhibits semiconducting-like properties has recently been discovered. A typical representative of this family is the system ${\mathrm{Ce}}_3$ ${\mathrm{Bi}}_4$ ${\mathrm{Pt}}_3$. Behavior similar to these rare-earth materials has also been observed in the compound FeSi. Using a two-band model and treating the electronic correlations beyond the mean-field level, we calculate the thermodynamic properties of this model for the case of two electrons per unit cell. We find that a hybridization gap appears above a critical value of the mixing term which depends on the ratio between the effective masses of the electrons in the large and narrow bands. The two-band model provides a unified description of the rare-earth and transition-metal-based systems. The results obtained for the temperature-dependent susceptibility are in good qualitative agreement with experimental data.