CeRuGe and CeRuSi: Heavy fermion systems with some unusual features

We have investigated the physical properties of CeRuSi, its homologue CeRuGe and some doped La alloys. All of them present similar properties: large paramagnetic temperatures θp ≈ −100 K and the level off of χ(T) below ≈ 10 K in coincidence with the maximum decrease of p(T), which starts to drop below 25 K. The respective γ(CeRuSi) ≊ 0.18 and γ(CeRuGe) ≊ 0.15 J/molK² coefficients reveal a heavy fermion character, with a Kondo temperature T_K ≊ 50 K comparable to the crystal field splitting. In both compounds a broad maximum in C_P(T)/T is observed around 5 K, whose origin is yet not clear. In contrast to the usual behavior observed in Kondo lattice systems, when Si is replaced by Ge T_K ∝ θ_p and 1/γ practically does not decreases despite the unit cell volume of CeRuGe is about 3% larger than that of CeRuSi. We discuss possible origins for these unusual features.