Specific heat of CexLa1-xB6 in the low cerium concentration limit (x ≤ 0.03)

The specific heat of high-quality Ce (x) La1 - x B-6 (x = 0, 0.01, 0.03) single crystals is studied in the temperature range 0.4-300 K. LaB6 samples with various boron isotope compositions (B-10, B-11, (nat) B) are analyzed to estimate the effect of boron vacancies. The experimental data are used to take into account the electron component correctly under the renormalization of the density of states at T < 8 K, the contribution of the quasi-local vibrational mode of a rare-earth ion with the Einstein temperature I similar to(E) a parts per thousand 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature I similar to(D) a parts per thousand 1160 K, and the low-temperature Schottky contribution related to the presence of 1.5-2.3% boron vacancies in the rare-earth hexaborides. The detected low-temperature anomalies in the specific heat are shown to be interpreted in terms of the formation of two-level systems with an energy Delta E = 92-98 K caused by the displacement of rare-earth ions from their centrosymmetric positions. A scenario of heavy fermion formation that is alternative to the Kondo mechanism is proposed for the systems with a magnetic impurity.