Specific heat of Ce _x La_{1 − x}B₆ in the low cerium concentration limit (x ≤ 0.03)

Abstract

The specific heat of high-quality Ce _x La_{1 − x}B₆ (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 (¹⁰B, ¹¹B, ^natB) 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 Θ_E ≈ 152 K, the Debye contribution from the rigid cage of boron atoms with the Debye temperature Θ_D ≈ 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 ΔE = 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.