Specific heat study of La_(1-x)Ca_xMnO_3 (0.5 ≤ × ≤ 0.9) with antiferromagnetic ground state

摘要

Specific heat data for La_(1-) _xCa_xMnO_3 compounds (1/2 ≤ x ≤ 0.9) are reported. For (1/2 ≤ x ≤ 0.8), the electronic specific heat displays two peaks. The first peak (at high temperatures) is related to the charge/orbital ordering and the second to the spin ordering transition. For x ≥ 0.85, the two peaks merge. Based on the behavior of low temperature specific heat, the low-lying excitations display a drastic change for x > 0.6. For samples with 1/2 ≤ x ≤ 0.6, the low temperature specific heat can be analyzed using a cubic term (C ∼ T~3) arising from phonons and antiferromagnetic magnon excitations and an anomalous square term C ∼ T~2 of unknown origin. For 0.6 ≤ x ≤ 0.9, the cubic term remains, while the anomalous term disappears. An unexpected linear term is required to fit the low temperature specific heat data. This novel finding is experimental proof that the low-lying CE, 2/3, 3/4, C and G states are different. Using reasonable values for the lattice specific heat, the temperature variation of the electronic entropy has been estimated.