Specific Heat Study of Energy Gap Formation in CeNiSn and CeRhSb

摘要

Specific heat of dense-Kondo compounds CeNiSn, CeRhSb and alloys CeNi_(1-x)Pt_xSn (x = 0.03, 0.06, 0.12, 0.20 and 0.33) and CeNi_(1-y)Co_ySn (y = 0.03 and 0.10) has been measured between 1.3 and 60 K in order to elucidate the mechanism of the energy gap formation in these compounds. The magnetic specific heat divided by temperature C_m/T of CeNiSn and CeRhSb marks a maximum at T_(max) and substantially decreases at low temperatures. This characteristic behavior is attributed to the energy gap opening in the heavy quasiparticle band developed by the dense Kondo effect. Here, T_(max) is proportional to the gap energy which is revealed by fitting C_m/T based on a model of the density of states with a V-shaped gap. On the other hand, C_m/T of CeNi_(1-x)Pt_xSn and CeNi_(1-y)Co_ySn shows that the energy gap is gradually suppressed with increasing both x and y. We estimated the Kondo temperature T_K of these materials from the magnetic entropy S_m estimated by integrating C_m/T with T. obtained values of T_K for CeNiSn and CeRhSb are 51 and 89 K, respectively. For CeNi_(1-x)Pt_xSn and CeNi_(1-y) Co Sn, T_K decreases with increasing x whereas increases with increasing y. C_m/T was analyzed using the reduced temperature T = T/T_K. A C_m/T versus T plot of the data was found to be nearly identical for CeNiSn and CeRhSb. The maximum in C_m/T is characterized by (C_m/T)_(max) ～πR/3 and T_(max) ～0.1. This implies that the gap energy is determined by a single characteristic temperature T_K. The same plot for alloys indicates that the energy gap suppression occurs similarly both in CeNi_(1-x)Pt_xSn and CeNi_(1-y)Co_ySn; T_(max) decreases with increasing x and y while (C_m/T)_(max) remains almost constant. The decrease of T_(max) indicates the reduction of the gap energy due to the disturbance of coherent scattering in the Kondo lattice. CeNiSn and CeRhSb are classified as a new type of semiconductors with the coherence gap, which opens at low temperatures.