Thermoelectric Power and Thermal Transport of Anomalous Rare-Earth Kondo Compounds

Abstract

This paper reviews the thermoelectric power (TEP) of cerium and ytterbium Kondo systems. We present first the experimental situation regarding the Kondo compounds and describe briefly the intermediate valence (IV) systems. We then define the Kondo temperature TK and summarize the theoretical approaches that explain the behavior of Kondo systems much above, and much below, the Kondo temperature. The results obtained for T>TK, explain the high-temperature TEP data of cerium (ytterbium) compounds which exhibit a broad positive (negative) maximum at T~Δ /a, where Δ is given by the crystalline field (CF) splitting and a≥ 1 is a numerical constant. For T < TK, the theory yields a small positive (negative) TEP peak at about TK/2 which explains the low-temperature TEP data on cerium (ytterbium) compounds. Combining these theoretical results, we can account for the overall temperature dependence of the TEP in most Kondo and IV compounds. We also explain the anomalous pressure dependence of the TEP in some cerium and ytterbium compounds and show that the charge transfer is of crucial importance for pressure experiments. We discuss in some detail the hydrogenation effects for cerium compounds and show that one should distinguish between the “ negative pressure” effects and the chemical effects which are due to the cerium-hydrogen bonding. Finally, we present briefly the experiments of the thermal conductivity in anomalous rare-earth Kondo compounds and show that the data can be explained by the Kondo scattering in the presence of CF splitting.