The thermal conductivity X and the specific heat c o of Ba2YCu307-x and La2Cu04 have been measured in the temperature range 0.03 < T < 30 K.-The thermal conductivity of sintered Ba2YCu307_x samples varies as T ~ between 0.7 and 8 K, while below 0.3 K X(T) is described as a sum of two terms where one is proportional to T and the other varies like T 3. For sintered La2CuOh, X(T) is distinctly smaller than in sintered Ba2YCu307_ x in the whole temperature range. The specific heat of both compounds shows a non-vanishing linear term yT between 1.4 K and 15 K, where y in sintered Ba2YCu307_ x is nearly an order of magnitude larger than in crystalline La2CuO 4.

For some time it was believed that heavy-mass electrons at low temperatures, in special rare-earth and actinide compounds, were due to a particular suppression of magnetic order in these materials. Various experiments reveal, however, that magnetic order may occur from a heavy-electron state and that also a heavy-electron state may develop in the presence of a magnetically-ordered matrix. Even in materials with no sign of a cooperative phase transition, microscopic experimental probes give evidence for strong magnetic correlations, involving only very tiny (< 10 -1 μn) ordered moments, however. All these experimental observations indicate that the heavy-electron state cannot always be regarded as a simple Fermi liquid in the usual sense. Various interactions of similar strength need to be considered. It is suggested that the balance of these interactions may lead to electronic inhomogeneities in k space. Both in magnetically-ordered or superconducting systems, unexpected electronic degrees of freedom appear to exist. In other cases experimental observations suggest magnetic inhomogeneities in real space which are not simply due to chemical inhomogeneities. Different examples of these various possibilities are demonstrated and discussed.

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