HEAT-CAPACITY DATA OF DOPED NDBA2CU3OX

Low-temperature specific-heat measurements were performed between 0.4 and 4 K on Nd-Ba-Cu-O-samples doped with Ca and additional Nd. In these samples, the specific-heat anomaly due to magnetic ordering of the Nd3+ ions as a function of hole concentration behaves similarly to that of oxygen-deficient NdBa2Cu3Ox. In samples with 0 < T(c) < 70 K, a broad anomaly due to short range magnetic interaction was found but no indication for long range magnetic ordering could be observed. In the semiconducting samples, 3D-Ising-like anomalies with T(N) around 1.6 K were measured. In order to explain the behavior of the magnetic anomaly in the oxygen-deficient undoped samples, one could assume that the magnetic interaction depends on lattice parameters or interatomic distances. Our newest results clearly show, however, that the position and shape of the anomaly is mainly controlled by the density of carriers in the superconducting CuO2 planes, and not by structural properties. Thus, the magnetic rare-earth ions are isolated enough from the CuO2 planes not to influence superconductivity (except for Pr), but are not totally decoupled from the rest of the unit cell and the carrier concentration.