Investigation of novel high T<SUB>c</SUB> superconducting oxides

Our results of microscopic and macroscopic studies of novel high Tc superconductors are discussed. They permit a straightforward comparison of the nature of the superconductivity in 40 K range (La2CuO4 type) and 90 K range (YBa2Cu3O7type) superconductors

Heat capacity and magnetic susceptibility of mixed valent YbPt<SUB>2</SUB>Si<SUB>2</SUB>

From heat capacity and magnetic susceptibility results, we conclude that YbPt2Si2 is a homogeneously mixed valent compound

Characterization of the valence transition in Yb<SUB>x</SUB>In<SUB>1−x</SUB>Cu<SUB>2</SUB> and anomalous <SUP>115</SUP>In Knight-shift behavior

115In nuclear-magnetic-resonance Knight shift studies in YbxIn1&#8722;xCu2 microscopically confirm the existence of an anomalous valence transition of Yb ions for some values of x. The rate of variation, as well as the overall variation of the Knight shifts is the largest ever observed in any Yb system. The Knight shifts do not track the susceptibility at low temperatures, the possible implications of which are discussed. Specific-heat results prove the intermediate-valence character of Yb ions in the ground state of these compounds

The effect of Ni and Pt substitution in CeRh<SUB>2</SUB>Si<SUB>2</SUB>

A small replacement of Rh by Ni or Pt (about 10%) results in the destruction of anomalous magnetism (TN=36 K) of CeRh2Si2, leading to a heavy fermion ground state. Further substitution of Ni for Rh stabilizes the strong mixed valent state