K. Andres

Magnetic Ordering in the Rare-Earth Hexaborides

Magnetic ordering in the rare-earth hexaborides has been studied by a variety of methods, and a clear correlation with the presence of conduction electrons has been found.

Superconductivity of Alpha-Uranium and the Role of 5f Electrons

A much sharper and lower superconducting transition has been found for α-uranium than any reported previously. A model that explains the unusual volume dependence of α-uranium below 43�K and the unusual pressure dependence of its superconducting transition temperature is presented.

Jahn‐Teller Transition in PrCu2

Previous specific heat measurements1) on PrCu2 indicated that below 7 K the two lowest lying singlet states of the 3H4 manifold of Pr3+ spontaneously increase their splitting due to a Jahn‐Teller effect. PrCu2 has an orthorhombic structure which can be viewed as a distortion of the hexagonal AlB2 structure. Further measurements on single crystals yield the following results: Large anisotropies are observed in the magnetic susceptibility, the electric resistivity and the elastic constants. At the transition the susceptibility shows relatively small anomalies, while marked anomalies are observed in the resistivity and thermal conductivity. The anisotropy of the resistivity remains unchanged through the transition, contrary to what one would expect from single ion anisotropic Coulomb charge scattering. Ultrasonic measurements above the transition temperature show softening of certain shear modes, and thermal expansion measurements down to 1,5 K show that the crystal distorts below 10 K.

Induced Jahn Teller Transition in PrCu2

We have investigated the low temperature phase transition of PrCu2 by means of several experimental methods on single crystal specimens. We have measured the magnetic susceptibility, the electrical resistivity, the thermal expansion and the magnetostriction, several elastic constants and the thermal conductivity. We conclude that PrCu2 undergoes an induced type Jahn-Teller effect at 7.5 K.