M.H. van Maaren

Oscillatory dependence of superconductive critical temperature on number of valency electrons in Cu3Au-type alloys

Abstract The superconducting critical temperature, the magnetic susceptibility and the thermo-electric power of a number of pseudo-binary B 3 A -type alloys with Cu 3 Au-structure have been investigated. An oscillatory dependence of the properties on the number of valence electrons per atom was observed. The constituents of the alloys were mainly non-transition metals: for the A -elements Ca, Y, La and Th and for the B -elements In, Sn, Tl and Pb. The results can be explained by taking into account the influence of the scattering of electrons by Bragg-planes near the Fermi surface on the electron density of states and on the superconductive interaction parameter. In addition 13 other Cu 3 Au-type compounds with mainly non-transition metals have been prepared. The superconductivity of 19 out of the 25 Cu 3 Au-type compounds investigated, can be fitted into the general scheme of oscillatory behaviour of T c with n . The absence of superconductivity for 5 of the compounds can be ascribed to magnetic interactions.

High electronic specific heat of some cubic UX3 intermetallic compounds

Abstract The low temperature specific heat of cubic U X 3 intermetallic compounds with X = Al, Ga, In, Si, Ge and Sn have been measured. High values for the coefficient of the electronic specific heat have been found, ranging from 14 to 169 mJ/mol K 2 .

An energy band model of Nb - and Ta -dichalcogenide superconductors

Abstract An energy band model for Nb- and Ta-dichalcogenide superconductors is proposed, based on experimental data of Hall effect and specific heat measurements. The results indicate an overlapping valence- and d-band system.

Superconductivity in group Va dichalcogenides

Abstract Group Va dichalcogenides were investigated for superconductivity down to 1.0°K. Besides NbSe2, NbS2 was found to become superconducting at 6.2°K. Measurements were made on single-crystal specimens.

Some new superconducting group Va dichalcogenides

Abstract Group Va dichalcogenides were investigated for superconductivity down to 0.05°K. The following new superconducting compounds were found: NbTe2, TaS2 and TaSe2, with maximum transition temperature of 0.74, 2.10 and 0.22 °K, respectively. No superconductivity was found in TaTe2 and in the non-stoichiometric compounds VSe2 and VTe2.

Superconductivity in tin -group Va trichalcogenides

Abstract Superconductivity has been found for the tetragonal compounds SnNbS 3 , SnTaS 3 and SnNbSe 3 at 2.75, 2.90 and 3.02, respectively. Crystallographic data and results of low temperature specific heat measurements are reported.

Low-temperature specific heat of CeAl3 and related compounds

Abstract Low-temperature measurements of the specific heat are reported for Ce 1−x La x Al 3 and Y 1−x Th x Al 3 . The observed maxima for Ce 1−x La x Al 3 are attributed to crystalline field effects. The marked variations in the electronic density of states for Y 1−x Th x Al 3 are ascribed to influences of Brillouin zones. Results are related to the Kondo sideband model.

Superconductivity of compounds having the Si2U3-type structure

Abstract Three compounds out of thirteen having the Si 2 U 3 -type structure were found to become superconducting above 0.1°K, viz. Be 2 Nb 3 , Be 2 Ta 3 and Al 2 Th 3 at 2.3, 1.0 and 2.6°K, respectively. Another new superconductor reported is Ga 3 Nb 5 with T c = 1.35°K.

Critical carrier concentration for superconductivity in a CuRh2Se4 - based system

Abstract In the system CuRh 2− x Sn x Se 4 (0 ≤ x ≤ 1) electronic properties vary from metallic to semiconducting. The disappearance of superconductivity at x ≈ 0.5 coincides with substantial changes in the dependence of the Hall coefficient and the electronic density of states upon composition.

Brillouin zone effects on the critical temperature of superconductors and some related normal metal properties

Abstract A Fermi surface passing through a set of Bragg planes gives rise to variations of several normal metal properties, e.g. the electronic density of states N(0), the magnetic susceptibility, the Seebeck coefficient, the Debye temperature and the electron-phonon interaction strength Vph, and consequently also to variation of the critical temperature for superconductivity, Tc. This behaviour is analysed on the basis of a nearly free electron model and a comparison is made with experimental results in various alloy systems. These systems include alloys of non-transition metals and pseudo-binary alloyed compounds with Cu3Au- and CuAl2-type structure. The observed variations of normal metal properties are in reasonable agreement with the theoretical estimates. The variations in Tc indicate that enhancements not only of N(0) but probably also of Vph occur near Bragg planes.