MO6SE8-CLUSTER-BASED SUPERCONDUCTING COMPOUNDS CS2MO12SE14 AND RB4MO18SE20 - EVIDENCE FOR A STRONGLY CORRELATED AND ANISOTROPIC ELECTRON-SYSTEM

We studied the normal and superconducting states of the title compound s by measuring the conductivity and magnetization of single crystals a nd powder samples. From the upper and lower critical fields we deduced the characteristic lengths and thermodynamical fields. These results are borne out by our specific-heat measurements. We recognize in these compounds many features of the Chevrel-phase superconductors, includi ng very small coherence lengths and strong-coupling-like effects. Howe ver, we show that the electron system is much more anisotropic and sti ll less delocalized in these materials where the Mo6Se8 clusters have condensed in Mo6nSe6n+2 finite chains. This condensation is accompanie d by an enhancement of the magnetic response whereas the lengthening o f the chains leads to a counteracting reduction of the density of carr iers. This indicates that superconductivity is built upon highly corre lated molecular states. Reviewing the available data on the other Chev rel-cluster-based superconductors confirms this picture and suggests t hat the small coherence lengths reflect the local character of the ele ctron pairing. This comparison also shows that forming finite chains o f Mo6Se8 clusters makes the electron correlations more repulsive and p ushes the electron system near the borderline between superconductivit y and magnetism. In this respect these compounds could provide valuabl e complementary information on issues which are at the center of the r esearch upon high-T-c superconductivity.