Single-crystal studies of the Chevrel-phase superconductor LaxMo6Se8 - II: Physical and superconducting properties

摘要

Single crystals of LaxMo6Se8 have been grown and some of their magnetic, transport, and superconducting properties studied, The electrical resistivity is characterized by its high value at room temperature, its low residual resistivity ratio, and a pronounced negative curvature at high temperatures, Comparison with the isostructural compounds Mo3Se4 (Mo6Se8) and LaMo6S8 shows that this behavior is due to structural as well as to extrinsic features (e.g., brittleness due to weak intercluster bondings), The position of the Fermi level near a peak of the density of states plays an important role in the normal-state physical properties, fixing the functional forms of both resistivity and magnetic susceptibility. The superconducting state is mainly characterized by a strong lanthanum concentration dependence of the critical temperature T-c, by quite definite granular effects, and by a very high critical field (H-c2(0) similar to 55 T), The intragrain critical current density, as estimated by magnetic mesurements, is relatively high (4 x 10(4) A/cm(2) at zero field and 1.7 K), three times larger than the one obtained for the void compound Mo3Se4. This fact is due to a higher density of pinning centers in the ternary compound because of microstructural features such as microcracks or crystal defects caused by the extreme brittleness of the crystals. (C) 1998 Academic Press. [References: 32]