SYNTHESIS, SUPERCONDUCTIVITY, X-RAY STRUCTURE AND ELECTRONIC BAND-STRUCTURE OF LAMBDA-(BETS)(2)GACL4

The first superconductor derived from bis(ethylenedithio)tetraselenafulvalene (BETS), lambda-(BETS)(2)GaCl4, possesses a relatively sharp resistive transition with an onset of about 7.5 K and a midpoint of 6 K. Several samples had much broader transitions with higher onsets (> 9 K). Superconductivity was confirmed by AC susceptibility (midpoint 4.5 K, Delta T = 1 K). lambda-(BETS)(2)GaCl4 crystallizes in the monoclinic space group P ($) over bar 1, with four BETS units stacked in a zig-zag fashion in the unit cell. Tight-binding band calculations suggest that lambda-(BETS)(2)GaCl4 has both 1-D and 2-D Fermi surfaces, the most prominent feature being a closed hole pocket centered at X accounting for similar to 33% of the first Brillouin zone. These results confirm and extend the recent findings of Kobayashi and coworkers.