Resistive transition in an extremely two-dinff-nsional superconductor, /spl alpha/-(BEDT-TTF)/sub 2/NH/sub 4/Hg(SCN)/sub 4/

Summary form only given. Among many BEDT-TTF based superconductors, the titled compound is unique due to highly two-dimensional nature of the electronic state. In the present work, the superconductivity of this compound has been investigated by in-plane and out-of-plane resistive measurements down to 0.5 K. The transition at a zero field is characterized by an onset of decrease of resistivity around 2.3 K and gradual decrease at lower temperatures, followed by an appearence of zero-resistance state around 0.9 K. This characteristics were reproduced for several samples measured, impking that the broadeness of the transition is intrinsic, not due to inhomogeneity in a crystal. The transition to a zero resistance was very sensitive to the field perpendicular to the layer although the 2.3-K-transition was insensitive to the field. The two-dimensionality and broad transition observed in this compound lead us to speculate on free vortex-antivortex excitation below the mean-field transition, inherent in two-dimensional superconductivity. We report further characterization of the superconductivity such as anisotropy, temperature and field dependence of resistivity, current-voltage characteristics. The possibility of the Kosteritz-Thouless transition in this compound is discussed.