PHASE-DIAGRAM OF VORTICES IN THE QUASI-2-DIMENSIONAL ORGANIC SUPERCONDUCTOR ALPHA-(BEDT-TTF)(2)NH4HG(SCN)(4) - A SYSTEM OF PANCAKE VORTICESWITH OUT-OF-PLANE COUPLING DOMINATED BY THE ELECTROMAGNETIC ENERGY

Anisotropy and field-temperature (H-T) phase diagram of the quasi-two- dimensional superconductor, alpha-(BEDT-TTF)(2)NH4Hg(SCN)(4), were inv estigated through the specific heat, ac complex susceptibility chi=chi '-i chi '', and resistivity measurements under zero field and de field s normal to the conducting plane. The anisotropy parameter gamma was d etermined to be as large as similar to 10(3). In the vortex state of t his highly anisotropic superconductor, it was found that the onset of the in-plane and out-of-plane ac susceptibility forms a characteristic line well below the H-c2 line determined by the specific-heat measure ments. The absence of the frequency dependence in the out-of-plane sus ceptibility suggests that this is a melting line of vortices, rather t han a crossover of vortex dynamics. The out-of-plane resistivity measu rements show that the interlayer coherence between pancake vortices is established well above the melting line. Considering the anisotropy p arameter obtained in the present study, the pancake vortices in this s uperconductor is viewed as an assembly coupled mainly through the elec tromagnetic energy in the out-of-plane direction. According to this fe ature, the two characteristic lines in the H-T diagram were qualitativ ely explained by the recent melting and decoupling theories taking acc ount of the electromagnetic coupling.