IV-H-1 Low-Temperature Phase Diagram of (lambda)-Type Organic Superconductor

Hayao KOBAYASHI, Hiroki AKUTSU, Emiko ARAI, Hisashi TANAKA (Univ. Tokyo) and Akiko KOBAYASHI (Univ. Tokyo)

[Phys. Rev. B 56, R8526 (1997)]

The physical properties of a new series of organic superconductors with fourfold quasi-stacking structures, (lambda)-BETS₂GaX_zY_4-z (BETS=bis(ethylene-dithiotetraselena-fulvalene; X, Y=F, Cl, Br; 0<z<2) are changed continuously by tuning z-value. (lambda)-BETS₂GaBr_zCl_4-z, takes a superconducting ground state at 0<z<zc (~~0.8) and a semiconducting state at z_c<z<2.0. The maximum T_c is about 10 K, showing the existence of "10 K-class non-ET organic superconductors" (ET= bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF)). The temperature dependence of the resistivity resembles closely that of (kappa)-type ET superconductor but the susceptibility behavior is different. The susceptibility of (lambda)-BETS₂GaBr_1.5C_l2.5 is isotropic and decreases fairly sharply below 30 K. No indication of spin flop transition was observed at 10 K, suggesting that the insulating phase neighboring on the superconducting phase is not antiferromagnetic down to at least 10 K.

Figure 1. Pressure dependence of the resistivity and T_c (inset) of (lambda)-BETS₂GaBr_1.5C_l2.5

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