Organic superconductivity enhanced by asymmetric-anion random potential in (MDT-TS)I_0.85Br_0.41 [MDT-TS = 5H-2-(l,3-diselenole-2- ylidene)-1,3,4,6-tetrathiapentalene]

X-ray investigation and conducting and magnetic properties of a novel organic superconductor composed of an organic donor MDT-TS and an asymmetric I₂Br^- anion are reported, where MDT-TS is 5H-2-(l,3-diselenol-2-ylidene)-l,3,4,6-tetrathiapentalene. The X-ray oscillation photograph and the energy dispersion spectroscopy (EDS) measurements determine the composition to be (MDTTS)I_0.85Br_0.41, where the anion composition is approximately I₂Br. The donor and the anion construct an incommensurate lattice similar to (MDT-TS)(AuI₂)_0.441. The magnetic susceptibility of the complex behaves anisotropically below 50 K, indicating an antiferromagnetic order, while a metal-insulator (M-I) transition is observed at a lower temperature, T_MI = 30 K, Application of hydrostatic pressure suppresses the M-I transition, and a superconducting phase appears above 9.8 kbar. The midpoint T_c rises to 6.1 K under 13.8 kbar, which is the highest T_c among the MDT-TSF series salts. The asymmetric I-I-Br^- anion reduces T_MI by considerably influencing the incommensurate antiferromagnetic insulating state but does not destroy the superconductivity because the incommensurate lattice originally has a kind of random potential on the donors. Consequently, this is the first organic superconductor with an asymmetric linear anion.