Local Spin Susceptibility Characteristic of Zero-Gap State of α-(BEDT-TTF)2I3 under Pressure

We performed C-13-NMR measurements in the metallic state of alpha-(BEDT-TTF)(2)I-3 under pressure to determine the local spin susceptibility at each BEDT-TTF molecular site. We found that the local susceptibility becomes quite different from molecule to molecule in a unit cell with approach to the zero-gap state (ZGS) under pressure. This is related to the charge disproportionation among molecular sites [named A (A'), B, and C]. We confirmed that the most/least positively charged site, the B/C molecule, carries the smallest/largest local susceptibility. We also confirmed the linear temperature dependence of the local susceptibility at low temperatures. These results are consistent with the theoretical calculation [Eur. Phys. J. B 67 (2009) 139] and should be considered as the characteristics of the ZGS.