¹³C NMR study on zero-gap state in the organic conductor θ-(BEDT-TTF)₂I₃ under pressure

We present the results of our ¹³C NMR study of the quasi-two-dimensional organic conductor θ-(BEDT-TTF)₂I ₃ under pressure, which is suggested to be a zero-gap conductor by transport measurements. We found that NMR spin shift is proportional to T and that spin-lattice relaxation rate follows the power law T^α (α = 3-4), where T is the temperature. This behavior is consistent with the cone-like band dispersion and provides microscopic evidence for the realization of the zero-gap state in the present material under pressure.