Multicritical Phenomena of Superconductivity and Antiferromagnetism in Organic Conductor $κ$-(BEDT-TTF)$_2$X

We study theoretically the multicritical phenomena of the superconductivity (SC) and antiferromagnetism (AF) in organic conductors $\kappa$-BEDT salts. The phase diagram and the experimental data on the NMR relaxation rate $1/T_1$ is analysed in terms of the renormalization group method. The bicritical phenomenon observed experimentally indicates the rotational symmetry, i.e., SO(5) symmetry, of the SC and the AF. The critical exponent $x$ for the divergence of $1/T_1$ is well explained by $ x = \nu (z - 1 - \eta)$ with the dynamical exponent $z = 3/2$ for the AF region while $z = \phi/\nu \sim 1.84 $ at the bicritical point. These results strongly suggest that the origin of the SC is in common with that of the AF and that its symmetry is d-wave.