Multicritical phenomena of superconductivity and antiferromagnetism in organic conductor κ-(BEDT-TTF)2X

We study theoretically the multicritical phenomena of the superconductivity (SC) and antiferromagnetism (AF) in organic conductors kappa-BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene) salts. The phase diagram and the experimental data on the nuclear magnetic resonance (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 similar to 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.