Multicritical Phenomena of Superconductivity and Antiferromagnetism in Organic Conductorκ-(BEDT-TTF)2X

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Abstract
We study theoretically the multicritical phenomena of the superconductivity (SC) and antiferromagnetism (AF) in organic conductors κ-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 = ν( z - 1 - η) with the dynamical exponent z = 3/2 for the AF region while z = φ/ν∼ 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.