Effects of Zeroline and Ferrimagnetic Fluctuation on Nuclear Magnetic Resonance for Dirac Electrons in Molecular Conductor alpha-(BEDT-TTF)2I3

We re-examine the wave function of two-dimensional massless Dirac electron in alpha-(BEDT-TTF)2I3 consisting of four molecules A, A', B and C in a unit cell, using a tight-binding model. We find zerolines in the Brillouin zone, on which the component of the wave function becomes zero for B or C sites. The zerolines, which are bounded by two Dirac points at k0 and pass through the M- or Y-points, result in a fact that the density of states of the B site exhibits no the Van Hove singularity near the energy of the Dirac points. By taking account of the on-site Coulomb interaction within the random phase approximation, we examine the spin fluctuation in order to investigate properties of the nuclear magnetic resonance for temperatures T > 50K. In the region for 100 < T < 300K, it is shown that the Knight sift for B-site monotonously decreases with decreasing temperature, owing to lack of the Van Hove singularity, while it shows a maximum for the other sites (A, A' and C sites). In the region for 50 < T < 100K, it is shown that the Knight sift is convex downward and the Korringa ratio increases with decreasing temperature for B-site. Such a behavior originates from the ferrimagnetic spin fluctuation related to the zerolines. These results are consistent with those of the nuclear magnetic resonance experiments.