Effects of Zero Line and Ferrimagnetic Fluctuation on Nuclear Magnetic Resonance for Dirac Electrons in Molecular Conductor \alpha-(BEDT-TTF)

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For massless Dirac electrons in two-dimensional conductor \alpha-(BEDT-TTF)<inf>2</inf>I<inf>3</inf>with four molecules (A, A<sup>\prime</sup>, B, and C) per unit cell, we found a zero line, on which the B (or C) component of the wave function becomes zero in the Brillouin zone. Within a random phase approximation, we investigated the nuclear magnetic resonance (NMR) at high temperatures (T > T_{\text{D}} \sim 50 K) where T_{\text{D}} corresponds to the maximum energy of the Dirac cone. The zero line, which is bounded by two Dirac points \pm \mathbf{k}_{0} through the time reversal symmetric point, provides notable behavior in the site-dependent NMR. With decreasing temperature (T > 100 K), the Knight shift of the B site monotonically decreases, while those of the other sites show a maximum. For 50 < T < 100 K, the Knight shift of only the B site is convex downward. The latter behavior originates from the ferrimagnetic spin fluctuation related to the zero line. NMR experiments are discussed in terms of these results.
2013-05-15