We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor -(BEDT-TTF) 2 I 3 salt, which emerges under uniaxial pressure along the a-axis (the stacking axis of the BEDT-TTF molecule). The ZGS state is the state in which a Dirac cone with the band spectrum of a linear dispersion exists around the Fermi point connecting an unoccupied (electron) band with an occupied (hole) band. The spectrum exhibits a large anisotropy in velocity, which depends on the direction from the Fermi point. By varying the magnitude of several transfer energies of a tight-binding model with four sites per unit cell, it is shown that the ZGS state exists in a wide pressure range, and is attributable to the large anisotropy of the transfer energies along the stacking axis.

The electronic states in two-dimensional organic conductor -(BEDT-TTF) 2 I 3 have been investigated to show the noticeable property of the massless fermions, i.e., the linear dispersion which exists on the contact point between the conduction band and the valence band. These fermions are well known in bismuth and graphite, where the former are described by the Dirac equation and the latter obeys the Weyl equation corresponding to the massless fermion. In the present study, we show that the effective Hamiltonian describing the massless fermions in -(BEDT-TTF) 2 I 3 contains intrinsically new terms of Pauli matrices z and 0 in addition to the Weyl equation which consists of x and y . The new massless fermions are robust against the charge disproportionation, and induce the anomalous momentumdependence in the charge density.

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Abstract. A zero-gap state (ZGS) has been found in a bulk system of two-dimensional organic conductor, α-(BEDT-TTF)2I3 salt which consists of four sites of donor molecules in a unit cell. In the present paper, the characteristic of the ZGS is analyzed in detail and the electronic properties are examined in the vicinity of the Dirac point where the conduction and valence bands degenerate to form the zero-gap. The eigenvectors of the energy band have four components of respective sites, where two of them correspond to inequivalent sites and the other two correspond to equivalent sites. It is shown that the former exhibits an exotic momentum dependence around the contact point and the latter shows almost a constant dependence. The density of states of each site close to the Dirac point is calculated to demonstrate the temperature dependence of the local magnetic susceptibility and the local nuclear magnetic relaxation rate. Further, the robust property of the ZGS against the anion potential is also shown by using the second-order perturbation.PACS. 71.10.-w Theories and models of many-electron systems -72.80.Le Polymers; organic compounds (including organic semiconductors)

The apparent zerogap fermions in -(BEDT-TTF) 2 I 3 are recently proposed to obey the tilted Weyl equation which is very unusual. The Hall conductivity, the conductivity and the orbital susceptibility are investigated theoretically for electrons described by this tilted Weyl equation. It is found that these quantities are enhanced due to tilting but keeping the same dependence on chemical potential as in the absence of tilting. Based on these results and expected temperature dependence of the chemical potential in -(BEDT-TTF) 2 I 3 , it is found that there will be the strong enhancement of the Hall coefficient toward low temperatures. Moreover, it is proposed that the sharp but continuous reversal of the sign of the Hall coefficient is possible at low temperatures if the extremely small amount of electron doping, about 1 ppm, exists. These results may explain very anomalous features of Hall coefficient observed in -(BEDT-TTF) 2 I 3 and can be an unambiguous indication of the existence of the zerogap fermions in -(BEDT-TTF) 2 I 3 . In addition, it is shown that the orbital diamagnetism can have a peak at the temperatures where the Hall coefficient changes sign. All these strong temperature dependences of both the Hall coefficient and the orbital diamagnetism are due to the inter-band effects of magnetic field.

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Single-component molecular conductors can provide a variety of electronic states. We demonstrate here that the Dirac electron system emerges in a single-component molecular conductor under high pressure. First-principles density functional theory calculations revealed that Dirac cones are formed in the single-component molecular conductor [Pd(dddt)] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate), which shows temperature-independent resistivity (zero-gap behavior) at 12.6 GPa. The Dirac cone formation in [Pd(dddt)] can be understood by a tight-binding model. The Dirac points originate from the HOMO and LUMO bands, each of which is associated with different molecular layers. Overlap of these two bands provides a closed intersection at the Fermi level (Fermi line) if there is no HOMO-LUMO coupling. Two-step HOMO-LUMO couplings remove the degeneracy on the Fermi line, resulting in gap formation. The Dirac cones emerge at the points where the Fermi line intersects with a line on which the HOMO-LUMO coupling is zero.