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.

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)

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The quasi-two-dimensional molecular conductor α-(BEDT-TTF) 2 I 3 exhibits anomalous transport phenomena where the temperature dependence of resistivity is weak but the ratio of the Hall coefficient at 10 K to that at room temperature is of the order of 10 4 . These puzzling phenomena were solved by predicting massless Dirac fermions, whose motions are described using the tilted Weyl equation with anisotropic velocity. α-(BEDT-TTF) 2 I 3 is a unique material among several materials with Dirac fermions, i.e. graphene, bismuth, and quantum wells such as HgTe, from the view-points of both the structure and electronic states described as follows. α-(BEDT-TTF) 2 I 3 has the layered structure with highly two-dimensional massless Dirac fermions. The anisotropic velocity and incommensurate momenta of the contact points, ±k 0 , originate from the inequivalency of the BEDT-TTF sites in the unit cell, where ±k 0 moves in the first Brillouin zone with increasing pressure. The massless Dirac fermions exist in the presence of the charge disproportionation and are robust against the increase in pressure. The electron densities on those inequivalent BEDT-TTF sites exhibit anomalous momentum distributions, reflecting the angular dependences of the wave functions around the contact points. Those unique electronic properties affect the spatial oscillations of the electron densities in the vicinity of an impurity. A marked behavior of the Hall coefficient, where the sign of the Hall coefficient reverses sharply but continuously at low temperatures around 5 K, is investigated by treating the interband effects of the magnetic field exactly. It is shown that such behavior is possible by assuming the existence of the extremely small amount of electron doping. The enhancement of the orbital diamagnetism is also expected. The results of the present research shed light on a new aspect of Dirac fermion physics, i.e. the emergence of unique electronic properties owing to the structure of the material.

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The electric conductivity which reveals the zero gap semiconducting (ZGS) state has been investigated as the function of temperature T and life time τ in order to understand the ZGS state in quarter-filled α-(BEDT-TTF)2I3 salt with four sites in the unit cell. By treating τ as a parameter and making use of the one-loop approximation, it is found that the conductivity is proportional to T and τ for kBT ≫ /τ and independent of T and τ for kBT ≪ /τ . Further the conductivity being independent of T in the ZGS state is examined in terms of Born approximation for the impurity scattering.

F-ATPase forms the membrane-associated segment of FF-ATP synthase - the fundamental enzyme complex in cellular bioenergetics for ATP hydrolysis and synthesis. Here, we report a crystal structure of the central F subcomplex, consisting of the rotary shaft γ subunit and the inhibitory ε subunit, from the photosynthetic cyanobacterium BP-1, at 1.98 Å resolution. In contrast with their homologous bacterial and mitochondrial counterparts, the γ subunits of photosynthetic organisms harbour a unique insertion of 35-40 amino acids. Our structural data reveal that this region forms a β-hairpin structure along the central stalk. We identified numerous critical hydrogen bonds and electrostatic interactions between residues in the hairpin and the rest of the γ subunit. To elaborate the critical function of this β-hairpin in inhibiting ATP hydrolysis, the corresponding domain was deleted in the cyanobacterial F subcomplex. Biochemical analyses of the corresponding αβγ complex confirm that the clinch of the hairpin structure plays a critical role and accounts for a significant interaction in the αβ complex to induce ADP inhibition during ATP hydrolysis. In addition, we found that truncating the β-hairpin insertion structure resulted in a marked impairment of the interaction with the ε subunit, which binds to the opposite side of the γ subunit from the β-hairpin structure. Combined with structural analyses, our work provides experimental evidence supporting the molecular principle of how the insertion region of the γ subunit suppresses F rotation during ATP hydrolysis.