arXiv subjects

Shinya Katayama

Publications and source records attributed to Shinya Katayama.

4 recordsLinked to original sources

arXiv2009

Electronic Properties Close to Dirac Cone in Two-Dimensional Organic Conductor $α$-(BEDT-TTF)$_2$I$_3$

A zero-gap state (ZGS) has been found in a bulk system of two-dimensional organic conductor, $α$-(BEDT-TTF)$_2$I$_3$ 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 contact 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 Fermi 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.

cond-mat.str-el↗

arXiv2007

Role of Interlayer Electron Hopping for Spin Density Wave State in the Zero-Gap Organic Conductor

We investigate the formation of density waves in the zero-gap state (ZGS) which has been found in the quasi-two-dimensional organic conductor $α$-(BEDT-TTF)$_2$I$_3$ salt under the hydrostatic pressure. The ZGS exhibits the cone-like dispersion for both the conduction band and the valence band which degenerate each other at the two-dimensional wave vectors, $\pm\mib{k}_0$ forming a zero gap. By using the extended Hubbard model with repulsive interaction we calculate the onset temperature of the spin density wave (SDW) as a function of the interlayer electron hopping, which by itself does not break the cone-like dispersion. It is shown that the SDW with wave number $2\mib{k}_0$ is induced by the combined effect of the interaction, the inter-band excitation across the zero-gap and the interlayer hopping.

cond-mat.str-el↗

arXiv2006

Pressure-Induced Zero-Gap Semiconducting State in Organic Conductor $α$-(BEDT-TTF)$_2$I$_3$ Salt

We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor $α$-(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.

cond-mat.mtrl-sci↗

arXiv2006

Electric Conductivity of the Zero-gap Semiconducting State in Alpha-(BEDT-TTF)2I3 Salt

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)$_2$I$_3$ 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 $k_B\gg\hbar/τ$ and independent of $T$ and $τ$ for $k_B T\ll\hbar/τ$. Further the conductivity being independent of $T$ in the ZGS state is examined in terms of Born approximation for the impurity cattering.

cond-mat.mtrl-sci↗