We have discovered two-dimensional zero-gap material with a layered structure in the organic conductor $alpha$-(BEDT-TTF)$_2$I$_3$ under high hydrostatic pressure. In contrast to graphene, the electron-hole symmetry is not good except at the vicinity of the Dirac points. Thus, temperature dependence of the chemical potential, $mu$, plays an important role in the transport in this system. The experimental formula of $mu$ is revealed. We succeeded in detecting the inter-band effects of a magnetic field on the Hall conductivity when $mu$ passes the Dirac point.
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