Thermal transport of Dirac fermions in multiband system

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The purpose of this work is to clarify the thermal transport of Dirac fermions in multiband system. The target material is the organic conductor θ-(BEDT-TTF)_2I_3, which has both Dirac-cone band and usual parabolic band under pressure. To begin with, we study the thermal transport of α-(BEDT-TTF)_2I_3 in which only Dirac-cone band exists.We have measured the thermopower and Nernst effect of α-(BEDT-TTF)_2I_3 under pressure. Because we obtained surprisingly interesting results in this system, we concentrated our research on this material. In the quantum limit region, the extremely large Nernst effect (>1mV/K) rather than the Seebeck effect was observed. This unusual behavior results from the intrinsic nature of the zero-mode (N=0) Landau level characteristic of the Dirac fermion system. The possible origin of the giant Nernst signal compared with that of graphene is the sharp width of the Landau level and the Zeeman gap in high-puritygα-(BEDT-TTF)_2I_3 crystal.