Universal Behavior of Magnetoresistance in Organic Dirac Electron Systems

In-plane magnetoresistance for organic massless Dirac electron system (OMDES) α-(BEDT-TTF)2I3 and θ-(BEDT-TTF)2I3 in addition to possible candidates of the OMDES α-(BETS)2I3 and α-(BEDT-STF)2I3, was investigated under hydrostatic pressure. We have found the universal behavior of the in-plane magnetoresistance under a low magnetic field perpendicular to two-dimensional plane. As for α-(BEDT-TTF)2I3, the universality was examined with the parameters of temperature, magnetic field and its direction. We suggest that the universal magnetoresistance behavior is found even for the gapped state of α-(BEDT-TTF)2I3 under intermediate pressure, when the thermal energy exceeds the gap.