Universal Behavior of Magnetoresistance in Organic Dirac Electron Systems

In-plane magnetoresistance for organic massless Dirac electron system (OMDES) alpha-(BEDT-TTF)(2)I-3 and o-(BEDT-TTF)(2)I-3 in addition to possible candidates of the OMDES alpha-(BETS)(2)I-3 and alpha-(BEDT-STF)(2)I-3, 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 alpha-(BEDT-TTF)(2)I-3, 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 alpha-(BEDT-TTF)(2)I-3 under intermediate pressure, when the thermal energy exceeds the gap.