Pressure-induced phase switching of the Shubnikov de Haas oscillations in molecular Dirac fermion system $\alpha -$(BETS)${}_2$I${}_3$

We report on the Shubnikov de Haas (SdH) oscillations in the quasi two-dimensional molecular conductor $\alpha -$(BETS)${}_2$I${}_3$ [BETS: bis(ethylenedithio)tetraselenafulvalene] laminated on polyimide films at 1.7 K. From the SdH phase factor, we verified experimentally that the material is in the Dirac fermion phase under pressure. $\alpha -$(BETS)${}_2$I${}_3$ is in the vicinity of the phase transition between strongly correlated insulating and Dirac fermion phases, and is a possible candidate for an ambient-pressure molecular Dirac fermion system. However, the SdH oscillations indicate that the Berry phase is zero at ambient pressure. Under pressure, a $\pi$ Berry phase emerges when the metal-insulator crossover is almost suppressed at $\sim$0.5 GPa. The results contrast those for the pioneering molecular Dirac fermion system $\alpha -$(BEDT-TTF)${}_2$I${}_3$ [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene] in which Dirac fermions and semiconducting behavior are simultaneously observed.