Observations of $\nu =1$ Quantum Hall Effect and Inter-Band Effects of Magnetic fields on Hall Conductivity in Organic Massless Dirac Fermion System $\alpha$-(BETS)${}_2$I${}_3$ under Pressure

We investigated the magnetoresistance and the Hall effect in an organic massless Dirac fermion system $\alpha$-(BETS)${}_2$I${}_3$ under pressure. The Fermi energy of this system is slightly far away from the Dirac points, and thus the $\nu =1$ quantum Hall state is realized in a low magnetic field at low temperatures. Moreover, the experimental formula for chemical potential as a function of temperature is clarified. We succeeded in detecting the inter-band effects of the magnetic field on the Hall conductivity when the chemical potential passes the Dirac points.