Quantum Hall Effect In Multilayered Massless Dirac Fermion Systems With Tilted Cones

A massless Dirac fermion ststem was realized in alpha-(BEDT-TTF)(2)I-3 [BEDT-TTF=bis(ethylenedithio) tetrathiafulvalene] under high pressure. In contrast to graphene, this is the first bulk (multilayered) massless Dirac fermion material. Another important difference from graphene is that this system has highly tilted Dirac cones. In this case, conventional chiral symmetry is broken under a magnetic field. Here we experimentally addressed the following question: Is the Landau level structure of the system with tilted Dirac cones the same as that of graphene [conventional two-dimensional (2D) Dirac fermion system] under a magnetic field? The answer is yes. We succeeded in injecting holes into alpha-(BEDT-TTF)(2)I-3 under high pressure. The detection of Shubnikov-de Haas oscillations whose phase was modified by Berry's phase pi is direct evidence that this system is truly a 2D Dirac fermion system. In addition, we revealed the energy diagram of this device and characterized the multilayered quantum Hall effect.