T. Konoike

Specific Heat of the Multilayered Massless Dirac Fermion System

We have measured the specific heat of the multilayered massless Dirac fermion system α-(BEDT-TTF) 2 I 3 under pressure. The temperature dependence of the specific heat is almost quadratic at low temperature, suggesting massless Dirac dispersion in this system. In a magnetic field, the specific heat shows anomalous behavior, which can be understood by assuming the zero-mode Landau level characteristic of the Dirac fermion system and its spin splitting. These results are in good agreement with the reported magnetoresistance.

Observation of Angle-Dependent Stark Cyclotron Resonance in a Layered Organic Conductor

We report novel angle-dependent magnetotransport phenomena in quasi-two-dimensional layered conductors under interlayer electric fields. Interlayer conduction shows the Stark cyclotron resonance when the orbital motion of an electron becomes periodic in k -space, and the amplitude of the reasonance oscillates depending on magnetic field orientations. A conventional angle-dependent magnetoresistance oscillation switches to this oscillation at high electric fields. Using the organic conductor α-(BEDT-TTF) 2 NH 4 Hg(SCN) 4 , we have successfully observed an angle-dependent Stark cyclotron resonance for the first time.

Magnetotransport of Massless Dirac Fermions in Multilayer Organic Conductors

Magnetotransport has been considered in multilayer massless Dirac fermion systems, in which two-dimensional (2D) layers with a pair of Dirac-cone dispersion stack with weak interlayer coupling. The Fermi level is assumed to be fixed at the Dirac point resulting in the zero-gap conductor. At the high-field quantum limit, where only the n = 0 Landau level at the Dirac point is partially occupied, all elements of conductivity tensor have been evaluated by the lowest order contribution of interlayer coupling. We have deduced saturation of in-plane resistance, local maxima of in-plane Hall resistance, negative interlayer magnetoresistance, and cotθ-type unusual angle-dependence of interlayer Hall resistance. These results propose good explanations for mysterious transport features observed in organic zero-gap conductors, α-(BEDT-TTF)2I3 and its related compounds.

Quantum Hall Transport across Monolayer-Bilayer Boundary in Graphene

Magnetotransport across monolayer-bilayer boundary on a graphene piece has been studied in the quantum Hall state. The edge channel transport in monolayer-bilayer junctions has been discussed based on the Landauer-Buttiker picture following p-n bipolar junctions of monolayer graphene. We considered two extreme cases of transmission from input edge channels to output channels at the junction, the maximum transmission model and the full-mixing model. They give different two-terminal conductance as a function of carrier filling. We measured the two-terminal conductance in monolayer-bilayer junction devices, and the measured conductance agreed better with the full-mixing model. This result suggests that edge channels of monolayer and bilayer regions are almost fully mixed up at the monolayer-bilayer boundary.