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Magnetotransport in organic Dirac fermion system at the quantum limit: Interlayer Hall effect and surface transport via helical edge states

We discuss two anomalous features of interlayer magnetotransport observed in α‐(BEDT‐TTF)2I3 in terms of massless Dirac fermions: the interlayer Hall effect and high‐field interlayer magnetoresistance (MR). The former is the anomalous Hall effect, which does not depend on magnetic field strength. It depends only on field orientation showing cotθ‐type angle dependence. We have derived the lowest order contribution of interlayer coupling to conductivity and successfully reproduced the interlayer Hall effect as a transport phenomenon in the massless Dirac fermion system at the quantum limit. The latter is an exponential increase of interlayer MR and its follow‐on saturation in higher magnetic fields. The exponential increase is evidence of the existence of a mobility gap at the Fermi level. It strongly suggests that the ν = 0 quantum Hall state is realized by spin splitting. In this case, two edge states with opposite spin and chirality are formed on the edge surface. The surface transport via edge states becomes dominant in interlayer transport in high enough fields, where bulk transport becomes exponentially small. The observed saturation of interlayer MR is well explained with this scenario.