View larger image

Recently, topological materials have received considerable attention. Based on the topology concept, if a shape or space can be continuously deformed, then any shape changed by this continuous deformation can be considered identical. Well-known examples include the donut and mug, which are topologically identical in shape to a single hole, and whose number of holes characterizes their shape as topologically invariant.

A typical example of a topology is the quantum Hall effect (QHE). The QHE is a phenomenon observed primarily in two-dimensional (2D) electron systems under strong magnetic fields. The Hall conductivity is quantized as σ_xy＝νe²/h in the region where the conductivity σ_xx＝0, where the integer is the filling factor and can be regarded as topological invariant. The essence of the topological state in the quantum hall (QH) state is that the energy differences between the Landau levels result in an energy difference in the excitation of the bulk electrons. Another important feature is the appearance of a metallic state at the sample edge.

The fractional QHE has been discovered, in which is not an integer but a fraction with odd denominators (ν＝1/3,2/3,1/5, ⋅⋅⋅). The interaction between electrons is crucial, and the fractional QHE is realized in 2D systems with extremely high mobility. In addition, in systems with Dirac fermions, a half-integer QHE with ν＝N +1/2 per single spin and valley occurs, thus reflecting the -berry phase. This was first observed in monolayer graphene.

The QHE is typically observed in 2D systems at extremely low temperatures and high magnetic fields, whereas it rarely occurs in bulk (multilayered) crystals. Novel (fractional) QH states induced by interlayer interactions are expected in systems with strong electron correlations. Thus, the QHE in multilayered crystals with strong correlation should be investigated.

We successfully observed a QHE with ν＝1 in an organic massless Dirac fermion system α-(BETS)₂I₃ under high pressure at low temperatures and a magnetic field of approximately 1 T.

One of the remarkable features of the Dirac fermion in solids is the emergence of a peculiar 𝑁 = 0 Landau levels, which do not undergo orbital motion even when a magnetic field is applied. All electrons are accommodated in the zero-energy state in a strong magnetic field. However, the Zeeman and correlation effects split the 𝑁 = 0 Landau levels into four levels; thus, ν＝ 0,±1 QH states appear.

The observation of the QHE with ν＝1 a magnetic field of approximately 1 T suggests the high mobility of the system and that electron-electron interactions are important. A novel fractional QHE that reflects the interactions between layers is expected in this system.

(Written by N. Tajima on behalf of all authors)