Spin and valley degree of freedom in a bulk massless-Dirac electron system, α-(BEDT-TTF)₂I₃ under magnetic fields

Since the discovery of massless-Dirac Fermions (MDF) in graphene, MDF states have found in a
range of materials. A quasi-2D organic conductor, α-(BEDT-TTF)₂I₃, in which a MDF phase appears under pressure. The bulk nature of α-(BEDT-TTF)₂I₃ enables one to investigate the properties by experimental probes that require sample volume, such as NMR.
When the magnetic field is applied normal to the 2D planes of MDFs, the orbital-quantized Landau levels (LLs) contains the n=0 state (zero mode LL). The MDF system has two degrees of freedom, spin and valley. In case of non-zero fields, there are two possible states, depending on which of spin- or valley-splitting dominates over the other in energy, that is, spin polarized (QHF) and a non-magnetic state (QHI).
We measured ¹³C NMR with a magnetic field applied perpendicular to the 2D layers. With decreasing temperature, the shift of spectra decreases, since the thermal-averaged density of state characteristic of Dirac cones decreased. At low T, the NMR shift increases because of the formation of the zero-mode LL. Upon further cooling, a large NMR shift is observed, which indicates the generation of local fields. The value of the shift agrees with the value estimated from the Zeeman spin splitting of the zero-mode LL, indicating the QHF state.