Graphene is commonly considered a zero-gap semiconductor with a Dirac-like band dispersion at the K and K′ points of the Brillouin zone. Yet, at low temperature, the spin–orbit interaction should open a tiny bandgap in the bulk. Only the sample edges would then hold spin-polarized bands connecting the electron bands at K with the hole bands at K′ and vice versa. Sichau et al. now provide experimental evidence for this predicted topologically non-trivial state of graphene.
The researchers produce a graphene device that minimizes sources of extrinsic spin–orbit coupling. Then they measure the sample resistance as a function of applied microwave excitation frequency in a magnetic field. In this electron spin resonance (ESR) experiment, the resonant microwave excitation of transitions that couple carriers of opposite spin changes the sample resistance. At zero magnetic field, no transition is allowed, but at higher field, Sichau et al. detect two ESR lines. The frequency difference between these lines corresponds to the energy gap (42.2 μeV) induced by the spin–orbit coupling, in agreement with some of the earlier theoretical predictions.
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Heinrich, B. A spin–orbit gap in graphene. Nat. Nanotechnol. 14, 194 (2019). https://doi.org/10.1038/s41565-019-0409-y