Suggestion for Direct Observation of Orbital Accumulation

The generation of orbital current is an intriguing research topic not only for developing energy-efficient control of spintronic devices, but also for observing new emerging phenomena, such as orbital-to-spin conversion. During the last two decades, many innovative measurement techniques have been developed for discoveries related to conversions between spin flow and natural phenomena such as light, heat, vibration, and charge flow. Observation of the orbital current also requires efforts that should result in many other state-of-the-art discoveries. However, a direct experimental way to observe the orbital current is still missing. In this article, we suggest that X-ray magnetic circular dichroism (XMCD) measurements may be a good candidate for directly observing the orbital current because it is the only way to detect orbital moment selectively. Just like spin accumulation, orbital current can also accumulate at the edge of a system, giving rise to a non-zero orbital magnetic moment. Although the orbital moment generated by orbital accumulation is expected to have a very small value, ~10‒5 μB, precise measurement with high sensitivity will allow direct observation of the orbital current.

Emergence of orbital angular moment at van Hove singularity in graphene/h-BN moiré superlattice

Bloch electrons lacking inversion symmetry exhibit orbital magnetic moments owing to the rotation around their center of mass; this moment induces a valley splitting in a magnetic field. For the graphene/h-BN moiré superlattice, inversion symmetry is broken by the h-BN. The superlattice potential generates a series of Dirac points (DPs) and van Hove singularities (vHSs) within an experimentally accessible low energy state, providing a platform to study orbital moments with respect to band structure. In this work, theoretical calculations and magnetothermoelectric measurements are combined to reveal the emergence of an orbital magnetic moment at vHSs in graphene/h-BN moiré superlattices. The thermoelectric signal for the vHS at the low energy side of the hole-side secondary DP exhibited significant magnetic field-induced valley splitting with an effective g-factor of approximately 130; splitting for other vHSs was negligible. This was attributed to the emergence of an orbital magnetic moment at the second vHS at the hole-side.