Electric-field independent spin-orbit coupling gap in hBN-encapsulated bilayer graphene

Abstract

The weak spin-orbit coupling (SOC) in bilayer graphene (BLG) is essential for encoding spin qubits while bringing technical challenges for extracting the opened small SOC gap Δ_SO in experiments. Moreover, in addition to the intrinsic Kane-Mele term, extrinsic mechanisms also contribute to SOC in BLG, especially under experimental conditions including encapsulation of BLG with hexagonal boron nitride (hBN) and applying an external out-of-plane electric displacement field D. Although measurements of Δ_SO in hBN-encapsulated BLG have been reported, the relatively large experimental variations and existing experimental controversy make it difficult to fully understand the physical origin of Δ_SO. Here, we report a combined experimental and theoretical study on Δ_SO in hBN-encapsulated BLG. We use an averaging method to extract Δ_SO in gate-defined single quantum dot devices. Under D fields as large as 0.57-0.90 V/nm, Δ_SO=53.4-61.8 μeV is obtained from two devices. Benchmarked with values reported at lower D field regime, our results support a D field-independent Δ_SO. This behavior is confirmed by our first-principle calculations, based on which Δ_SO is found to be independent of D field, regardless of different hBN/BLG/hBN stacking configurations. Our calculations also suggest a weak proximity effect from hBN, indicating that SOC in hBN-encapsulated BLG is dominated by the intrinsic Kane-Mele mechanism. Our results offer insightful understandings of SOC in BLG, which benefit SOC engineering and spin manipulations in BLG.