[Paper Review] Gate-tunable Spin-Orbit-Coupling in Bilayer Graphene-WSe$_2$-heterostructures

Julia Amann, Tobias Völkl+6 more|arXiv (Cornell University)|Dec 10, 2020

Graphene research and applications4 citations

TL;DR

This study demonstrates gate-tunable spin-orbit coupling in bilayer graphene by exfoliating it onto WSe₂, enabling electric-field control of Rashba and intrinsic spin-orbit parameters via dual gating. Weak antilocalization measurements reveal strong enhancement or suppression of spin-orbit scattering, confirming layer-polarization-dependent spin-orbit coupling tunable across a wide range of electric fields.

ABSTRACT

Spin-orbit coupling in graphene can be increased far beyond its intrinsic value by proximity coupling to a transition metal dichalcogenide. In bilayer graphene, this effect was predicted to depend on the occupancy of both graphene layers, rendering it gate-tunable by an out-of-plane electric field. We experimentally confirm this prediction by studying magnetotransport in a dual-gated WSe$_2$/bilayer graphene heterostructure. Weak antilocalization, which is characteristic for phase-coherent transport in diffusive samples with spin-orbit interaction, can be strongly enhanced or suppressed at constant carrier density, depending on the polarity of the electric field. From the spin-orbit scattering times extracted from the fits, we calculate the corresponding Rashba and intrinsic spin-orbit parameters. They show a strong dependence on the transverse electric field, which is well described by a gate-dependent layer polarization of bilayer graphene.

Motivation & Objective

• To experimentally verify the theoretical prediction that spin-orbit coupling in bilayer graphene can be tuned via an out-of-plane electric field.
• To investigate how layer-specific carrier occupancy in bilayer graphene modulates spin-orbit coupling through proximity to WSe₂.
• To measure and quantify the gate-dependent evolution of Rashba and intrinsic spin-orbit parameters in a dual-gated heterostructure.
• To establish a link between transverse electric field, layer polarization, and spin-orbit scattering times in bilayer graphene.

Proposed method

• Fabrication of dual-gated WSe₂/bilayer graphene heterostructures using mechanical exfoliation and hBN encapsulation.
• Application of dual-gate voltages to independently control carrier density and transverse electric field in bilayer graphene.
• Measurement of magnetotransport properties, particularly weak antilocalization, as a function of magnetic field and gate voltages.
• Fitting the weak antilocalization data to extract spin-orbit scattering times and deduce Rashba and intrinsic spin-orbit coupling parameters.
• Analysis of the dependence of spin-orbit parameters on the transverse electric field to confirm layer polarization effects.

Experimental results

Research questions

• RQ1 Can spin-orbit coupling in bilayer graphene be electrically tuned via an out-of-plane field in a WSe₂ heterostructure?
• RQ2 How does the transverse electric field influence the relative contributions of Rashba and intrinsic spin-orbit coupling?
• RQ3 To what extent does layer polarization in bilayer graphene modulate spin-orbit scattering?
• RQ4 Is the gate-tunable spin-orbit coupling consistent with theoretical models of layer-polarized bilayer graphene?

Key findings

• Weak antilocalization in the heterostructure can be strongly enhanced or suppressed at constant carrier density by tuning the gate voltage, indicating electric-field control of spin-orbit coupling.
• The extracted spin-orbit scattering times vary significantly with the transverse electric field, demonstrating tunability of spin-orbit interaction.
• Rashba and intrinsic spin-orbit parameters exhibit strong dependence on the gate-induced electric field, consistent with theoretical predictions.
• The observed field dependence is well described by a gate-tuned layer polarization model in bilayer graphene.
• The results confirm that spin-orbit coupling in bilayer graphene can be engineered via electrostatic gating in van der Waals heterostructures.