Large tunable valley splitting in edge-free graphene quantum dots on boron nitride

TL;DR

This paper demonstrates that stacking graphene on boron nitride creates a platform where valley splitting in quantum dots can be precisely tuned and inverted, advancing the potential for graphene-based quantum bits.

Contribution

It introduces a method to achieve and control valley splitting in graphene quantum dots using van-der Waals stacking and scanning tunneling microscopy.

Findings

Valley splitting tunable from -5 to +10 meV.

Valley inversion achieved by sub-10-nm displacement.

Range of valley control increased by about tenfold.

Abstract

Coherent manipulation of binary degrees of freedom is at the heart of modern quantum technologies. Graphene offers two binary degrees: the electron spin and the valley. Efficient spin control has been demonstrated in many solid state systems, while exploitation of the valley has only recently been started, yet without control on the single electron level. Here, we show that van-der Waals stacking of graphene onto hexagonal boron nitride offers a natural platform for valley control. We use a graphene quantum dot induced by the tip of a scanning tunneling microscope and demonstrate valley splitting that is tunable from -5 to +10 meV (including valley inversion) by sub-10-nm displacements of the quantum dot position. This boosts the range of controlled valley splitting by about one order of magnitude. The tunable inversion of spin and valley states should enable coherent superposition of these degrees of freedom as a first step towards graphene-based qubits.