Hyperfine-induced valley mixing and the spin-valley blockade in carbon-based quantum dots

TL;DR

This paper theoretically investigates how hyperfine interactions in carbon nanotube and graphene quantum dots induce valley mixing and influence the spin-valley blockade, revealing that both spin and valley degeneracies must be lifted for transport blockade.

Contribution

It introduces the concept that short-range hyperfine interactions couple valley and nuclear spins, affecting Pauli blockade in carbon-based quantum dots, a novel insight.

Findings

Hyperfine interaction couples valley and nuclear spins.

Transport blockade requires lifting both spin and valley degeneracies.

Four supertriplet states cause blockade in (1,1) charge configuration.

Abstract

Hyperfine interaction (HFI) in carbon nanotube and graphene quantum dots is due to the presence of 13C atoms. We theoretically show that in these structures the short-range nature of the HFI gives rise to a coupling between the valley degree of freedom of the electron and the nuclear spin, in addition to the usual electron spin-nuclear spin coupling. We predict that this property of the HFI affects the Pauli blockade transport in carbon-based double quantum dots. In particular, we show that transport is blocked only if both the spin and the valley degeneracies of the quantum dot levels are lifted, e.g., by an appropriately oriented magnetic field. The blockade is caused by four "supertriplet" states in the (1,1) charge configuration.