Double Quantum Dots in Carbon Nanotubes

TL;DR

This paper investigates the two-electron energy spectrum in carbon nanotube double quantum dots, revealing how electron interactions, dot configuration, and magnetic fields influence ground states and quantum transport properties.

Contribution

It provides an exact and intuitive analysis of single-particle and interaction effects in carbon nanotube double quantum dots, including the impact on ground states and potential qubit stability.

Findings

Ground state is antisymmetric in spin-valley for symmetric dots and weak magnetic fields.

Coulomb interactions induce strong correlations and orbital mixing at high double occupation.

Configuration changes can lead to ferromagnetic ground states and affect Pauli blockade.

Abstract

We study the two-electron eigenspectrum of a carbon-nanotube double quantum dot with spin-orbit coupling. Exact calculation are combined with a simple model to provide an intuitive and accurate description of single-particle and interaction effects. For symmetric dots and weak magnetic fields, the two-electron ground state is antisymmetric in the spin-valley degree of freedom and is not a pure spin-singlet state. When double occupation of one dot is favored by increasing the detuning between the dots, the Coulomb interaction causes strong correlation effects realized by higher orbital-level mixing. Changes in the double-dot configuration affect the relative strength of the electron-electron interactions and can lead to different ground state transitions. In particular, they can favor a ferromagnetic ground state both in spin and valley degrees of freedom. The strong suppression of the energy gap can cause the disappearance of the Pauli blockade in transport experiments and thereby can also limit the stability of spin-qubits in quantum information proposals. Our analysis is generalized to an array of coupled dots which is expected to exhibit rich many-body behavior.