Spin-Dependent Transport Through a Colloidal Quantum Dot: The Role of Exchange Interactions

TL;DR

This paper investigates how exchange interactions influence spin states and transport properties in colloidal quantum dots, revealing conditions for spin blockade and implications for quantum computing at relatively high temperatures.

Contribution

It combines atomistic electronic structure calculations with quantum master equations to analyze spin-dependent transport and the role of exchange interactions in colloidal quantum dots.

Findings

Spin singlet is lowest energy for electrons due to low density of states.

High density of states and exchange lead to spin triplet being lowest for holes.

Spin blockades can persist up to 77K in strongly confined quantum dots.

Abstract

The study of charge and spin transport through semiconductor quantum dots is experiencing a renaissance due to recent advances in nano-fabrication and the realization of quantum dots as candidates for quantum computing. In this work, we combine atomistic electronic structure calculations with quantum master equation methods to study the transport of electrons and holes through strongly confined quantum dots coupled to two leads with a voltage bias. We find that a competition between the energy spacing between the two lowest quasiparticle energy levels and the strength of the exchange interaction determines the spin states of the lowest two quasiparticle energy levels. Specifically, the low density of electron states results in a spin singlet being the lowest energy two-electron state whereas, in contrast, the high density of states and significant exchange interaction results in a spin triplet being the lowest energy two-hole state. The exchange interaction is also responsible for spin blockades in transport properties, which could persist up to temperatures as high as 77K for strongly confined colloidal quantum dots from our calculations. Lastly, we relate these findings to the preparation and manipulation of singlet and triplet spin qubit states in quantum dots using voltage biases.