Introduction to Spintronics and Quantum Dots

Spintronics, a new and innovative realm of research, is essentially the study of spin degrees of freedom in solids and molecules. Traditional electronics utilize electric charges whereas spintronics draw upon quantum information science, particularly for quantum computing. This is because the spin carries paramount importance for quantum computations. Transport of spins in solids and molecular systems stems from the transfer or tunneling of charged electrons. By this virtue, it is always accompanied by electric currents.

However, the advantages of spin over charge cannot be understated. Contrary to charge or mass, spin, or angular momentum to be precise, can be transferred without the need for tunneling or ballistic transport. There are mechanisms for the spin transfer without tunneling, especially in the case of optically excited semiconductor Quantum Dots (QDs). In such QDs, spin-polarized excitons can be transferred via long-range, non-contact Coulomb interaction. An important note here is that Coulomb (Förster) transfer of spin in QDs succeeds due to strong spin-dependent interactions in semiconductors, such as spin-orbit and exchange couplings.

The Mechanisms of Energy and Spin Transfer in Quantum Dot Pairs

This article delves into a theoretical exploration of the microscopic mechanisms of spin-dependent Förster transfer in a molecular pair of self-assembled QDs. The fundamental scheme of Förster transfer enlightens that an optically excited exciton in donor QD1 is transferred to acceptor QD2 via Coulomb interaction. This transfer can often be identified through time-resolved photoluminescence spectroscopy and photon correlations. It has also been discovered that in cases of resonant transfer in self-assembled quantum dots (QDs), the spin selection rules are governed by the electron-hole exchange interaction and the spin-orbit interaction.

In QD molecules with perfect symmetry, the transfer preserves the exciton spin configuration. However, in cases where the symmetry is disrupted, the exciton spin could be partially lost during the transfer process. The rate of transfer reflects a robust dependency on the exciton energy discrepancy in a QD pair. In cases where the resonant regime ∆E≈0 holds true, exciton and spin transfer occur rapidly. But in non-resonant regimes, the transfer can still be facilitated by acoustic phonons, heavily contingent upon ∆E.

The Dipole-Dipole Approximation and Beyond

Earlier discussions on spin transfer have frequently incorporated the dipole-dipole approximation. This approximation, however, is not trustable for the typical inter-dot distances in experimental structures. As a result, this article describes a method to compute Coulomb matrix elements beyond the dipole approach. This method is significant because it obtains validity whenR≫alattice, whereR is in the inter-dot distance and alattice is the lattice period.

Returning to the mechanism of Förster transfer tackled in the paper, it is essential to note its electrostatic, near-field nature. There has been a recent surge in discussions about the radiative coupling between quantum dots (QDs), but despite the surge, the Förster transfer mechanism presented here stands in contrast.

Future Implications and Applications

In conclusion, the Förster transfer of excitons can be studied using time-resolve photoluminescence or the photon cross-correlation method. Recent studies have delved into energy transfer in nano-structures with colloidal nano-crystals and self-assembledInAsQDs. A future boon of study could also be to navigate Coulomb transfer in QDs which could potentially be coupled via tunneling.

Moreover, the spin current of mobile polarized electrons is invariably accompanied by electric charge flow. This brings into contention the issue of energy dissipation in electronic devices, indicated by the Joule heat. However, in the case of Coulomb-induced transfer between QDs, the energy dissipation is of a different character and is a result of phonon-assisted relaxation.

The approaches discussed in this article could open new avenues for the exploration of spintronics and the manipulation of quantum dots, especially those related to the transfer of energy without dissipation.