Spin-selective transport in semiconductor spintronics and single-defect quantum technology

Typically when one thinks of information and technology they reflect on the tremendous progress made in controlling the flow of electric current - a quantity proportional to the transport of electronic charge. The conservation of charge makes it a reliable quantity for encoding information and provides a sound foundation for technological devices. However, the electron’s charge is not the only useful conserved quantity. The intrinsic angular momentum or “spin” of the electron provides an additional handle for calculation and information storage. Unlike charge, the spin is a quantum mechanical quantity and therefore allows one to imagine devices that skirt the limitations imposed by classical systems, e.g. through the inherent ambiguity in the state of a spin, known as coherence, that allows it to exist as more than one physical outcome at once, until measured. While useful, this coherent state is easily destroyed through interactions between the spin and its environment. As a result, probing the delicate evolution of these spin states is a serious challenge, especially at the single-spin level. In addition, any engineering that prolongs the lifetime of the spin is highly desirable.

This thesis focuses on methods of probing and even controlling the coherent dynamics of spin through consideration of how they evolve in the presence of forbidden transitions. These forbidden transitions result from linking the spin state to additional characteristics like spatial orientation or the number of available final states during transport. Through this linking, the transport of certain spin states is not possible. As a first example, the electron spin is linked to spatial orientation through the influence of its motion around a nucleus. If transport is the result of discrete hops from one site to another, the relative orientation of neighboring sites can determine the lifetime of a given spin’s polarization. We predict that modification to the relative molecular orientation in organic conjugated semiconductors leads to ten-fold increases in the spin lifetime.

In addition to linking the spin to spatial orientation, one can link the spin of an isolated defect in a semiconductor to available states in a contact by building the contact from ferro-magnetic materials. Contacts of this type will preferentially admit a certain spin orientation restricting current flow through the defect when the resident spin does not match this orientation. The coherent evolution of the spin will rotate the orientation periodically, opening transport channels for states initially in an orientation with limited transport. We predict, in this case, that modification of the coherent evolution rate of the defect spin will lead to changes in the current measured through the defect.