Control of spin currents and thermoelectric properties in graphene

Abstract

Spintronics requires the efficient injection, transport, control and detection of electronic spins with the aim to provide faster classical computers and potentially become an enabling technology for quantum computing applications. Graphene has been identified as an excellent material for spin transport, however, there are difficulties found with efficient injection and detection of spin and also with its control; the latter due to graphene's lack of a band gap and small spin orbit coupling. In this thesis I present my research, focusing on bilayer graphene devices, which possesses a small but tunable band gap. The device architecture applied in this work, bilayer graphene fully encapsulated with hexagonal boron nitride (hBN) and 1D edge electrical/ferromagnetic contacts, enables a method to minimise the `conductance mismatch problem' which has a deleterious effect on efficient injection and detection of spins. I show the transport and control of spin in such a device at room temperature which has been fabricated with a top gate such that the application of a potential difference between this and the back gate is shown to open a bandgap and modulate the device's spin parameters. In addition, the same device also exhibits effects at room temperature which we propose are attributed to universal conductance fluctuations which have previously only been seen at low temperature and high magnetic field. A spin transport simulator was designed, implemented and verified as part of this research work which provides the ability to simulate a device with any number of spin injection contacts (and regions) with definable spin relaxation properties, where the transport channel is of finite or infinite length. This simulator goes beyond those that are documented in the literature and enables a more realistic and extensible device spin transport analysis. It is expected that this simulator could be used to assist with device design, post-measurement analysis and as an educational tool for new members of the team. A complementary aspect of this technology is that of thermoelectric effects which we propose could be used as a tool for probing the electronic band structure of the heterostructure. We show that the additional features, that are the signature of charge transport in an aligned Moire superlattice, are reflected very clearly in the thermoelectric measurements. We find however that the standard theory by Mott does not provide a good description of the thermoelectric response of either a non-aligned or aligned device, suggesting that a more refined or different theory may be necessary.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Irina Grigorieva (Supervisor) & Ivan Vera Marun (Supervisor)