All-spin logic operations: Memory device and Reconfigurable computing

Exploiting spin degree of freedom of electron a new proposal is given to characterize spin-based logical operations using a quantum interferometer that can be utilized as a programmable spin logic device (PSLD). The ON and OFF states of both inputs and outputs are described by {\em spin} state only, circumventing spin-to-charge conversion at every stage as often used in conventional devices with the inclusion of extra hardware that can eventually diminish the efficiency. All possible logic functions can be engineered from a single device without redesigning the circuit which certainly offers the opportunities of designing new generation spintronic devices. Moreover we also discuss the utilization of the present model as a memory device and suitable computing operations with proposed experimental setups.Comment: 6 pages, 7 figure

Graphene spin circuits and spin-orbit phenomena in van der Waals heterostructures with topological insulators

Spintronics offers an alternative approach to conventional charge-based information processing by using the electron spin for next-generation non-volatile memory and logic technologies. To realize such technologies, it is necessary to develop spin-polarized current sources, spin interconnects, charge-to-spin conversion processes, and gate-tunable spintronic functionalities. The recently emerged two-dimensional (2D) and topological materials represent a promising platform to realize such spin-based phenomena. Due to its small spin-orbit coupling (SOC), graphene was predicted to preserve electron spin coherence for a long time, making it an ideal material for spin communication. In contrast, topological insulators (TIs) have high SOC and develop a nontrivial band structure with insulating bulk but conducting spin-polarized surface states. Combining these materials in van der Waals heterostructures has been predicted to give rise to unique proximity-induced spin-orbit phenomena that may be used for electrical control of spin polarization.In this thesis, we experimentally prove that the large-area chemical vapor deposited (CVD) graphene is an excellent material choice for the realization of robust spin interconnects, which are capable of spin communication over channel lengths exceeding 34 μm. Utilizing such graphene, we realize a spin summation operation in multiterminal devices and employ it to construct a prototype spin majority logic gate operating with pure spin currents. In topological insulators, we electrically detect the spin-momentum locking and reveal how the bulk and surface conducting channels affect the charge-to-spin conversion efficiency. Finally, by combining graphene and TIs in hybrid devices, we confirm the emergence of a strong proximity-induced SOC with a Rashba spin texture in graphene. We further show that in such heterostructures a spin-charge conversion capability is induced in graphene via the spin-galvanic effect at room temperature and reveal its strong tunability in magnitude and sign by the gate voltage. These findings demonstrate the robust performance of graphene as a spin interconnect for emerging spin-logic architectures and present all-electrical and gate-tunable spintronic devices based on graphene-TI heterostructures, paving the way for next-generation spin-based computing

Transport Theory for Materials with Spin-Orbit Coupling: Physics to Devices

Materials with spin-orbit coupling (SOC) exhibiting spin-momentum locking (SML) are of great current interest in spintronics because of their ability to efficiently convert charge signals into spin signals and vice versa. This dissertation develops a generalized diffusion equation with four electrochemical potentials starting from the standard Boltzmann transport equation and maps it to a transmission line model. This model applies to diverse materials with SOC including topological insulators, transition metals, narrow bandgap semiconductors, perovskite oxides, etc. and presents a new viewpoint suggesting that materials with low Fermi wave vector lead to larger spin voltages. The model has been used to make a number of predictions some of which have later received experimental confirmation up to room temperature. We also use it to propose new devices for writing and reading information to and from magnets. Specifically, we show using experimentally established phenomena that magnetic state can be read without conventional magnetoresistive devices. We analyze the proposals with SPICE compatible multi-physics framework along with a new model developed in this dissertation for pure spin conduction by magnon diffusion in ferromagnetic insulators

Giant Resistance Switch in Twisted Transition Metal Dichalcogenide Tunnel Junctions

Resistance switching in multilayer structures are typically based on materials possessing ferroic orders. Here we predict an extremely large resistance switching based on the relative spin-orbit splitting in twisted transition metal dichalcogenide (TMD) monolayers tunnel junctions. Because of the valence band spin splitting which depends on the valley index in the Brillouin zone, the perpendicular electronic transport through the junction depends on the relative reciprocal space overlap of the spin-dependent Fermi surfaces of both layers, which can be tuned by twisting one layer. Our quantum transport calculations reveal a switching resistance of up to $10^6 \%$ when the relative alignment of TMDs goes from $0^{\circ}$ to $60^{\circ}$ and when the angle is kept fixed at $60^{\circ}$ and the Fermi level is varied. By creating vacancies, we evaluate how inter-valley scattering affects the efficiency and find that the resistance switching remains large ($10^4 \%$) for typical values of vacancy concentration. Not only this resistance switching should be observed at room temperature due to the large spin splitting, but our results show how twist angle engineering and control of van der Waals heterostructures could be used for next-generation memory and electronic applications.Comment: Feedback is appreciate