Resumen de Acoblament espín-òrbita en dispositius de grafè/dicalcogenurs de metalls de transició
The presented work is within the fields of spintronics and spin-orbitronics, whose final aim is to control the electron's spin degree of freedom via the spin-orbit coupling (SOC) in solid-state systems. The quest for greater control of the electron's spin has taken an exciting direction with the isolation and later demonstration of spin injection in graphene. Because of the graphene intrinsic low SOC, spins can travel over long distances through its crystal lattice, resulting in an ideal channel for the spin transport. But at the same time, the graphene's low SOC inhibits the manipulation and the generation of spin currents, which are the cornerstone for implementing spin-based devices successfully. We can get over this limitation by enhancing graphene's SOC by proximity with transition metal dichalcogenides (TMDCs) in van der Waals heterostructures. TMDCs are two-dimensional semiconductors that possess a strong intrinsic SOC. In this thesis, using spin transport measurements, we investigate the induced SOC in graphene by the proximity to TMDCs to achieve two main objectives:
1. Obtain signatures of an enhanced spin-orbit and spin-valley coupling in graphene by proximity to a semiconducting TMDC using spin transport measurements.
2. Study the possibility to generate spin currents using the induced spin-orbit in graphene, which would be a building block for spin generation and detection free from magnetic materials.
To achieve such objectives, many efforts have been devoted to fabricating carefully designed samples, adapting and proposing experimental protocols based on spin precession measurements, and in the analysis and modeling of the signals. As a result, several relevant contributions to the understanding of spin-orbit phenomena in graphene/TMDCs have been achieved, which summarized according to the objectives are:
1. The unambiguous demonstration of anisotropic spin dynamics in heterostructures comprising graphene, WS2, MoS2, and WSe2. Using out-of-plane and oblique spin precession measurements, we show that the spin lifetime is largest when the spins point out of the graphene plane. We observe that the spin lifetime changes over an order of magnitude depending on the spin orientation, indicating that the strong spin-valley coupling of the TMDC is imprinted in the graphene and felt by the propagating spins. Moreover, we show that such anisotropic spin relaxation can be electrically controlled, changing from a highly anisotropic to a nearly isotropic regime. These findings provide a rich platform to explore coupled spin-valley phenomena and offer novel spin manipulation strategies based on spin relaxation anisotropy in two-dimensional materials.
2. The demonstration of strongly enhanced room-temperature spin-to-charge interconversion in graphene driven by the proximity of WS2. By performing spin precession experiments in Hall bars, we separate the contributions of the spin Hall and the spin galvanic effects. Remarkably, their corresponding conversion efficiencies can be tailored by electrostatic gating in magnitude and sign, peaking nearby the charge neutrality point with an equivalent magnitude that is comparable to the largest efficiencies reported to date. Such an electric-field tunability provides a building block for a spin-generation free from magnetic materials, and for ultra-compact magnetic memory technologies.
