Estudi teòric de dinàmica d'espin en materials quantics bidimensionals

Resumen

The field of spintronics aims at using the spin degree of freedom to store, transport and manipulate information in next-generation electronic devices. In this thesis, I use quantum transport methodologies to simulate spin dynamics in devices made of two-dimensional materials. The first part of the thesis focuses on spin transport in graphene, while the second part deals with charge-to-spin interconversion effects and topological phenomena in low-symmetry transition metal dichalcogenides (TMDs). The Landauer-Büttiker formalism has been employed, as implemented in the open-source Kwant package, to simulate different kinds of electronic devices, including nonlocal spin valves. In graphene, I reveal that the full geometry of nonlocal spin valves should be taken into account when analyzing experiments in the diffusive regime when the spin transport is very efficient; otherwise spin diffusion lengths might be underestimated. Furthermore, I predict the experimental outcome of a Hanle spin precession measurement when the material quality drives the system towards a (quasi)ballistic transport regime, a regime that is not captured by the typical spin diffusion theory used to interpret experiments. For TMDs, I show that the low symmetry common in some phases of this class of materials directly affects their spin texture, which in turn impacts the spin transport, as well as charge-to-spin interconversion processes such as the spin Hall effect. The spin polarization of electrons in these TMDs displays a momentum invariant (persistent) spin texture fixed in a direction along the yz plane, and as a result, anisotropic spin relaxation is found. The spin Hall effect exhibits an unconventional component, with spin accumulation generated in the plane, which together with the conventional out-of-plane polarization, forms an oblique or canted spin Hall effect. Near the band gap region, the charge-to-spin interconversion efficiency reaches values as large as 80% and, when the Fermi level is placed in the topologically nontrivial gap, a canted quantum spin Hall effect is predicted. The corresponding topologically protected edge states are robust to disorder and carry spins polarized in the same direction as the persistent spin texture found at the bottom of the conduction bands. The findings presented in this thesis open a new perspective to predict and scrutinize spin transport in high-quality graphene devices and topological, low-symmetry two-dimensional materials.