Harvesting the Manifold Quantum Degrees of Freedom in Dirac Fermions and their Interlinking for Controlling the Transport Mechanisms and Information Processing

February 8th, 2018 STEPHAN ROCHE Catalan Institute of Nanoscience Nanotechnology, UAB
ICREA, Institució Catalana de Recerca i Estudis Avancats

Graphene has been heralded as the ideal material to achieve long spin propagation and further control the spin degree of freedom, in the quest of advancing non-charge-based information processing and computing, and for creating a new generation of active (CMOS compatible) spintronic devices together with non-volatile low energy MRAM memories. Despite ultralow intrinsic and Rashba spin-orbit couplings (SOC) in clean graphene (μeV range), measured spin lifetimes reach the range of ten nanoseconds, which is orders of magnitude shorter than initially predicted theoretically, but largely enough to envision disruptive room-temperature applications. Besides, the physics of graphene “can be enriched and manipulated” by harvesting the large amount of possibilities of proximity effects with magnetic insulators, strong SOC materials (like metal-transition dichalcogenides-TMDC), topological insulators, etc. Simultaneously, the presence of extra quantum degrees of freedom (sublattice pseudospin, valley isospin) of low energy Dirac fermions, points towards new directions for information processing, extending the playground to valleytronics, multifunctional electronic devices or even disruptive quantum computing by harnessing all these degrees of freedom in combination with electromagnetic fields or other external fields (strain, nano-patterning, etc) [1]. One challenge is to endow a sizable spin-to-current conversion efficiency by enhancing spin-orbit interaction (say up to meV), or to understand if similar valley Hall currents can be generated. Claims have been made that very large spin Hall effect, as well as topological valley Hall currents could be generated by using chemical functionalization with hydrogen or Au/Cu ad-atoms, or interfacing graphene with metal transition dichalcogenides (TMDC) or hBN substrates. Those results are however fiercely questioned and the understanding of spin/valley dynamics for Dirac fermions in all those heterostructures has been elusive for a while.

In this talk, based on our recent theoretical breakthroughs, I will overview the foundations of charge and spin transport for Dirac fermions propagating in graphene supported onto substrates (SiO2, hBN) or interfaced with strong SOC materials (TMDC and topological insulators). The role of “valley and sublattice pseudospins” in tailoring the spin dephasing and relaxation mechanisms will be introduced in the ultraclean graphene limit while the impact of strong SOC proximity effects on spin lifetime anisotropy weak antilocalization and spin Hall effects will be presented and compared to experimental literature. New horizons for quantum computing will be discussed if time allows…

Thursday, February 8, 2018, 12:00. Blue Lecture Room

Hosted by Prof. Javier García de Abajo

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