Theoretical and Computational Nanoscience

Group Leader: Stephan Roche

ICN2 Theoretical and Computational Nanoscience Group

Main Research Lines

  • Leading-edge theoretical research on quantum transport phenomena in graphene and 2D mateirals

  • Spin dynamics and entanglement properties in Dirac matter (graphene, topological insulators)

  • Molecular dynamics, thermal transport properties and thermoelectricity in nanomaterials

  • Predictive modelling and multiscale numerical simulation of complex nanomaterials and quantum nanodevices

In 2019 the group published the following relevant works:

Universal spin diffusion length in polycrystalline graphene

Together with researchers from the Université Catholique de Louvain (Belgium), we have used first-principles simulations to study the impact of grain boundaries on spin transport in polycrystalline graphene. Our theoretical study shows that spin diffusion length in polycrystalline graphene is independent of grain size and depends only on the strength of the substrate-induced spin-orbit coupling. Moreover, this is valid not only for the diffusive regime of transport, but also for the weakly localized one, in which quantum phenomena begin to prevail. This is the first quantum simulation confirming that the same expression for spin diffusion length holds in both regimes. Such results highlight the fact that single-domain graphene may not be a requirement for spintronics applications, and that polycrystalline CVD-grown graphene may be as operational. This puts the focus on other aspects to enhance in graphene production, such as the elimination of magnetic impurities.

Nonvolatile memories based on graphene and related 2D materials

In collaboration with various groups from the Graphene Flagship consortium, we have reviewed how graphene and related 2D materials can be integrated into different types of non-volatile memory technologies, including resistive random-access, flash, magnetic and phase-change memories. We emphasized that the main challenges facing scientists today is to create durable devices that can run for over one billion switching cycles and achieving data retention times of over 10 years. We also pointed out that augmenting resistive RAM with graphene materials results in highly stable devices with very promising performance.

Room temperature spin Hall effect in graphene/MoS2 van der Waals heterostructures

Together with an experimental group at CIC-NANOGUNE, we have unambiguously demonstrated the formation of spin Hall effect in graphene induced by MoS2 proximity and for varying temperatures up to room temperature. The fact that spin transport and the spin Hall effect occur in different parts of the same material gives rise to some unprecedented efficiency for the spin-to-charge voltage output. Additionally, for a single graphene/MoS2 heterostructure-based device, a superimposed spin-to-charge current conversion was found but shown to be indistinguishably associated with either the proximity-induced Rashba–Edelstein effect in graphene or the spin Hall effect in MoS2. By a comparison of our theoretical calculations, the latter scenario was found to be the most plausible one. Such findings pave the way toward the combination of spin information transport and spin-to-charge conversion in two-dimensional materials, opening exciting opportunities in a variety of future spintronic applications.

Tunable circular dichroism and valley polarization in the modified Haldane model

In collaboration with the Japanese group of Prof. R. Saito from Tohoku University, we have studied the polarization dependence of optical absorption for a modified Haldane model, which exhibits so-called “antichiral edge modes” in the presence of sample boundaries and has been argued to be realizable in transition metal dichalcogenides or Weyl semimetals. A rich optical phase diagram has been revealed from our theoretical analysis, in which the correlations between perfect circular dichroism, pseudospin and valley polarization have been shown to be tuneable upon varying the charge density. Remarkably, perfect circular dichroism and valley polarization can be achieved simultaneously, a property hitherto unseen in all known materials. This combination of optical properties suggests some appealing photonic device functionality (e.g., light polarizer) which could be combined with valleytronics applications (e.g., generation of valley currents).

Group Leader

Stephan Roche

ICREA Research Professor

Prof. Stephan Roche is a theoretician with more than 25 years’ experience in the study of transport theory in low-dimensional systems, including graphene, carbon nanotubes, semiconducting nanowires, organic materials and topological insulators.

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