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)

  • Thermal transport properties and thermoelectricity

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

In 2018 the group published the following four publications of note:

Ballistic tracks in graphene nanoribbons

In collaboration with an experimental group in Germany (Technische Universität Chemnitz), and the Danish Graphene Center from DTU we have theoretically analyzed unprecedented results obtained and asymmetric terminations at opposite ribbon edges due to the underlying SiC structure morphology. Our findings demonstrate a precise control of transport through multiple, independent, ballistic tracks in graphene-based devices, opening intriguing pathways for quantum information device concepts.

Origin of nonlocal resistance in multiterminal graphene on hexagonal-boron-nitride: Fermi surface edge currents rather than Fermi sea topological valley currents

Combining ab initio with quantum transport calculations, we have demonstrated that G/hBN wires with zigzag edges host low-energy dispersive edge states that are absent in theories based on the simplistic Hamiltonian, and are strongly resilient to disorder effects. Such edge states resolve the long-standing puzzle of why the highly insulating state of G/ hBN is rarely observed and allow to conclude that intruiguing non local resistance signals obtained in experiments are unrelated to Fermi sea topological valley currents conjectured for gapped Dirac spectra, as wrongly proposed by some theory.

Tailoring emergent spin phenomena in Dirac material heterostructures

Together with an experimental group in Chalmers University of Technoloy, We have investigated the spin transport properties of heterostructures combining graphene with topological insulators (TIs) in van der Waals heterostructures, and have demonstrated the emergence of a strong proximity-induced spin-orbit coupling in graphene. By performing spin transport and precession measurements supported by ab initio simulations, we discover a strong tunability and suppression of the spin signal and spin lifetime due to the hybridization of graphene and TI electronic bands. The enhanced spin-orbit coupling strength is estimated to be nearly an order of magnitude higher than in pristine graphene. These findings in graphene-TI heterostructures could open interesting opportunities for exploring exotic physical phenomena and new device functionalities governed by topological proximity effects.

Spin Proximity Effects in Graphene/Topological Insulator Heterostructures

Enhancing the spin-orbit interaction in graphene, via proximity effects with topological insulators, could create a novel 2D system that combines nontrivial spin textures with high electron mobility. To engineer practical spintronics applications with such graphene/ topological insulator (Gr/TI) heterostructures, an understanding of the hybrid spin-dependent properties is essential. However, to date, despite the large number of experimental studies on Gr/TI heterostructures reporting a great variety of remarkable (spin) transport phenomena, little is known about the true nature of the spin texture of the interface states as well as their role on the measured properties. By using ab initio simulations and tight-binding models, we have determined the precise spin texture of electronic states in graphene interfaced with a Bi2Se3 topological insulator. Our calculations predict the emergence of a giant spin lifetime anisotropy in the graphene layer, which should be a measurable hallmark of spin transport in Gr/TI heterostructures and suggest novel types of spin devices.

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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