Theoretical and Computational Nanoscience

Group Leader: Stephan Roche

Main Research Lines

  • Theoretical research on quantum transport phenomena in topological quantum matter (topological insulators, Weyl semimetals)

  • Spin dynamics and entanglement properties in Dirac matter (graphene, two-dimensional materials) and van der Waals heterostructures

  • Molecular dynamics, thermal transport properties and thermoelectricity in nanomaterials of interest for microelectronics (amorphous Graphene and boron nitride)

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

Theoretical and Computational Nanoscience

In 2021 the group published the following relevant works:

Linear scaling quantum transport methodologies and launch of LSQUANT

In a fundamental review on the topic of linear scaling quantum transport techniques, which have been pioneered and led by the group over the past 25 years, we have described and compared different order-computational methods to explore quantum transport phenomena in disordered media. A particular focus was given to the zero-frequency electrical conductivities derived within the Kubo-Greenwood and Kubo-Streda formalisms, while detailed illustrations of the capabilities of these methods to tackle the quasi-ballistic, diffusive, and localization regimes of quantum transport in the noninteracting limit were provided. The fundamental issue of computational cost versus accuracy of the various proposed numerical schemes was addressed in depth, and the usefulness of these methods was shown on several examples of transport in disordered materials, such as polycrystalline and defected graphene models, 3D metals and Dirac semimetals, carbon nanotubes, and organic semiconductors. Finally, we extended the review to the study of spin dynamics and topological transport, for which efficient approaches for calculating charge, spin, and valley Hall conductivities. In parallel a new flagship initiative LSQUANT was launched by the group (lsquant.org), to take these methods to the next level, i.e. enlarging the user and developer community, enhancing international networking, engaging young researchers interested in quantum science and quantum technologies, and connecting new developments to technology challenges of global concern.

Low-symmetry topological materials for large charge-to-spin interconversion: The case of transition metal dichalcogenide monolayers

Together with the Center for Advanced 2D Materials from the National University of Singapore and the University of Grenoble Alpes (France), we have predicted a very large gate-tunable SHE figure of merit λsθxy ≈ 1-50 nm in MoTe2 and WTe2 monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length (λs) and charge-to-spin interconversion efficiency as large as θxy ≈ 80%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation have been proposed, paving the way toward exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities.

Manipulation of spin transport in graphene/transition metal dichalcogenide heterobilayers upon twisting

In collaboration with Utrecht University and Prof Pablo Ordejón’s group at ICN2, we have performed large-scale first principles calculations to demonstrate that strain and twist-angle strongly vary the spin-orbit coupling in graphene/transition metal dichalcogenide heterobilayers. Such a change results in a modulation of the spin relaxation times by up to two orders of magnitude. Additionally, the relative strengths of valley-Zeeman and Rashba spin-orbit coupling can be tailored upon twisting, which can turn the system into an ideal Dirac-Rashba regime or generate transitions between topological states of matter. These results shed new light on the debated variability of spin-orbit coupling and clarify how lattice deformations can be used as a knob to control spin transport. Our outcomes also suggest complex spin transport in polycrystalline materials, due to the random variation of grain orientation, which could reflect in large spatial fluctuations of spin-orbit coupling fields.

Janus monolayers of magnetic transition metal dichalcogenides as an all-in-one platform for spin-orbit torque

In collaboration with KAUST from South Arabia, we theoretically predicted that vanadium-based Janus dichalcogenide monolayers constitute an ideal platform for spin-orbit torque memories. Using first-principles calculations, we demonstrated that magnetic exchange and magnetic anisotropy energies are higher for heavier chalcogen atoms, while the broken inversion symmetry in the Janus form leads to the emergence of Rashba-like spin-orbit coupling. The spin-orbit torque efficiency is evaluated using optimized quantum transport methodology and found to be comparable to heavy nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in such materials with tunable Fermi-level opens new possibilities for monitoring magnetization dynamics in the perspective of nonvolatile magnetic random-access memories.

Hinge spin polarization in magnetic topological insulators revealed by resistance switch

As part of our contribution of the European project TOCHA, led by the ICN2, we have discovered the possibility to detect hinge spin polarization in magnetic topological insulators by resistance measurements. By implementing a three-dimensional model of magnetic topological insulators into a multi-terminal device with ferromagnetic contacts near the top surface, local spin features of the chiral edge modes have been unveiled. We also found that local spin polarization at the hinges inverts sign between top and bottom surfaces. At the opposite edge, the topological state with inverted spin polarization propagates in the reverse direction. As a consequence, we could predict large resistance switch between forward and backward propagating states, as a result of the matching between the spin polarized hinges and the ferromagnetic contacts. This feature is general to the ferromagnetic, antiferromagnetic and canted-antiferromagnetic phases, and enables the design of spin-sensitive devices, with the possibility of reversing the hinge spin polarization of the currents.


Group Leader

Stephan Roche

ICREA Research Professor
stephan.roche@icn2.cat

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.

He has published more than 250 papers in journals such as Nature, Review of Modern Physics, Nature Physics, Nano Letters and Physical Review Letters and he is the co-author of the book titled “Introduction to Graphene-Based Nanomaterials: From Electronic Structure to Quantum Transport” (Cambridge University Press, 2020-second edition).

He received the qualification to supervise PhD students from the Université Joseph Fourier (Grenoble, France) in 2004, and since then he has supervised more than ten PhD students and about 25 postdoctoral researchers in France, Germany and Spain. In 2009 Prof. Roche was awarded the Friedrich Wilhelm Bessel Research Award by the Alexander Von-Humboldt Foundation (Germany) and, since 2011, he has been actively involved in the European Graphene Flagship project as deputy leader of the Spintronics Work Package (WP). He is serving as leader of this WP since April 2020 and will continue until March 2023. He is also Division Leader of the Graphene Flagship.

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