Theoretical and Computational Nanoscience Group

Group Leader: Stephan Roche

Main Research Lines

  • Leading-edge theoretical research on quantum transport phenomena in graphene
  • Spin dynamics in Dirac matter (graphene, topological insulators)
  • Thermal properties and thermoelectricity in 2D materials
  • Predictive modelling and multiscale numerical simulation of complex nanomaterials and quantum nanodevices

1) Spin Manipulation in Graphene by Chemically-Induced Sublattice Pseudospin Polarisation

Spin manipulation is one of the most critical challenges to realising spin-based logic devices and spintronic circuits. Graphene has been heralded as an ideal material to achieve spin manipulation, but so far new paradigms and demonstrators are limited. We have shown that certain impurities such as fluorine ad-atoms, which locally break sublattice symmetry without the formation of strong magnetic moments, could result in a remarkable variability of spin transport characteristics.

In 2014 our group discovered a novel spin relaxation mechanism in non-magnetic graphene samples connected to the unique spin-pseudospin entanglement occurring near the Dirac point. Such a finding has inspired new directions towards the control of the spin degree of freedom modifying the pseudospin or vice versa. In 2016 we have shown how a chemical functionalisation of graphene with certain types of ad-atoms such as fluorine, by breaking the sublattice symmetry and by inducing a SOC without the formation of strong magnetic moment, could provide an innovative technique to monitor spin transport properties for spintronic applications. Our theory also allows current experimental controversies to be revisited and brings an enabling building block for graphene spintronics.

2) Spin Hall effect in decorated graphene

Although graphene has attractive properties in spintronics, such as long room temperature spin diffusion length, it is inactive for the spin Hall effect (SHE), a spin transport phenomenon mediated by strong spin-orbit coupling in which opposite spins are deviated in contrary directions while propagating inside a channel. Several experiments reporting an unexpectedly large SHE in graphene decorated with ad-atoms, locally enhancing the spin-orbit coupling effects in graphene, have raised fierce controversy. Indeed to date, measured values for the spin Hall angle range from 0.0001  in semiconductors to 0.3 in some metals, which are finding important applications in the magnetic memory market. The measurements on decorated graphene indicate a spin Hall angle of about 0.2, which would make modified graphene technologically relevant.

We developed a fully quantum simulation of this phenomenon to analyse such intriguing experimental results and found that multiple background contributions to non-local resistance, which was argued to be the smoking gun of SHE, could resolve these controversies. A novel device geometry to suppress these contributions and quantify the upper limit for the SHE in 2-dimensional materials has been also proposed. Such results are opening new directions for experiments in this field and give some hope for the efficient engineering of the spin Hall effect in graphene-based materials.

3) Simulation CVD graphene devices 

Major roadblocks towards high-performance graphene devices are the nanoscale variations of graphene polycrystalline morphologies (grain boundaries, grain sizes), which strongly impact on all macroscopic physical properties (mechanical, electrical and thermal). The requirements in terms of device quality and uniformity are very demanding, and major roadblocks to the high-performance of many graphene devices stem from the complex structural morphologies of large-scale graphene (CVD, reduced graphene oxides, etc.), which are detrimental to their optimal macroscopic properties. 

We have clarified the impact of edges and grain boundaries on a large spectrum of properties, including charge mobility, Seebeck coefficient thermal conductivity and the thermoelectric figure of merit of CVD graphene. In particular we have reported on the scaling properties of polycrystalline graphene and hybrid graphene/hBN heterostructures, providing guidance for the optimisation of materials for a desired application.

4 Spin lifetime in ultraclean graphene devices 

We have clarified theoretically the fundamental properties of spin dynamics in ultraclean spin-orbit-coupled materials, by considering the quasiballistic limit, and introducing small broadening of electronic states due to thermal effects or electrical bias. In the ballistic limit, the spin lifetime was demonstrated to be dictated by dephasing effects arising from energy broadening plus a non-uniform spin precession, which is very unique to Dirac materials such as graphene and topological insulators. For the case of clean graphene, we find a strong anisotropy with spin lifetimes that can be short even for modest energy scales, on the order of a few nanoseconds. These results offer deeper insight into the nature of spin dynamics in graphene, and are also applicable to the investigation of other systems where spin-orbit coupling plays an important role.

Group Leader

Stephan Roche

ICREA Prof Stephan Roche

Prof. Stephan Roche is a theoretician with more than 20 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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