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 two-dimensional Materials
  • Predictive Modelling and Multiscale numerical simulation of complex nanomaterials and quantum nanodevices

1) The publication of the Spintronic roadmap for graphene and 2D Materials. We have reviewed current challenges and perspectives in graphene spintronics, which is one of the most promising directions of innovation, given its room-temperature long-spin lifetimes and the ability of graphene to be easily interfaced with other classes of materials (ferromagnets, magnetic insulators, semiconductors, oxides, etc), allowing proximity effects to be harvested. This work is the result of the first two-years of active research conducted within the Graphene Spintronics Work Package consortium within the Graphene Flagship project, and co-supervised by ICN2 and University of Gröningen. Based on such progress, which establishes the state of the art, several novel opportunities for spin manipulation such as the generation of pure spin current (through spin Hall effect) and the control of magnetization through the spin torque phenomena are already seen in the horizon. Practical applications are within reach, but will require the demonstration of wafer-scale graphene device integration, and the realization of functional prototypes employed for determined applications such as magnetic sensors or nano-oscillators. This is the ambition of the new research phase which has started in April 2016.

2) Efficient linear scaling approach for computing the Kubo Hall conductivity. We have developed an order-N approach to compute the Kubo Hall conductivity for disordered two-dimensional systems reaching tens of millions of orbitals, and realistic values of the applied external magnetic fields (as low as a few Tesla). A time-evolution scheme is employed to evaluate the Hall conductivity σxy using a wave-packet propagation method and a continued fraction expansion for the computation of diagonal and off-diagonal matrix elements of the Green functions. The validity of the method is demonstrated by comparison of results with brute-force diagonalization of the Kubo formula, using (disordered) graphene as the system of study. This approach to mesoscopic system sizes is opening an unprecedented perspective for so-called reverse engineering in which the available experimental transport data are used to get a deeper understanding of the microscopic structure of the samples. Besides, this will not only allow addressing subtle issues in terms of resistance standardization of large-scale materials (such as wafer scale polycrystalline graphene), but will also enable the discovery of new quantum transport phenomena in complex two-dimensional materials, out of reach with classical methods. The results were published in Physical Review B.

3) Spin transport in hydrogenated graphene. We have presented the multifaceted problem of spin transport in hydrogenated graphene from a theoretical perspective in 2D Materials journal. The current experimental findings suggest that hydrogenation can either increase or decrease spin lifetimes, which calls for clarification. We first discuss the spin-orbit coupling induced by local sigma-pi re-hybridization and sp3 C-H defect formation together with the formation of a local magnetic moment. First-principles calculations of hydrogenated graphene unravel the strong interplay of spin-orbit and exchange couplings. The concept of magnetic scattering resonances has been revisited by describing the local magnetism through the self-consistent Hubbard model in the mean field approximation in the dilute limit, while spin relaxation lengths and transport times are computed using an efficient real space order N wavepacket propagation method. Typical spin lifetimes on the order of 1 ns are obtained for 1 ppm of hydrogen impurities (corresponding to a transport time of about 50 ps), and the scaling of spin lifetimes with impurity density is described by the Elliott-Yafet mechanism. This reinforces the statement that local defect-induced magnetism can be at the origin of the substantial spin polarization loss in the clean graphene limit.

 

Group Leader

Stephan Roche

ICREA Prof Stephan Roche

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

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Highlighted Publication

Localized electronic states at grain boundaries on the surface of graphene and graphite
Luican-Mayer A., Barrios-Vargas J.E., Falkenberg J.T., Autès G., Cummings A.W., Soriano D., Li G., Brandbyge M., Yazyev O.V., Roche S., Yandrei E. 2D Materials (2016)

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