第14回　物理工学科教室談話会(講師：Prof. Stephan Roche)

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概要：
Graphene has been heralded as the ideal material to achieve long spin propagation and further control the spin degree of freedom, in the quest of advancing non-charge-based information processing and computing, and for creating a new generation of active (CMOS compatible) spintronic devices together with non-volatile low energy MRAM memories. Despite ultralow μeV intrinsic and Rashba spin-orbit couplings (SOC) in clean graphene, measured spin lifetimes remain in the range of few nanoseconds, which is orders of magnitude shorter than initially predicted theoretically [1]. Besides, the physics of graphene “can be enriched and manipulated” by harvesting the large amount of possibilities of proximity effects with magnetic insulators, strong SOC materials (like transition metal dichalcogenides-TMDC, topological insulators, etc.). Simultaneously, the presence of extra quantum degrees of freedom (sublattice pseudospin, valley isospin) points towards new directions for information processing [2], extending the playground to valleytronics, multifunctional electronic devices or even disruptive quantum computing by harnessing all these degrees of freedom in combination with electromagnetic fields or other external fields (strain, nano-patterning, etc) [1]. One challenge is to endow a sizable spin-to-current conversion efficiency by enhancing spin-orbit interaction (up to meV), or to understand if similar valley Hall currents can be generated. Claims have been made that very large spin Hall effect-SHE, as well as topological valley Hall currents could be generated by using chemical functionalization with hydrogen or Au/Cu ad-atoms, or interfacing graphene with TMDC or hBN substrates [3]. Those results are however fiercely questioned [4] and the understanding of spin/valley dynamics for Dirac fermions in all those heterostructures remains elusive.
In this talk, will present the foundations of spin transport for Dirac fermions propagating in supported graphene devices or interfaced with strong SOC materials. The role of “valley and sublattice pseudospins” in tailoring the spin dephasing and relaxation mechanisms will be explained as well as the impact of strong SOC proximity effects on spin lifetime anisotropy, weak antilocalization and SHE [4].

[1] S. Roche et al, 2D Materials 2, 030202 (2015)
[2] J. Balakrishnan et al., Nature Comm.5, 4748 (2014). A. Avsar et al., Nat. Comm. 5, 4875 (2014).
[3] Y. Wang, X. Cai, J. Reutt-Robey, and M. S. Fuhrer, Phys. Rev. B 92, 161411 (2015). A. A. Kaverzin and B. J. van Wees, Phys. Rev. B 91, 165412 (2015). Z. Wang et al, Nat. Comm. 6, 8339 (2015)
[4] D. Van Tuan et al, Nature Physics, 10, 857–863 (2014); D. Van Tuan et al., Sc. Rep. 6, 21046 (2016); A.W. Cummings and S. Roche, Phys. Rev. Lett 116, 086602 (2016).
[5] A. Cresti, D. Van Tuan, D. Soriano, A. W. Cummings, S. Roche, Phys. Rev. Lett 113, 246603 (2014). D. Van Tuan and S. Roche, Phys. Rev. Lett. 116, 106601 (2016); D. Van Tuan et al., Phys. Rev. Lett 117, 176602 (2016); A. Cresti, B.Nikolic, J.H. Garcia, S.Roche, Riv. Nuov. Cim. 39,587-667 (2016); J.H. Garcia, A.W. Cummings, S. Roche, Nano Lett. 17 (8), 5078–5083 (2017). A. W. Cummings, J.H. García, J. Fabian, S. Roche, arXiv:1705.10972 (Phys. Rev. Lett. in press)
ICN2 – Institut Catala de Nanociencia i Nanotecnologia, Campus UAB, 08193 Bellaterra (Barcelona), Spain ICREA – Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
stephan.roche@icn2.cat
紹介教員：岩佐教授、求教授