[15p-2D-1~8] 4.6 Nanocarbon and 2D materials photonics

▲ [15p-2D-1] [JSAP-OSA Joint Symposia 2015 Invited Talk] Valleytronic properties and devices in 2D crystals

〇Yoshihiro Iwasa^1,2 (1.University of Tokyo, 2.RIKEN)

Keywords:transition metal dichalcogenides,valleytronics,transistor

Layered transition metal dichalcogenides (TMDs) are fruitful platform for electronics, spintronics, and opto-valleytronics. The monolayer TMDs have a similar crystal structure as staggered graphene and thus various physics predicted for inversion asymmetric graphene also inheres in monolayer TMDs. Especially, valley related physics are of particular importance. The broken inversion symmetry splits six Fermi pockets, locating at the first Brillouin zone edges, into two inequivalent groups (±K). The existence of valley degree of freedom is the base requirement for valleytronics. The broken inversion symmetry also lead to finite and valley-depended Berry curvature, which leads to valley-depended optical selection rule (valley circular dichroism), Zeeman-type spin splitting, and valley Hall effect [1]. After the fundamental investigation of valley circular dichroism in TMDs by polarization-resolved photoluminescence, valley-dependent spin splitting [2] and light-induced valley Hall effect [3] were experimentally observed.
We have investigated p-n junctions embedded in TMDs, in terms of opto-electronic applications. Taking advantage of the ambipolar transport characteristics, p-n junctions can be electrostatically formed in channel TMD materials using field effect transistor (FET) geometry [4]. Among various FETs, electric double layer transistor (EDLT), a FET using liquid dielectrics, have been manifested their potentials upon TMDs by field-induced superconductivity [5] or control of spin relaxation [6]. For opto-electronic devices, we observed electrically controllable helical electroluminescence from TMD p-n junction formed by EDLTs [7]. In a stark contrast, such a functionality is absent in junctions formed by conventional FETs [8]. The origin of this phenomenon lies in the anisotropic band dispersion (trigonal warping) that, under in-plane electric field, leads to valley-dependent carrier transport and electron-hole recombination.

References
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[4] Y. J. Zhang, J. T. Ye, Y. Yomogida, T. Takenobu, and Y. Iwasa, Nano Lett. 13, 3023 (2013).
[5] J. T. Ye et al., Science 338, 1193 (2012).
[6] H. T. Yuan et al., Nat. Phys. 9, 563 (2013).
[7] Y. J. Zhang, T. Oka, R. Suzuki, J. T. Ye, and Y. Iwasa, Science 344, 725 (2014).
[8] A. Pospischil, M. M. Furchi, and T. Mueller, Nat. Nanotechnol. 9, 257 (2014).