Berlin 2024 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 53: 2D Materials and Heterostructures: (Twisted) Bilayers (joint session HL/TT)

HL 53.8: Talk

Friday, March 22, 2024, 11:45–12:00, EW 201

Valley transport assisted by Fermi surface warping — Josep Ingla-Aynés¹, •Antonio L. R. Manesco¹, Talieh S. Ghiasi¹, Serhii Volosheniuk¹, Kenji Watanabe², Takashi Taniguchi³, and Herre S. J. van der Zant¹ — ¹Kavli Institute of Nanoscience, Delft University of Technology, Netherlands — ²Research Center for Electronic and Optical Materials, National Institute for Materials Science, Japan — ³Research Center for Materials Nanoarchitectonics, National Institute for Materials Science, Japan

Valleys are distinct energy extrema in a band structure. Graphene has a band structure with two valleys at distinct high-symmetry points. The large momentum separation of these points suppresses intervalley scattering in ballistic devices. However, even in ballistic devices, valley coherence is limited by atomically-sharp edge disorder. Gate-defined bilayer graphene devices overcome this limitation and enable the observation of valley coherent phenomena. In this work, we explored ballistic electron transport in multiterminal bilayer graphene devices. We observed specular electron-focusing between gate-defined quantum point contacts, suggesting that smooth edges preserve valley polarization. Moreover, trigonal warping of the Fermi surface causes valley-dependent electron jetting, which we detected with magnetic collimation. Our results show two current peaks in the collector signal at opposite magnetic fields, consistent with the injection of two valley-polarized electron jets. Since the valley polarization of the electron current depends on the magnetic field, collimation devices are current sources with tuneable valley polarization.

Keywords: Valley degrees of freedom; Graphene; Trigonal warping; Fermi surface; Mesoscopic transport