Session G67: Quantum Valley Hall Edge Channels: Highways for Electrons, Photons, and Surface Plasmons

Topological valleytronics using kink states in bilayer graphene: a valley valve and beam splitter

Two-dimensional honeycomb lattices possess a valley degree of freedom, the control of which offers opportunities to realize topologically protected edge modes to transport electrons without dissipation, and potentially contribute to new paradigms of electronics, namely valleytronics. In this talk, I will describe how we realize valley-momentum locked topological 1D channels, i.e. quantum valley Hall kink states, at internal line boundaries of two artificial domains of Bernal stacked bilayer graphene with opposite band gaps via asymmetrical gating [1]. These kink states possess mean free path of a few µm at B=0, and their helicity (±1) can be conveniently controlled by changing the gating polarity. In an all electrically defined “cross” device with 4 channels, I will demonstrate reconfigurable ballistic waveguides of the kink states, and valleytronic operations such as valley valve and electron beam splitter that exploit topological protection [2]. Kink states can ballistically transmit through two channels with aligned helicity (on state), while get blocked between channels with opposite helicity (off state). The on/off ratio reaches 800% at B=0, and it doesn’t require valley-polarized current to operate, instead relying on the control of topology. In a B-field, Fermi energy becomes a knob to continuously tune the current splitting ratio of kink states in two propagation directions, resembling the function of a quantum point contact for quantum Hall edge states. The versatile control and potential scalability of the kink states open the door to many exciting possibilities in low dimensional topological applications.
[1] J. Li, K. Wang, K. J. McFaul, Z. Zern, Y. F. Ren, K. Watanabe, T. Taniguchi, Z. H. Qiao, J. Zhu, “Gate-controlled topological conducting channels in bilayer graphene”, Nat. Nano. 11, 1060 (2016)
[2] J. Li, R. Zhang, Z. Yin, J. Zhang, K. Watanabe, T. Taniguchi, C. Liu, J. Zhu, “A valley valve and electron beam splitter", Science 362, 1149 (2018)   

Polaritons in Moire Superlattices of Graphene and Boron Nitride

Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moiré superlattice. The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus-obtained twisted bilayer graphene (TBG). We studied the propagation of plasmon polaritons in TBG by infrared nano-imaging. The atomic reconstruction occurring at small twist angles transforms the TBG into a network of conducting channels that act as efficient reflectors of plasmon polaritons. The analysis of plasmonic images allows one to infer the conductivity of the domain walls in moire superlattices. TBG at small twist angles effectively acts a natural plasmon photonic crystal for propagating nano-light. We observed similar nano-optical effects in other twisted van der Waals crystals including hexagonal boron nitride. In hBN, domain walls modify the response of phonon polaritons.   

Topological photonics: slow light and solitons

In this talk we will describe the realization of chiral edge states for photons in a photonic Floquet topological insulator, employing optical waveguide arrays. We will describe how such topological states can be used to simultaneously overcome two shortcomings of slow-light systems, namely disorder-induced scattering (and Anderson localization) as well as low bandwidth. We will show how optical nonlinearity can be used to realize Gross-Pitaevskii-type interactions, and a resulting soliton that spectrally resides in a topological band gap.   

The electron thickness of graphene

The van-der-Waals stacking technique enables the fabrication of heterostructures, where two conducting layers are atomically close. In this case, the finite layer thickness matters for the interlayer electrostatic coupling. Here we investigate the electrostatic coupling of two graphene layers, twisted by 22 degrees such that the layers are decoupled by the huge momentum mismatch between the K and K' points of the two layers. We observe a splitting of the zero-density lines of the two layers with increasing interlayer energy difference. This splitting is given by the ratio of single-layer quantum capacitance over interlayer capacitance C and is therefore suited to extract C. We explain the large observed value of C by considering the finite dielectric thickness d of each graphene layer and determine d=2.6 Angstrom. In a second experiment we map out the entire density range with a Fabry-Pérot resonator. We can precisely measure the Fermi-wavelength in each layer, showing that the layers are decoupled. We find that the Fermi wavelength exceeds 600nm at the lowest densities and can differ by an order of magnitude between the upper and lower layer. These findings are reproduced using tight-binding calculations.
This work was done in collaboration with Peter Rickhaus, Ming-Hao Liu, Marcin Kurpas, Annika Kurzmann, Yongjin Lee, Hiske Overweg, Marius Eich, Riccardo Pisoni, Takashi Tamaguchi, Kenji Wantanabe, Klaus Richter, and Thomas Ihn   

Atomic and electronic reconstruction at van der Waals interface in twisted 2-D materials

Controlling the interlayer twist angle in two-dimensional (2-D) van der Waals (vdW) heterostructures offers an experimental route to create moire superlattices. One can realize exotic electronic states by adjusting the width of the electronic bands with a tunable moire length scale. However, in the small twist angle regime, vdW interlayer interaction can cause significant structural reconfiguration at the interface, creating the arrays of domain structures. In this talk, we will discuss the atomic reconstruction at twisted vdW interfaces and its effect on electronic structure and electrical transport behavior. Moreover, one can tune the domain topology at the reconstructed interface using 2-D vdW crystals that has lower symmetry. We will discuss the connection between crystal symmetry and tunable domain topology.