Interacting multi-channel topological boundary modes in a quantum Hall valley system

Mallika T. Randeria, Kartiek Agarwal, Benjamin E. Feldman, Hao Ding, Huiwen Ji, R. J. Cava, S. L. Sondhi, Siddharth A. Parameswaran and Ali Yazdani ()
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Mallika T. Randeria: Princeton University
Kartiek Agarwal: Princeton University
Benjamin E. Feldman: Princeton University
Hao Ding: Princeton University
Huiwen Ji: Princeton University
R. J. Cava: Princeton University
S. L. Sondhi: Princeton University
Siddharth A. Parameswaran: University of Oxford
Ali Yazdani: Princeton University

Nature, 2019, vol. 566, issue 7744, 363-367

Abstract: Abstract Symmetry and topology are central to understanding quantum Hall ferromagnets (QHFMs), two-dimensional electronic phases with spontaneously broken spin or pseudospin symmetry whose wavefunctions also have topological properties1,2. Domain walls between distinct broken-symmetry QHFM phases are predicted to host gapless one-dimensional modes—that is, quantum channels that emerge because of a topological change in the underlying electronic wavefunctions at such interfaces. Although various QHFMs have been identified in different materials3–8, interacting electronic modes at these domain walls have not been probed. Here we use a scanning tunnelling microscope to directly visualize the spontaneous formation of boundary modes at domain walls between QHFM phases with different valley polarization (that is, the occupation of equal-energy but quantum mechanically distinct valleys in the electronic structure) on the surface of bismuth. Spectroscopy shows that these modes occur within a topological energy gap, which closes and reopens as the valley polarization switches across the domain wall. By changing the valley flavour and the number of modes at the domain wall, we can realize different regimes in which the valley-polarized channels are either metallic or develop a spectroscopic gap. This behaviour is a consequence of Coulomb interactions constrained by the valley flavour, which determines whether electrons in the topological modes can backscatter, making these channels a unique class of interacting one-dimensional quantum wires. QHFM domain walls can be realized in different classes of two-dimensional materials, providing the opportunity to explore a rich phase space of interactions in these quantum wires.

Date: 2019
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DOI: 10.1038/s41586-019-0913-0

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