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Symmetry protected Luttinger liquids on the surface of Quantum Hall Nematics

Quantum Hall Ferromagnets are a unique platform for studying the confluence of two pillars of condensed matter physics - symmetry-breaking phenomena,
and topological physics. The consequence of this is a rich phenomenology
of ground states and excitations which derive their properties from
both these pillars.  In this talk, I will describe a class of quantum
Hall ferromagnets that have been observed very recently on the surface
of Bi(111) by Feldman et al. [1].  These are different from previously
explored quantum Hall ferromagnets both theoretically in that they arise
in multi-valley systems and ferromagnetism occurs by the spontaneous
breaking of symmetry between these valleys. More recent STM experiments
[2] directly probe the excitations residing between ferromagnetic domains
of opposite polarization. Curiously, domain walls appear to host low
energy excitations that are gapped/gapless depending upon the filling
fraction of the quantum Hall states. I will discuss how we explain [3]
these observations both qualitatively and quantitatively, and highlight
the crucial role of interactions and symmetries specific to multi-valley
systems in engendering such excitations.

[1] B. Feldman et al., Science 2016
[2] M. T. Randeria, K. Agarwal et al., Nature 566 (2019)
[3] K. Agarwal et al., Phys Rev. B 100, 165103 (2019)