Pomeranchuk instability of the surface state of a topological insulator

Strong interactions in a canonical Fermi liquid can drive a Pomeranchuk instability, in which the Fermi surface spontaneously deforms and lowers its symmetry. Experimentally, however, unambiguous identification of this instability has been challenging, since most systems host additional degrees of freedom or intrinsic structural anisotropies that obscure its signature. Here we show that the metallic surface state of the topological insulator grey arsenic—whose bulk exhibits an energy gap over part of momentum space—provides a clean platform to expose a paradigmatic Fermi liquid state to electron–electron interactions, offering an unambiguous route to establish the Pomeranchuk instability. Using scanning tunneling microscopy and photoemission spectroscopy, we directly visualize a deformation of the surface-state Fermi surface. Near the Fermi energy, the originally circular Fermi surface becomes elliptical, marking the onset of the Pomeranchuk instability. Moreover, our experiments demonstrate that this instability drives the system toward a nematic state, providing a clear example of fermiology-driven nematicity.