  The topology and spin-orbital polarization of two-dimensional (2D) surface
electronic states have been extensively studied in this decade. One major
interest in them is their close relationship with the parities of the bulk (3D)
electronic states. In this context, the surface is often regarded as a simple
truncation of the bulk crystal. Here we show breakdown of the bulk-related
in-plane rotation symmetry in the topological surface states (TSSs) of the
Kondo insulator SmB$_6$. Angle-resolved photoelectron spectroscopy (ARPES)
performed on the vicinal SmB$_6$(001)-$p$(2$\times$2) surface showed that TSSs
are anisotropic and that the Fermi contour lacks the fourfold rotation symmetry
maintained in the bulk. This result emphasizes the important role of the
surface atomic structure even in TSSs. Moreover, it suggests that the
engineering of surface atomic structure could provide a new pathway to tailor
various properties among TSSs, such as anisotropic surface conductivity,
nesting of surface Fermi contours, or the number and position of van Hove
singularities in 2D reciprocal space.
