  The surface states of 3D topological insulators can exhibit Fermi surfaces of
arbitrary area when the chemical potential is tuned away from the Dirac points.
We focus on topological Kondo insulators and show that the surface states can
acquire a finite Fermi surface even when the chemical potential is pinned to
the Dirac point energy. We illustrate how this can occur when the crystal
symmetry is lowered from cubic to tetragonal in a minimal two-orbital model. We
label such surface modes as `shadow surface states'. We also show that for
certain bulk hybridization the Fermi surface of the shadow states can become
comparable to the extremal area of the unhybridized bulk bands. The `large'
Fermi surface of the shadow states is expected to lead to large-frequency
quantum oscillations in the presence of an applied magnetic field.
Consequently, shadow surface states provide an alternative to mechanisms
involving bulk Landau-quantized levels or surface Kondo breakdown for anomalous
magnetic quantum oscillations in topological Kondo insulators with tetragonal
crystal symmetry.
