  Topological interface states in multivalley systems are studied to unravel
their valley sensitivity. For this purpose, multivalley IV-VI topological
crystalline insulator (TCI) heterostructures are explored using magneto-optical
Landau level spectroscopy up to 34 teslas. We characterize the topological
interface states emerging from the distinct L-valleys in Pb1-xSnxSe multi
quantum wells grown along the [111] direction. It is shown that the shape of
the 2D Fermi surfaces of topological interface states residing at the
TCI/trivial insulator interfaces are strongly affected by the valley anisotropy
of topologically trivial Pb1-yEuySe barriers. This phenomenon is shown to be
due to the deep penetration of the topological interface states into the
barriers. For the valleys tilted with respect to the confinement direction, a
significant interaction between topological states and the conventional massive
quantum well states is observed, evidenced by the resulting large
anti-crossings between Landau levels. These are theoretically well-described by
a k.p model that takes into account tilt and anisotropy of the valleys in two
dimensions. Therefore, our work provides a precise characterization of the
topological interface state valley splitting, as well as an accurate
determination of the anisotropy of their Dirac cone dispersion.
