The first models of 3D topological insulators were proposed by B. A. Volkov and O. A. Pankratov in 1985, and subsequently by Pankratov, S. V. Pakhomov, and Volkov in 1987. Gapless 2D Dirac states were shown to exist at the band inversion contact in PbTe / SnTe and HgTe / CdTe heterostructures. Existence of interface Dirac states in HgTe/CdTe was experimentally verified by Laurens W. Molenkamp's group in 2D topological insulators in 2007.