Scanning tunnelling microscopy of cleaved surfaces at low temperatures revealed the existence of identical thin twin inter-layers that were embedded in the bulk crystal. The inter-layers were strictly ordered along the surface atomic rows, and had lengths which were of the order of 1. The width of the inter-layers was about 7nm. This corresponded roughly to the distance at which the height that was gained due to the tilt of the twin inter-layer, with respect to the main crystal surface, reached the interplanar spacing in the [00•1] direction. It was therefore proposed that the width quantization was related to the matching of atomic planes on the two sides of the twin inter-layer.

V.S.Edelman, D.J.Sharvin, I.N.Khlyustikov, A.M.Troyanovskii: Europhysics Letters, 1996, 34[2], 115-20