The 2 x 1-reconstructed cleavage surface of diamond: a challenging test for experiment and theory

We have recently measured for the first time - by using Reflectance Anisotropy spectroscopy- the optical gap between dangling-bond states on a single-domain diamond C(111)-2 x 1 surface. The experimental result we have obtained (1.47 +/- 0.02 eV) represents the conclusion of the long-standing effort to determine if this surface is metallic or semiconducting. The large surface gap (even larger for possible excitonic effects) opens a new scientific question: at the moment neither state-of-the-art band calculations have succeeded in obtaining it, nor has its physical origin been explained.