Evolution of Fermi surface by electron filling into a free-electronlike surface state

We have studied the evolutions of surface electronic structure (Fermi surfaces and valence bands) by electron filling into a two-dimensional free-electronlike surface state, during adsorptions of monovalent metal atoms (noble metal; Ag, and alkali metal; Na) on the Si(111)root 3 x root 3-Ag surface. The Fermi surfaces (Fermi rings) of a small electron pocket grow continuously with the adsorption. Eventually, when the root 21 x root 21 superstructure was formed by 0.1-0.2 monolayer adsorption of Ag or Na, the Fermi ring is found to be larger than the root 21 x root 21-surface Brillouin zone (SBZ), and to be folded by obeying the root 21 x root 21 periodicity. As a result, the Fermi surface is composed of a large hole pocket at the (Gamma) over bar point and small electron pockets at the (K) over bar point in each reduced root 21 x root 21 SBZ, meaning that the behavior of surface-state carriers becomes hole-like. Despite a sharp chemical distinction between the adsorbates, a very similar surface electronic structure is found for both the Ag-induced and Na-induced root 21 x root 21 phases. Based on the Boltzmann equation, surface-state conductivites of these surfaces are obtained from the measured Fermi surfaces, reproducing successfully the results of previous surface transport measurements.