Threshold Phenomena in Nonlinear Currents upon Metallization of Si(001)

Using a novel combination of fundamental and harmonic polarizations, we have used optical second-harmonic (SH) generation to probe the nonlinear currents induced parallel to the surface plane of a Si(001) crystal upon adsorption of Na, K, and Cs. For all three alkalis, we observe abrupt thresholds in SH intensity and relative SH phase shift at the same coverage of very nearly one-sixth of a monolayer. We speculate that these thresholds correspond to insulator-metal transitions, and that a recently observed low-coverage (2 × 3) structure is a Mott-Hubbard-like insulator.