THRESHOLD PHENOMENA IN NONLINEAR CURRENTS UPON METALLIZATION OF SI(001)

Using a novel combination of fundamental and harmonic polarizations, w e have used optical second-harmonic (SH) generation to probe the nonli near currents induced parallel to the surface plane of a Si(001) cryst al 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 th at these thresholds correspond to insulator-metal transitions, and tha t a recently observed low-coverage (2 x 3) structure is a Mott-Hubbard -like insulator.