NONLINEAR-OPTICAL STUDY OF THE SI(111)7X7 TO 1X1 PHASE-TRANSITION - SUPERHEATING AND THE NATURE OF THE 1X1

The reversible 7x7 <----> 1x1 phase transition of Si(111) is investiga ted by means of optical second-harmonic (SH) generation. The SH respon se increases sharply at T-c similar or equal to 1130 K. The symmetry a nd magnitude of the SH signal are compatible with an adatom gas model for the 1x1 phase in which the number of dangling bonds is increased b y roughly 25% with respect to the 7x7 structure. The behavior observed under conditions of laser heating provides clear evidence of superhea ting of the Si(111)7x7 phase by more than 50 K above T-c on the nanose cond time scale.