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 investigated by means of optical second-harmonic (SH) generation. The SH response increases sharply at T-c similar or equal to 1130 K. The symmetry and 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 by roughly 25% with respect to the 7x7 structure. The behavior observed under conditions of laser heating provides clear evidence of superheating of the Si(111)7x7 phase by more than 50 K above T-c on the nanosecond time scale.