The reversible 7 x 7  1 x 1 phase transition of (111) was investigated by means of optical secondary harmonic generation. The latter response increased sharply at a critical temperature of 1130K. The symmetry and magnitude of the secondary harmonic signals were compatible with an adatom gas model, for the 1 x 1 phase, in which the number of dangling bonds was increased by some 25% with respect to the 7 x 7 structure. The behavior under laser heating conditions provided clear evidence for a superheating, of the (111) 7 x 7 phase, to more than 50K above the critical temperature; at the ns time-scale.

U.Höfer, L.Li, G.A.Ratzlaff, T.F.Heinz: Physical Review B, 1995, 52[7], 5264-8