  A new structural phase transition has been observed at low temperatures for
the one third of a monolayer (alpha phase) of Pb on Ge(111) using a
variable-temperature scanning tunneling microscope. The well-known (r3xr3)R30
to (3x3) transition is accompanied by a new structural phase transition from
(3x3) to a disordered phase at ~76 K. The formation of the new disordered phase
is incompatible with the belief that the (3x3) phase is the ground state. The
mechanism of the phase transition in this triangular lattice can be mapped onto
antiferromagnetic spin Ising model, with the geometric frustration.
