Au(110) (1 × 2)-to-(1 × 1) Phase Transition: A Physical Realization of the Two-Dimensional Ising Model

We report the first measurements of the critical exponents of a phase transition at the surface of a clean metal, the Au(110) (1 × 2) ↔ (1 × 1) transition. The exponents $\beta$, $\gamma$, and $\nu$ and the amplitudes were measured by LEED and place the phase transition in the two-dimensional Ising universality class. The critical scattering is observed to peak above $T_c$, signaling the breakdown of the Ornstein-Zernike theory, and is in numerical accord with predictions of Fisher and Burford.