The coupling energies between the buckled dimers of the Si(001) surface were determined through analysis of the anisotropic critical behavior of its order-disorder phase transition. Spot profiles in high-resolution low-energy electron diffraction as a function of temperature were analyzed within the framework of the anisotropic two-dimensional Ising model. The validity of this approach is justified by the large ratio of correlation lengths, ${\xi }_{\parallel }^{+}/{\xi }_{\perp }^{+}=5.2$ of the fluctuating $c(4$$\times$$2)$ domains above the critical temperature $T_c=(190.6\pm 10)\mathrm{K}$. We obtain effective couplings $J_{\parallel }=(-24.9\pm 1.3)\mathrm{meV}$ along the dimer rows and $J_{\perp }=(-0.8\pm 0.1)\mathrm{meV}$ across the dimer rows, i.e., antiferromagneticlike coupling of the dimers with $c(4$$\times$$2)$ symmetry.

• Received 19 May 2022
• Accepted 2 February 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.126203