We consider an extension of the classical Ising model to a nonequilibrium situation including a source of spin-flip excitations localized on the scale of individual spins. The scenario is realized in scanning tunneling microscopy of the Si(100) surface at low temperatures. The reconstructions of Si(100) have long been analyzed within the confines of an Ising model, where the spin variable describes the tilt of the buckled silicon dimers. The surface structure is dictated by strong antiferromagnetic correlations along dimer rows and a weak antiferromagnetic interaction between rows. An STM allows the introduction of a very localized source of spin-flips, through excitation of dimer rocking vibrations along the chain. Remarkable details are detected in the tunnel current for particular rows, corresponding to the passage of domain walls through the tunnel junction. Kinetic Monte Carlo simulations capture the essential features of the experimental observations. The study reveals general characteristics of a classical Ising chain subject to a localized excitation.

Reference: Y. Pennec et al., Phys. Rev. Lett. 96 (2006) 026102.