Ferromagnetism in the thermodynamically stable monolayer Fe(110) on W(110), coated by Ag

Optical reflectance anisotropy of the growth of Fe monolayers on W(110)

We report measurements of the optical anisotropy of Fe layers grown on the W(110) surface using reflection anisotropy spectroscopy (RAS). As the first monolayer of Fe is deposited onto W(110), the resonance-like RAS profile of the clean surface is reduced in intensity. We find evidence for the surface state on W(110) surviving as an interface state following Fe deposition. We observe an anisotropic optical response from Fe layers grown on top of the first two monolayers, where a broad peak at 3 eV dominates the RAS response. The results are simulated in terms of a layered Fresnel reflection model incorporating either a strained Fe overlayer or an Fe overlayer whose dielectric properties are approximated by a simple Lorentzian oscillator. Both approaches are found to produce simulated RA spectra that are in good agreement with experiment. The former approach provides evidence that RAS can detect anisotropy in strained overlayers and that 7 ML films have bulk-like electronic and optical properties.

Universal window for two-dimensional critical exponents

Two-dimensional condensed matter is realized in increasingly diverse forms that are accessible to experiment and of potential technological value. The properties of these systems are influenced by many length scales and reflect both generic physics and chemical detail. To unify their physical description is therefore a complex and important challenge. Here we investigate the distribution of experimentally estimated critical exponents, β, that characterize the evolution of the order parameter through the ordering transition. The distribution is found to be bimodal and bounded within a window ∼0.1≤β≤0.25, facts that are only in partial agreement with the established theory of critical phenomena. In particular, the bounded nature of the distribution is impossible to reconcile with the existing theory for one of the major universality classes of two-dimensional behaviour-the XY model with four-fold crystal field-which predicts a spectrum of non-universal exponents bounded only from below. Through a combination of numerical and renormalization group arguments we resolve the contradiction between theory and experiment and demonstrate how the 'universal window' for critical exponents observed in experiment arises from a competition between marginal operators.