Experimental verification of critical exponents in the two-dimensional four-state Potts universality class: Oxygen on Ru(0001)

Three-dimensional Baxter-Wu model

A classic three-dimensional spin model, based upon the Baxter-Wu scheme, is presented. It is found, by entropic sampling simulations, that the behavior of the energy and magnetization fourth-order cumulants points to a first-order phase transition. A finite-size procedure was performed, confirming that the system scales with the dimensionality d=3, yielding a high-resolution estimate of the transition temperature as T_{c}=11.377485(29).

Oxygen orders differently under graphene: new superstructures on Ir(111)

Using scanning tunneling microscopy, the oxygen adsorbate superstructures on bare Ir(111) are identified and compared to the ones formed by intercalation in between graphene and the Ir(111) substrate. For bare Ir(111) we observe O-(2 × 2) and O-(2 × 1) structures, thereby clarifying a persistent uncertainty about the existence of these structures and the role of defects for their stability. For the case of graphene-covered Ir(111), oxygen intercalation superstructures can be imaged through the graphene monolayer by choosing proper tunneling conditions. Depending on the pressure, temperature and duration of O2 exposure as well as on the graphene morphology, O-(2 × 2), O-(√3×√3)-R30°, O-(2 × 1) and O-(2√3 × 2√3)-R30° superstructures with respect to Ir(111) are observed under the graphene cover. Two of these structures, the O-(√3 × √3)-R30° and the (2√3 × 2√3)-R30° structure are only observed when the graphene layer is on top. Phase coexistence and formation conditions of the intercalation structures between graphene and Ir(111) are analyzed. The experimental results are compared to density functional theory calculations including dispersive forces. The existence of these phases under graphene and their absence on bare Ir(111) are discussed in terms of possible changes in the adsorbate-substrate interaction due to the presence of the graphene cover.

High density gradients in the (3×3)R30°-CO layer on Ru(0001)

The coverage regime just beyond 0.33 ML, representative of a perfectly ordered (square root 3 x square root 3)R30 degrees-CO layer on Ru(0001), has been investigated using infrared-absorption spectroscopy. Different isotopic mixtures of CO have been employed to derive a profound understanding of structural properties of such layers. It is found that extra CO molecules incorporated into the (square root 3 x square root 3)R30 degrees-CO layer affect their nearest neighbor molecules only, and the associated density gradient extends over no more than a few angstroms. Contrary to existing belief, the model system CO on Ru(0001) does not represent a case of an unusually shallow adsorption potential corrugation. Rather, CO experiences an exceptionally strong site preference when adsorbed on Ru(0001). Annealing causes the local distortion of the overlattice to propagate laterally, most probably in a density wave-like manner. Incipient motion on the atomic scale thereby has been detected by means of isotopic labeling of inequivalent molecules within the high density areas. All major conclusions are based on observations of (isotopically labeled) minority CO species which feature negligible dynamical lateral coupling. The majority CO species, on the other hand, provide laterally averaged, unspecific information on the status of the layer.