The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.

No abstract

The effect of microscopic Mn cluster distribution on the Curie temperature (Tc) is studied using density-functional calculations. We find that the calculated Tc depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental Tc values from a few K to well above room temperature. The partially dimerized Mn_2-Mn_1 distribution is found to give the highest Tc > 500 K, and in general, the presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.Comment: To appear in Applied Phyiscs Letter

Articles you may be interested inThe hydrophobic effect in a simple isotropic water-like model: Monte Carlo study J. Chem. Phys. 140, 144904 (2014) We use the three-dimensional Mercedes-Benz model for water and Monte Carlo simulations to study the structure and thermodynamics of the hydrophobic interaction. Radial distribution functions are used to classify different cases of the interaction, namely, contact configurations, solvent separated configurations, and desolvation configurations. The temperature dependence of these cases is shown to be in qualitative agreement with atomistic models of water. In particular, while the energy for the formation of contact configurations is favored by entropy, its strengthening with increasing temperature is accounted for by enthalpy. This is consistent with our simulated heat capacity. An important feature of the model is that it can be used to account for well-converged thermodynamics quantities, e.g., the heat capacity of transfer. Microscopic mechanisms for the temperature dependence of the hydrophobic interaction are discussed at the molecular level based on the conceptual simplicity of the model.

In this study, we use first-principles simulations to study the adsorption of copper onto H-terminated and partially OH-terminated silicon surfaces. We show that, in contrast to previous studies, copper adsorbs strongly to the H-terminated silicon surface and that the adsorption energy is significantly dependent on the local bonding environment. The addition of a hydroxide group increases the average adsorption energy while reducing the range of adsorption energies due to the strong interaction between copper and oxygen. Our results predict that copper will generally prefer to adsorb at dihydride sites on the surface, agreeing with experimental studies of copper nucleation. The adsorption energy hierarchy predicted by the calculations strongly supports the suggestion that copper acts as a micromask in wet chemical etching, blocking reactive sites.

In order to understand how the doping with self-assembled nanorods of different sizes and concentrations as well as applied magnetic fields affect the critical current anisotropy in YBa 2 cu 3 o 7−x (YBCO) thin films close to YBCO c-axis, we present an extensive and systematic computational study done by molecular dynamics simulation. the simulations are also used to understand experimentally measured J c (θ) curves for BaHfO 3 , BaZrO 3 and BaSno 3 doped YBCO thin films with the help of nanorod parameters obtained from transmission electron microscopy measurements. our simulations reveal that the relation between applied and matching field plays a crucial role in the formation of J c (θ)-peak around YBCO c-axis (c-peak) due to vortex-vortex interactions. We also find how different concentrations of different size nanorods effect the shape of the c-peak and explain how different features, such as double c-peak structures, arise. In addition to this, we have quantitatively explained that, even in an ideal superconductor, the overdoping of nanorods results in decrease of the critical current. our results can be widely used to understand and predict the critical current anisotropy of YBco thin films to improve and develop new pinscapes for various transport applications.High temperature superconductors (HTS) are expected to have large number of applications in different fields of technology and power industry in the future 1-3 . Since all known HTS are of type II, the critical current passed through them is highly dependent on the surrounding magnetic field due to the movement of vortices. Thus, to enhance and widen the usability of HTS, the dynamics of vortices need to be well understood.Among the high temperature superconductors, YBa 2 Cu 3 O 7−x (YBCO) seems the most practical choice when thinking for the applications 1 . The intrinsic anisotropy of the critical current, in thin films and coated conductors, can be modified by adding impurities within the lattice of YBCO which pin the vortices restricting their movement. Based on growth conditions and lattice mismatch between the YBCO and the dopant as well as their elastic properties 4,5 , impurities such as Y 2 O 3 6 , BaCeO 3 7-9 and BaZrO 3 (BZO) 10,11 can form uncorrelated randomly distributed nanoparticles within the YBCO lattice. Under optimized deposition conditions, via a spontaneous phase-separation and strain-driven self-assembly process during film deposition 12 , self-assembly of nanorods of BaHfO 3 (BHO) 1 , BaZrO 3 (BZO) 4,13,14 , BaSnO 3 (BSO) 15,16 , Ba 2 YTaO 3 (BYTO) 17 or Ba 2 YNbO 6 (BYNO) 18 within the YBCO lattice can be realized.Recently, a topic of interest has been to add both point-like nanodots and nanorods within the YBCO lattice simultaneously. This has been achieved by doping YBCO simultaneously with both BYTO and BYNO (referred as BYNTO) with an additional rare earth oxide, leading to continuous niobiate/tantalate nanorods and rare-earth oxide nanoparticles 19 . A lot of experimental research has been done in order to understand the...

Noncontact atomic force microscopy (nc-AFM), Kelvin probe force microscopy (KPFM) and first principle calculations show that the nanostructured (001) Suzuki surface of Cd(2+) doped NaCl can be used to confine the growth of palladium clusters and functionalized brominated pentahelicene molecules into only the Suzuki regions, which contain the impurities. The Suzuki surface is an ideal model surface for nanostructuring metal clusters and molecules.

A molecular dynamics (MD) simulation to simulate the vortices in superconductors with artificial pinning sites is presented. The simulation reproduces the correct anisotropic behavior in angular dependence of critical current. We also show that the shape of the [Formula: see text] curve depends on the size of the pinning sites and the change from p  =  0.5 to [Formula: see text] is due to the breaking of the vortex lattice to individually acting vortices. The results beautifully correspond to experimental data. Furthermore, we found that the size and shape of the c-axis peak observed with columnar pinning sites in [Formula: see text] also depends on the size of the rods, larger pinning sites leading to wider peaks. The results obtained from the MD-simulation are similar to those of the much more computationally intensive Ginzburg-Landau simulations. Furthermore, the MD-simulations can provide insight to the vortex dynamics within the samples.