Publications from the group

2006

1. Farid El Gabaly, 24-nov-2006, PhD thesis
   Ultrathin Films on Ruthenium: Growth, Structure, and Magnetism


2. Farid El Gabaly, Silvia Gallego, Carmen Munoz, Laszlo Szunyogh, Peter Weinberger, Christof Klein, Andreas K. Schmid, Kevin F. McCarty, Juan de la Figuera
   "Imaging Spin Reorientation Transitions in Consecutive Atomic Co layers"
   Phys. Rev. Lett. 96 147202 (2006);(cond-mat/051220)
   

By means of spin-polarized low-energy electron microscopy (SPLEEM) we show that the magnetic easy-axis of one to three atomic-layer thick cobalt films on ruthenium crystals changes its orientation twice during deposition: one-monolayer and three-monolayer thick films are magnetized in-plane, while two-monolayer films are magnetized out-of-plane, with a Curie temperature well above room temperature. Fully-relativistic calculations based on the Screened Korringa-Kohn-Rostoker (SKKR) method demonstrate that only for two-monolayer cobalt films the interplay between strain, surface and interface effects leads to perpendicular magnetization.

3. J. de la Figuera, J. M. Puerta, J. I. Cerda, F. El Gabaly, K. F. McCarty
   "Determining the structure of Ru(0001) from low-energy electron diffraction of a single terrace"
   Surf. Sci. 600 L105-L109 (2006)
   DOI: 10.1016/j.susc.2006.02.027; (cond-mat/0601475)
   

While a perfect hcp (0001) surface has three-fold symmetry, the diffraction patterns commonly obtained are six-fold symmetric. This apparent change in symmetry occurs because on a stepped surface, the atomic layers on adjacent terraces are rotated by 180 degrees. Here we use a Low-Energy Electron Microscope to acquire the three-fold diffraction pattern from a single hcp Ru terrace and measure the intensity-vs-energy curves for several diffracted beams. By means of multiple scattering calculations fitted to the experimental data with a Pendry R-factor of 0.077, we find that the surface is contracted by 3.5(+-0.9) at 456 K.

4. W.L. Ling, J.C. Hamilton, K. Thürmer, G.E. Thayer, J. de la Figuera, R.Q. Hwang, C.B. Carter, N.C. Bartelt and K.F. McCarty
   "Herringbone and triangular patterns of dislocations in Ag, Au, and AgAu alloy films on Ru(0001)"
   Surf. Sci. 600 1735-1757 (2006)
   

We have studied the dislocation structures that occur in films of Ag, Au, and Ag0.5Au0.5 alloy on a Ru(0 0 0 1) substrate. Monolayer (ML) films form herringbone phases while films two or more layers thick contain triangular patterns of dislocations. We use scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) to determine how the film composition affects the structure and periodicity of these ordered structures. One layer of Ag forms two different herringbone phases depending on the exact Ag coverage and temperature. Low-energy electron microscopy (LEEM) establishes that a reversible, first-order phase transition occurs between these two phases at a certain temperature. We critically compare our 1 ML Ag structures to conflicting results from an X-ray scattering study [H. Zajonz et al., Phys. Rev. B 67 (2003) 155417]. Unlike Ag, the herringbone phases of Au and AgAu alloy are independent of the exact film coverage. For two layer films in all three systems, none of the dislocations in the triangular networks thread into the second film layer. In all three systems, the in-plane atomic spacing of the second film layer is nearly the same as in the bulk. Film composition does, however, affect the details of the two layer structures. Ag and Au films form interconnected networks of dislocations, which we refer to as “trigons.” In 2 ML AgAu alloy, the dislocations form a different triangular network that shares features of both trigon and moiré structures. Yet another well-ordered structure, with square symmetry, forms at the boundaries of translational trigon domains in 2 ML Ag films but not in Au films.
5. J. de la Figuera, N. C. Bartelt, K. F. McCarty
   "Electron reflectivity measurements of Ag adatom concentrations on W(110)"
   Surf. Sci. 600 4062-4066 (2006)
   cond-mat/0601406

The density of two-dimensional Ag adatom gases on W(110) is determined by monitoring local electron reflectivity using low energy electron microscopy (LEEM). This method of adatom concentration measurement can detect changes in adatom density at least as small as 10$^{-3}$ ML for a $\mu$m size region of the surface. Using this technique at high temperatures, we measure the sublimation rates of Ag adatoms on W(110). At lower temperatures, where Ag adatoms condense into monolayer islands, we determine the temperature dependence of the density of adatoms coexisting with this condensed phase and compare it with previous estimates.

2005

1. Farid El Gabaly, Wai Li W. Ling, Kevin F. McCarty, Juan de la Figuera.
   "The Importance of Threading Dislocations on the Motion of Domain Boundaries in Thin Films"
   Science 308,1303 (2005).
   
   
Thin films often present domain structures whose detailed evolution is a subject of debate. We analyze the evolution of copper films, which contain both rotational and stacking domains, on ruthenium. Real-time observation by low-energy electron microscopy shows that the stacking domains evolve in a seemingly complex way. Not only do the stacking boundaries move in preferred directions, but their motion is extremely uneven and they become stuck when they reach rotational boundaries. We show that this behavior occurs because the stacking-boundary motion is impeded by threading dislocations. This study underscores how the coarse-scale evolution of thin films can be controlled by defects.

2004

1. R. Otero, F. Calleja, V.M. García-Suárez, J.J. Hinarejos, J. de la Figuera, J. Ferrer, A. L. Vázquez de Parga and R. Miranda.
   "Tailoring surface electronic states vie strain to control adsorption: O/Cu/Ru(0001)"
   Surf. Sci. 550 65 (2004)
   A possible relationship between population of the surface electronic state and chemical reactivity for Cu layers on Ru(0001) is revealed by tunneling spectroscopy and microscopy. The surface state shifts down in energy with increasing thickness. Ab initio calculations indicate that the energy shift can be assigned to the decreasing tensile strain of the deposited film. The reactivity of the surface towards oxygen correlates with the population of this state, reaching a maximum at Cu thicknesses where the surface state is empty. These data provides an indication of the effect of strain on the reactivity of metal surfaces.
2. W. L. Ling, J. de la Figuera, N. Bartelt, R. Q. Hwang, A. K. Schmid, G. E. Thayer and J. C. Hamilton.
   "Strain Relief through Heterophase Interface Reconstruction: Ag(111)/Ru(0001)"
   Phys. Rev. Lett. 92 116102 (2004).
   We report an experimental (scanning tunneling microscopy) and theoretical (embedded atom method) study of a heterophase interface reconstruction between Ag(111) and Ru(0001). Despite the large 7% mismatch, the second layer of Ag from the Ru exhibits a hexagonal structure with Ag bulk spacing, providing a close match to bulk Ag. The first layer of Ag (next to Ru) is reconstructed in a highly symmetrical and regular structure containing monolayer long threading dislocations. We argue that this structure may generally occur to relieve strain in a certain class of heterophase interfaces
3. F. El Gabaly, R. Miranda, J. de la Figuera.
   "Properties of dislocation half-loops in Au(100): Structure, formation energy and diffusion barrier"
   Phys. Rev. B, 70 12102 (2004).
   Dislocation half-loops intersecting the surface have been detected by Scanning Tunneling Microscopy after mild sputtering on Au(100). We determine their structure and formation energy by means of atomistic simulations. The Peierls barrier is also estimated from the simulations and found to be a few meV. This represents a very efficient way of moving mass parallel to the surface under applied stress, where a net movement of hundreds of atoms can take place with barriers smaller than that for the diffusion of a single adatom. Thermal diffusion is not observed.
4. W. L. Ling, N. C. Bartelt, K. Pohl, J. de la Figuera, R. Q. Hwang, and K. F. McCarty.
   "Enhanced Self-Diffusion on Cu(111) by Trace Amounts of S: Chemical-Reaction Limited Kinetics"
   Phys. Rev. Lett. 93 166101 (2004).
   We find that less than 0.01 monolayer of S can enhance surface self-diffusion on Cu(111) by several orders of magnitude. The measured dependence of two-dimensional island decay rates on S coverage (S) is consistent with the proposal that Cu3S3 clusters are responsible for the enhancement. Unexpectedly, the decay and ripening are diffusion limited with very low and very high S but not for intermediate S. To explain this result we propose that surface mass transport in the intermediate region is limited by the rate of reaction to form Cu3S3 clusters on the terraces.

2003

1. J. de la Figuera, C. B. Carter, N. C. Bartelt and R. Q. Hwang.
   "Interplay between gas adsorption and dislocation structure on a metal surface"
   Surf. Sci. 531, 29 (2003).
   The influence of oxygen and sulfur on the dislocation patterns of the strained, two monolayer Cu film on Ru was observed by scanning tunneling microscopy. Both oxygen and sulfur adsorption lead to the formation of vacancies that aggregate over existing dislocations in the film, thereby modifying the dislocation structure. With increasing adsorbate coverage the overall dislocation structure and pattern are transformed. The atomic mechanisms and general nature of this transformation can be explained in terms of generic dislocation reactions. This interpretation is also supported by atomistic simulations.
   
2. J. de la Figuera, K. A. K. Schmid, K. Pohl, N. C. Bartelt, C. B. Carter and R. Q. Hwang.
   "Glide and Climb of Dislocations in Ultra-Thin Metal Films"
   Mat. Sci. Forum 426-432, 3421 (2003).
   Near-surface misfit dislocations can be directly observed by Scanning Tunneling Microscopy through the distortion on the positions of the atoms of the surface layer. In this way, the behavior of misfit dislocations can be followed in detail. Experiments performed in monolayer films of Cu on Ru(0001) show both climb and glide of the misfit dislocations with atomic resolution. The motion of the dislocations represents a novel pathway for surface diffusion in ultra-thin films.