5,100 research outputs found
Tuning surface metallicity and ferromagnetism by hydrogen adsorption at the polar ZnO(0001) surface
The adsorption of hydrogen on the polar Zn-ended ZnO(0001) surface has been
investigated by density functional {\it ab-initio} calculations. An on top
H(1x1) ordered overlayer with genuine H-Zn chemical bonds is shown to be
energetically favorable. The H covered surface is metallic and spin-polarized,
with a noticeable magnetic moment at the surface region. Lower hydrogen
coverages lead to strengthening of the H-Zn bonds, corrugation of the surface
layer and to an insulating surface. Our results explain experimental
observations of hydrogen adsorption on this surface, and not only predict a
metal-insulator transition, but primarily provide a method to reversible switch
surface magnetism by varying the hydrogen density on the surface.Comment: 4 pages, 3 figure
Reversible enhancement of the magnetism of ultrathin Co films by H adsorption
By means of ab initio calculations, we have investigated the effect of H
adsorption in the structural, electronic and magnetic properties of ultrathin
Co films on Ru(0001). Our calculations predict that H occupies hollow sites
preserving the two-dimensional 3-fold symmetry. The formation of a complete H
overlayer leads to a very stable surface with strong H-Co bonds. H tends to
suppress surface features, in particular, the enhancement of the magnetic
moments of the bare film. The H-induced effects are mostly confined to the Co
atoms bonded to H, independent of the H coverage or of the thickness and the
structure of the Co film. However, for partial H coverages a significant
increase occurs in the magnetic moment for the surface Co atoms not bonded to
H, leading to a net enhancement of surface magnetism.Comment: 6 pages, 4 figures, 3 table
Surface roughness and thermal conductivity of semiconductor nanowires: going below the Casimir limit
By explicitly considering surface roughness at the atomic level, we
quantitatively show that the thermal conductivity of Si nanowires can be lower
than Casimir's classical limit. However, this violation only occurs for deep
surface degradation. For shallow surface roughness, the Casimir formula is
shown to yield a good approximation to the phonon mean free paths and
conductivity, even for nanowire diameters as thin as 2.22 nm. Our exact
treatment of roughness scattering is in stark contrast with a previously
proposed perturbative approach, which is found to overpredict scattering rates
by an order of magnitude. The obtained results suggest that a complete
theoretical understanding of some previously published experimental results is
still lacking.Comment: 11 pages, 4 figure
Renormalization Group and Grand Unification with 331 Models
By making a renormalization group analysis we explore the possibility of
having a 331 model as the only intermediate gauge group between the standard
model and the scale of unification of the three coupling constants. We shall
assume that there is no necessarily a group of grand unification at the scale
of convergence of the couplings. With this scenario, different 331 models and
their corresponding supersymmetric versions are considered, and we find the
versions that allow the symmetry breaking described above. Besides, the allowed
interval for the 331 symmetry breaking scale, and the behavior of the running
coupling constants are obtained. It worths saying that some of the
supersymmetric scenarios could be natural frameworks for split supersymmetry.
Finally, we look for possible 331 models with a simple group at the grand
unification scale, that could fit the symmetry breaking scheme described above.Comment: 18 pages. 3 figures. Some results reinterpreted, a new section and
references added. Version to appear in International Journal of Modern
Physics
Evolución cuaternaria del drenaje en un corredor intramontano: La Canal de Navarrés (Valencia, Spain)
[Resumen] Este artÃculo analiza la evolución actual y del Cuaternario reciente de la red de
drenaje en una fosa intramontana del macizo del Caroig (Valencia). De la información bibliográfica, el análisis geomorfológico y el registro sedimentario obtenido por sondeo mecánico, acompañado de dataciones absolutas C14 y TL, se concluye que claros signos de precariedad en la organización de la red han perdurado, si bien a menor escala, hasta la actualidad[Abstract] This paper is about the drainage network evolution during the Holocene and Late Quaternary in the rift valley of the Caroig massif (Valencia). Geological and geomorphological studies showed halocinesis movements in this graben. Sorne other processes, for instance, aIluvial and coIluvial deposition, moreover lithochemical and biochemical sedimention occured at the same time. The core sediment analysis and C14 and TL datations, allow us to conclude that in this zone, the disruption of the quaternary drainage network persists at the present time, eventhough it is less evident.Dirección General de Investigación CientÃfica y Técnica; PB91-089
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