Adsorption of Cs on InAs(111) surfaces

Caesiated InAs(111)B (1 x 1) and InAs(111)A (2 x 2) surfaces have been studied by photoelectron spectroscopy. On the InAs(111)B a new (root 3 x root 3)R30 degrees reconstruction was observed. During Cs evaporation remarkably small changes are observed in the lone pair states, and no sign of an accumulation layer at the surface can be observed. Instead, the additional charge provided by Cs is rapidly transported towards the bulk. On the InAs(111)A cesium behaves as a typical electropositive alkali metal donator that enhances the already existing accumulation layer. (c) 2005 Elsevier B.V. All rights reserved

FINE-STRUCTURE OF THE CA 2P X-RAY-ABSORPTION EDGE FOR BULK COMPOUNDS, SURFACES, AND INTERFACES

The fine structure of the Ca 2p soft-x-ray-absorption edge is studied for a variety of bulk compounds (Ca metal, CaSi2, CaO, and CaF2), for surfaces and interfaces [CaF2(111), BaF2 on CaF2(111), Ca and CaF2 on Si(111)], and for defects (F centers in CaF2). The observed multiplet structure is explained by atomic calculations in a crystal field [cubic O(h) for the bulk and threefold C3-nu for the (111) surfaces and interfaces]. While the bulk spectra are isotropic, the surface and interface spectra exhibit a pronounced polarization dependence, which is borne out by the calculations. This effect can be used to become surface and/or interface selective via polarization-modulation experiments, even for buried interfaces. A change in valence from Ca2+ to Ca1+ causes a downwards energy shift and extra multiplet lines according to the calculation. The energy shift is observed for F centers at the CaF2 surface and for the CaF2/Si(111) interface

Low-energy acoustic plasmons at metal surfaces RID B-5938-2008 RID C-8602-2009

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers(1) and artificial layered materials(2,3) permit the existence of low-energy collective excitations(4,5), called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons(3), and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors(6). Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons(7). Rather, metallic surfaces should support only conventional surface plasmons(8) - higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy(9,10) and sub-wavelength optics(11-13), but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic ( linear) dispersion, different to the q(parallel to)(1/2) dependence of a 2D plasmon, and was observed on Be( 0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications