Dynamical properties of clean and H-covered W(110) surfaces

In recent years, the availability of high-resolution surface-sensitive experimental techniques such as Helium atom scattering (HAS) and electron energy loss spectroscopy (EELS) has opened new perspectives in the study of the dynamical properties of solid surfaces. In particular, a great deal of experimental data are now available on the phonon dispersions of surfaces of insulator, semiconductors, and metals [1]. Among transition metals, the vibrational properties of the hydrogenated W(llO) surface have attracted much attention because they display unusual and unexpected features which are still poorly understood [2-6]. When a full monolayer of hydrogen is adsorbed, the surface phonon spectrum undergoes a dramatic change and an anomalous behavior appears

Negotiating the Local and the Global: SomeUneasyConjecturesonPostcolonial Studies and Pedagogy

Since the Nineties, postcolonial literature has become an increasingly popular specialism in academic institutions in the UK. The growing critical respect afforded to the cultural production of previously marginalised Anglophone nations is, of course, to be celebrated. However, the increasing institutionalisation within English departments of postcolonial studies ironically risks reinforcing the centrality of \u27white\u27, metropolitan English culture, and presenting the Anglophone world as peripheral and monolithic. If postcolonialism is nothing more than a means to revising canons and reading texts in departments of English, it might be viewed merely in terms of changes in the structure and constituencies of universities; but the claims of postcolonialism reach much further than curricular matters. Thus the question of changing constituencies within universities points to larger forces at work

Ab initio study of phonons in wurtzite AlxGa1-xN alloys

We present a theoretical study of the zone-center optical phonons in wurtzite AlxGa1-xN alloys over the whole compositional range from pure GaN to pure AlN. The phonon modes are broadened upon alloying and their frequencies display a blue shift with increasing Al concentration. The E-2 and E-1(TO) modes display a two-mode like behavior and do not preserve a well-defined symmetry in the alloy but rather are mixed and have a large broadening. The LO modes, instead, display a one-mode behavior and have a well-defined symmetry, small broadening, and a pronounced dependence of the frequency upon alloy composition. Therefore, we propose them as the best candidates for the compositional characterization of these materials. (C) 2000 American Institute of Physics. [S0003-6951(00)04815-4]

Coexistence of antiferrodistortive and ferroelectric distortions at the PbTiO3_3 (001) surface

The c(2$\times$2) reconstruction of (001) PbTiO$_3$ surfaces is studied by means of first principles calculations for paraelectric (non-polar) and ferroelectric ([001] polarized) films. Analysis of the atomic displacements in the near-surface region shows how the surface modifies the antiferrodistortive (AFD) instability and its interaction with ferroelectric (FE) distortions. The effect of the surface is found to be termination dependent. The AFD instability is suppressed at the TiO$_2$ termination while it is strongly enhanced, relative to the bulk, at the PbO termination resulting in a c(2x2) surface reconstruction which is in excellent agreement with experiments. We find that, in contrast to bulk PbTiO$_3$, in-plane ferroelectricity at the PbO termination does not suppress the AFD instability. The AFD and the in-plane FE distortions are instead concurrently enhanced at the PbO termination. This leads to a novel surface phase with coexisting FE and AFD distortions which is not found in PbTiO$_3$ bulk

A critical discussion of calculated modulated structures, Fermi surface nesting and phonon softening in magnetic shape memory alloys Ni2_2Mn(Ga, Ge, Al) and Co2_2Mn(Ga, Ge)

A series of first principles calculations have been carried out in order to discuss electronic structure, phonon dynamics, structural instabilities and the nature of martensitic transformations of the Heusler alloys Ni$_2$Mn(Ga, Ge, Al) and Co$_2$Mn(Ga, Ge). The calculations show that besides electronic pecularities like Fermi--surface nesting, hybridizing optical and acoustic phonon modes are important for the stabilization of the modulated martensitic structures.Comment: 3 pages, 4 figures, JEMS-200