231 research outputs found
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 PbTiO (001) surface
The c(22) reconstruction of (001) PbTiO 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 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, 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 bulk
A critical discussion of calculated modulated structures, Fermi surface nesting and phonon softening in magnetic shape memory alloys NiMn(Ga, Ge, Al) and CoMn(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 NiMn(Ga, Ge,
Al) and CoMn(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
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