Showing papers by "John B. Pendry published in 1983"

Quantum limits to the flow of information and entropy

Abstract: The quantum limits to information flow are explored developing the central concept of a medium comprising several channels through which the information flows. In each channel there is an inequality between information flow I and energy flow E I2

Determination of Local Atomic Arrangements at Surfaces from Near-Edge X-Ray-Absorption Fine-Structure Studies: O on Ni(100)

Abstract: Near-edge x-ray-absorption fine-structure (NEXAFS) spectra for oxygen adsorption on Ni(100) are found to exhibit pronounced variations with oxygen exposure and x-ray incidence angle. The measured spectra are compared to calculations of the NEXAFS with use of a multiple-scattering formalism. The theory shows considerable sensitivity to changes in the O-Ni coordination and x-ray incidence angle. By comparison of experimental and theoretical spectra NEXAFS is shown to be a new technique for determining the local structure of adsorbates on surfaces.

Removing the limits to accurate band-structure determination by photoemission

Abstract: Present methods of measuring electronic band structure of solids by photoemission suffer from reduced resolution in kz because the escaping electron has a finite escape depth which broadens its momentum. Thin films grown epitaxially on substrates are predicted to show fine oscillations in their spectra caused by quantisation of hole states. The spacing of peaks is determined by the number of layers and by the group velocity of the band concerned and the peaks are broadened only by the intrinsic lifetime of the hole state. Realistic calculations are presented for silver grown epitaxially on a palladium (001) surface. They show strong differences from pure silver up to thicknesses of at least 7 layers due to the quantisation effects.

X-ray absorption near edge structure (XANES) for CO, CN and deoxyhaemoglobin: geometrical information.

Abstract: We use the recently developed multiple scattering theory to give a quantitative analysis of the X-ray absorption near edge structure (XANES) of haemoglobin and some of its substituents. We demonstrate that the XANES may contain information not provided by the extended X-ray absorption fine structure (EXAFS) part of the spectrum about the coordination geometry around the Fe atom, and in particular discuss the sensitivity of the XANES to the orientation of the CN group in HbCN. The anisotropy of the system leads to a strong dependence of the calculated spectrum on the polarisation of the X-rays. We show how this effect can be exploited in further XANES structural studies.

The Transition Region Between XANES and EXAFS

Abstract: X-ray absorption experiments have in recent years assumed an important role as a technique of structure determination [1,2]. The active ingredient for structural sensitivity is an electron emitted from an inner shell: the scattering of this electron modulates the absorption cross section. Figure 1 shows a typical X-ray absorption cross section and makes the somewhat arbitrary distinction between X-ray absorption near edge structure (XANES) and the extended structure (EXAFS). The distinction between these two regimes is in strength of electron scattering: at low electron energies cross sections are high and strong modulations in the cross section result. At higher energies figure 1 clearly shows much weaker modulations indicating weaker scattering. In fact, the experiments are misleading guides to the overall strength of scattering because the intensity of modulation is primarily sensitive to the backscattering factor. The electron is also forward scattered, resulting in changes to the phase of modulations in the EXAFS spectrum, a particularly insidious effect because it interferes with determinations of atomic coordinates. For classical EXAFS theory to be valid [3,4], the ejected electron must be only weakly scattered. Hence, a theory of XANES [5,6,7,8,9,10) must address the problem of multiple scattering.

New Perspectives in Surface Crystallography

Abstract: The ideal balance between theoretical and experimental techniques occurs where comparable amounts of time and money are spent on each side of the problem. In LEED structure analysis this is true to within an order of magnitude for the sorts of structures currently being studied, but in the future we can already see a serious imbalance appearing. Experiments scale linearly in time with N, the number of beams (assuming that all are measured) and theory scales as N3. The moral is that we should be working to increase the speed of computation and if possible to devise new methods that scale in time as N, so that a competitive position, once established, remains so for a wide class of experiments. Clearly N must be the optimum scaling because even the printing of all beams scales as N.

The Adsorption of Oxygen on Nickel (001) Studied by XANES

Abstract: We report the results of multiple scattering calculations for the X-ray absorption near-edge structure (XANES) for oxygen adsorbed on Ni(001), and compare them with experimental results. The theory shows considerable sensitivity to changes in adsorption site or distance, while there is no change in the experimental spectra for a wide range of oxygen coverages. We conclude that there is no significant difference between the local coordination sites for adsorption in the p(2x2) or c(2x2) configurations, contrary to recent speculation. It is shown that XANES is a promising technique for studying adsorbates on surfaces.