John B. Pendry

Bio: John B. Pendry is an academic researcher from Imperial College London. The author has contributed to research in topics: Metamaterial & Plasmon. The author has an hindex of 100, co-authored 536 publications receiving 88802 citations. Previous affiliations of John B. Pendry include University of California, San Diego & Duke University.

LEED, XANES and the Structure of Disordered Surfaces

Abstract: Surface crystallography is now a well-established discipline, but, like its bulk counterpart, high quality surface crystals tend to be required for a complete structural analysis. This has been particularly true where lowenergy electron diffraction has been concerned. However, there are techniques for studying surfaces which are insensitive to long-range order. I want to talk about two of them: X-ray absorption near edge structure (XANES) and a new technique only recently proposed: diffuse low-energy electron diffraction (DLEED).

Xanes and the determination of bond angles

Abstract: It is argued that conventional weak scattering probes provide no direct information beyond the radial distribution function. In contrast, X-ray Absorption Near Edge Structure is produced by the multiple scattering of low energy electrons and has sensitivity to bond angles and symmetry of the environment 1. WEAK SCATTERING AND PAIR CORRELATIONS The traditional probe of structure is a wave that is weakly scattered by the material under consideration,such as X-rays or neutrons. Weak scattering has the great virtue of a simple interpretation. For example, a plane wave, incident on the solid is scattered to an outgoing wave, where A(g) = s fs(g) exp(iq.Rs), ( 3 ) and R is the position of the çth atom, withscatteriqfactor fS; ki and k ar8 the incident and scattered wave vectors respectively. Thë egperiment observes an expression in which the atomic coordinates occur in pairs, and only in pairs. Such an expression can only ever tell us about the pair correlation function in the material but in ordered, crystalline materials this information is usually sufficient to give us the higher order correlation functions by implication / 1 / 2 / . In the case of an orientated crystal measurement of IAl for a full set Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985911 C9-94 JOURNAL DE PHYSIQUE of 3D vectors, q, provides enough information to fix the higher order correlations. By contrast, in a liquid there is much less information available because the scattering is on average isotropie so that ( A ( ~ is a function only of (q(. Occasionally this provides enough information to determine al1 correlation functions, but in most instances this is not the case. In this sense disordered materials differ in a fundamental way £rom their ordered counterparts. Their structure is much more subtle, as indicated by their finite entropy content, and demands new probes to reveal its details to us. II. MULTIPLE SCATTERING A PROBLEM FOR THE THEORIST When scattering is strong, the probe can be expected to interact with more than one scattering centre in the solidland equation ( 3 ) must be generalised to ~ ~ , ( k ~ , & ~ i = C exp(iSf.Rsi fs(kf,ki) exp(ik. .R ) + -1 -S exp(ikf.Rs) fs Gst ft exp(iki.Rt) + exp(ikf.Rs) fs GsU fU Gut ft exp(iki.Rt) (plus higher order terms) (6 The new ingredients in this formula are (i) the scattering factors, f, are now generally complex, reflecting multiple scattering events 'within individual atoms. (ii) The scattered amplitude, A , is no longer a function of ( & -Li( aloneysbut depends on kf and k . independentl5. -1 (iii) Several atomic coordinates appear in a typical term through the propagators, G, which represent the wave travelling between atoms. This last point becomes more explicit if we take the case of point scatterers for which so that the overall dependence on atomic coordinates of the last term in equation (6) would be As the wave explores the solid, scattering from successive atoms, it measures the path length through the phase which it acquires. We might imagine that in principle we could find al1 possible path lengths for multiple scatterings in the medium by Fourier transforming the scattered intensities. This information could be interpreted in terms of pair correlations. However the program is not a practicable one, partly because the propagators, G, are rarely simple phase factors, and partly because the individual atomic scattering factors, f, introduce further complications which are difficult to correct for. The only practical way of proceeding that has been used to interpret multiple scattering data has been to postulate a trial structure, calculate the implied spectrum, compare with experiment, and then successively refine the structure. The passage from trial structure to calculated spectrum is non-trivial and is one of the limiting factors to interpretation of data which in principle contain the information we require. One multiple scattering probe that has been much used in structure determination is the low energy electron/3/. Traditionally low energy electron diffraction (LEED) has been used as a surface structure probe. Here the strong scattering interaction is required to give surface sensitivity, and the multiple scattering processes are regarded as something of a nuisance. LEED experiments are now routinely interpreted to give surface structures, and essentially the same theory can be used in another context to give higher order correlation functions. X-ray absorption near edge structure (XANES, or NEXAFS as some cal1 it) /1/4/ results £rom the ejection of an electron £rom an inner core of the absorbing atom. The ability of the electron to escape £rom the atom determines the absorption cross section and in the near edge region, where the electron has, Say, less than 50eV of kinetic energy, multiple scattering is dominant; in contrast to the region of extended X-ray absorption fine structure (EXAFS) /5/6/, more than 50 eV above the edge, in which cross sections are small enough for most multiple scattering processes to be neglected. Here we have a probe which can be tuned to the environment of a particular atom by selecting the appropriate absorption edge. The ejected electron having a finite elastic lifetime explores only the irnmediate surroundings of that atom before it vanishes in incoherent processes which in turn limits the number of multiple scattering events that we have to interpret. Fig. 1 The relative absorption coefficient for X-rays in the vicinity of the iron K edge. The division of the spectrum into XANES and EXAFS is somewhat arbitrary. K4Fe(CN)6.3H20 and K3Fe(CN)6. Ar: l

Nanostructure sensors and sensing systems

Abstract: Various sensors and arrays of sensors that utilize nanostructures or carbon structures, such as nanotubes, nanotube meshes, or graphene sheets, are disclosed. In some arrangements, at least a pair of contacts are electrically coupled with a given nanostructure or carbon structure to sense a change.

疟原虫var基因转换速率变化导致抗原变异[英]／Paul H, Robert P, Christodoulou Z, et al//Proc Natl Acad Sci U S A

Abstract: 抗原变异可使得多种致病微生物易于逃避宿主免疫应答。表达在感染红细胞表面的恶性疟原虫红细胞表面蛋白1（PfPMP1）与感染红细胞、内皮细胞、树突状细胞以及胎盘的单个或多个受体作用，在黏附及免疫逃避中起关键的作用。每个单倍体基因组var基因家族编码约60种成员，通过启动转录不同的var基因变异体为抗原变异提供了分子基础。

Surface plasmon subwavelength optics

Abstract: Surface plasmons are waves that propagate along the surface of a conductor. By altering the structure of a metal's surface, the properties of surface plasmons--in particular their interaction with light--can be tailored, which offers the potential for developing new types of photonic device. This could lead to miniaturized photonic circuits with length scales that are much smaller than those currently achieved. Surface plasmons are being explored for their potential in subwavelength optics, data storage, light generation, microscopy and bio-photonics.

Experimental Verification of a Negative Index of Refraction

Abstract: We present experimental scattering data at microwave frequencies on a structured metamaterial that exhibits a frequency band where the effective index of refraction (n) is negative. The material consists of a two-dimensional array of repeated unit cells of copper strips and split ring resonators on interlocking strips of standard circuit board material. By measuring the scattering angle of the transmitted beam through a prism fabricated from this material, we determine the effective n, appropriate to Snell's law. These experiments directly confirm the predictions of Maxwell's equations that n is given by the negative square root of epsilon.mu for the frequencies where both the permittivity (epsilon) and the permeability (mu) are negative. Configurations of geometrical optical designs are now possible that could not be realized by positive index materials.

Magnetism from conductors and enhanced nonlinear phenomena

Abstract: We show that microstructures built from nonmagnetic conducting sheets exhibit an effective magnetic permeability /spl mu//sub eff/, which can be tuned to values not accessible in naturally occurring materials, including large imaginary components of /spl mu//sub eff/. The microstructure is on a scale much less than the wavelength of radiation, is not resolved by incident microwaves, and uses a very low density of metal so that structures can be extremely lightweight. Most of the structures are resonant due to internal capacitance and inductance, and resonant enhancement combined with compression of electrical energy into a very small volume greatly enhances the energy density at critical locations in the structure, easily by factors of a million and possibly by much more. Weakly nonlinear materials placed at these critical locations will show greatly enhanced effects raising the possibility of manufacturing active structures whose properties can be switched at will between many states.

Plasmonics for improved photovoltaic devices

Abstract: The emerging field of plasmonics has yielded methods for guiding and localizing light at the nanoscale, well below the scale of the wavelength of light in free space. Now plasmonics researchers are turning their attention to photovoltaics, where design approaches based on plasmonics can be used to improve absorption in photovoltaic devices, permitting a considerable reduction in the physical thickness of solar photovoltaic absorber layers, and yielding new options for solar-cell design. In this review, we survey recent advances at the intersection of plasmonics and photovoltaics and offer an outlook on the future of solar cells based on these principles.