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We present a simple method to incorporate electrostatic forces into the extended Huckel scheme, thereby extending its range of application to ionic systems. A series of applications to alkali halide molecules and clusters shows the importance of including the Madelung energy in such calculations. The amount of charge transfer, the position of the valence levels, and the band gap are found to depend on the inclusion of the electrostatic potential. The calculated bulk and surface electronic structure compares well with experimental findings.

Heats of solution and impurity-induced lattice relaxations are calculated for hydrogen in all the transition metals. The hydrogen-metal interaction is described using the effective-medium theory and the metal-lattice relaxations are included in a harmonic-continuum model. The authors find that the trends in the heats of solution can be related to the magnitude of the electron density of the interstitial metal. The trends in the hydrogen trapping energies for vacancies are estimated from the difference between the calculated heats of solution and chemisorption energies. A very simple conceptual picture of the hydrogen-metal interaction results, which can easily be extended to treat for instance hydrogen in metal alloys.

We present an Anderson-Newns Hamiltonian model of inverse photoemission spectra (IPES) of adsorbates on metals. We find that besides image screening, another screening mechanism may operate in IPES when intra-adsorbate electron correlation is strong and the adsorbate has filled levels near the Fermi energy of the metal. While image screening results in spectral shifts and the appearance of plasmon satellites, this new screening mechanism gives a doublet structure to the IPES. We also find that the hybridization of the adsorbate affinity levels with the unoccupied metal levels plays a crucial role in determining both the width and energy of the spectra.

Calculations of the physisorption interaction between a H 2 -molecule and simple metal surfaces are reported. The physisorption wells of simple and noble metals having the same s-electron densities are markedly different. The former are shallower because there is no contribution of d-electrons to the Van der Waals force. The physisorption energies are found to depend strongly on the bulk density, varying from 25 meV (Al) to 2.7 meV (K). Results for three Al surfaces show a face dependence due to the different work functions that may be measurable.

Atom-beam scattering experiments, which provide useful information on the surface structure, are now being performed at a level that makes it crucially important to know the atom-surface potential accurately. As one of the recent attempts to provide atom-surface scattering potentials, that are simple in form, that apply for different atom-surface combinations by just changing some parameters, and that can include lattice effects, Harris and Liebsch have proposed a pseudo-potential perturbation approach. They show that already the first-order term agrees within 15% with the accurate result of Zaremba and Kohn for jellium. The present calculation shows that inclusion of the second-order term gives almost complete agreement with the latter in the case of He on Al, thus providing a simple and practically useful method to get accurate atom-surface scattering potentials. Application has also been made to the He-Ag and He-Mg systems.

We investigate how the initial formation of an oxide layer on a metal surface can effect the further incorporation of oxygen into the metal. In particular, we show that the formation of NbO on a Nb surface dramatically increases the potential barrier for oxygen incorporation. On Cu the formation of an oxide layer reduces the barrier. On Cr and on Ni the barriers on the metal and on the oxide are similar. It is shown that the height of the barrier can be significantly reduced by the relaxation of the atoms in the surface oxide. The gain in energy in this process is shown to depend on the magnitude of the elastic constants of the oxide on the surface. We also show that oxides which grow by Cabrera-Mott Kinetics have a barrier to oxygen penetration while others do not.

The binding of multiple deuterium (D) atoms to vacancies in iron (Fe) is investigated both experimentally, by use of ion‐beam techniques, and theoretically, by use of the effective‐medium scheme. The experimental D‐release stages are accounted for by trapping at vacancies with a binding enthalpy that depends on occupancy. It is found that for 1–2 D in a vacancy, the trap strength is 0.63 eV, whereas the binding enthalpy for 3–6 D in a vacancy is 0.43 eV. These results are in good agreement with predictions from the effective‐medium theory.

The binding energies of up to six deuterium atoms into monovacancies of Pd, Ni, Fe, Cu, Nb, and Mo have been calculated using the effective-medium theory. For all metals it is found that at least six D atoms can be accommodated in the vacancy. Of the systems studied the deuterium-deuterium interaction is found to be least repulsive for Pd. This multiple occupancy of the vacancy defect can have potentially significant implications for the recently proposed fusion of D atoms in Pd.

Theoretical analysis of the defect trapping of ion-implanted hydrogen in metals has been extended in two respects. A new transport formalism has been developed which takes account not only of the diffusion, trapping, and surface release of the hydrogen, which were included in earlier treatments, but also the diffusion, recombination, agglomeration, and surface annihilation of the vacancy and interstitial traps. In addition, effective-medium theory has been used to examine multiple hydrogen occupancy of the vacancy, and, for the fcc structure, appreciable binding enthalpies relative to the solution site have been found for occupancies of up to six. These extensions have been employed to model the depth distribution of ion-implanted hydrogen in Ni and Al during linear ramping of temperature, and the results have been used to interpret previously published data from these metals. The agreement between theory and experiment is good for both systems. In the case of Ni, the two experimentally observed hydrogen-release stages are both accounted for in terms of trapping at vacancies with a binding enthalpy that depends upon occupancy in accord with effective-medium theory.

We estimated surface penetration barriers for interstitial oxygen using a simple calculational scheme, the effective-medium theory for a single atom. In this theory the single parameter describing the host is the electron density. Since the electron density depends primarily on the interatomic distances, i.e. the structure of the host, we found that the barrier to oxygen penetration in oxides depends primarily on the structure of the oxide, the closest-packed isotropic oxides with the smallest lattice constants having the highest barriers.

Trabajo presentado a: "Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference", celebradas en Munich (Alemania) del 12 al 16 de Mayo de 2013., The optical response of metallic nanoparticles shows antenna resonance effects at optical frequencies derived from the collective excitation of the conduction electrons at the boundaries of the particles, so called surface plasmons. This response is commonly described in the framework of classical electrodynamics by means of a linear polarizability obtained within a local or nonlocal approach of the excitations. Alternatively, quantum mechanics within time-dependent density functional theory (TDDFT) offers an appropriate framework to fully address the complex exchange and correlations of the electron gas in the metal together with an accurate description of the geometrical boundaries of the particles through the corresponding potential barriers. In Fig. 1 we show the differences between a classical and a quantum treatment of the optical response for the two lowest bonding plasmon modes in a Na nanodimer. Quantum effects derived from the spill out of the electrons at the boundaries and from the electron tunnelling between particles are properly addressed within the TDDFT showing clear differences with respect to a classical treatment. However the number of electrons that can be considered in the quantum calculation is typically limited to a few thousands of electrons, therefore far from realistic plasmonic systems that usually involve millions or billions of electrons. Here we introduce a quantum-corrected model (QCM) that integrates the quantum response of a metallic cavity with the macroscopic response of the rest of the plasmonic system. This hybrid response accounts properly for quantum effects derived from the coherent tunnelling across the cavity while tracing the macroscopic plasmonic modes.

Plasmonic modes with long radiative lifetimes combine strong nanoscale light confinement with a narrow spectral line width carrying the signature of Fano resonances making them very promising for nanophotonic applications such as sensing lasing and switching. Their coupling to incident radiation also known as radiance determines their optical properties and optimal use in applications. In this work we theoretically and experimentally demonstrate that the radiance of a plasmonic mode can be classified into three different regimes. In the weak coupling regime the line shape exhibits remarkable sensitivity to the dielectric environment. We show that geometrical displacements and deformations at the Ångström scale can be detected optically by measuring the radiance. In the intermediate regime the electromagnetic energy stored in the mode is maximal with large electric field enhancements that can be exploited in surface enhanced spectroscopy applications. In the strong coupling regime the interaction can result in hybridized modes with tunable energies. © 2012 American Chemical Society.

A general discussion of radiative interference processes in plasmonic nanostructures is presented. It will be shown that the interference between subradiant and superradiant plasmon modes can induce pronounced Fano resonances it the optical spectra.

Optical Nanoengineering Optics and electronics operate at very different length scales. Surface plasmons are light-induced electronic excitations that are being pursued as a route to bridge the length scales and bring the processing speed offered by optical communication down to the size scales of electronic chip circuitry. Now, Fan et al. (p. 1135 ) describe the self-assembly of nanoscale dielectric particles coated with gold. Functionalization of the gold surface with polymer ligands allowed controlled production of clusters of nanoparticles. The optical properties of the self-assembled nanostructures depended on the number of components within the cluster and each structure could be selected for its unique optical properties. Such a bottom-up approach should help in fabricating designed optical circuits on the nanoscale.

Plasmonic nanostructures can serve as model systems for a variety of fascinating classical oscillator physics. For example, in nanostructures with broken symmetry, Fano resonances can arise due to the interaction of narrow dark modes with broad bright modes. We have recently identified non-concentric ring/disk cavities as a system with a highly tunable Fano resonance and exceptionally large refractive index sensitivity and localized surface plasmon resonance figure of merit [1]. Using numerical electromagnetic simulations, Zhang and co-workers have further shown that for dolmen-shaped slab structures with optimized geometries a full plasmonic-induced optical transparency can develop [2]. This constitutes a classical analogue [3] to the well-known phenomenon of electromagnetically induced transparency (EIT). Here we present an experimental demonstration of Fano resonances in two plasmonic systems: firstly the dimer/monomer dolmen slab structure proposed by Zhang et al [2], and secondly for a side-by-side arrangement of a Au ring and a Au disk (ring near disk cavity, RNDC), both fabricated using e-beam lithography. We analyze the dependence of the Fano lineshapes on the polarization of the incident light, and of the strength of the feature on the separation between individual nanoscale sub-units. We further show that for significantly broad dipolar modes, multiple Fano resonances can arise.

We obtain the optical response of plasmonic nanoantennas showing subnanometric gaps with use of a quantum-corrected model. A redistribution of plasmonic modes and a quenching of the field-enhancement are produced due to quantum tunneling.

We calculated the potential of interstitial oxygen in YBa 2 Cu 3 O 7 using effective medium theory. We found that there is no potential energy barrier for oxygen motion in the [½, b,0] tunnel. Oxygen motion in the a and c directions requires overcoming barriers of 1.7 and 1.6 eV. Implications for superconductivity and oxygen diffusion are given.