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In the $GW$ approximation, the screened interaction $W$ is a non-local and dynamical potential that usually has a complex frequency dependence. A full description of such dependence is possible but often computationally demanding. For this reason, it is still common practice to approximate $W(\omega)$ using a plasmon pole (PP) model. Such approach, however, may deliver an accuracy limited by its simplistic description of the frequency dependence of the polarizability, i.e. of $W$. In this work we explore a multi-pole approach (MPA) and develop an effective representation of the frequency dependence of $W$. We show that an appropriate sampling of the polarizability in the frequency complex plane and a multi-pole interpolation can lead to a level of accuracy comparable with full-frequency methods at much lower computational cost. Moreover, both accuracy and cost are controllable by the number of poles used in MPA. Eventually we validate the MPA approach in selected prototype systems, showing that full-frequency quality results can be obtained with a limited number of poles., Comment: 7 figures, 18 pages + 2 supplementary pages

Rocksalt $\mathrm{Z}{\mathrm{n}}_{x}\mathrm{M}{\mathrm{g}}_{1\ensuremath{-}x}\mathrm{O}$ alloys are theoretically and experimentally investigated for near- and deep-UV optoelectronics with a tunable band gap of 4.2--7.8 eV. Regarding the key question about the composition $x$, at which there is a transition between the direct and indirect gaps, we performed ab initio calculations for various Zn concentrations and all possible atomic arrangements in eight- to 64-atom supercells. We show that, depending on the detailed Zn distribution (clustered, random, or uniform distribution), the alloy band gap can vary by as much as 1.27 eV. The band gap is indirect for clustered and random Zn arrangements in the supercell. For uniform Zn arrangements, the gap is also indirect, except for $xl0.5$ and atom uniform arrangements excluding Zn-O-Zn nearest neighbor bridges, for which the direct gap can be lowered below the indirect gap by about 0.1 eV. The mechanisms of band-gap fluctuation, Zn clustering, and direct-indirect band-gap transitions are analyzed and explained in terms of atomic contributions to band structures by projecting Bloch functions onto localized Wannier functions. Simultaneously, cathodoluminescence measurements were performed on a set of $\mathrm{Z}{\mathrm{n}}_{x}\mathrm{M}{\mathrm{g}}_{1\ensuremath{-}x}\mathrm{O}$ multiquantum wells grown by molecular beam epitaxy on MgO substrates. We observed strong and broad emission bands, redshifting with increasing Zn concentration but featuring no clear-cut evidence for any direct to indirect band-gap crossover. We argue that these alloys are well suited for deep-UV optoelectronics, thanks to the rare combination of strong exciton binding energy, coupling to phonons, and carrier localization, which is favored by the marked flattening of the top valence bands by both short-range and long-range Zn-Zn interactions.

Mie's theory of light scattering on spherical particles is being increasingly used in nanophotonics, and these demanding applications have laid bare some shortcomings of Mie theory in its standard formulation. One problem that deserves special attention is the electron spill-out in small metallic nanoparticles, which invalidates the assumption of an abrupt interface. Here we present an alternative derivation of Mie theory without this assumption. To avoid the usual electromagnetic boundary conditions suitable for a hard-wall interface, we set up equations for the electromagnetic potentials instead of the electric and magnetic field. We show that in the limit of a hard-wall interface, the results of the standard Mie theory are recovered. Additionally, a numerical solution scheme is proposed for the equations for the vector potential and the scalar potential. Analysis of the optical cross sections of soft-interface nanospheres shows that the absorption increases and occurs at lower frequencies as compared to hard-walled nanospheres. This effect is rather dramatic in large spheres with large spill-out, due to the disappearance of high-frequency resonance peaks.

Ab initio band-structure calculations were performed for bulk, single slab, and thin films of TiX2 (X=S, Se) using the localized spherical wave method. According to these calculations, bulk TiS2 and TiSe2 are semimetallic. The calculations show that TiS2 thin films are semiconductors, but thin films of TiSe2 are semimetallic. The indirect gap for single slab TiS2 is about 1.0 eV, and the gap becomes smaller with increasing number of layers. When the number of layers increases to 11, the TiS2 thin films are semimetallic. All but the surface layers are found to be electrically neutral. The density of states as a function of the energy for the surface layer is different from that of the bulk. The Madelung energy of the Ti atoms on the surface is about 0.35 eV lower than that for the Ti atoms in the bulk. The calculations are compared with photoemission spectra, reported in the literature.

The structure of the technologically important---but still mostly unknown---GaSb(001)--$c(2\ifmmode\times\else\texttimes\fi{}6)$ surface reconstruction is investigated by means of ab initio simulations of reflectance anisotropy spectroscopy (RAS) and total energy calculations. A large number of reconstruction models for the GaSb(001) surface in the Sb-rich coverage regime are considered. The influence of each single surface structural motif on the RAS spectra is studied in detail, as well as their role in the surface stability with regard to application of the electron counting rule (ECR). We interpret the features of the RAS data measured for this reconstruction and suggest a new model for the $c(2\ifmmode\times\else\texttimes\fi{}6)$ phase. In this model a few Sb atoms in the second layer are randomly substituted by Ga, forming surface antisite defects. When used to fulfill the ECR, this ``doping'' effect considerably lowers the total energy of the long chain $c(2\ifmmode\times\else\texttimes\fi{}6)$ reconstruction model, making it competitive with the more stable short-chain $(4\ifmmode\times\else\texttimes\fi{}3)$ reconstructions. Formation of the surface antisites occurs spontaneously in the presence of dynamical negative charge fluctuations and is favored by the excellent matching between GaSb(001) and metallic Sb and by the natural softness of the Ga--Sb bonds. Calculations of the reflectance anisotropy spectra confirm that this structure is a major component of a largely disordered surface, where motifs of the stable $(4\ifmmode\times\else\texttimes\fi{}3)$ reconstructions are also present.

Detailed investigation of the luminescence features due to the recombination of the exciton-free-carrier process in GaSe is presented. This process shows a well-resolved luminescence line and it is an efficient recombination process in this semiconductor, even at low temperatures. We report accurate photoluminescence spectra measured at 2 K as a function of the excitation intensity, of the excitation energy, and of the lattice temperature from 2 to 200 K. To discriminate between the different kinds of free carriers participating in exciton-carrier scattering in GaSe, we have computed the temperature dependence of the scattering probability of the direct excitons with different carriers: direct electrons, indirect electrons, and holes; we have found that the most probable exciton-carrier scattering process is that involving the coexistence of direct electron and hole collisions. Line shape of the exciton-carrier scattering as a function of the carrier temperature has been calculated and fitted to the experimental luminescence spectra from 2 to 200 K and a very good agreement is found. From the absorption spectra of GaSe, we obtained a direct estimate of the excitonic polarizability.

In Al0.3Ga0.7As/GaAs quantum wells bordered by ultrathin (one or two monolayers) AlAs layers, the AlAs layer has been shown to provide a well for electrons at high pressure [Phys. Rev. B 45, 11 846 (1992)]. In this work, the same type of structures but grown on (001) substrates misoriented towards (111)Ga, are studied by photoluminescence as a function of pressure, and a comparison is made with those grown on nominal substrates in order to investigate the influence of surface steps on electronic levels. Particular emphasis is given to indirect type-II transitions. An important blueshift of indirect transitions is observed in the case of two AlAs monolayers grown on a vicinal substrate. The size of this shift may be explained considering diffusion of carriers by the interface steps, or also lateral confinement. Inversely, a redshift is observed in the one AlAs monolayer sample. We suggest that this last point may be due to terrace length fluctuations.

The Purcell effect, acceleration of a spontaneous emission recombination rate, has been observed in InN/In nanocomposites with buried nanoparticles of metallic In. This effect, associated with localized plasmons, is characterized by the averaged Purcell factor as high as 30--40 in the structures with large enough particles. This high value is indicative of a noticeable contribution from the emitting dipoles polarized normally to the nanoparticle surface in this system. The experimental observation of shortening of the emission lifetimes with increasing the amount of In is supported by calculations performed in a semiclassical approximation.

The electronic and spin dynamics in thin iron films have been investigated by means of time-resolved reflectivity and time-resolved magneto-optical Kerr effect. Combining the two techniques, it is possible to clarify the role of electron-phonon and electron-magnon interactions on the ultrafast magnetization dynamics. In particular, we show that the rapid $(\ensuremath{\sim}100\text{ }\text{fs})$ demagnetization is established at the electronic level through electron-magnon excitation, while the subsequent recovery of the spin order is attributed to the Elliott-Yafet spin-flip scattering process on a time scale slightly shorter than a picosecond. Both processes have characteristic time constants that undoubtedly differ from the measured electron-phonon coupling time of 240 fs.

International audience; The interaction between small unsaturated hydrocarbon molecules of C2H4 with a vicinal silicon 001 surface is studied by means of reflectance anisotropy spectroscopy and analyzed with first principles calculations. Our results confirm that ethylene adsorbs without breaking the silicon dimers. Comparison of theoretical optical spectra with experimental data shows that the C2H4 molecules lay on top of the silicon dimers from low to high coverage. This occurs even though, from a purely energetic point of view, a bridge configuration would be favorable at 1 monolayer coverage.

In this paper, we present results of calculations of linear and second-order nonlinear polarizabilities of sector-shaped metallic nanoparticles (hemisphere is a special case) using free electron theory. The dependences of the ground state electron density distribution and polarizabilities on various shape parameters of sector are analyzed. The ground state electron densities near the corners and edges of sector-shaped nanoparticle are very low and do not contribute to the linear and second-order polarizabilities. The second-order polarizability is found to depend strongly on the angle of the sector and is shown to be proportional to the product of an appropriately defined asymmetric volume of the particle and the third power of the electron cloud length.

We propose a smart nanoparticle, a ``regulatron,'' that exploits a cycle of dynamic plasmonic feedback to self-regulate its temperature to a fixed range. One kind of regulatron can be conceived from $\mathrm{V}{\mathrm{O}}_{2}$ and Au; the temperature of this particle when illuminated by laser irradiation of suitable intensities and wavelengths is $\ensuremath{\sim}340\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We expect that regulatrons of this and other compositions will be useful in applications such as photothermal medical therapeutics.

We extend the three-layer model for energy loss calculation to include many-body effects in an ab initio framework. The electron energy loss spectrum of the $\mathrm{C}(100)2\ifmmode\times\else\texttimes\fi{}1$ surface is calculated and compared with the existing experimental results. We show how many-body effects, namely self-energy, local-fields, and electron-hole interaction, deeply influence the dielectric response and how their inclusion is essential to have good agreement with the experiment. A strong anisotropic behavior of local-field effects on the dielectric response has been observed, while the inclusion of the electron-hole attractive interaction produces a surface-state exciton with binding energy of about $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, confirming a recent theoretical-experimental study of the optical spectra of this surface.

Sensitization of Er emission by Si nanoclusters (Si-nc) is investigated with pulsed and continuous optical pumping, in and off resonance with excited states of ${\text{Er}}^{3+}$ ion. We show that under high-power pulsed pumping, the excitation process is limited by the finite energy transfer time from Si-nc to ${\text{Er}}^{3+}$ ions. By comparison between pulsed and steady-state excitation, the concentration of sensitizers and average number of ${\text{Er}}^{3+}$ ions coupled to a single nanocluster are independently determined in an experiment. The results clarify conditions needed for efficient sensitization of ${\text{SiO}}_{2}:\text{Er}$ by Si-nc.

Recent measurements surprisingly show that the lowest valence-to-conduction confined transitions in narrow $(\mathrm{InAs}{)}_{8}/(\mathrm{GaSb}{)}_{n}$ and $(\mathrm{InAs}{)}_{6}/(\mathrm{GaSb}{)}_{n}$ superlattices increase in energy as the barrier thickness n increases. We show that in addition to the mesoscopic geometric quantities (well and barrier sizes), an atomic-scale description of interdiffused interfaces is needed to correctly reproduce the observed spectroscopic trend. The interdiffused interface is modeled via diffusion equations. We compare our atomistic empirical pseudopotential calculation in which only the bulk binary data are fit to experiment, with contemporary methods in which agreement with experiment is forced using ideally abrupt interfaces.

A reordered valence band in GaN/AlxGa1-xN quantum wells with respect to GaN epilayers has been found as a result of the observation of an enhanced g factor (g*similar to3) in magnetoluminescence spectra in fields up to 55 T. This has been caused by a reversal of the states in the strained AlxGa1-xN barriers thus giving different barker heights for the different quantum well hole states. From k-p calculations in the quasicubic approximation, a change in the valence band ordering will account for the observed values for the g factors. We have also observed the well-width dependence of the in-plane extent of the excitonic wave function from which we infer an increase in the exciton binding energy with the reduction of the well width in general agreement with theoretical calculations of Bigenwald et al. [Phys. Status Sokidi B 216, 371 (1999)] that use a variational approach in the envelope function formalism that includes the effect of the electric field in the wells.

Insight into the nature of the pairing of charge carriers in high- Tc superconductors may be provided by a systematic investigation of the condensation energy. In this work we report on studies of the electronic kinetic energy across the complex phase diagram of these materials. The c-axis component of the electronic kinetic energy ~determined from an analysis of the optical constants! is shown to be reduced below Tc , primarily in those compounds in which the superconducting transition at Tc is preceded by the formation of the partial gap in the density of states ~pseudogap! at T*.Tc . An examination of the doping dependence of the infrared conductivity in conjunction with the results of photoemission spectroscopy suggests that the lowering of the kinetic energy is a property of the electronic states close to the intersection of the two-dimensional Fermi surface with the boundary of the Brillouin zone. We contrast the c-axis results with the energetics associated with the nodal quasiparticles probed through the in-plane conductivity.