Alfonso San Miguel

Direct measurement of the absolute absorption spectrum of individual semiconducting single-wall carbon nanotubes.

The optical properties of single-wall carbon nanotubes are very promising for developing novel opto-electronic components and sensors with applications in many fields. Despite numerous studies performed using photoluminescence or Raman and Rayleigh scattering, knowledge of their optical response is still partial. Here we determine using spatial modulation spectroscopy, over a broad optical spectral range, the spectrum and amplitude of the absorption cross-section of individual semiconducting single-wall carbon nanotubes. These quantitative measurements permit determination of the oscillator strength of the different excitonic resonances and their dependencies on the excitonic transition and type of semiconducting nanotube. A non-resonant background is also identified and its cross-section comparable to the ideal graphene optical absorbance. Furthermore, investigation of the same single-wall nanotube either free standing or lying on a substrate shows large broadening of the excitonic resonances with increase of oscillator strength, as well as stark weakening of polarization-dependent antenna effects, due to nanotube-substrate interaction.

First-principles study of lithium-doped carbon clathrates under pressure

We present a theoretical study of the behavior under pressure of the two hypothetical C46 and Li8C46 type-I carbon clathrates in order to obtain new information concerning their synthesis. Using ab initio calculations, we have explored the energetic and structural properties under pressure of these two carbon based cage-like materials. These low-density metastable phases show large negative pressure transitions compared to diamond, which represent a serious obstacle to their synthesis. However, we show that a minimum energy barrier can be reached close to 40 GPa, suggesting that synthesis of the Li-clathrate under extreme conditions of pressure and temperature may be possible. The electronic band structure with related density of states behavior under pressure, as well as the dependence of the active Raman modes with pressure are also examined.

Phonon density of states, anharmonicity, electron-phonon coupling, and possible multigap superconductivity in the clathrate superconductors Ba 8 Si 46 and Ba 24 Si 100 : Factors behind large difference in T c

We report a detailed study of specific heat, electrical resistivity, and thermal expansion in combination with inelastic neutron and inelastic x-ray scattering to investigate the origin of superconductivity in the two silicon clathrate superconductors Ba8Si46 and Ba24Si100. Both compounds have a similar structure based on encaged barium atoms in oversized silicon cages. However, the transition temperatures are rather different: 8 and 1.5 K, respectively. By extracting the superconducting properties, phonon density of states, electron-phonon coupling function, and phonon anharmonicity from these measurements, we discuss the important factors governing Tc and explain the difference between the two compounds.

Full Multiple Scattering Calculations on HgTe Under High Pressure at the Mercury L3 X-Ray Absorption Edge

HgTe crystallizes in the zincblende structure at ambient pressure, transforming to a cinnabar structure at 1.5 GPa, at 300K, and then to a rocksalt structure at 8 GPa. Simultaneously to pressure-induced structural changes, transitions from semi-metal → semiconductor → metal occur. The zincblende → cinnabar → rocksalt transition leads to important changes in the shape of the XANES spectra1 at the Hg L3 edge. The goal of these Full Multiple Scattering (FMS) calculations based on a real-space cluster method is threefold: we first want to reproduce the XANES modifications observed under increasing pressure, then confirm the atomic positions determined elsewhere by angular dispersive X-ray diffraction (ADX) and XAS1,2 by using these data for the construction of the cluster and finally address XANES resonances as fingerprints of the different structures.

High-Pressure Effect on the Optical Extinction of a Single Gold Nanoparticle

When reducing the size of a material from bulk down to nanoscale, the enhanced surface-to-volume ratio and the presence of interfaces make the properties of nano-objects very sensitive not only to confinement effects but also to their local environment. In the optical domain, the latter dependence can be exploited to tune the plasmonic response of metal nanoparticles by controlling their surroundings, notably applying high pressures. To date, only a few optical absorption experiments have demonstrated this feasibility, on ensembles of metal nanoparticles in a diamond anvil cell. Here, we report a nontrivial combination between a spatial modulation spectroscopy microscope and an ultraflat diamond anvil cell, allowing us to quantitatively investigate the high-pressure optical extinction spectrum of an individual nano-object. A large tuning of the surface plasmon resonance of a gold nanobipyramid is experimentally demonstrated up to 10 GPa, in quantitative agreement with finite-element simulations and an analytical model disentangling the impact of metal and local environment dielectric modifications. High-pressure optical characterizations of single nanoparticles allow for the accurate investigation and modeling of size, strain, and environment effects on physical properties of nano-objects and also enable fine-tuned applications in nanocomposites, nanoelectromechanical systems, or nanosensing devices.

Raman study of the electron-phonon interaction in light alkali metal intercalated metallic fullerides.

By laser-irradiating polymeric Li(4)C(60) and Na(4)C(60), we have obtained pure monomeric metallic phases stable at ambient conditions. Based on a systemic Raman analysis, we have determined the electron-phonon coupling constant for both metallic phases. The e-p coupling constants of Li- and Na-intercalated metallic fullerides are smaller than those of superconductive K(3)C(60) and Rb(3)C(60) and comparable to or slightly higher than that of ambient-pressure non-superconductive Cs(3)C(60). We predict that Na-doped fulleride could exhibit superconductivity with T(c) ∼ 10 K. Much lower T(c) or even no superconductivity can be expected for the Li-doped fulleride which exhibits a strong Li(+)-C interaction. These results contribute to the understanding of superconductivity in light alkali metal intercalated fullerides.