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F. J. Garcia-Vidal

Publications and source records attributed to F. J. Garcia-Vidal.

64 records · Page 4Linked to original sources
arXiv1998

Silver Filled Carbon Nanotubes used as Spectroscopic Enhancers

We analyse from a theoretical point of view the optical properties of arrays of carbon nanotubes filled with silver. Dependence of these properties on the different parameters involved is studied using a Transfer Matrix formalism able to work with tensor-like dielectric functions and including the full electromagnetic coupling between the nanotubes. We find these structures exhibit very strong linear optical response and hence could be used as spectroscopic enhancers or chemical sensors in the visible range. Very localised surface plasmons, created by the electromagnetic interaction between the capped silver cylinders, are responsible of this enhancing ability. Enhancements of up to $10^6$ in the Raman signal of molecules absorbed on these arrays could be obtained.

cond-mat.mtrl-sci↗
arXiv1998

Quantum mechanical analysis of the elastic propagation of electrons in the Au/Si system: application to Ballistic Electron Emission Microscopy

We present a Green's function approach based on a LCAO scheme to compute the elastic propagation of electrons injected from a STM tip into a metallic film. The obtained 2D current distribution in real and reciprocal space furnish a good representation of the elastic component of Ballistic Electron Emission Microscopy (BEEM) currents. Since this component accurately approximates the total current in the near threshold region, this procedure allows --in contrast to prior analyses-- to take into account effects of the metal band structure in the modeling of these experiments. The Au band structure, and in particular its gaps appearing in the [111] and [100] directions provides a good explanation for the previously irreconcilable results of nanometric resolution and similarity of BEEM spectra on both Au/Si(111) and Au/Si(100).

cond-mat.mtrl-sci↗
arXiv1998

Hot electron transport in Ballistic Electron Emission Spectroscopy: band structure effects and k-space currents

Using a Green's function approach, we investigate band structure effects in the BEEM current distribution in reciprocal space. In the elastic limit, this formalism provides a 'parameter free' solution to the BEEM problem. At low temperatures, and for thin metallic layers, the elastic approximation is enough to explain the experimental I(V) curves at low voltages. At higher voltages inelastic effects are approximately taken into account by introducing an effective RPA-electron lifetime, much in similarity with LEED theory. For thick films, however, additional damping mechanisms are required to obtain agreement with experiment.

cond-mat.mtrl-sci↗
arXiv1998

Effective electronic response of a system of metallic cylinders

The electronic response of a composite consisting of aligned metallic cylinders in vacuum is investigated, on the basis of photonic band structure calculations. The effective long-wavelength dielectric response function is computed, as a function of the filling fraction. A spectral representation of the effective response is considered, and the surface mode strengths and positions are analyzed. The range of validity of a Maxwell-Garnett-like approach is discussed, and the impact of our results on absorption spectra and electron energy-loss phenomena is addressed.

cond-mat.mtrl-sci↗
arXiv1997

A theoretical analysis of Ballistic Electron Emission Microscopy: k-space distributions and spectroscopy

We analyze BEEM experiments. At low temperatures and low voltages, near the threshold value of the Schottky barrier, the BEEM current is dominated by the elastic component. Elastic scattering by the lattice results in the formation of focused beams and narrow lines in real space. To obtain the current injected in the semiconductor, we compute the current distribution in reciprocal space and, assuming energy and $k_{\parallel}$ conservation. Our results show an important focalization of the injected electron beam and explain the similarity between BEEM currents for Au/Si(111) and Au/Si(100).

cond-mat.mtrl-sci↗
arXiv1997

Effective medium theory of the optical properties of aligned carbon nanotubes

We present an effective medium theory in order to analyze the reported optical properties of aligned carbon nanotube films (W.A. de Heer et al., Science 268, 845 (1995)). This methodology is based on photonic band structure calculations and allows treatment of complex media consisting of particles that interact strongly. We also develop a simple Maxwell-Garnett type approach for studying this system. In comparing the results of both mean field theories, we demonstrate that the inclusion of the full electromagnetic coupling between the nanotubes, as our numerical scheme does, is necessary for a complete explanation of the experimental data.

cond-mat.mtrl-sci↗
arXiv1995

Thermal noise in superconducting quantum point-contacts

We present a theory for the frequency-dependent current fluctuations in superconducting quantum point-contacts (SQPC) within the dc transport regime. This theory is valid for any barrier transparency between the tunnel and ballistic limits, yielding an analytical expression for the fluctuations spectrum in the subgap region. It is shown that the level of noise in a quasi-ballistic SQPC may have a huge increase in comparison with the case of a normal contact carrying the same average current. The effect of this high level of noise on the actual observability of the current-phase relation for a ballistic point-contact is discussed in connection with recent experimental measurements.

cond-mat↗
arXiv1994

Self-Consistent Theory of Superconducting Mesoscopic Weak Links

A microscopic model for describing a superconducting mesoscopic weak link is presented. We consider a model geometry consisting of a narrow channel coupled to wider superconducting electrodes which act as reservoirs fixing the asymptotic values of the complex order parameter. For this model, the Bogoliubov-de Gennes equations are discretized and solved self-consistently using a non-equilibrium Green functions formalism. The transport properties and the electronic excitation spectra of this system are studied for the different regimes that can be reached by varying parameters like coherence length, constriction length, normal transmission coefficient and temperature. We study in detail the transition from the point contact limit to the infinite channel length case, analyzing the maximum Josephson current that can be sustained by the weak link as a function of its transmission coefficient and length.

cond-mat↗
arXiv1993

Microscopic Theory of Josephson Mesoscopic Constrictions

We present a microscopic theory for the d.c. Josephson effect in model mesoscopic constrictions. Our method is based on a non-equilibrium Green function formalism which allows for a self-consistent determination of the order parameter profile along the constriction. The various regimes defined by the different length scales (Fermi wavelength $λ_F$, coherence length $ξ_0$ and constriction length $L_C$) can be analyzed, including the case where all these lengths are comparable. For the case $λ_F \tilde{<} (L_C,ξ_0)$ phase oscillations with spatial period $λ_F/2$ can be observed. In the case of $L_C>ξ_0$ solutions with a phase-slip center inside the constriction can be found, in agreement with previous phenomenological theories.

cond-mat↗