We study the resonant tunneling effects through double barrier graphene systems (DBGSs). We have considered two types of DBGSs in order to take into account or rule out Klein tunneling effects: (1) the well-known and documented electrostatic-barrier structures (EBSs) created by means of electrostatic probes that act perpendicularly to the graphene sheet; and (2) substrate-barrier structures (SBSs) built sitting the graphene layer on alternating substrates, such as SiO2 and SiC, which are capable of non-open and open an energy bandgap on graphene. The transfer matrix approach is used to obtain the transmittance, linear-regime conductance, and electronic structure for different set of parameters such as electron energy, electron incident angle, barrier, and well widths. Particular attention is paid to the asymmetric characteristics of the DBGSs, as well as to the main differences between Klein and non-Klein tunneling structures. We find that: (1) the transmission properties can be modulated readily changing the energy and angle of the incident electrons, the widths of the well and barrier regions; (2) the linear-regime conductance is easily enhancing, diminishing, and shifted changing from symmetric to asymmetric DBGSs configuration overall in the case of non-Klein tunneling structures; (3) the conductance shows an oscillatory behavior as function of the well width, with peaks that are directly related to the opening and opening-closure of bound-state subbands for EBSs and SBSs, respectively. Finally, it is important to mention that electrostatic DBGSs or substrate DBGSs could be more suitable depending on a specific application, and in the case of non-Klein tunneling structures, they seem possible considering the sophistication of the current epitaxial growth techniques and whenever substrates that open an energy bandgap on graphene, without diminishing the carrier's mobility, be experimentally discovered.

No abstract

Electron transmission through different non-conventional (non-uniform barrier height) gated and gapped graphene superlattices (GSLs) is studied. Linear, Gaussian, Lorentzian and Pöschl-Teller superlattice potential profiles have been assessed. A relativistic description of electrons in graphene as well as the transfer matrix method have been used to obtain the transmission properties. We find that it is not possible to have perfect or nearly perfect pass bands in gated GSLs. Regardless of the potential profile and the number of barriers there are remanent oscillations in the transmission bands. On the contrary, nearly perfect pass bands are obtained for gapped GSLs. The Gaussian profile is the best option when the number of barriers is reduced, and there is practically no difference among the profiles for large number of barriers. We also find that both gated and gapped GSLs can work as omnidirectional band-pass filters. In the case of gated Gaussian GSLs the omnidirectional range goes from −50° to 50° with an energy bandwidth of 55 meV, while for gapped Gaussian GSLs the range goes from −80° to 80° with a bandwidth of 40 meV. Here, it is important that the energy range does not include remanent oscillations. On the light of these results, the hole states inside the barriers of gated GSLs are not beneficial for band-pass filtering. So, the flatness of the pass bands is determined by the superlattice potential profile and the chiral nature of the charge carriers in graphene. Moreover, the width and the number of electron pass bands can be modulated through the superlattice structural parameters. We consider that our findings can be useful to design electron filters based on non-conventional GSLs.

We present a spectroscopic ellipsometry study of Mo-doped VO2 thin films deposited on silicon substrates for the mid-infrared range. The dielectric functions and conductivity were extracted from analytical fittings of Ψ and Δ ellipsometric angles showing a strong dependence on the dopant concentration and the temperature. Insulator-to-metal transition (IMT) temperature is found to decrease linearly with increasing doping level. A correction to the classical Drude model (termed Drude-Smith) has been shown to provide excellent fits to the experimental measurements of dielectric constants of doped/undoped films and the extracted parameters offer an adequate explanation for the IMT based on the carriers backscattering across the percolation transition. The smoother IMT observed in the hysteresis loops as the doping concentration is increased, is explained by charge density accumulation, which we quantify through the integral of optical conductivity. In addition, we describe the physics behind a localized Fano resonance that has not yet been demonstrated and explained in the literature for doped/undoped VO2 films.

No abstract

Using the transfer matrix method we calculate the frequency dependence of the transmission of longitudinal elastic waves for a layered structure where the specific acoustic impedance of the layers with odd numbering follows a Gaussian distribution, while the inserted even layers have the same impedance as the propagation medium. The structure presents intervals of low-pass, bandstop, and band-pass. The characteristics of the bands depend on the number of layers, on the contrast between the maximum and minimum impedances of the structure, and on the ratio of the width of the inserted layers to the width of the layers with a Gaussian distribution of impedances.

No abstract

An optical spectroscopy, based upon the determination of the difference between the reflectivity for p-polarized light (Rp) and the squared reflectivity for s-polarized light (Rs2) with an angle of incidence of 45°, is proposed to study the interaction of excitons with the surface potential in semiconductors. It is found that Rp−Rs2 as a function of the wave frequency is quite sensitive to the form of the surface potential. In addition, the generation of longitudinal modes such as the quantized polarization waves of excitons produces resonant dips well-identified in the new spectrum. On the other hand, the transverse resonances present in Rp and Rs disappear in the difference Rp−Rs2.