Non-conventional graphene superlattices as electron band-pass filters

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

Propiedades de transmisión de electrones de Dirac a través de superredes Cantor en grafeno

En este trabajo usamos el método de la matriz de transferencia para estudiar el tunelamiento de los electrones de Dirac a través de superredes aperiodicas en grafeno. Consideramos una hoja de grafeno depositada encima de bloques de sustratos de Óxido de Silicio (SiO2) y Carburo de Silicio (SiC), en los cuales aplicamos la serie de Cantor. Calculamos la transmitancia para diferentes parámetros fundamentales tales como: ancho de partida, energía de incidencia, ángulo de incidencia y número de generación de la serie de Cantor. En este caso, la transmitancia como función de la energía presenta rasgos autosimilares al variar el número de generación. También computamos la distribución angular de la transmitancia para energías fijas econtrando un patrón autosimilar entre generaciones. Por último, calculamos los factores de escala para algunos espectros de la transmitancia, los cuales efectivamente muestran escalabilidad.In this work we use the transfer matrix method to study the tunneling of Dirac electrons through aperiodic monolayer graphene superlattices. We consider a graphene sheet deposited on top of slabs of Silicon-Oxide (SiO2) and Silicon-Carbide (SiC) substrates, in which we applied the Cantor's series. We calculate the transmittance for different fundamental parameters such as: starting width, incident energy, incident angle and generation number of the Cantor's series. In this case, the transmittance as function of energy presents self-similar features as a function of the generation number. We also compute the angular distribution of the transmittance for fixed energies finding a self-similar patterns between generations. Finally, we calculate the scaling factor for some transmittance spectra, which effectively show scalability

Bandgap engineering in aperiodic Thue-Morse graphene superlattices

The lack of bandgap in graphene is the main factor that prevents that this outstanding material be implemented in optoelectronics. In this work, we show that by nanostructuring graphene aperiodically it is possible to have an efficient transmission bandgap engineering. In particular, we are considering aperiodic graphene superlattices in which electrostatic barriers are arranged following the basic construction rules of the Thue-Morse sequence. We find that the transmission bandgap can be modulated readily by changing the angle of incidence as well as by appropriately choosing the generation of the Thue-Morse superlattice. Even, this angle-dependent bandgap engineering is more effective than the corresponding one for periodic graphene superlattices

Pseudospin-dependent Zitterbewegung in monolayer graphene

We propose a spintronic device based on a narrow nanoribbon patterned from a monolayer graphene (MLG) sheet, embedded between a film of hexagonal boron nitride and a SiO2 substrate, all comprised under a three top-gated structure, to explore spin-dependent quantum transport of Dirac fermions. We developed a theoretical procedure for describing the pseudospin-related effects and the dynamics of Dirac fermions represented by a one-dimensional Gaussian wave packet (1DGWP), which is electrostatically confined in the device. The free-space 1DGWP time evolution follows expected features. Meanwhile, due to the weak breakdown of the real-spin degeneracy, the 1DGWP barely splits inside the under-barrier region governed by the extrinsic Rashba spin–orbit interaction (SOI-R). Most importantly, departing from the pristine MLG, we have found evidence of trembling antiphase oscillations in the probability density time-distribution for each sublattice state, which we have called the pseudospinorial Zitterbewegung effect (PZBE). The PZBE appears modulated with robust transient character and with a decay time in the femtosecond scale. Interestingly, several features of the PZBE become tunable, even its complete disappearance at the vicinity of the Dirac points or at a symmetric pseudospin configuration. For the proposed quasi-1D MLG device, we have captured evidence of the familiar Klein tunneling and the unusual anti-Klein tunneling, whose interplay for 2D MLG under tunable SOI-R was reported recently

Concentration and band offset dependence of the electronic basic transition of cubic InxGa1−xN/InyGa1−yN quantum wells

We calculate the transition energy from the first level of holes to the first level of electrons for cubic InxGa1−xN/InyGa1−yN quantum wells. We employ the empirical tight binding approach with an sp3s* orbital basis, nearest neighbour interactions and the spin–orbit coupling, together with the surface Green function matching method. For the alloy, we use the virtual crystal approximation. We take into account the strain in the well. We assume a value of 0.65 eV for the InN bandgap and 3.3 eV for the GaN gap. Using a value of 20% for the valence band offset, we study the transition energy behaviour varying the well width for the sets of concentrations x=0.3, y=0.02 and y=0.05; x=0.15, y=0.05; and x=0.16, y=0. For the concentrations x=0.16, y=0, we also study the influence of the band offset using values of 20%, 50% and 80% for the valence band offset. We compare our calculations with experimental data from hexagonal and cubic quantum wells, and with other theoretical calculations for cubic quantum wells. The comparison of the calculations with the experimental results from hexagonal quantum wells is good. The theoretical energy transitions are 0.35–0.5 eV higher than those obtained experimentally for cubic quantum wells

Enhancement of Fano-resonance response in bilayer graphene single and double barriers induced by bandgap opening

Fano resonances in bilayer graphene arise due to the coupling between extended and discrete electrons states, and represent an exotic phenomenon in graphene akin to Klein and anti-Klein tunneling, atomic collapse and negative refraction to mention a few. The hallmark of these resonances is identifiable in the conductance curves of bilayer graphene barrier structures. Furthermore, the Fano line-shape can be presented in the conductance by reducing the angular range in the computation of the transport properties. In this work, we explore the possible consequences that bandgap opening in the band structure of bilayer graphene can have over Fano resonances. We have used a four-band Hamiltonian to taking into account the mentioned band structure modifications. The hybrid matrix method and the Landauer–Büttiker formalism have been implemented to obtain the transmittance and the conductance, respectively. We find that the signatures of the Fano resonances on the conductance are enhanced by the opening of the bandgap. In fact, the Fano profile is manifested in the conductance without the need of reducing the angular range. This enhancement results from the improvement of the chirality matching between extended and discrete states induced by the bandgap opening. The main characteristics of the impact of the bandgap opening on the transmission and transport properties of single and double barriers are presented. So, the bandgap opening far from hamper the Fano resonance response promotes it and can be used as modulation parameter to prove the exotic phenomenon of Fano resonances in bilayer graphene barrier structures

Controlling the optical absorption properties of d-FETs by means of contact voltage and hydrostatic pressure effects

The effects of contact voltage and hydrostatic pressure on subband structure and optical transitions in GaAs delta-Field Effect Transistor (δ-FET) are theoretically studied. The electronic structure of δ-FET under hydrostatic pressure is determined by solving the Schrödinger equation using a theoretical model at low pressure. It is found that the subband energies and intersubband optical absorption on δ-FET are quite sensitive to the contact voltage and applied hydrostatic pressure. Wherein, a blue-shifting as hydrostatic pressure increases and a red-shifting as the contact potential increases, are shown. Our results could be important for infrared optical device applications and useful in the design of devices based on contact voltage and hydrostatic pressure-dependent optical processes

Refractive index changes in n-type delta-doped GaAs under hydrostatic pressure

The effect of hydrostatic pressure on the refractive index changes (RIC) is studied in δ-doped quantum well (DDQW) in GaAs. Based on the effective mass approximation we implement an algebraic formalism to calculate the electronic structure and RIC. Our results obtained with this model show that the position and the magnitude of the linear, nonlinear and total RIC are sensitive to hydrostatic pressure and bidimensional density. The incident optical intensity has a great effect on these optical quantities

Study of the optical properties of dielectric-graphene-dielectric multilayer quasi-periodic structures: Thue-Morse case

Potential applications in optoelectronics had generated a great interest on the study of graphene optical properties. Along with this, graphene has exceptional properties such as high mobility and optical transparency, flexibility, mechanical robustness, etc. Due to these properties, graphene could be used in different devices such as transparent conductors, organic light-emitting diodes, photodetectors, touch screens, saturable absorbers and ultrafast lasers. A transfer-matrix method is used in order to calculate graphene optical properties, such as transmission, and absorption in the infrared region. The quasi-periodic structure consists in intercalated graphene sheets between two consecutives dielectrics. The dielectric materials follow the Thue-Morse sequence (ThMo). The graphene sheets are described by the optical conductivity considering interband and intraband transitions. The structure of the spectra depends strongly on the number of sequence generation, width of the different dielectrics and dielectric permittivity. In our case, the infrared region corresponds to a chemical potential greater than kT. In the calculated spectra, the geometrical properties of the Thue-Morse sequence can be observed. We obtain absorption bands well defined.En este estudio se usó el método de Matriz de Transferencia para calcular las propiedades ópticas de Transformación y Absorción en la región infrarroja. La estructura estudiada consiste en un arreglo cuasi-periódico intercalando láminas de grafeno entre dos dieléctricos consecutivos. Los materiales dieléctricos siguen una secuencia Thue-Morse (ThMo). Las láminas de grafeno son descritas por medio de su conductividad óptica considerando las transiciones interbanda e intrabanda. La estructura de los espectros depende fuertemente del número de generación, espesor de los diferentes dieléctricos y de la permitividad de los dieléctricos. En nuestro caso, en la región infrarroja corresponde a una región donde el potencial químico del grafeno es mucho mayor que kT. En los espectros obtenidos, las propiedades geométricas de la secuencia Thue-Morse puede ser observada. Nosotros obtenemos bandas de absorción bien definidas

Transmittance and Absorption Properties of Graphene Multilayer Quasi-periodic Structure: Period-Doubling case

Graphene is a two dimensional material of special interest due to its unusual electronic, mechanical, chemical, optical among other properties, which suggest a wide range of applications in optoelectronics, computer, ecology, etc. The study of the optical properties of graphene is important due to its potential applications such as ultrafast photonics, optical filters, composite materials, photovoltaics and energy storage device. In this work we study the transmission and absorption properties of a quasi-regular multilayer dielectric-graphene- dielectric system. The multilayer structure is built on the quasi-regular Period-Doubling (PD) sequence. The optical response of graphene takes into account intra-band and inter-band transitions. We use the transfer-matrix method to calculate the transmission and absorption spectra. It is obtained a strong dependence on the number of layers in the system, the width of dielectric media and the optical contrast. Furthermore, we calculate the spectra for both transverse magnetic (TM) and transverse electric (TE) polarization in the infrared region.En este trabajo nosotros estudiamos las propiedades de transmisión y Absorción de un sistema multicapa cuasiregular dieléctrico-grafeno-dieléctrico. La estructura multicapa está construida en base a una secuencia cuasi-regular Period-Doubling (PD). La respuesta óptica de el grafeno toma en cuenta las transiciones intra-banda e inter-banda. Nosotros usamos el método de matriz de transferencia para calcular los espectros de transmisión y absorción. Esto mostró una fuerte dependencia con el número de capas de sistema, el espesor de los medios dieléctricos y el contraste óptico. Además nosotros calculamos los espectros para la polarización transversal Magnética (TM) y transversal eléctrica (TE) en la región de frecuencia del infrarojo