Godfrey Gumbs

Magnetoplasmons in layered graphene structures

We calculate the dispersion equations for magnetoplasmons in a single layer, a pair of parallel layers, a graphite bilayer, and a superlattice of graphene layers in a perpendicular magnetic field. We demonstrate the feasibility of a drift-induced instability of magnetoplasmons. The magnetoplasmon instability in a superlattice is enhanced compared to a single graphene layer. The energies of the unstable magnetoplasmons could be in the terahertz (THz) part of the electromagnetic spectrum. The enhanced instability makes superlattice graphene a potential source of THz radiation.

Dipole-dipole interaction between a quantum dot and a graphene nanodisk

We study theoretically the dipole-dipole interaction and energy transfer in a hybrid system consisting of a quantum dot and graphene nanodisk embedded in a nonlinear photonic crystal. In our model, a probe laser field is applied to measure the energy transfer between the quantum dot and graphene nanodisk, while a control field manipulates the energy transfer process. These fields create excitons in the quantum dot and surface plasmon polaritons in the graphene nanodisk which interact via the dipole-dipole interaction. Here, the nonlinear photonic crystal acts as a tunable photonic reservoir for the quantum dot, and is used to control the energy transfer. We have found that the spectrum of power absorption in the quantum dot has two peaks due to the creation of two dressed excitons in the presence of the dipole-dipole interaction. The energy transfer rate spectrum of the graphene nanodisk also has two peaks due to the absorption of these two dressed excitons. Additionally, energy transfer between the quantum dot and the graphene nanodisk can be switched on and off by applying a pump laser to the photonic crystal or by adjusting the strength of the dipole-dipole interaction. We show that the intensity and frequencies of the peaks in the energy transfer rate spectra can be modified by changing the number of graphene monolayers in the nanodisk or the separation between the quantum dot and graphene. Our results agree with existing experiments on a qualitative basis. The principle of our system can be employed to fabricate nanobiosensors, optical nanoswitches, and energy transfer devices.

Anomalous photon-assisted tunneling in graphene

We investigated the transmission of Dirac electrons through a potential barrier in the presence of circularly polarized light. An anomalous photon-assisted enhanced transmission is predicted and explained. It is demonstrated that the perfect transmission for nearly head-on collision in infinite graphene is suppressed in gapped dressed states of electrons, which is further accompanied by a shift of peaks as a function of the incident angle away from head-on collision. In addition, the perfect transmission is partially suppressed by a photon-induced gap in illuminated graphene. After the effect of rough edges of the potential barrier or impurity scattering is included, the perfect transmission with no potential barrier becomes completely suppressed and the energy range for the photon-assisted transmission is reduced at the same time.

Enhanced current quantization in high-frequency electron pumps in a perpendicular magnetic field

We present experimental results of high-frequency quantized charge pumping through a quantum dot formed by the electric field arising from applied voltages in a GaAs/AlGaAs system in the presence of a perpendicular magnetic field B. Clear changes are observed in the quantized current plateaus as a function of applied magnetic field. We report on the robustness in the length of the quantized plateaus and improvements in the quantization as a result of the applied B field.

Tunable band structure effects on ballistic transport in graphene nanoribbons

Abstract Graphene nanoribbons (GNR) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNRs. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry E k = E − k of the subband dispersion. When applied together, they reverse the dispersion parity to be odd and gives E e , k = − E h , − k and mix the electron and hole subbands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range.

Measurement of the GaSb surface band bending potential from the magnetotransport characteristics of GaSb–InAs–AlSb quantum wells

Low-temperature magnetotransport measurements on GaSb∕InAs∕AlSb coupled quantum well structures with a GaSb cap layer and self-consistent calculations of their electronic structure have led to the determination of the Fermi level at the surface, EFS, of undoped molecular-beam-epitaxy-grown GaSb. EFS is pinned around 0.2eV above the top of the GaSb valence band when the GaSb cap layer width is greater than around 900A. For smaller GaSb cap widths, EFS decreases with the GaSb width. The undoped GaSb∕InAs∕AlSb heterostructure’s Fermi level is determined by bulk donor defects in the AlSb layer adjacent to the InAs quantum well.

Self-consistent electronic subband structure of undoped InAs/GaSb-based type II and broken-gap quantum well systems

Motivated by a very recent experimental work on investigating electronic properties of InAs/GaSb-based type II and broken-gap quantum well structures, in this article we present a simple and transparent theoretical approach to calculate electronic subband structure in such device systems. The theoretical model is developed on the basis of solving self-consistently the Schrodinger equation for the eigenfunctions and eigenvalues coupled with the Poisson equation for the confinement potentials, in which the effects such as charge distribution and depletion are considered. In particular, we examine the effect of a GaSb cap layer on electronic properties of the quantum well systems in conjunction with experiments and experimental findings. The results obtained from the proposed self-consistent calculation can be used to understand important experimental findings and are in line with those measured experimentally.

Band hybridization and spin-splitting in InAs/AlSb/GaSb type II and broken-gap quantum wells

We present a detailed theoretical study on the features of band hybridization and zero-field spin-splitting in InAs/AlSb/GaSb quantum wells (QWs). An eight-band k⋅p approach is developed to calculate the electronic subband structure in such structures. In the absence of the AlSb layer, the hybridized energy gaps can be observed at the anticrossing points between the lowest electron subband and the highest heavy-hole subband in the InAs and GaSb layers respectively. In such a case, the position and magnitude of the gaps are spin-dependent. When a thin AlSb layer is inserted between the InAs and GaSb layers, we find that the lowest electron subband in the InAs layer is only hybridized with the highest light-hole subband which is also hybridized with the highest heavy-hole subband in the GaSb layer. The hybridized energy gaps and spin-splitting in the InAs/AlSb/GaSb QWs are reduced significantly. These results can be used to understand why electrons and holes can be well separated and why relatively high mob...

Plasmon Excitations of Multi-layer Graphene on a Conducting Substrate

We predict the existence of low-frequency nonlocal plasmons at the vacuum-surface interface of a superlattice of N graphene layers interacting with conducting substrate. We derive a dispersion function that incorporates the polarization function of both the graphene monolayers and the semi-infinite electron liquid at whose surface the electrons scatter specularly. We find a surface plasmon-polariton that is not damped by particle-hole excitations or the bulk modes and which separates below the continuum mini-band of bulk plasmon modes. The surface plasmon frequency of the hybrid structure always lies below , the surface plasmon frequency of the conducting substrate. The intensity of this mode depends on the distance of the graphene layers from the conductor’s surface, the energy band gap between valence and conduction bands of graphene monolayer and, most importantly, on the number of two-dimensional layers. For a sufficiently large number of layers the hybrid structure has no surface plasmon. The existence of plasmons with different dispersion relations indicates that quasiparticles with different group velocity may coexist for various ranges of wavelengths determined by the number of layers in the superlattice.

Plasma excitations of dressed Dirac electrons in graphene layers

Collective plasma excitations of optically dressed Dirac electrons in single and double graphene layers are calculated in the RPA. The presence of circularly polarized light gives rise to an energy gap Eg between the conduction and valence energy bands. Its value may be adjusted by varying the frequency and intensity of the light, and may reach values of the gap reported for epitaxially grown graphene and far exceeding that caused by spin-orbit coupling. We report plasmon dispersion relations for various energy gaps and separations between graphene layers. For a single graphene sheet, we find that plasmon modes may be excited for larger wave vector and frequency when subjected to light. For double layers, we obtained an optical and phononlike mode and found that the optical mode is not as sensitive as the phononlike mode in the long wavelength limit when the layer separation is varied, for a chosen Eg. The dressed electron plasma—although massive—still has Dirac origin, giving rise to anomalous plasmon be...