Conventional Two-dimensional Electron Gas Research Articles

Temperature effect on plasmon dispersions in double-layer graphene systems

We investigate the plasmon dispersion relation and damping rate of a double-layer graphene system consisting of two separated monolayer graphenes with no interlayer tunneling at finite temperature. We use the temperature dependent RPA dielectric function which is valid for graphene systems to obtain the plasmon frequencies and damping rates at different temperatures, interlayer correlation parameters and electron densities and then compare them with those obtained from the zero temperature calculations. Our results show that by increasing the temperature, the plasmon frequencies decrease and the decay rate increases. Furthermore, we find that the behavior of a double-layer graphene system at small and large correlation parameters is different from the conventional double-layer two-dimensional electron gas system. Finally, we obtain that in a density imbalanced double-layer graphene system, the acoustic plasmons are more affected by temperature than the equal electron densities one.

Magnetic quantum oscillations for the surface states of topological insulatorBi2Se3

We study quantum oscillations of the magnetization in Bi$_{2}$Se$_{3}$(111) surface system in the presence of a perpendicular magnetic field. The combined spin-chiral Dirac cone and Landau quantization produce profound effects on the magnetization properties that are fundamentally different from those in the conventional semiconductor two-dimensional electron gas. In particular, we show that the oscillating center in the magnetization chooses to pick up positive or negative values depending on whether the zero-mode Landau level is occupied or empty. An intuitive analysis of these new features is given and the subsequent effects on the magnetic susceptibility and Hall conductance are also discussed.

Thermodynamic properties of a weakly modulated graphene monolayer in a magneticfield

Theoretical investigation of thermodynamic properties of an electricallymodulated graphene monolayer in the presence of a perpendicular magnetic fieldB is presented. This work is aimed at determining the modulation-induced effectson the thermodynamic properties of graphene. The results obtained arecompared with those of conventional two-dimensional electron gas (2DEG)systems realized in semiconductor heterostructures. The one-dimensional periodicpotential, due to electric modulation lifts the degeneracy of the Landaulevels and converts them into bands whose width oscillates as a function ofB. We find commensurability (Weiss) oscillations for small values ofB and de Haas–van Alphen (dHvA)-type oscillations at larger values ofB. We find that the modulation-induced effects on the thermodynamic properties areenhanced and less damped with temperature in graphene compared with conventional2DEG systems. Furthermore, we have derived analytic asymptotic expressions whichallow us to determine the critical temperature and critical magnetic field for thedamping of magnetic oscillations in the thermodynamic quantities considered here.

Local density of states of electron-crystal phases in graphene in the quantum Hall regime

We calculate, within a self-consistent Hartree-Fock approximation, the local density of states for different electron crystals in graphene subject to a strong magnetic field. We investigate both the Wigner crystal and bubble crystals with ${M}_{\text{e}}$ electrons per lattice site. The total density of states consists of several pronounced peaks, the number of which in the negative energy range coincides with the number of electrons ${M}_{\text{e}}$ per lattice site, as for the case of electron-solid phases in the conventional two-dimensional electron gas. Analyzing the local density of states at the peak energies, we find particular scaling properties of the density patterns if one fixes the ratio ${\ensuremath{\nu}}_{N}/{M}_{\text{e}}$ between the filling factor ${\ensuremath{\nu}}_{N}$ of the last partially filled Landau level and the number of electrons per bubble. Although the total density profile depends explicitly on ${M}_{\text{e}}$, the local density of states of the lowest peaks turns out to be identical regardless the number of electrons ${M}_{\text{e}}$. Whereas these electron-solid phases are reminiscent of those expected in the conventional two-dimensional electron gas in GaAs heterostructures in the quantum Hall regime, the local density of states and the scaling relations we highlight in this paper may be, in graphene, directly measured by spectroscopic means, such as, e.g., scanning tunneling microscopy.

Orbital Reconstruction and the Two-Dimensional Electron Gas at theLaAlO3/SrTiO3Interface

In 2004, Ohtomo and Hwang discovered that an electron gas is created at the interface between insulating LaAlO3 and SrTiO3 compounds. Here we show that the generation of a conducting electron gas is related to an orbital reconstruction occurring at the LaAlO3/SrTiO3 interface. Our results are based on extensive investigations of the electronic properties and of the orbital structure of the interface using x-ray absorption spectroscopy. In particular, we find that the degeneracy of the Ti 3d states is fully removed and that the Ti 3d xy levels become the first available states for conducting electrons.

Inter-band magnetoplasmons in mono- and bilayer graphene

Collective excitation spectra of Dirac electrons in mono- and bilayer graphene inthe presence of a uniform magnetic field are investigated. Analytical results forinter-Landau-band plasmon spectra within the self-consistent-field approach are obtained.Shubnikov–de Haas (SdH) type oscillations that are a monotonic function of themagnetic field are observed in the plasmon spectrum of both mono- and bilayergraphene systems. The results presented are also compared with those obtained ina conventional two-dimensional electron gas (2DEG). The chiral nature of thequasiparticles in mono- and bilayer graphene systems results in the observation ofπ and 2π Berry’s phase in the SdH-type oscillations in the plasmon spectrum.

Quantum transport of Dirac electrons in graphene in the presence of a spatially modulated magnetic field

We have investigated the electrical transport properties of Dirac electrons in a monolayer graphene sheet in the presence of a perpendicular magnetic field that is weakly and periodically modulated along one direction. We find that the Landau levels broaden into bands and their width oscillates as a function of the band index and the magnetic field. We determine the ${\ensuremath{\sigma}}_{yy}$ component of the magnetoconductivity tensor for this system, which is shown to exhibit Weiss oscillations. We also analytically determine the asymptotic expressions for ${\ensuremath{\sigma}}_{yy}$. We compare these results to recently obtained results for electrically modulated graphene, as well as those for a magnetically modulated conventional two-dimensional electron gas (2DEG) system. We find that in the magnetically modulated graphene system considered in this work, Weiss oscillations in ${\ensuremath{\sigma}}_{yy}$ have a reduced amplitude compared to that in the 2DEG but are less damped by temperature, while they have a higher amplitude than in the electrically modulated graphene system. We also find that these oscillations are out of phase by $\ensuremath{\pi}$ with those of the electrically modulated system while they are in phase with those in the 2DEG system.

Theory of Weiss oscillations in the magnetoplasmon spectrum of Dirac electrons in graphene

We present the collective excitations spectrum (magnetoplasmon spectrum) of Dirac electrons in a weakly modulated single graphene layer in the presence of a uniform magnetic field. We consider electric modulation in one dimension and the magnetic field applied perpendicular to graphene. We derive analytical results for the intra-Landau-band plasmon spectrum within the self-consistent-field approach. We find Weiss oscillations in the magnetoplasmon spectrum which is the primary focus of this work. Results are presented for the intra-Landau-band magnetoplasmon spectrum as a function of inverse magnetic field. These results are also compared with those of conventional two-dimensional electron gas (2DEG). We have found that the Weiss oscillations in the magnetoplasmon spectrum are larger in amplitude compared to those in conventional 2DEG for the same modulation strength, period of modulation, and electron density.

Quantum anti-dot arrays and quantum wire transistors fabricated on heterostructures

We report on a new fabrication process of anti-dot and multiple quantum wire transistors based on InAs/AlGaSb heterostructures with conventional photolithography and wet chemical etching. Starting from the square arrays of a mask, anti-dot arrays of circular shape were fabricated utilizing the unique etching properties of this material system, which are quite different from those of its counterpart GaAs/AlGaAs. Successful formation of the anti-dot structures was confirmed by magnetotransport measurements. For the fabrication of multiple quantum wire transistors, we introduced a GaAs regrowth as a hole barrier between the gate electrode and the InAs channel layer to eliminate the inherent gate leakage current for this material system and to obtain good device performance. By optimizing the fabrication process, we succeeded in forming quantum wires with a width as narrow as 100 nm. The device performance of the quantum wire transistor is compared with that of conventional two-dimensional electron gas transistors.

Simulation of lateral confinement in very narrow channels.

A theoretical study is presented of the lateral confinement potential (CP) in the very narrow mesa channels fabricated in the conventional two-dimensional (2D) electron gas in GaAs-AlxGa1-xAs heterostructures. The ID electronic structures are calculated in the framework of the confinement potential: V(x) = m* omega0(2)x2/2 for Absolute value of x <d0/2 and V(x) = infinity at Absolute value of = d0/2 where do and omega0, the parameters characterizing such a composite potential well, are the effective width and the characteristic frequency, respectively. Calculations suggest that, for wires having structural widths W(str) = 1500, 550, and 500 nm, the CP models consisting of flat bottoms and soft walls are more realistic than this complex potential. The experimental result for a wire of W(str) = 250 nm cannot be explained within the composite-well model including these two cases. Thus how to explain the experiment for this wire remains an open question.

One-dimensional transport in quantum well wire-high electron mobility transistor

A novel one-dimensional electron gas field-effect transistor (FET) is proposed with the advantages of higher electron mobility and higher carrier concentration than conventional two-dimensional electron gas FET. The FET structure, device operation, and the low-field mobility of impurity scattering, which takes the screening effect into account, are discussed.

A high-current drivability i-AlGaAs/n-GaAs doped-channel MIS-Like FET (DMT)

A high-current drivability doped-channel MIS-like FET (DMT) has been proposed. The DMT takes advantage of high saturation current with large transconductance and high breakdown voltage, in regard to its operating principle. The fabricated 0.5-µm gate DMT showed 310-mS/mm (410-mS/mm) transconductance and 650-mA/mm (800-mA/mm) maximum saturation current at room temperature (at 77 K). Output current values are about three or four times those for conventional two-dimensional electron gas (2DEG) FET's. Estimated average electron velocity is rather high, 1.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s (2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm/s) at room temperature (77 K). In addition, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{\max}</tex> is as high as 41 GHz. f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</inf> is 45 GHz, which is the best data ever reported in 0.5-µm gate FET's.