Non-linear Doppler shift of the plasmon resonance in a grating-coupled drifting 2DEG

We report experimental measurements and computer calculations of the plasmon resonances of two dimensional electron gases in the far-infrared which show the effects of laterally drifting the 2DEG. Coupling to radiation is achieved using an overlaid metal grating of submicron period, and its periodic screening effect splits the plasmon into upper and lower energy modes. For a symmetric grating profile the higher energy mode is non-radiative for a stationary 2DEG and a splitting is not observable, but when the 2DEG is laterally drifted under the grating, coupling to both modes can occur, and their Doppler shifts produce an observable splitting which increases with drift velocity. These Doppler shifts are not linear with drift velocity for low velocities, but approach asymptotically the expected linear shift with increasing drift velocity. Experimental results on 2DEGs at GaAs/AlGaAs heterojunctions compare well with theoretical calculations.     

Thermodynamic properties of a magnetically modulated graphene monolayer

The effect of magnetic modulation on thermodynamic properties of a graphene monolayer in the presence of a constant perpendicular magnetic field is reported here. One-dimensional spatial electric or magnetic modulation lifts the degeneracy of the Landau levels and converts into bands and their bandwidth oscillates with magnetic field, leading to Weiss-type oscillations in the thermodynamic properties. The effect of magnetic modulation on the thermodynamic properties of a graphene sheet is studied and then compared with electrically modulated graphene and magnetically modulated conventional two-dimensional electron gas (2DEG). We observe Weiss-type and de Haas-van Alphen oscillations at low and high magnetic fields, respectively. There is a definite phase difference in Weiss-type oscillations in thermodynamic quantities of magnetically modulated graphene compared to electrically modulated graphene. On the other hand, the phase remains the same and the amplitude of the oscillation is large when compared with the magnetically modulated two-dimensional electron gas (2DEG). Explicit asymptotic expressions of the density of states and the Helmholtz free energy are provided to understand the phase and amplitude of the Weiss-type oscillations qualitatively. We also study thermodynamic properties when both electric and magnetic modulations are present. The Weiss-type oscillations still exist when the modulations are out-of-phase.     

Conductance oscillations and transport spectroscopy of a quantum dot

Results are reported for low temperature measurements of the conductance through small regions of a two-dimensional electron gas (2 DEG). An unconventional GaAs heterostructure is used to form a 2 DEG whose density can be tuned by the gate voltage applied to its conductive substrate. Electron beam lithography is used to pattern a narrow channel in the 2 DEG interrupted by two constrictions, defining a small 2 DEG island between them. The conductance is found to oscillate periodically with the gate voltage, namely with electron density. Calculations of the capacitance between the substrate and the island show that the period of oscillation corresponds to adding one electron to the island. The oscillatory behavior results primarily from the discreteness of charge and the Coulomb interaction between electrons. However, the observed temperature dependence of these oscillations requires a more sophisticated treatment which includes the quantized electron energy levels as well. The magnetic field dependence of the oscillations allows us to extract the discrete energy spectrum of the quantum dot in the quantum-Hall regime.     

Zero-field spin splitting in a two-dimensional electron gas with the spin-orbit interaction revisited

We consider a two-dimensional electron gas (2DEG) with the Rashba spin-orbit interaction (SOI) in the presence of a perpendicular magnetic field. We derive analytical expressions of the density of states (DOS) of a 2DEG with the Rashba SOI in the presence of a magnetic field by using the Green's function technique. The DOS allows us to obtain the analytical expressions of the magnetoconductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and give rise to the beating patterns in the amplitude of the Shubnikov-de Haas (SdH) oscillations. We find a simple equation which determines the zero-field spin splitting energy if the magnetic field corresponding to any beat node is known from the experiment. Our analytical results reproduce well the experimentally observed non-periodic beating patterns, number of oscillations between two successive nodes and the measured zero-field spin splitting energy.     

二维电子气等离激元太赫兹波器件

固态等离激元太赫兹波器件正成为微波毫米波电子器件技术和半导体激光器技术向太赫兹波段发展和融合的重要方向之一。本综述介绍AlGaN/GaN异质结高浓度和高迁移率二维电子气中的等离激元调控、激发及其在太赫兹波探测器、调制器和光源中应用的近期研究进展。通过光栅和太赫兹天线实现自由空间太赫兹波与二维电子气等离激元的耦合,通过太赫兹法布里-珀罗谐振腔进一步调制太赫兹波模式,增强太赫兹波与等离激元的耦合强度。在光栅-谐振腔耦合的二维电子气中验证了场效应栅控的等离激元色散关系,实现了等离激元模式与太赫兹波腔模强耦合产生的等离极化激元模式,演示了太赫兹波的调制和发射。在太赫兹天线耦合二维电子气中实现了等离激元共振与非共振的太赫兹波探测,建立了太赫兹场效应混频探测的物理模型,指导了室温高灵敏度自混频探测器的设计与优化。研究表明,基于非共振等离激元激发可发展形成室温高速高灵敏度的太赫兹探测器及其焦平面阵列技术。然而,固态等离激元的高损耗特性仍是制约基于等离激元共振的高效太赫兹光源和调制器的主要瓶颈。未来的研究重点将围绕高品质因子等离激元谐振腔的构筑,包括固态等离激元物理、等离激元谐振腔边界的调控、新型室温高迁移率二维电子材料的运用和高品质太赫兹谐振腔与等离激元器件的集成等。     

Electric field driven plasmon dispersion in AlGaN/GaN high electron mobility transistors

We present a theoretical study on the electric field driven plasmon dispersion of the two-dimensional electron gas(2DEG)in AlGaN/GaN high electron mobility transistors(HEMTs).By introducing a drifted Fermi–Dirac distribution,we calculate the transport properties of the 2DEG in the AlGaN/GaN interface by employing the balance-equation approach based on the Boltzmann equation.Then,the nonequilibrium Fermi–Dirac function is obtained by applying the calculated electron drift velocity and electron temperature.Under random phase approximation(RPA),the electric field driven plasmon dispersion is investigated.The calculated results indicate that the plasmon frequency is dominated by both the electric field and the angle between wavevector and electric field.Importantly,the plasmon frequency could be tuned by the applied source–drain bias voltage besides the gate voltage(change of the electron density).     

Magnetic phase diagram in a quasi-two-dimensional electron gas

Using spin density functional theory within the framework of the local spin density approximation with Perdew-Zunger type exchange-correlation energy, ferromagnetism in a quasi-two-dimensional electron gas (Q-2DEG) is studied. The electronic and magnetic structures of a thin film are calculated as a function of film thickness and electron density. Ferromagnetism in the Q-2DEG is found to appear at a higher electron density than in the three-dimensional electron gas. Unless a film is very thin, with decreasing electron density, a magnetic phase transition occurs from a spin-unpolarized fluid to a Wigner film with surface magnetism, in which the spin polarization localizes only in the neighborhood of surfaces. Further decreasing density induces another transition to a fully spin-polarized ferromagnetic Wigner film.     

Transient radiation induced by a modulated flow of charged particles on a perfectly conducting surface with random roughness

The excitation of surface waves by a modulated electron flow in a medium whose surface has random inhomogeneities is studied. It is shown that the energy flux density of a surface wave (polariton) exhibits oscillations determined by the ratio of the period of oscillations of the electron beam and the time of flight of a charged particle in the space between the beam modulation plane and the interface between the media.     

Transport properties of a 2DEG in the presence of tilted magnetic field: the influence of a 1D modulation

The effect of 1D periodic modulation on the transport and thermodynamic properties of a non-interacting two-dimensional electron gas (2DEG) is investigated. The Hamiltonian used also includes a tilted magnetic field, Rashba, and Dresselhauss spin-orbit interactions. The 1D periodic modulation introduces non-quantized regions on Hall conductivity and a non-zero diffusive conductivity. A method to estimate the modulation periodicity is given using the periodicity of Weiss oscillations on the diffusive conductivity.     

Fermi edge polaritons in a microcavity containing a high density two-dimensional electron gas

Sharp, near band gap lines are observed in the reflection and photoluminescence spectra of GaAs/AlGaAs structures consisting of a modulation doped quantum well (MDQW) that contains a high density two-dimensional electron gas (2DEG) and is embedded in a microcavity (MC). The energy dependence of these lines on the MC-confined photon energy shows level anticrossings and Rabi splittings very similar to those observed in systems of undoped QW's embedded in a MC. The spectra are analyzed by calculating the optical susceptibility of the MDQW in the near band gap spectral range and using it within the transfer matrix method. The calculated reflection spectra indicate that the sharp spectral lines are due to k{ parallel}=0 cavity polaritons that are composed of e-h pair excitations just above the 2DEG Fermi edge and are strongly coupled to the MC-confined photons.     

Plasmon-interband coupling in gallium compounds

Electro energy loss spectra have been measured for metallic Ga and the semiconductor compounds GaSb, GaAs, GaP and GaN. The intensity at the measured plasmon energy decreases with increasing ionicity of the compound semiconductor. This is explained by a simple model involving the coupling of plasmon oscillations and interband transitions.     

The effect of adsorption of various gas molecules on the EEEL Sof the (100) face of tungsten

The effect of adsorption on the EELS of the tungsten (100) surface plane, for loss energies in the range of 2–25 eV, is investigated for a series of adsorbates (i.e. O₂, N₂, NO, CO and H₂). Low-lying sharp structures observed in the energy loss spectra seem to be associated with prominent structures in the local surface density of states. A growth of a peak at a loss energy of about 7.5 eV, observed after exposing the surface to any one of the gases studied, is attributed to the breakdown of optic-like selection rules by the adsorption process. The effect of adsorption on the various surface plasmon peaks seems to be adsorbate-specific.     

Plasmons in the modulated and confined 2DEG: A Raman scattering study

Lateral wire arrays have been fabricated from a single modulation-doped GaAs quantum well employing reactive ion etching. Depending on the etch depth, the two-dimensional electron gas (2DEG) in the well acquires different degrees of modulation up to complete confinement. The different regimes are identified by their unique photoluminescence and Raman spectra. Deep-etched wires show plasmon resonances down to a width of 100nm.     

Raman scattering from a two-dimensional electron gas in a weakly and periodically modulated magnetic field

An analysis of light scattering from a two-dimensional electron gas (2DEG) subject to aweak periodicmagnetic modulation of strengthB_mand to a perpendicular uniform magnetic fieldBof arbitrary strength is presented. Raman spectra are calculated for (i) electron inter-Landau-level transitions, which result in a Raman shift ∼  2ω_c, and (ii) inter-Landau-level collective electron excitations at a hybridized magnetoplasmon frequency and at anewlow-frequency, intra-Landau-level plasmon mode induced by the modulation. The dependence of the Raman cross sections on the strength ofBandB_mis assessed. The effect of a weakB_mon the Raman spectrum is similar to that of a weak electric modulation but the signals from the magnetically modulated 2DEG are much stronger. The combined effect of the electric and magnetic modulations is also considered.     

Plasmon loss structure in synchrotron radiation photoemission from Mg films

Synchrotron radiation excitation has been used to vary the kinetic energy of electrons photoemitted from the 2p core level of polycrystalline magnesium films and the energy-dependence of the accompanying plasmon loss structure has been observed. Difficulties associated with background subtraction are discussed, and it is shown that the method of background subtraction profoundly affects the conclusions drawn from such an experiment on the relative importance of plasmon and single particle excitation mechanisms for electron scattering. Very thin films of Mg have also been studied in an attempt to separate the intrinsic and extrinsic modes of plasmon creation. Although the plasmon loss structure is dependent on film thickness implying a considerable contribution from extrinsic processes, we are not able to observe plasmons excited in the thin film by electrons originating in the substrate core level. The implications of these results are discussed and further experiments suggested.     

Influence of a two-dimensional electron gas on current-voltage characteristics of Al_0.3Ga_0.7 N/GaN high electron mobility transistors

The J-V characteristics of AltGa1 tN/GaN high electron mobility transistors(HEMTs) are investigated and simulated using the self-consistent solution of the Schro¨dinger and Poisson equations for a two-dimensional electron gas(2DEG) in a triangular potential well with the Al mole fraction t = 0.3 as an example.Using a simple analytical model,the electronic drift velocity in a 2DEG channel is obtained.It is found that the current density through the 2DEG channel is on the order of 1013 A/m2 within a very narrow region(about 5 nm).For a current density of 7 × 1013 A/m2 passing through the 2DEG channel with a 2DEG density of above 1.2 × 1017 m-2 under a drain voltage Vds = 1.5 V at room temperature,the barrier thickness Lb should be more than 10 nm and the gate bias must be higher than 2 V.     

AlGaN/GaN HEMT器件电流坍塌和膝点电压漂移机理的研究

考虑了势垒层、缓冲层体陷阱及表面电荷的浓度变化对电流坍塌和膝点电压的影响,发现表面电荷和势垒层体陷阱浓度的变化对沟道电子的浓度影响较小,表面电荷浓度变化下的膝点电压的偏移和坍塌强度的大小与势垒层势阱能量的变化有着主要的关系.缓冲层有着比势垒层更强的局域作用,势垒层和缓冲层的体陷阱浓度在一定范围变化时的膝点电压偏移主要是由沟道电子浓度的变化而引起的,但偏移量却比表面电荷浓度变化的情况下小很多.势阱能量的变化是造成膝点电压偏移的重要原因,坍塌强度主要取决于势阱能量和沟道的电子浓度. 关键词： AlGaN/GaN高电子迁移率晶体管 电流坍塌 膝点电压 陷阱俘获     

Autoignition delay modulation by high-frequency thermoacoustic oscillations in reheat flames

This paper investigates the sensitivity of the autoignition delay in reheat flames to acoustic pulsations associated with high-frequency transverse thermoacoustic oscillations. A reduced order model for the response of purely autoignition-stabilised flames to acoustic disturbances is compared with experimental observations. The experiments identified periodic flame motion associated with high-amplitude transverse limit-cycle oscillations in an atmospheric pressure reheat combustor. This flame motion was assumed to be the result of a superposition of two flame-acoustic coupling mechanisms: autoignition delay modulation by the oscillating acoustic field and displacement and deformation of the flame by the acoustic velocity. The reduced order model coupled to reaction kinetics calculations reveals that a significant portion of the observed flame motion can be attributed to autoignition delay modulation. The ignition position responds instantaneously to the acoustic pressure at the time of ignition, as observed experimentally. The model also provides insight into the importance of the history of acoustic disturbances experienced by the fuel-air mixture prior to ignition. Due to the high-frequency nature of the instability, a fluid particle can experience multiple oscillation cycles before ignition. The ignition delay responds in-phase with the net-acoustic perturbation experienced by a fluid particle between injection and ignition. These findings shed light on the underlying mechanisms of the flame motion observed in experiments and provide useful insight into the importance of autoignition delay modulation as a driving mechanism of high-frequency thermoacoustic instabilities in reheat flames.     

Magnetotransport properties of a magnetically modulated two-dimensional electron gas with the spin-orbit interaction

We study the electrical transport properties of a two-dimensional electron gas (2DEG) with the Rashba spin-orbit interaction in the presence of a constant perpendicular magnetic field (B(0)( ?z) which is weakly modulated by B1 = B1 cos(qx) ?z, where B(1) ? B(0) and q = 2π/a with a the modulation period. We obtain the analytical expressions of the diffusive conductivities for spin-up and spin-down electrons. The conductivities for spin-up and spin-down electrons oscillate with different frequencies and produce beating patterns in the amplitude of the Weiss and Shubnikov-de Haas oscillations. We show that the Rashba strength can be determined by analyzing the beating pattern in the Weiss oscillation. We find a simple equation which determines the Rashba spin-orbit interaction strength if the number of Weiss oscillations between any two successive nodes is known from the experiment. We compare our results with the electrically modulated 2DEG with the Rashba interaction. For completeness, we also study the beating pattern formation in the collisional and the Hall conductivities.