Two-dimensional Electron Research Articles

New state of a 2D electron crystal above liquid helium

A transition to a new, presumably unstable, state of a two-dimensional (2D) electron crystal above liquid helium has been discovered at temperatures well below the melting point. The transition is manifested as an abrupt increase in the active component of the inverse conductivity of the crystal with a decrease in the potential pressing of the electrons to the surface. The state can be destroyed by the in-plane electric field of a sufficiently high amplitude. The new state is supposed to be a 2D electron glass.

Theory of charge transport in two-dimensional electron gas/noncentrosymmetric superconductor junction

Based on the extended Blonder–Tinkham–Klapwijk (BTK) approach, we have investigated the charge transport in two-dimensional electron gas/noncentrosymmetric superconductor (2DEG/NCS) junction. The effects of the Rashba spin–orbit coupling (RSOC) and the interface barrier on the conductance are taken into account. It is found that there is a peak of the conductance with the Rashba strength β in 2DEG for the nonzero Rashba strength χ in NCS. The peak feature of the conductance can be tuned largely by the interface barrier. These phenomena are essentially different from those found in FM/NCS tunneling junctions where the zero bias conductance is suppressed by χ, independent of the barrier strength.

Two-dimensional electron or hole gas at ZnO/6H-SiC interface

Electronic structures of ZnO(0001)/6H-SiC(0001) interfaces are investigated using first-principles method. Two-dimensional charge carriers are found at the interfaces. Depending on the interface structure, the type of charge carriers can be n-type if oxygen terminated ZnO(0001) is grown on SiC and p-type when the interface is formed with Zn-terminated ZnO and C-terminated SiC. The interface formed with Zn-terminated ZnO and Si-terminated SiC is found to be half-metallic. Intrinsic charge carriers at the interface of the two wide gap semiconductors could be useful for future oxide-based electronics and spintronics.

QUASIPARTICLE LIFETIME OF A TWO-DIMENSIONAL ELECTRON GAS

We calculate the inelastic scattering lifetime of an excited quasiparticle at low (or zero) temperature, due to electron-electron interaction for a clean two-dimensional (2D) electron gas within the random-phase approximation (RPA) and compare it with the lifetime measured from the tunnelling experiment. Our result obtained by direct numerical calculation increases the electron relaxation rate considerably, hence decreases the size of discrepancy (roughly by a factor of 4) between theory and experiment which exists in the literature. We also show that including local-field factors in the effective electron interaction yields small correction to the result calculated within the RPA for rs ~ 1, corresponding to electron density of the sample in the tunnelling experiment. This result suggests that the RPA is reasonably accurate for a 2D electron gas in weak coupling limit.

The study of the two-dimensional position sensitive gas electron multiplier based on strips readout

To meet the need of Beijing Synchrotron Radiation Facility (BSRF) in two-dimensional detectors, a two-dimensional position sensitive X-ray gas electron multiplier(GEM)detector based on PCB strips and anolog readout were designed and constructed. Through the test of two kinds of strip pitchs and based on the system of multi-channel charge sensitive preamplifier and VME buffer-ADC, the spatial resolution of soft X-ray was measured and pretty good spatial resolutions of about 84 μm and 75μm respectively were obtained. The effect of the sum of the voltages applied on the three GEM foils on the spatial resolution was studied and position veracity and linearity of GEM was tested. The principles of GEM based on anode readout and the anolog electronic system were clarified meanwhile.

Exchange energy for two closed shells generated by a bare Coulomb potential energy −Ze 2/r in the limit of large Z, in two dimensions

Two-dimensional (2D) inhomogeneous electron assemblies are becoming increasingly important in Condensed Matter and associated technologies. Here, therefore, we contribute to the Density Functional Theory of such 2D electronic systems by calculating, analytically, (i) the idempotent Dirac density matrix γ(r, r′) generated by two closed shells for the bare Coulomb potential −Ze2/r and (ii) the exchange energy density \({\varepsilon_x({\bf r})}\) . Some progress is also possible concerning the exchange potential Vx(r), one non-local approximation being the Slater potential \({2\varepsilon_x(r)/n(r)}\) , with n(r) the ground state electron density. However, to complete the theory of Vx(r), the functional derivative of the single-particle kinetic energy per unit area δt(s)/δn(r) is still required.

Disorder effect of resonant spin Hall effect in a tilted magnetic field

We study the disorder effect of resonant spin Hall effect in a two-dimension electron system with Rashba coupling in the presence of a tilted magnetic field. The competition between the Rashba coupling and the Zeeman coupling leads to the energy crossing of the Landau levels, which gives rise to the resonant spin Hall effect. Utilizing the Streda's formula within the self-consistent Born approximation, we find that the impurity scattering broadens the energy levels, and the resonant spin Hall conductance exhibits a double peak around the resonant point, which is recovered in an applied titled magnetic field.

The role of electron-electron and electronphonon interactions in the processes of destruction of landau quantization in InAs/AlSb nanostructures

A nonmonotonous temperature dependence of the electron quantum relaxation time is found. The results are interpreted, taking into account the competition between the intrasubband and intersubband electron-electron interaction channels. The experimental nonlinear τ q (T) and T D (T) dependences are explained by the appearance of an additional channel of two-dimensional (2D) electron scattering by deformation acoustic phonons under interaction with piezoacoustic phonons.

GA-BASED OPTIMIZATION OF TAPERING WINDOWS FOR ARTIFACT REDUCTION IN FOURIER ELECTRON MAGNETIC RESONANCE IMAGES

Optimization of tapering windows for artifact reduction in two-dimensional (2D) Fourier electron magnetic resonance (EMR) tomography using genetic algorithm (GA) is presented. EMR imaging (EMRI) is a fast emerging functional imaging modality for studying free radicals in biological systems. EMRI by single point imaging (SPI) modality is a Fourier imaging technique. The bioclearance of the imaging agent as well as the need to minimize the radio frequency power deposition on the live animals, dictate reduced k-space sampling. This leads to ringing (Gibbs) artifacts in both directions of the 2D image, because, unlike the conventional MRI, SPI is phase encoding in both directions. To dampen the high-frequency components, data tapering windows are multiplicatively applied to provide tolerable blurred resultant image with reduced Gibbs ringing. To find a compromise between blur and ringing artifact, in this paper a method of optimizing the window functions by using GA is proposed. Our experiments suggest GA-based Kaiser window shows better performance by visual as well as quantitative evaluation.

PHOTO-INDUCED QUANTUM HALL CURRENTS IN 2D ELECTRON SYSTEMS

A novel photocurrent effect induced by Terahertz (THz) radiation in two-dimensional (2D) electron systems is reported. It was found that when the system is tuned to electron cyclotron resonance conditions, THz radiation induces a visible signal related to currents flowing in the plane of 2D electron systems without any external bias. This effect is employed as a technique for the study of the integer Quantum Hall samples in the absence of external bias. Our data indicate that at the integer ν the photocurrents are circulating currents in the dissipationless quantum Hall regime.

Magnetoresistance of two-dimensional tight-binding electrons in a weak magnetic field

We study the Anderson model on a two-dimensional square lattice with an applied weak magnetic field B which causes the hopping matrix elements to have Peierls phase factors. The recursion method is applied and B dependent conductivity σ(B) is calculated from the Kubo formula for different system sizes N and degree of disorder W. For large W there is no appreciable change of σ(B) with B, but its system size dependence is first an increasing and then a decreasing behavior.

Simulations of two-dimensional magnetic electron drift vortex mode turbulence in plasmas

AbstractSimulations are performed to investigate the turbulent properties of nonlinearly interacting two-dimensional magnetic electron drift vortex (MEDV) modes in a non-uniform unmagnetized plasma. The relevant nonlinear equations governing the dynamics of the MEDV modes are the wave magnetic field and electron temperature perturbations in the presence of the equilibrium density and temperature gradients. The important nonlinearities come from the advection of the electron fluid velocity perturbation and the electron temperature, as well as from the nonlinear electron Lorentz force. Computer simulations of the governing equations for the nonlinear MEDV modes reveal the generation of streamer-like electron flows, such that the corresponding gradients in the direction of the inhomogeneities tend to flatten out. In contrast, the gradients in an orthogonal direction vary rapidly. Consequently, the inertial range energy spectrum in decaying MEDV mode turbulence exhibits a much steeper anisotropic spectral index. The magnetic structures in the MEDV mode turbulence produce non-thermal electron transport in our non-uniform plasma.

Formation of a silicon micropore array of a two-dimension electron multiplier by photo electrochemical etching

A semiconductor PEC etching method is applied to fabricate the n-type silicon deep micropore channel array. In this method, it is important to arrange the direction of the micropore array along the crystal orientation of the Si substrate. Otherwise, serious lateral erosion will happen. The etching process is also relative to the light intensity and HF concentration. 5% HF concentration and 10–15 cm distance between the light source and the silicon wafer are demonstrated to be the best in our experiments. The n-type silicon deep micropore channel array with aperture of 3 μm and aspect ratio of 40–60, whose inner walls are smooth, is finally obtained.

Coherent quantum transport in two-dimensional electron gas/superconductor double junctions with Rashba spin-orbit coupling

Based on the extended Blonder-Tinkham-Klapwijk (BTK) approach, we have investigated the coherent quantum transport in two-dimensional electron gas/superconductor (2DEG/SC) double tunneling junctions in the presence of the Rashba spin-orbit coupling (RSOC). It is found that all the reflection coefficients in BTK theory as well as conductance spectra oscillate with the external voltage and energy. The oscillation feature of conductance can be tuned largely by the RSOC for low insulating barriers, while for high insulating barriers it is almost independent of the RSOC. These phenomena are essentially different from those found in ferromagnet/superconductor double tunneling junctions.

Nonlinear electromagnetic response of graphene: frequency multiplication and theself-consistent-field effects

Graphene is a recently discovered carbon-based material with unique physical properties.This is a monolayer of graphite, and the two-dimensional electrons and holes in it aredescribed by the effective Dirac equation with a vanishing effective mass. As a consequence,the electromagnetic response of graphene is predicted to be strongly nonlinear. Wedevelop a quasi-classical kinetic theory of the nonlinear electromagnetic response ofgraphene, taking into account the self-consistent-field effects. The response ofthe system to both harmonic and pulse excitation is considered. The frequencymultiplication effect, resulting from the nonlinearity of the electromagnetic response,is studied under realistic experimental conditions. The frequency upconversionefficiency is analyzed as a function of the applied electric field and parameters of thesamples. Possible applications of graphene in terahertz electronics are discussed.

Intrinsic Spin-Hall Effect: Topological Transitions in Two-Dimensional Systems

Topological transitions are the discontinuous changes of the Berry phase caused by continuous variation of parameters. These transitions can be detected by investigating the spin-Hall effect in two-dimensional electron and hole systems with a spin-obrit interaction when the density of carriers or the structural asymmetry are varied. The discontinuous behavior and the universality features of the spin-Hall conductivity are determined by the topological properties of the spin-orbit field.

Near-field radiative heat transfer and noncontact friction

All material bodies are surrounded by a fluctuating electromagnetic field because of the thermal and quantum fluctuations of the current density inside them. Close to the surface of planar sources (when the distance $d⪡{\ensuremath{\lambda}}_{T}=c\ensuremath{\hbar}∕{k}_{B}T$), thermal radiation can be spatially and temporally coherent if the surface can support surface modes like surface plasmon polaritons, surface phonon polaritons, or adsorbate vibrational modes. The fluctuating field is responsible for important phenomena such as radiative heat transfer, the van der Waals interaction, and the van der Waals friction between bodies. A general formalism for the calculation of the power spectral density for the fluctuating electromagnetic field is presented and applied to the radiative heat transfer and the van der Waals friction using both the semiclassical theory of the fluctuating electromagnetic field and quantum field theory. The radiative heat transfer and the van der Waals friction are greatly enhanced at short separations $(d⪡{\ensuremath{\lambda}}_{T})$ between the bodies due to the evanescent electromagnetic waves. Particularly strong enhancement occurs if the surface of the body can support localized surface modes like surface plasmons, surface polaritons, or adsorbate vibrational modes. An electromagnetic field outside a moving body can also be created by static charges which are always present on the surface of the body due to inhomogeneities, or due to a bias voltage. This electromagnetic field produces electrostatic friction which can be greatly enhanced if on the surface of the body there is a two-dimensional electron or hole system, or an incommensurate adsorbed layer of ions exhibiting acoustic vibrations. Applications of radiative heat transfer and noncontact friction to scanning probe spectroscopy are discussed. The theory gives a tentative explanation for the experimental noncontact friction data.

Theory of electron-phonon interaction in a nonequilibrium open electronic system

We study the effects of time-independent nonequilibrium drive on an open two-dimensional (2D) electron gas system coupled to 2D longitudinal acoustic phonons using the Keldysh path integral method. The layer electron-phonon system is defined at the two-dimensional interface between a pair of three-dimensional Fermi liquid leads, which act both as a particle pump and an infinite bath. The nonequilibrium steady state is achieved in the layer by assuming the leads to be thermally equilibrated at two different chemical potentials. This subjects the layer to an out-of-plane voltage $V$ and drives a steady-state charge current perpendicular to the system. We compute the effects of small voltages $(V⪡{\ensuremath{\omega}}_{D})$ on the in-plane electron-phonon scattering rate and the electron effective mass at zero temperature. We also find that the obtained nonequilibrium modification to the acoustic phonon velocity and the Thomas-Fermi screening length reveal the possibility of tuning these quantities with the external voltage.

Feynman propagator, density matrices and Green functions for the inhomogeneous electron liquid generated by a bare Coulomb potential in two dimensions

For a two-dimensional homogeneous electron gas, the canonical density matrix is well-known. This object is related to the Feynman propagator K(r,r′;t), where t is the time, by the transform β→it. From the free electron form of C(r,r′;β), the Green function follows in terms of the Bessel function K 0. When a bare Coulomb potential -Ze2/r is now “switched on”, one known property is the local density of states at the nucleus. This enables the imaginary part Im, G of the Green function at the nucleus to be determined as an explicit function of energy E and nuclear charge Ze. Off-diagonal information on Im, G will yield the real part of the Green function by using the Kramers–Krönig relation. The analysis of the two-dimensional Green function G into partial waves characterized by angular momentum quantum number ℓ is then considered. The imaginary part of G for ℓ = 0 is determined in terms of a hypergeometric function. The real part is again in principle accessible by invoking the Kramers–Krönig relation. From the relation between G and the Laplace transform of C with respect to β, information is also obtained on the ℓ = 0 partial wave component of the Slater sum and hence the Feynman propagator on the diagonal, in the limiting case Z→0.

QUASIEXCITONS IN PHOTOLUMINESCENCE OF INCOMPRESSIBLE QUANTUM LIQUIDS

The problem of a trion (charged exciton) immersed in an incompressible quantum liquid formed by two-dimensional electrons in a high magnetic field is studied theoretically by combining the composite fermion model and exact numerical diagonalization. Laughlin correlation of the trion with the surrounding electrons is shown to lead to the formation of a family of novel many-body excitations called "quasiexcitons" (QXs). The residual charge of the most stable QX can be either zero or the fractional quantum ±e/3, depending on the presence of negative or positive Laughlin quasiparticles in the electron liquid (controlled by the Landau level filling factor ν), but also on the width of the quantum well, Zeeman spin splitting, etc. The prediction of the magnetic-field driven transition from negative to positive QXs is argued to explain the recently observed minute discontinuity in the photoluminescence spectra of sufficiently wide (w ≥ 20 nm ) asymmetric GaAs quantum wells at ν = 1/3. Conversely, the observed discontinuity is hence indirectly linked to the emergence of a fractional charge quantum in the electron liquid.