Indirect evidence for the existence of viscoelastic shear modes in a two-dimensional classical electron liquid

It has recently been predicted that viscoelastic shear modes should propagate in a two-dimensional (2D) classical electron liquid above a minimum temperature dependent frequency. A 2D electron layer held above a liquid helium substrate is considered in this paper. The contribution of scattering between viscoelastic shear modes of the electron fluid and ripplons on the liquid helium surface to the mobility of electrons is calculated. This contribution explains the differences observed between the existing experimental data and single electron-ripplon scattering calculations quite well. This may be taken as indirect evidence for the existence of viscoelastic shear modes in a 2D electron liquid.     

Quantum Hall effects in graphene-based two-dimensional electron systems

In this article we review the quantum Hall physics of graphene-based two-dimensional electron systems, with a special focus on recent experimental and theoretical developments. We explain why graphene and bilayer graphene can be viewed respectively as J D 1 and 2 chiral two-dimensional electron gases (C2DEGs), and why this property frames their quantum Hall physics. The current status of experimental and theoretical work on the role of electron-electron interactions is reviewed at length with an emphasis on unresolved issues in the field, including the role of disorder in current experiments. Special attention is given to the interesting low magnetic field limit, and to the relationship between quantum Hall effects and the spontaneous anomalous Hall effects that might occur in bilayer graphene systems in the absence of a magnetic field.     

Collective modes in the two-dimensional electron liquid near the Wigner phase transition

We have investigated the properties of the collective modes of the two dimensional electron liquid in regions of phase space near the Wigner phase transition. We used a new method which we have recently developed for dealing with the strongly correlated electron liquid. We find that the dynamical correlations cause the dispersion curve of the 2D plasmon to pass through a maximum, leading to negative dispersion for largerq values and a much reduced value for the plasmon cut-offq _c. It was of interest to discover, on comparing the plasmon dispersion curve with the corresponding curve for the phonon in the Wigner crystal, that the negative dispersion of the plasmon duplicated the turnover of the phonon dispersion curve at the edge of the Brillouin zone,q=1.2G, and that the two curves were in close agreement for allq less than the plasmon cut-off. A new collective mode in the liquid phase on the highq side of the single-particle excitation spectrum was also found to match up with the phonon dispersion curve nearqG.Australian Research Council Fellow.     

Creation and control of a two-dimensional electron liquid at the bare SrTiO3 surface

Many-body interactions in transition-metal oxides give rise to a wide range of functional properties, such as high-temperature superconductivity, colossal magnetoresistance or multiferroicity . The seminal recent discovery of a two-dimensional electron gas (2DEG) at the interface of the insulating oxides LaAlO(3) and SrTiO(3) (ref. 4) represents an important milestone towards exploiting such properties in all-oxide devices. This conducting interface shows a number of appealing properties, including a high electron mobility, superconductivity and large magnetoresistance, and can be patterned on the few-nanometre length scale. However, the microscopic origin of the interface 2DEG is poorly understood. Here, we show that a similar 2DEG, with an electron density as large as 8×10(13) cm(-2), can be formed at the bare SrTiO(3) surface. Furthermore, we find that the 2DEG density can be controlled through exposure of the surface to intense ultraviolet light. Subsequent angle-resolved photoemission spectroscopy measurements reveal an unusual coexistence of a light quasiparticle mass and signatures of strong many-body interactions.     

Dynamic measurement of the surface tension of liquid helium with a two-dimensional electron probe

Electrons deposited above the free surface of liquid helium exhibit coupled electron-ripplon modes whose frequencies are simply related to the surface tension of helium. The hydrodynamic expression for the ripplon dispersion relation for pure⁴He is shown to be valid up to the wavevector range 10⁵ cm^–1<q<2×10⁶ cm^–1 of the coupled modes. For a small amount of³He (X ₃10^–6–10^–8) added to pure⁴He, we measure the variation of the surface tension in the 60–600 mK temperature range. These experiments, which cover the frequency range v10–300 MHz, agree fairly well with previous static (capillary) or low-frequency (surface sound velocity) experiments.     

Magnetization of a two-dimensional electron gas

The oscillations of magnetization of a two-dimensional free electron gas with field are derived first in the ideal situationT=0 and sharp Landau levels without electron spin and then for finiteT and for broadened levels. Explicit results are obtained in the limiting approximations that the broadening (or kT) is small or large compared with the separation of Landau levels, and the special situations of a field-independent Fermi energy and a field-independent electron density are discussed. The modifications introduced by taking account of electron spin are then considered, and finally the steady magnetic susceptibility superimposed on the oscillations is discussed.     

Frustration-mediated superconductivity in two-dimensional classical Heisenberg model

The two-dimensional frustrated classical Heisenberg model on a square lattice is used to describe the magnetic interaction in La₂CuO₄. In the slow-hopping regime, the model suggests anXY-like ground state. An analysis of the effective interaction between frustrated bonds shows the possibility of pair formation and superconductivity.     

Reduction of the classical electromagnetism to a two-dimensional curved surface

The reduction of the three-dimensional classical electromagnetism to a two-dimensional curved surface is performed in a twofold way. In the first case, the ordinary two-dimensional electromagnetism is obtained with sources in the form of conserved electric currents flowing along the surface. The electric field is a two-vector tangent to the surface and magnetic field is a scalar quantity. In the second approach, the reduced theory is that of the two-vector magnetic field and a scalar electric one. The only source coupled to the fields is now a scalar, subject to no conservation law. In the redefined theory, this scalar source may be converted into an eddy magnetic current flowing in the surface. No magnetic monopoles appear. Our results can find some applications in the electrodynamics of thin layers and of metal-dielectric interfaces.     

Viscoelastic effects in the propagation of a central crack

The integral solutions for the propagation of a central crack in a standard linear viscoelastic solid with constant Poisson's ratio and small loss are obtained as sums of associated static solutions and linear combinations of pure decay with non-exponential functions. An integral equation is obtained to determine the crack shape function for a prescribed crack pressure, where the crack is propagating at constant or varying speed. For a crack of constant speed, the crack shape function and the stress intensity factor are obtained as the sums of the associated static elastic solutions and the dynamic and viscoelastic effect terms. Numerically calculated values of the crack shape function normalized by the maximum value of its associated static solution and the stress intensity factor normalized by its associated static value are presented as a function of crack speed and material properties.

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Résumé Les équations intégrales pour la propagation d'une fissure centrale dans un solide standard linéairement visco-élastique avec un module de Poisson constant et de faibles pertes ont été obtenues comme la somme de solutions statiques associées et de combinaisons linéaires d'une altération pure exprimée par des fonctions non-exponentielles. L'équation intégrale est obtenue en vue de déterminer la fonction de forme de la fissure dans le cas d'une pression de fissuration fixée, pour laquelle la fissure se propage à une vitesse constante ou à une vitesse variable. Dans le cas d'une fissure se propageant à vitesse constante, la fonction de forme de la fissure et le facteur d'intensité de contrainte sont obtenus en procédant à la somme des solutions élastiques statiques associées et des termes faisant apparaître l'effet dynamiqueet visco-élastique. Les valeurs numériques calculées de la fonction de forme de la fissure normalisée par la valeur maximum de la solutions statique associée, et du facteur d'intensité de contrainte normalisé par la valeur statique associée, sont présentées en fonction de la vitesse de propagation et des propriétés du matériau.

     

Modeling of a ferromagnetic two-dimensional electron gas device

We present a realistic modeling of ballistic electron transport in a hybrid ferromagnetic (FM) two-dimensional electron gas (2DEG) device, consisting of an FM gate on an AlGaAs-GaAs or AlSb-InAs high electron mobility transistor (HEMT) heterostructure. The carriers within the 2DEG are spin-polarized by a combination of magnetic and electrostatic barriers. The magnetic barriers are supplied by a composite FM gate, consisting of two domains made of magnetically hard and soft materials. This gate arrangement breaks the antisymmetry of the fringe B field, and results in a finite spin polarization of the 2DEG current. The B field strength is calculated by considering the pole strength at the gate surfaces and domain boundary, and is significantly weaker than normally assumed. We obtain parameters such as the electrostatic barrier height, Fermi level, and carrier concentration within the 2DEG by a finite-element Poisson calculation, which is self-consistent with the Fermi-Dirac distribution. We calculate the transmission probability and conductance through the 2DEG from these parameter values, assuming a single particle effective mass Hamiltonian and purely ballistic transport. We show that the spin polarization ratio P/sub G/ is extremely sensitive to the gate bias and HEMT doping concentration. However, the maximum P/sub G/ is extremely low for AlGaAs-GaAs (0.003%) and even for AlSb-InAs (0.12%) devices, despite a large Lande g factor. These values are many orders of magnitude smaller than previous predictions of close to 100% polarization, obtained by using simpler models.     

Magnetic correlations and disorder in a two-dimensional fermi liquid: Possible important effects for high-T c superconductors

The influence of magnetic correlations and disorder in a two-dimensional Fermi liquid is considered in connection with the occurrence of the superconducting state. The disorder effect and magnetic correlations work against the superconducting state but at the same time are competitive phenomena. We show that the critical temperature can decrease due to the magnetic correlations and disorder, but the effect of correlations is reduced by the disorder. The electronic mass enhancement has been calculated taking into consideration the magnetic correlation effects and disorder. Its dependence on the concentration impurities, in agreement with the experimental data, shows the importance of the localization effects due to disorder. These results are in agreement with the experimental data obtained for the high critical temperature superconductors.     

Quantification in comprehensive two-dimensional liquid chromatography

Although the use of comprehensive two-dimensional liquid chromatography (LCxLC) as a powerful separation technique is continuously increasing, its employment in quantification experiments is rather limited. The present research is focused on the quantification of a series of standards, as well as real-world sample compounds, by using dedicated laboratory-constructed LCxLC software, developed through a novel approach. Moreover, the difficulties encountered during software operation, in various elution conditions, are described and discussed. The results attained were compared with those observed in conventional LC, and no statistically significant differences were observed in the determination of aurapten in grapefruit oil. However, a loss in sensitivity was observed when using LCxLC (limit of detection = 0.10 ppm) compared to conventional LC (limit of detection = 0.05 ppm) as a consequence of the sample dilution in comprehensive two-dimensional liquid chromatography.     

Plasmon resonances in a gated two-dimensional electron system with lateral contacts

It is shown that strong plasmon resonances can be excited in the totally gated two-dimensional electron gas with lateral contacts by incoming electromagnetic wave. In such a structure, the plasmon resonance linewidth shrinks down to the minimal theoretical value determined by the electron scattering in two-dimensional electron system.     

Non-linear effects in the forces acting on fast charged particles passing over two-dimensional electron gas

We study the interactions of fast charged particles with a two-dimensional electron gas describing its collective or plasmon excitations by a hydrodynamic model with the parameters characteristic of graphene. We solve the sets of first and second order equations in a perturbational treatment of the hydrodynamic model, and obtain numerical results for the stopping force and the image force on a particle moving parallel to the electron gas. Although the second order effects in these forces are relatively small, they could be observed in experiments on grazing scattering of, e.g., fast protons and antiprotons from the surface of graphene.     

Marginal fermi liquid behavior for a two-dimensional electronic system

The imaginary part of the self-energy of a two-dimensional Fermi system interacting with bosonic excitations has been calculated analytically and numerically. The result shows a marginal Fermi liquid behavior. The model can be a reasonable approach for studying the marginal behavior of a two-dimensional Millis-Monien-Pines phenomenological model.