Plasma waves in a two-dimensional electron system laterally screened by a metallic gate

The dispersion of magnetoplasma and plasma excitations in two-dimensional electron systems, whose edges are formed by a voltage applied to a metallic gate, has been studied. A substantial decrease in the plasma wave frequency as compared to the plasma frequency measured in the etched mesas with the same geometry, size, and electron density has been observed. The dependence of the observed frequency softening on the structure size has been studied and the laterally screened plasma excitation has been shown to violate the square-root dispersion relation.     

Minimum magnetocapacitance of a screened two-dimensional electron gas

A theory for the magnetocapacitance of a partially screened two-dimensional (2D) electron gas is proposed. The model investigated is sensitive to different types of screening in a 2D electron gas with an integer filling factor: the so-called conventional approach and the self-consistent approximation introduced in the present paper. The calculations point to the importance of the self-consistent treatment of the magnetocapacitance of a 2D electron gas under the conditions of an integer filling factor. The final self-consistent results are qualitatively consistent with the available experimental data. Fiz. Tverd. Tela (St. Petersburg) 39, 742–745 (April 1997)     

Electromagnetic renormalization of the plasmon spectrum in a laterally screened two-dimensional electron system

It is demonstrated theoretically that the effects of lateral electromagnetic screening of plasmons in a two-dimensional electron system by side metallic electrodes modify the plasmon spectrum. The effects of lateral screening become more pronounced at larger values of the plasmon wave vector. This leads to the deviation of the dispersion of laterally screened plasmons from the square-root dispersion law characteristic of unscreened plasmons. The results allow interpreting the recent experimental data of other authors on the dispersion of the plasma oscillations in a laterally screened two-dimensional electron system.     

Landau-level spectroscopy of a two-dimensional electron system by tunneling through a quantum dot

A single InAs self-assembled quantum dot is incorporated in the barrier of a tunnel diode and used as a spectroscopic probe of an adjacent two-dimensional electron system from the Fermi energy to the subband edge. We obtain quantitative information about the energy dependence of the quasiparticle lifetime. For magnetic field B, applied parallel to the current, we observe peaks in the current-voltage characteristics I(V) corresponding to the formation of Landau levels. Close to filling factor nu=1 we observe directly the exchange enhancement of the Lande g factor.     

Spin polarization of two-dimensional electron system in parabolic potential

We analyze the ground state of the two-dimensional quantum system of electrons confined in a parabolic potential with the system size around 100 at 0 K. We map the system onto a classical system on the basis of the classical-map hypernetted-chain (CHNC) method which has been proven to work in the integral-equation-based analyses of uniform systems and apply classical Monte Carlo and molecular dynamics simulations. We find that, when we decrease the strength of confinement keeping the number of confined electrons fixed, the energy of the spin-polarized state with somewhat lower average density becomes smaller than that of the spin-unpolarized state with somewhat higher average density. This system thus undergoes the transition from the spin-unpolarized state to the spin polarized state and the corresponding critical value of rs estimated from the average density is as low as rs∼0.4 which is much smaller than the rs value for the Wigner lattice formation. When we compare the energies of spin-unpolarized and spin-polarized states for given average density, our data give the critical rs value for the transition between unpolarized and polarized states around 10 which is close to but still smaller than the known possibility of polarization at rs∼27. The advantage of our method is a direct applicability to geometrically complex systems which are difficult to analyze by integral equations and this is an example.     

Real-space observation of drift States in a two-dimensional electron system at high magnetic fields

The local density of states of the adsorbate-induced two-dimensional electron system is studied in magnetic fields up to B=6 T. Landau quantization is observed and drift states with a width of about the magnetic length are found in agreement with theoretical predictions. At the tails of the Landau levels the states form closed paths indicating localization. These states show the expected energy dependence. A multifractal analysis applied to the data results in a nice parabolic shape of the characteristic f(alpha) spectra, but we find only a slight displacement of the origin from alpha=2.0 for the states in the center of the Landau level.     

Density of states of a two-dimensional electron gas measured by high-resolution photoelectron spectroscopy

We present high energy-resolution photoemission measurements of the spectral density at the discrete quantized electronic levels of a two-dimensional (2D) electron gas. The dynamical 2D electron gas has been obtained by generating a strong accumulation layer at the (110) surface of narrow-gap III–V semiconductors. Exploitation of a number of cases generating band bending (metallic chains or clusters, atomic structure, defects) demonstrates the generality of 2D electron gas formation at charge-accumulated semiconductor surfaces. A self-consistent solution of the Poisson and Schrödinger equations gives the potential well shape, the sub-band energy level position and the accumulated charge density, in excellent agreement with the present experimental data.     

Phase diagram of a two-dimensional electron system

The melting curve of the two-dimensional electron system is interpolated between the known classical and ground state limits. The coexistence curve encloses a finite solid-phase domain, as in the three-dimensional case.     

Inductive excitation of a two-dimensional electron system

The possibility of measuring the off-diagonal component of the magnetoconductivity tensor of a two-dimensional electron gas excited by a linear alternating current is discussed.     

Experimental observation of edge magnetoplasma excitations in a two-dimensional electron system in the quantum-hall-effect regime

New low-frequency modes corresponding to acoustic edge magnetoplasma excitations have been observed in the resonance microwave absorption spectra of a two-dimensional electron system in a transverse magnetic field. The additional excitation modes have been shown to appear only in the quantum-Hall-effect regime (in narrow magnetic-field regions near the integer values of the filling factor), when the resonance microwave absorption lines exhibit sharp narrowing. The absolute values of the resonance absorption frequencies and their dependence on the parameters of the electron system coincide (without any fitting parameters) with the respective theoretical predictions of the formula describing the properties of the acoustic modes of edge magnetoplasma excitations.     

Probing the potential landscape inside a two-dimensional electron Gas

We report direct observations of the scattering potentials in a two-dimensional electron gas using electron-beam diffraction experiments. The diffracting objects are local density fluctuations caused by spatial and charge-state distribution of donors in the GaAs-(Al,Ga)As heterostructures. The scatterers can be manipulated externally by sample illumination or by cooling the sample down under depleted conditions.     

Shock waves in capillary collapse of colloids: a model system for two-dimensional screened Newtonian gravity

Using Brownian dynamics simulations, density functional theory, and analytical perturbation theory we study the collapse of a patch of interfacially trapped, micrometer-sized colloidal particles, driven by long-ranged capillary attraction. This attraction is formally analogous to two-dimensional (2D) screened Newtonian gravity with the capillary length λ as the screening length. Whereas the limit λ→∞ corresponds to the global collapse of a self-gravitating fluid, for finite λ[over ^] we predict theoretically and observe in simulations a ringlike density peak at the outer rim of a disclike patch, moving as an inbound shock wave. Possible experimental realizations are discussed.     

Bichromatic microwave photoresistance of a two-dimensional electron system

We explore experimentally bichromatic (frequencies omega(1) and omega(2)) photoresistance of a two-dimensional electron system in the regimes of microwave-induced resistance oscillations and zero-resistance states. We find bichromatic resistance to be well described by a superposition of omega(1) and omega(2) and components, provided that both monochromatic resistances are positive. This relation holds even when the oscillation amplitudes are small and one could expect additive contributions from monochromatic photoresistances. In contrast, whenever a zero-resistance state is formed by one of the frequencies, such superposition relation breaks down and the bichromatic resistance is strongly suppressed.     

Experimental determination of the mean free path of screened edge magnetoplasmons in a two-dimensional electron gas

Magnetic oscillations of the photovoltage in a two-dimensional electron system with the back gate, exposed to microwave radiation, are studied. The oscillations result from the interference of screened edge magnetoplasmons (EMPs). The mean free path of the EMPs is quantitatively determined by analyzing the dependence of the oscillation amplitude on the electron density. The dependences of the mean free path of the EMPs on the two-dimensional electron density, microwave frequency, electron relaxation time, and the magnetic field are studied. It is found that the dependences agree qualitatively with the known theoretical calculations.     

Topographic study by scanning-tunneling microscopy of a two-dimensional graphite film on (10\bar 10)

Two-dimensional graphite films on $(10\bar 10)$ were produced in ultrahigh vacuum by adsorption of benzene vapor on the metal heated to T=1800 K. High-resolution Auger spectroscopy used for the film characterization showed the film indeed to have graphitic structure and monolayer thickness. The surface topography was studied in air by scanning-tunneling microscopy. The monolayer thickness was confirmed, and it was shown that a two-dimensional graphite film has a complex topography featuring numerous hillocks with linear dimensions of ～3000 Å and height differences of ～300 Å, while retaining graphitic structure on the atomic scale. The lack of planarity of such a film at room temperature is considered to be due to deformation occurring under cooling from the temperature of formation down to 300 K, which is caused by the difference in thermal expansion coefficients between the graphite sheet and rhenium.