Dynamic correlations in symmetric electron-electron and electron-hole bilayers

The ground-state behavior of the symmetric electron-electron and electron-hole bilayers is studied by including dynamic correlation effects within the quantum version of Singwi, Tosi, Land, and Sjolander (qSTLS) theory. The static pair-correlation functions, the local-field correction factors, and the ground-state energy are calculated over a wide range of carrier density and layer spacing. The possibility of a phase transition into a density-modulated ground state is also investigated. Results for both the electron-electron and electron-hole bilayers are compared with those of recent diffusion Monte Carlo (DMC) simulation studies. We find that the qSTLS results differ markedly from those of the conventional STLS approach and compare in the overall more favorably with the DMC predictions. An important result is that the qSTLS theory signals a phase transition from the liquid to the coupled Wigner crystal ground state, in both the electron-electron and electron-hole bilayers, below a critical density and in the close proximity of layers (d <~ r_sa_0^*), in qualitative agreement with the findings of the DMC simulations.Comment: 13 pages, 11 figures, 2 table

Metal-insulator transition in disordered 2DEG including temperature effects

We calculate self-consistently the mutual dependence of electron correlations and electron-defect scattering for a two dimensional electron gas at finite temperature. We employ an STLS approach to calculate the electron correlations while the electron scattering rate off Coulombic impurities and surface roughness is calculated using self-consistent current-relaxation theory. The methods are combined and self-consistently solved. We discuss a metal-insulator transition for a range of disorder levels and electron densities. Our results are in good agreement with recent experimental observations.Comment: 4 pages, RevTeX + epsf, 5 figure

Fine structure in the off-resonance conductance of small Coulomb blockade systems

We show how a fine, multiple-peak structure can arise in the off-resonance, zero-bias conductance of Coulomb blockade systems. In order to understand how this effect comes about one must abandon the orthodox, mean-field understanding of the Coulomb blockade phenomenon and consider quantum fluctuations in the occupation of the single-particle electronic levels. We illustrate such an effect with a spinless Anderson-like model for multi-level systems and an equation-of-motion method for calculating Green's functions that combines two simple decoupling schemes.Comment: 5 pages, 3 figures, postscript file also available at http://www.pa.uky.edu/~palacios/papers/eom.ps One figure added. Discussion of results extende

Excitonic condensation in a symmetric electron-hole bilayer

Using Diffusion Monte Carlo simulations we have investigated the ground state of a symmetric electron-hole bilayer and determined its phase diagram at T=0. We find clear evidence of an excitonic condensate, whose stability however is affected by in-layer electronic correlation. This stabilizes the electron-hole plasma at large values of the density or inter-layer distance, and the Wigner crystal at low density and large distance. We have also estimated pair correlation functions and low order density matrices, to give a microscopic characterization of correlations, as well as to try and estimate the condensate fraction.Comment: 4 pages, 3 figures, 2 table

Finite Temperature Correlations on Plasmon and Coulomb Drag in Coupled Quantum Wells

We calculate the temperature dependence of correlations in coupled quantum wells using an STLS formalism generalized to Þnite temperatures. We investigate the e¤ects of the temperature dependence of the correlations on the acoustic and optical plasmon dispersion curves and on Coulomb drag. Correlations are known to depress the energies of the two plasmons, while Þnite temperatures tend to increase the energies. We Þnd this can result in considerable cancellation between the two e¤ects at Þnite temperatures. We Þnd that at temperatures kBT<EF the local fields do not change appreciably with temperature so that in calculating the temperature dependence of the Coulomb drag using the zero temperature local fields suffices. The pair correlation function within the layers is also found to be insensitive to temperatures for kBT<EF. For higher temperatures than this the pair correlation function for small separations starts to increase as the exchange-correlation hole shrinks

Exciton and Charge Density Wave Formation in Spatially Separated Electron Hole Liquids

We present a microscopic many-body calculation investigating the competition between the charge density wave and exciton formation in coupled electron–hole layer systems at finite temperature. We find that as the layer separation is decreased there is a divergence at a finite wave vector in the static susceptibility, indicating an instability in the liquid to a coupled charge density wave. We also see evidence in the pair correlation function g(r) for the liquid phase of a divergence occurring at small r when the layer separation is reduced sufficiently, which would indicate the onset of a bound state instability. However, the charge density wave instability occurs in the liquid before g(r) can actually diverge. The unified picture which starts to emerge from our results is of a charge density wave instability occurring in the liquid phase at a layer separation larger than the exciton radius. This leaves open the possibility of a second transition from the charge density wave state to the excitonic phase

Static and dynamic properties of coupled electron-electron and electron-hole layers

We have investigated coupled layers of electron and hole liquids in semiconductor heterostructures in zero magnetic field for densities rs<~20 using the Singwi-Tosi-Land-Sjölander self-consistent formalism generalized for layers of unequal density. We calculate susceptibilities, local fields, pair correlation functions, and the dispersion of the collective modes for a range of layer spacings. We include cases where the densities in the two layers are not equal. We find generally that static correlations acting between layers do not have a large effect on the correlations within the layers. For coupled electron-hole layers we find that as the spacing between the layers decreases there is a divergence in the static susceptibility of the liquid that signals an instability towards a charge-density-wave ground state. When the layer spacing approaches the effective Bohr radius the electron-hole correlation function starts to diverge at small interparticle separations. This effect is a precursor to the onset of excitonic bound states but this is preempted by the charge-density-wave instability. The acoustic plasmon exhibits a crossover in behavior from a coupled mode to a mode that is confined to a single layer. Correlations sometimes push the acoustic plasmon dispersion curve completely into the single-particle excitation spectrum. For layers with different densities the Landau damping within the single-particle excitation region is sometimes so weak that the acoustic plasmon can exist inside the region as a sharp resonance. We find for the electron-hole case that proximity to the charge-density-wave instability has an unusual effect on the dispersion of the optical plasmon mode

Patterning of sub-1 nm dangling-bond lines with atomic precision alignment on H:Si(100) surface at room temperature

We have patterned sub-1 nm dangling-bond (DB) lines on a H-terminated Si(100)-2 × 1 surface aligned with atomic precision at room temperature using a scanning tunneling microscope (STM) to controllably desorb hydrogen atoms from a H:Si(100) surface. In order to achieve continuous and aligned DB lines, we have performed a detailed investigation of the effects of patterning parameters such as the writing voltage, writing current and electron dosage, as well as STM tip apex geometry on the fabrication and alignment of Si DB lines. We show that there exists an optimum set of patterning parameters which enables us to obtain near-perfect Si DB lines and align them with near atomic precision in a highly controllable manner. In addition, our results indicate that the pattern quality is weakly dependent on the STM tip apex quality when the patterning parameters are within the optimum parameter space