Gate Adjustable Coherent Three and Four Level Mixing in a Vertical Quantum Dot Molecule

We study level mixing in the single particle energy spectrum of one of the constituent quantum dots in a vertical double quantum dot by performing magneto-resonant-tunneling spectroscopy. The device used in this study differs from previous vertical double quantum dot devices in that the single side gate is now split into four separate gates. Because of the presence of natural perturbations caused by anharmonicity and anistrophy, applying different combinations of voltages to these gates allows us to alter the effective potential landscape of the two dots and hence influence the level mixing. We present here preliminary results from one three level crossing and one four level crossings high up in the energy spectrum of one of the probed quantum dots, and demonstrate that we are able to change significantly the energy dispersions with magnetic field in the vicinity of the crossing regions.Comment: 5 pages, 4 figures. MSS-14 conference proceedings submitted to Physica

Electron transport in gated InGaAs and InAsP quantum well wires in selectively-grown InP ridge structures

The purpose of this work is to fabricate ribbon-like InGaAs and InAsP wires embedded in InP ridge structures and investigate their transport properties. The InP ridge structures that contain the wires are selectively grown by chemical beam epitaxy (CBE) on pre-patterned InP substrates. To optimize the growth and micro-fabrication processes for electronic transport, we explore the Ohmic contact resistance, the electron density, and the mobility as a function of the wire width using standard transport and Shubnikov-de Haas measurements. At low temperatures the ridge structures reveal reproducible mesoscopic conductance fluctuations. We also fabricate ridge structures with submicron gate electrodes that exhibit non-leaky gating and good pinch-off characteristics acceptable for device operation. Using such wrap gate electrodes, we demonstrate that the wires can be split to form quantum dots evidenced by Coulomb blockade oscillations in transport measurements.Comment: 5 pages, 4 figures, additional references and improved Fig. 4c, MSS-14 conference, submitted to Physica

Theory of infrared conductivity from density waves: (TMTSF)2X

It Is argued that the Infrared conductivity due to a spin density wave (SDW) in (TMTSF)2X may in some cases be of a semiconductor form for a strongly pinned density wave or in other cases be of a more complicated form with phonon resonances due to charge density wave harmonics of a weakly pinned SDW.Peer reviewed: YesNRC publication: Ye

Inversion-layer width, electron-electron interactions, and the fractional quantum Hall effect

The current understanding of the fractional quantum Hall effect is based on the incompressible quantum fluid state proposed by Laughlin [Phys. Rev. Lett. 50, 1395 (1983)]. For a two-dimensional electron gas with Coulombic electron-electron interactions this proposal has been justified by demonstrating that Laughlin's state is lower in energy than the charge-density-wave state of the same system. In an actual inversion layer, the effective electron-electron interaction is strongly modified when the electron-electron separation becomes comparable with the inversion layer width. In this Rapid Communication we demonstrate that this modification results in a substantial reduction in the energy of both Laughlin and charge-density-wave states, but nevertheless leaves Laughlin's state lower in energy.Peer reviewed: YesNRC publication: Ye

Enclosed flux dependence of the eigenvalue spectrum: localisation and quantised Hall conductivity in a two-dimensional electron gas

The authors consider a two-dimensional electron gas in a strong magnetic field and confined by an infinite potential to a strip of finite width. The dependence of the eigenvalue spectrum on the choice of origin for a Landau-gauge vector potential has been examined numerically for the case in which a weak random potential is also present. For an electric potential difference across the system which is large compared with the Landau level width they find that the behaviour is similar to the semiclassical situation in which states may be associated with equal-potential contours and is in agreement with earlier work. However for an electric potential difference which is small compared with the Landau level width they find more complicated behaviour in disagreement with the recent study of Aoki (1982,1983).Peer reviewed: YesNRC publication: Ye

Determination of the pair potential and the ion-electron pseudopotential for aluminum from experimental structure-factor data for liquid aluminum

A method of inverting a given structure factor [S(k)]expt of a liquid metal using a hypernetted-chain equation containing bridge-diagram contributions is presented. Starting from parametrized local pseudopotential and a parametrized model local field (or a theoretical local field), a pair potential is constructed. The S(k) calculated from it is fitted to the given [S(k)]expt. The method is first applied to a molecular-dynamics-generated S(k) derived from an ab initio aluminum potential and shown to yield the pair potential, the pseudopotential, and the charge density in excellent quantitative agreement with the original ab initio potential and other quantities. The method is then applied to the experimental S(k) of aluminum from x-ray data at 943 K. The Al-Al pair potential, Al-electron pseudopotential, and electron charge densities as well as the electron-gas response function (i.e., the model local field) are obtained self-consistently, to within the accuracy of the experimental data. The calculated electrical resistivity is in excellent agreement with experiment. These investigations provide a comparative examination of the electron-gas local fields of Geldart-Taylor, Vashishta-Singwi, Ichimaru-Utsumi, and the density-functional local-density approximation. The hard-sphere parameter defining the bridge term is found to be essentially the same for the different ion-ion potentials determined from all but one of the different local fields, thus supporting the "universality" hypothesis of Rosenfeld and Ashcroft.Peer reviewed: YesNRC publication: Ye

Analysis of the structure factor of dense krypton gas: Bridge contributions and many-body effects

The pair-correlation function g(r) of the Kr-type model fluid with only pair interactions was calculated using the Rosenfeld-Ashcroft modification of the hypernetted-chain (HNC) equation which includes bridge diagrams, and gave results in excellent agreement with Monte Carlo g(r) data. These bridge functions and the known pair potential were used to analyze the neutron-diffraction structure-factor data of Teitsma and Egelstaff, to determine the effective strength of the three-body potential as a function of the density assuming it to be of the Axilrod-Teller (AT) form. The strength of the effective three-body contribution s=\u3bd\u3bdtheor, where \u3bdtheor is the theoretical value, decreases for higher densities, suggesting that the many-body terms (beyond the Axilrod-Teller form) screen the AT interaction as the density increases. The results are very sensitive to the uncertainties in the structure factor S(k) for small k if parameter optimization is used to determine the effective pair potential. However, prediction of the compressibility using s=1 allows us to conclude that \u3bdtheor is consistent with the experimental data for low densities, to within the uncertainties in the presently available pair potentials and in the structure-factor data.Peer reviewed: YesNRC publication: Ye

Model calculations of ellipsometric parameters for ligand field absorption by oxidized Ni and Co surfaces

Ligand field absorption of photons by such transition metal compounds as NiO and CoO, although weak, is known to be sensitive to the orientation of the O ions or ligands near the metal atoms. The wavelength dependence and intensity of the absorption could therefore in principle give information regarding the structure of surface oxides on Ni and Co. To estimate whether such absorption is detectable using ellipsometry, we have calculated the ellipsometric angles \u3c8 and \u394 for thin films of bulk-like octahedral NiO and CoO on Ni and Co respectively, and conclude that the changes in these angles due to ligand field absorption in very thin films detectable. In reality, the spectra will be sensitive to the chemical species of the metal ions and to the local ligand geometry, and we assess the importance of some these factors. We find changes from one stage of the oxidation process to the next, suggesting that ligand field absorption will make-high-resolution ellipsometry a sensitive probe of the oxidation process of Ni and Co, and possibly other transition metal ions which show ligand field absorption.Peer reviewed: YesNRC publication: Ye