Fano effect and Kondo effect in quantum dots formed in strongly coupled quantum wells

We present lateral transport measurements on strongly, vertically coupled quantum dots formed in separate quantum wells in a GaAs/AlGaAs heterostructure. Coulomb oscillations are observed forming a honeycomb lattice consistent with two strongly coupled dots. When the tunnel barriers in the upper well are reduced we observe the Fano effect due to the interfering paths through a resonant state in the lower well and a continuum state in the upper well. In both regimes an in plane magnetic field reduces the coupling between the wells when the magnetic length is comparable to the center to center separation of the wells. We also observe the Kondo effect which allows the spin states of the double dot system to be probed.Comment: 4 pages, 5 figure

Relative concentration and structure of native defects in GaP

The native defects in the compound semiconductor GaP have been studied using a pseudopotential density functional theory method in order to determine their relative concentrations and the most stable charge states. The electronic and atomic structures are presented and the defect concentrations are estimated using calculated formation energies. Relaxation effects are taken into account fully and produce negative-U charge transfer levels for VP and PGa. The concentration of VGa is in good agreement with the results of positron annihilation experiments. The charge transfer levels presented compare qualitatively well with experiments where available. The effect of stoichiometry on the defect concentrations is also described and is shown to be considerable. The lowest formation energies are found for PGa +2 in p-type and VGa −3 in n-type GaP under P-rich conditions, and for GaP −2 in n-type GaP under Ga-rich conditions. Finally, the finite size errors arising from the use of supercells with periodic boundary conditions are examined

Electron transport in Coulomb- and tunnel-coupled one-dimensional systems

We develop a linear theory of electron transport for a system of two identical quantum wires in a wide range of the wire length L, unifying both the ballistic and diffusive transport regimes. The microscopic model, involving the interaction of electrons with each other and with bulk acoustical phonons allows a reduction of the quantum kinetic equation to a set of coupled equations for the local chemical potentials for forward- and backward-moving electrons in the wires. As an application of the general solution of these equations, we consider different kinds of electrical contacts to the double-wire system and calculate the direct resistance, the transresistance, in the presence of tunneling and Coulomb drag, and the tunneling resistance. If L is smaller than the backscattering length l_P, both the tunneling and the drag lead to a negative transresistance, while in the diffusive regime (L >>l_P) the tunneling opposes the drag and leads to a positive transresistance. If L is smaller than the phase-breaking length, the tunneling leads to interference oscillations of the resistances that are damped exponentially with L.Comment: Text 14 pages in Latex/Revtex format, 4 Postscript figure

Point defects on the (110) surfaces of InP, InAs and InSb: a comparison with bulk

The basic properties of point defects, such as local geometries, positions of charge-transfer levels, and formation energies, have been calculated using density-functional theory, both in the bulk and on the 110 surface of InP, InAs, and InSb. Based on these results we discuss the electronic properties of bulk and surface defects, defect segregation, and compensation. In comparing the relative stability of the surface and bulk defects, it is found that the native defects generally have higher formation energies in the bulk. From this it can be concluded that at equilibrium there is a considerably larger fraction of defects at the surface and under nonequilibrium conditions defects are expected to segregate to the surface, given sufficient time. In most cases the charge state of a defect changes upon segregation, altering the charge-carrier concentrations. The photo-thresholds are also calculated for the three semiconductors and are found to be in good agreement with experimental data

Giant Thermoelectric Effect from Transmission Supernodes

We predict an enormous order-dependent quantum enhancement of thermoelectric effects in the vicinity of a higher-order `supernode' in the transmission spectrum of a nanoscale junction. Single-molecule junctions based on 3,3'-biphenyl and polyphenyl ether (PPE) are investigated in detail. The nonequilibrium thermodynamic efficiency and power output of a thermoelectric heat engine based on a 1,3-benzene junction are calculated using many-body theory, and compared to the predictions of the figure-of-merit ZT.Comment: 5 pages, 6 figure

Mode Spectroscopy and Level Coupling in Ballistic Electron Waveguides

A tunable quantum point contact with modes occupied in both transverse directions is studied by magnetotransport experiments. We use conductance quantization of the one-dimensional subbands as a tool to determine the mode spectrum. A magnetic field applied along the direction of the current flow couples the modes. This can be described by an extension of the Darwin-Fock model. Anticrossings are observed as a function of the magnetic field, but not for zero field or perpendicular field directions, indicating coupling of the subbands due to nonparabolicity in the electrical confinement.Comment: 4 pages, 3 figure

Direct observation of electron doping in La0.7Ce0.3MnO3 using x-ray absorption spectroscopy

We report on a X-ray absorption spectroscopic (XAS) study on a thin film of La0.7Ce0.3MnO3, a manganite which was previously only speculated to be an electron doped system. The measurements clearly show that the cerium is in the Ce(IV) valence state and that the manganese is present in a mixture of Mn2+ and Mn3+ valence states. These data unambiguously demonstrate that La0.7Ce0.3MnO3 is an electron doped colossal magnetoresistive manganite, a finding that may open up new opportunities both for device applications as well as for further basic research towards a better modelling of the colossal magnetoresistance phenomenon in these materials.Comment: 4 pages, 3 figures, revised versio

Resonant transmission through an open quantum dot

We have measured the low-temperature transport properties of a quantum dot formed in a one-dimensional channel. In zero magnetic field this device shows quantized ballistic conductance plateaus with resonant tunneling peaks in each transition region between plateaus. Studies of this structure as a function of applied perpendicular magnetic field and source-drain bias indicate that resonant structure deriving from tightly bound states is split by Coulomb charging at zero magnetic field.Comment: To be published in Phys. Rev. B (1997). 8 LaTex pages with 5 figure

The number of transmission channels through a single-molecule junction

We calculate transmission eigenvalue distributions for Pt-benzene-Pt and Pt-butadiene-Pt junctions using realistic state-of-the-art many-body techniques. An effective field theory of interacting $\pi$-electrons is used to include screening and van der Waals interactions with the metal electrodes. We find that the number of dominant transmission channels in a molecular junction is equal to the degeneracy of the molecular orbital closest to the metal Fermi level.Comment: 9 pages, 8 figure

Safe Design Suggestions for Vegetated Roofs

Rooftop vegetation is becoming increasingly popular because of its environmental benefits and its ability to earn green-building certification credits. With the exception of one international guideline, there is little mention of worker safety and health in vegetated-roof codes and literature. Observations and field investigations of 19 vegetated roofs in the United States revealed unsafe access for workers and equipment, a lack of fall-protection measures, and other site-specific hazards. Design for safety strategies and the integration of life-cycle safety thinking with green-building credits systems are the preferred methods to reduce risk to workers on vegetated roofs. Design suggestions have been developed to add to the body of knowledge. The findings complement several National Institute for Occupational Safety and Health (NIOSH) construction and prevention through design (PtD) goals and are congruent with NIOSH’s Safe Green Jobs initiative. Organizations that install and maintain vegetated roofs can utilize the findings to understand hazards, take precautions, and incorporate safety into their bids The published version of this article is available here: 10.1061/(ASCE)CO.1943-7862.0000500Support from the the Virginia Tech Occupational Safety and Health Research Center through the Kevin P. Granata Pilot Program funded by the Institute for Critical Technology and Applied Sciences