Investigation on single-electron dynamics in coupled GaAs-AlGaAs quantum wires

The aim of this paper is the study of the single-electron coherent propagation in a quantum-computing gate made of coupled quantum wires. The structure under investigation is based on a two-dimensional (2-D) electron gas realized in a modulation-doped GaAs-AlGaAs heterostructure. A number of surface electrodes are used to form one-dimensional channels. The profile of the conduction band at the heterojunction has been computed numerically by solving the three-dimensional Poisson equation on the whole structure at 300 mK. Finally, a single-electron wavefunction is propagated within the so-formed quantum wire geometry by means of a 2-D, time-dependent Schro/spl uml/dinger solver. Results are shown for a single-qubit rotation gate implementing a quantum-NOT transformation. This work is part of a feasibility study on a solid-state realization of a universal set of quantum gates.     

Numerical simulation of coherent transport in quantum wires for quantum computing

A solid-state implementation of a universal set of gates for quantum computation is proposed and analysed using a time-dependent 2D Schrödinger solver. The qubit is defined as the state of an electron propagating along a couple of quantum wires. The wires are suitably coupled through a potential barrier with variable height and/or width. It is shown how a proper design of the system allows the implementation of any one-qubit transformation. The two-qubit gate is realized through a Coulomb coupler able to entangle the quantum states of two electrons running in two wires of two different qubits. The simulated devices are GaAs—AlGaAs heterostructures that should be on the borderline of present semiconductor technology. An estimate of decoherence effects due to phonon scattering is also presented.     

Correlating electronic transport to atomic structures in self-assembled quantum wires

Quantum wires, as a smallest electronic conductor, are expected to be a fundamental component in all quantum architectures. The electronic conductance in quantum wires, however, is often dictated by structural instabilities and electron localization at the atomic scale. Here we report on the evolutions of electronic transport as a function of temperature and interwire coupling as the quantum wires of GdSi(2) are self-assembled on Si(100) wire-by-wire. The correlation between structure, electronic properties, and electronic transport are examined by combining nanotransport measurements, scanning tunneling microscopy, and density functional theory calculations. A metal-insulator transition is revealed in isolated nanowires, while a robust metallic state is obtained in wire bundles at low temperature. The atomic defects lead to electron localizations in isolated nanowire, and interwire coupling stabilizes the structure and promotes the metallic states in wire bundles. This illustrates how the conductance nature of a one-dimensional system can be dramatically modified by the environmental change on the atomic scale.     

Novel quantum interference effects in transport through molecular radicals

We investigate electronic transport through molecular radicals and predict a correlation-induced transmission node arising from destructive interference between transport contributions from different charge states of the molecule. This quantum interference effect has no single-particle analog and cannot be described by effective single-particle theories. Large errors in the thermoelectric properties and nonlinear current-voltage response of molecular radical junctions are introduced when the complementary wave and particle aspects of the electron are not properly treated. A method to accurately calculate the low-energy transport through a radical-based junction using an Anderson model is given.     

Electronic transport properties of coupled quantum dots on carbon nanotubes

We investigate the electronic transport properties of coupled quantum dots, controlled by local gates on carbon nanotubes. The inter-dot coupling can be tuned from weak to strong by changing gate voltages, and oscillates in short and long period with the distance between two gates. We introduce a one-dimensional scattering model to describe the mechanism of the electron transport through the carbon nanotube quantum dots. We show that pi and PI* channels contribute differently to the inter-dot coupling and the transport phase plays a key role in the oscillations of the coupling.     

X-ray diffraction from quantum wires and quantum dots

From the distribution of the scattered intensity in reciprocal space, information on the shape as well as on the strain distribution in nanostructured samples can be obtained. This is exemplified by applying this method to laterally patterned periodic Si/SiGe superlattices as well as to periodic SiGe dot arrays embedded in Si.     

Transition from two-dimensional to three-dimensional quantum confinement in semiconductor quantum wires/quantum dots

We report the photoluminescence (PL) and polarization-resolved PL characteristics of a novel GaAs/AlGaAs quantum wire/dot semiconductor system, realized by metalorganic vapor-phase epitaxy of site-controlled, self-assembled nanostructures in inverted tetrahedral pyramids. By systematically changing the length of the quantum wires, we implement a continuous transition between the regimes of two-dimensional and three-dimensional quantum confinement. The two main evidences for this transition are observed experimentally and confirmed theoretically: (i) strongly blue-shifted ground-state emission, accompanied by increase separation of ground and excited transition energies; and (ii) change in the orientation of the main axis of linear polarization of the photoluminescence, from parallel to perpendicular with respect to the wire axis. This latter effect, whose origin is shown to be purely due to quantum confinement and valence band mixing, sets in at wire lengths of only approximately 30 nm.     

Impurity scattering in quantum wires with Coulomb interaction

The Coulomb 1/r-interaction plays an important role in quantum wires. We study the interplay between this long range interaction and impurity. For a single band quantum wire, we find that the transport properties are strongly modified. The linear conductance G is found to vanish with temperature as G exp[- ln^3/2(1/T)]and the current-voltage characteristics acquires a threshold-like behavior.We are grateful to A. Dyugaev for interesting discussions.     

Colloidal CdSe quantum wires by oriented attachment

We report here a relatively low temperature (100-180 degrees C) synthetic route to high-quality and single-crystalline CdSe nanowires using air-stable and generic chemicals. The diameter of nanowires was controlled and varied in an exceptionally small size regime, between 1.5 and 6 nm. This was achieved by using alkylamines, a single type or a mixture of two different types of amines, with different chain lengths and varying the reaction temperature. The experimental results suggest the coexistence of two types of fragments in the prewire aggregates, known as "pearl-necklace" or "string-of-pearls" in the literature, which are loosely associated and chemically fused sections.     

High critical temperature in a superlattice of quantum wires

Here we report experimental evidence that the high T_c superconductivity in a cuprate perovskite occurs in a superlattice of Josephson coupled quantum wires. We show that this particular heterostructure provides the physical mechanism raising T_c from the low temperature range T_c<23K to the high temperature range 30Kc<150K by amplification of the critical temperature of the homogeneous CuO₂ plane by a factor 10. The structure of the high T_c superconducting CuO₂ plane in Bi₂Sr₂CaCu₂O_8+y (Bi2212) at the mesoscopic level (10–100 Å) has been determined. The superconducting plane is decorated by a plurality of parallel superconducting stripes of width L defined by the domain walls formed by stripes of width W characterized by a short Cu-O(apical) distance and large tilting 110° of the distorted square pyramids. We have measured the width of the domain walls W=11±1 Å and of the stripes L=14±1 Å. The critical parameters raising T_c are: 1) the Fermi level is near the bottom of the second subband of the stripes, with k_2y=2/L, formed by the quantum size effect and 2) W is of the order of the superconducting coherence length ₀.     

Suspended carbon nanotube quantum wires with two gates

Suspended single-walled carbon nanotube devices comprised of high-quality electrical contacts and two electrostatic gates per device have been prepared. Compared to nanotubes pinned on substrates, the suspended devices exhibit little hysteresis related to environmental factors and act as cleaner Fabry-Perot interferometers or single-electron transistors. The high-field saturation currents in the suspended nanotubes related to optical phonon or zone-boundary phonon scattering are significantly lower due to the lack of efficient heat sinking. The multiple-gate design may also facilitate future investigations into the electromechanical properties of nanotube quantum systems.     

Longitudinal photoconductivity of GaAs/AlGaAs quantum wires

The photoconductivity (PC) of undoped GaAs/AlGaAs quantum wires (QWRs) along the wire direction is studied for the first time. The PC spectrum reveals a strong substrate-related background as well as several small structures, some of which could be connected with the QWRs. This suggestion is confirmed by the observed polarization dependence of the PC and by photoluminescence (PL) and photoluminescence excitation (PLE) measurements on a similar sample. Prolonged pre-illumination of the sample with infrared light (hν=1.18 eV) considerably reduces the background in the PC spectrum, and makes the QWR structures better resolved.     

Bandlike transport in strongly coupled and doped quantum dot solids: a route to high-performance thin-film electronics

We report bandlike transport in solution-deposited, CdSe QD thin-films with room temperature field-effect mobilities for electrons of 27 cm(2)/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands. Temperature-dependent measurements show bandlike transport to 220 K on a SiO(2) gate insulator that is extended to 140 K by reducing the interface trap density using an Al(2)O(3)/SiO(2) gate insulator. The use of compact ligands and doping provides a pathway to high performance, solution-deposited QD electronics and optoelectronics.     

Electronic structure and spectroscopy of cadmium telluride quantum wires

The size-dependent electronic structure of CdTe quantum wires is determined by density functional theory using the local density approximation with band-corrected pseudopotential method. The results of the calculations are then used to assign the size-dependent absorption spectrum of colloidal CdTe quantum wires synthesized by the solution-liquid-solid mechanism. Quantitative agreement between experiment and theory is achieved. The absorption features comprise transitions involving the highest 25-30 valence-band states and lowest 15 conduction-band states. Individual transitions are not resolved; rather, the absorption features consist of clusters of transitions that are determined by the conduction-band energy-level spacings. The sequence, character, and spacing of the conduction-band states are strikingly consistent with the predictions of the simple effective-mass-approximation, particle-in-a-cylinder model. The model is used to calculate the size dependence of the electron effective mass in CdTe quantum wires.     

Nanoscale transport enables active self-assembly of millimeter-scale wires

Active self-assembly processes exploit an energy source to accelerate the movement of building blocks and intermediate structures and modify their interactions. A model system is the assembly of biotinylated microtubules partially coated with streptavidin into linear bundles as they glide on a surface coated with kinesin motor proteins. By tuning the assembly conditions, microtubule bundles with near millimeter length are created, demonstrating that active self-assembly is beneficial if components are too large for diffusive self-assembly but too small for robotic assembly.     

Graphene nanoribbons with smooth edges behave as quantum wires

Graphene nanoribbons with perfect edges are predicted to exhibit interesting electronic and spintronic properties, notably quantum-confined bandgaps and magnetic edge states. However, so far, graphene nanoribbons produced by lithography have had rough edges, as well as low-temperature transport characteristics dominated by defects (mainly variable range hopping between localized states in a transport gap near the Dirac point). Here, we report that one- and two-layer nanoribbon quantum dots made by unzipping carbon nanotubes exhibit well-defined quantum transport phenomena, including Coulomb blockade, the Kondo effect, clear excited states up to ～20?meV, and inelastic co-tunnelling. Together with the signatures of intrinsic quantum-confined bandgaps and high conductivities, our data indicate that the nanoribbons behave as clean quantum wires at low temperatures, and are not dominated by defects.     

Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires

Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or use reduced-dimensional conductors such as carbon nanotubes, nanowires and gated high-mobility two-dimensional electron systems. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomena such as Coulomb drag, where a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have demonstrated Coulomb electron drag in wires separated by a soft electrostatic barrier of width ?80?nm (ref.?12), which was interpreted as resulting entirely from momentum transfer. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by ～15?nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron sub-bands occupied in each wire, and interpret the results both in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different sub-band occupancies, the positive drag effect can be as large as 25%.     

Mobility asymmetry in InGaAs/InAlAs heterostructures with InAs quantum wires

Strong asymmetry of electron mobility in InGaAs/InAlAs heterostructures (lattice matched to InP) with the presence of InAs quantum wires was observed. Self-assembled InAs quantum wires, embedded in an InGaAs matrix close to the hetero-interface, has a strong effect in electron conduction in the interface channel. The low temperature mobility for electrons moving parallel to the quantum wires is much higher than that of electrons moving perpendicular to the wires. The asymmetry in mobility is attributed to the difference in scattering cross section of the quantum wires in these two directions.