Alex Marchi

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.

3D simulation of quantum-wire confining potential for a GaAs/AlGaAs 2DEG heterostructure

The aim of this work is the study of electrostatically confined single and coupled quantum wires realized within a high-mobility two-dimensional electron gas (2DEG) at a GaAs/AlGaAs heterointerface. The one-dimensional channels are formed by the potential created by suitably biased surface electrodes. The shape of the bottom of the conduction band energy at the heterojunction has been obtained by numerically solving the three-dimensional Poisson equation over the whole structure at 20 K. Special attention has been paid to the depletion condition for the 2DEG within the quantum wires. To this purpose, the 1D Schrodinger equation is solved along the growth direction consistently with the Poisson solution, to accurately calculate the local electron distribution at the heterojunction. Finally, a single-electron wavefunction is propagated within the structure by means of a two-dimensional time-dependent Schrodinger solver, and results are shown for a single-qubit gate able to split the wavepacket. This investigation is part of a feasibility study carried out to identify the experimental requirements for the realization of basic quantum-computing gates.

The R-Σ Approach to Tunnelling in Nanoscale Devices

The R-Σ method provides the time evolution of two dynamical variables extracted from a wave function, namely, the expectation value of the position and the dispersion. It overcomes the Ehrenfest approximation while keeping the Newtonian form of the equations, thus providing the basis for including quantum features into the description of the single-particle dynamics and for extending such features to the collective-transport case. Here the single-particle R-Σ equations are applied to the case of tunnelling, and the results are compared with a full-quantum calculation.

A Schrödinger-Poisson Solution of CNT-FET Arrays

In this work we investigate and compare the electrostatics of carbon-nanotube field-effect transistor (CNT-FET) arrays. To this purpose, we have developed a self-consistent Schrödinger-Poisson solver which fully takes into account quantum effects and the CNTs physical properties. We show that quantum effects have to be carefully taken into account in order to properly catch the electrostatic behavior of these devices. A further analysis is carried out in order to quantify the screening effects that arise when an array of nanotubes in parallel is used, showing that such effects play a fundamental role in the electrostatic performance of CNT-FET arrays.

Quantum-mechanical analysis of the electrostatics in silicon-nanowire and carbon-nanotube FETs

In this work, we investigate the electrostatics of the silicon-based /spl Pi/-gate FET and the top-gate carbon-nanotube FET at extreme miniaturization limits. In order to do so, we solve the coupled Schrodinger-Poisson equations within the two device cross sections, and compare the channel-charge and capacitance curves as functions of the gate voltage. This study shows that, for a fixed cross-sectional area, the quantitative differences between the two devices are small both in terms of charge and capacitance. The use of a classical model for the /spl Pi/-gate FET shows instead that the resulting discrepancies with respect to the quantum-mechanical (QM) model are very relevant using both the Boltzmann and Fermi statistics. Thus, accounting for quantum-mechanical effects is essential for a realistic prediction of the device on-current and transconductance at the feature sizes here considered. The effect of high-k dielectrics is also addressed. As opposed to planar-gate devices, the electrostatic performance of Si-nanowire and CNT-FETs is not adversely affected by the use of different insulating materials with the same equivalent oxide thickness. As a consequence, not only do high-k dielectrics relieve the gate leakage problem; they also improve the device performance in terms of the gate-control effectiveness over the channel.

Effect of topology on coherent transport through nanotube junctions

In this paper, we study the effects of topology on the coherent single-electron transport through junctions of nanotubes with different chiralities. The junctions are modeled as cylindrical surfaces with variable radius. The quantum dynamics of the particle bound to a curved surface is described by a modified Schrodinger equation depending on two curvilinear coordinates. The transmission coefficients are computed by numerically solving the equation for a number of different geometries. Our results show how the topology of the nanotube structure is reflected onto the coherent transport characteristics of the system.