Mahfuza Khatun

A Scalable Signal Distribution Network for Quantum-Dot Cellular Automata

The authors describe a signal distribution network (SDN) for quantum-dot cellular automata (QCA) devices. This network allows the distribution of a set of inputs to an arbitrary number of combinational functions, overcoming the challenges associated with wire crossings that have faced QCA systems for many years. As an additional benefit, the proposed SDN requires only four distinct clock signals, regardless of the number of inputs or outputs, and those clock signals each repeat a very simple pattern. Furthermore, this network is highly scalable, completing the distribution of inputs to an arbitrary number of distributed signals and an arbitrary number of outputs in 4 - 2 clock cycles. To illustrate its operation, the authors apply the SDN to a two-input/one-output exclusive OR operation, a three-input/two-output full adder, and a four-input/four-output multiplier.

Fault tolerance properties in quantum-dot cellular automata devices

We present a study of the joint influence of temperature and fabrication defects on the operation of quantum-dot cellular automata (QCA) devices. Canonical ensemble, a Hubbard-type Hamiltonian and the inter-cellular Hartree approximation were used, and a statistical model has been introduced to simulate defects in the QCA devices. Parameters such as success rate and breakdown displacement factor (BDF) were defined and calculated numerically. Results show the thermal dependence of BDF values of the QCA devices. The BDF values decrease with temperature. The joint influence of randomly missing dots and temperature was also studied.

Fault tolerance calculations for clocked quantum-dot cellular automata devices

We present a numerical study of fault tolerance properties in quantum-dot cellular automata (QCA) devices. A full-basis quantum method is used for calculations of the Hamiltonian, and a statistical model has been introduced to simulate the influence of position defects of the dots within cells on the logical output. Combined effects of temperature and cell defects on a shift register have been studied. Uniform and normal distributions have been used for the cell defect simulations. Normal distribution simulations produce realistic results compared to the uniform distribution. In order to show the operational limit of a device, parameters such as “displacement factor” and “success rate” are introduced. Results show that the fault tolerance of a QCA device is strongly dependent on temperature as well as on the cell defects. The robustness of a shift register is also dependent on the size of the device.

Random cellular structures generated from a 2D Ising ferromagnet

Topological models of 2D cellular structures are associated with distributions of spins given by a ferromagnetic Ising model with nearest-neighbour interactions on a square lattice. Every vertex of a square lattice is topologically unstable as it belongs to more than three polygons. There are two ways, associated with the spin states, to remove this degeneracy by replacing every vertex by one side. Topological properties such as the distribution P(n) of the number n of cell sides, the two-cell correlation Mk(n)=P(k)Akn which is the average number of k-sided neighbours of a cell with n sides (n-cell), the mean number m(n) of sides of the first neighbour cells of n-cells are characterized as a function of T/Tc, by direct calculations and by Monte Carlo and Q2R simulations. The correlations Akn of a biological tissue are compared with the Akn of a ferromagnetic Ising cellular structure and with the Akn expected from the maximum-entropy model.

Fault-tolerance and Thermal characteristics of quantum-dot cellular automata devices

We present fault tolerant properties of various quantum-dot cellular automata (QCA) devices. Effects of temperatures and dot displacements on the operation of the fundamental devices such as a binary wire, logical gates, a crossover, and an exclusive OR (XOR) have been investigated. A Hubbard-type Hamiltonian and intercellular Hartree approximation have been used for modeling, and a uniform random distribution has been implemented for the defect simulations. The breakdown characteristics of all the devices are almost the same except the crossover. Results show that the success of any device is significantly dependent on both the fabrication defects and temperatures. We have observed unique characteristic features of the crossover. It is highly sensitive to defects of any magnitude. Results show that the presence of a crossover in a XOR design is a major factor for its failure. The effects of temperature and defects in the crossover device are pronounced and have significant impact on larger and complicate...

Quasi-adiabatic clocking of quantum-dot cellular automata

We present a theoretical study of quasi-adiabatic clocking of quantum-dot cellular automata (QCA). Quasi-adiabatic clocking refers to periodical modulation of interdot potential barriers in order to keep the cells of a QCA device near their ground state throughout the entire switching process. The barrier modulation has been studied through the use of a trapezoidal-shaped, periodic, time-dependent, electric field. The time-dependent electric field has been calculated for arrays of linear charged rods. A continuous traveling maximum in the electric field represents the flow of information from one zone to the next. For a QCA device where the zones are set up, such that the flow of information is linear, a line of electrostatically charged rods can quasi-adiabatically clock the system.

Quantum conductance of zigzag graphene oxide nanoribbons

The electronic properties of zigzag graphene oxide nanoribbons (ZGOR) are presented. The results show interesting behaviors which are considerably different from the properties of the perfect graphene nanoribbons (GNRs). The theoretical methods include a Huckel-tight binding approach, a Greens function methodology, and the Landauer formalism. The presence of oxygen on the edge results in band bending, a noticeable change in density of states and thus the conductance. Consequently, the occupation in the valence bands increase for the next neighboring carbon atom in the unit cell. Conductance drops in both the conduction and valence band regions are due to the reduction of allowed k modes resulting from band bending. The asymmetry of the energy band structure of the ZGOR is due to the energy differences of the atoms. The inclusion of a foreign atoms orbital energies changes the dispersion relation of the eigenvalues in energy space. These novel characteristics are important and valuable in the study of quantum transport of GNRs.

Quantum transport anomalies in semiconductor nanosystems

We present quantum transport anomalies in the theoretical conductance of various semiconductor nanostructures. We first investigate a quantum channel with a chain of quantum boxes connected by slits, called a superlattice structure, and study the miniband and minigap effects associated with resonances and anti‐resonances in the conductance. We also report studies of electron transport in a quantum wire containing series or parallel slits and a detector slit. In these systems, strong conductance oscillations due to quantum interference effects are predicted as a detector slit is moved across the wire. In the case of a single and multi‐series slits, we attribute these effects to multiple reflections of the phase‐coherent electron along the quantum wire. The transmission coefficients and electronic phase shifts are examined, which provide insights into the origins of these conductance oscillations. In the case of multi‐parallel slits, peaks with two‐ (four‐) fold splitting in the conductance are exhibited du...

Quantum calculation of thermal effect in quantum‐dot cellular automata

We present a theoretical study of thermal effect in quantum-dot cellular automata (QCA). An extended Hubbard-type model for the Hamiltonian of the QCA arrays, and canonical distribution were used to obtain thermal average of polarization for the QCA cells. A full-basis quantum method has been used for the calculation of response function for a two- and a three-cell array system. Each cell is composed of four dots located at the corners of the cells. Results show that the nonlinear behavior of the response function functions decay with the temperature as well as with the number of cells in the array.

Exact two-cell correlations in random cellular structures generated from a 2D Ising ferromagnet

Random 2D cellular structures are generated from distributions of spins given by a ferromagnetic Ising model with nearest-neighbour couplings on a square lattice. Topological correlations between pairs of neighbouring cells, are expressed as rational functions of even-number spin correlations, involving two, four and six spins, which are exactly known for the square Ising model. The latter correlations have been calculated numerically in the vicinity of the Curie point. They agree with computer simulation results.