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Mixed-valence complexes are potential candidates for the molecular quantum-dot cellular automata (QCA) approach, which encodes binary information using the configuration of localized and mobile charges. In the Robin–Day classification of mixed-valence complexes, charge localization/delocalization is determined by the ratio of the nuclear reorganization energy and the electron transfer (ET) matrix element. We point out that the driving force for charge localization in molecular QCA is the Coulomb interaction between neighboring molecules rather than nuclear relaxation. This suggests a different criterion for the relevant charge localization, one based on the ET matrix element and the driving bias of a neighboring molecule.

Molecular quantum-dot cellular automata (QCA) paradigm is a promising approach to molecular electronics. QCA cells can be implemented using mixed-valence compounds. However, the existence of counterions can perturb the local electric field and thus is detrimental to information encoding and processing. Here we examine the feasibility of using charge neutral, zwitterionic mixed-valence complex as QCA cells in which the positive and negative charges are found at different yet fixed locations within the molecule and therefore counterion effects are more predictable and controllable. A double-dot model molecule based on the derivative of the 1-carba- closo -decaborate monoanion [ closo -1-CB 9 H 10 ] − is investigated using computational chemistry techniques. The model molecule demonstrates bistability and switchability.

Interaction between single electrons in coupled quantum dots is used to perform binary logic operations in Quantum-dot Cellular Automata (QCA). Clocked control over tunneling is necessary to achieve power gain and minimize power dissipation in QCA. The high temperature limit for clocked operation is set by the ability of the cells to store binary information when the input signal is removed (so-called ‘locked mode’). We present an experimental investigation of the temperature dependence of the locked mode in metal-dot based clocked QCA device. The experimental results are in very good agreement with the orthodox Coulomb blockade theory for thermally activated electron escape mechanism.

The quantum-dot cellular automata (QCA) computational paradigm provides a means to achieve ultimately low limits of power dissipation by replacing binary coding in currents and voltages with single-electron switching within arrays of quantum dots (“cells”). Clocked control over the cells allows the realization of power gain, memory and pipelining in QCA circuits. We present an experimental demonstration of a clocked QCA two-stage shift register (SR) and use it to mimic the operation of a multi-stage SR. Error-bit rates for binary switching operations in a metal tunnel junction device are experimentally investigated, and discussed for future molecular QCAs.

We present experimental demonstration of a basic cell of Quantum-dot Cellular Automata, a transistorless computation paradigm which addresses the issues of device density and interconnection. The devices presented consist of four and six-dot quantum-dot cellular systems where the metal dots are connected by capacitors and tunnel junctions. The operation of a basic cell is confirmed by the externally controlled polarization change of the cell. The cell exhibits a bistable response and voltage gain. We present an experimental technique which cancels the parasitic cross-talk capacitors in the system.

Several practical issues in the development and operation of quantum-dot cellular automata (QCA) cells and systems are discussed. The need for adiabatic clocking of QCA systems and modeling of electrostatic confinement of quantum dots are presented. Experimental data on dot coupling and applications to QCA detectors in a 2-dimensional electron gas (2DEG) are presented. We report a charge detection scheme where we observe strong modulation in the detector signal, in addition to the detector exhibiting minimal effect on the dot being measured. With this investigation, we demonstrate these two key components required for QCA in AlGaAs/GaAs materials, namely dot coupling and charge-state detection.

Recent experiments in transport through a ballistic constriction with a periodical corrugation on one wall to modulate the width [1] have revealed miniband structure in the presence of a magnetic field. We have previously explored the properties of edge states in a similar structure[2]. Here we present our calculation of bandstructure and edge states on a 1D channel having an array of antidots placed along its central axis. Our interest is to examine the allowed states in such a channel and, by direct calculating the particle current density, to visualize the current flow via magnetic edge states as well as localized Landau states.

We study local potential variations due to self-consistent space-charge effects in calculations of coherent transport in narrow split-gate structures. We present a numerical technique based on calculating the Hartree potential from the charge density obtained by solving the two-dimensional effective-mass Schrodinger equation for scattering states, and from the bound charge density obtained from a semi-classical Thomas-Fermi screening model. This method allows us to obtain the local self-consistent potential variations close to scattering centers exposed to an incident flux of electrons.

Quantum-dot Cellular Automata (QCA) is a promising architecture which employs quantum dots for digital computation. It is a revolutionary approach which addresses the issues of device density and power dissipation. With a dot size of 20 nm an entire full adder would occupy only one square micron, and the power delay product is as low as a few kT. A basic QCA cell consists of four quantum dots coupled capacitively and by tunnel barriers. The cell is biased to contain two excess electrons within the four dots, which are forced to opposite "corners" of the four-dot system by Coulomb repulsion. These two possible polarization states of the cell represent logic "0" and "1". Properly arranged, arrays of these basic cells can implement Boolean logic functions. We present experimental results from functional QCA devices built of nanoscale metal dots defined by tunnel barriers.

In electron or atom diffraction experiments on surfaces, the angular shapes of the diffracted beams depend upon the distribution of steps over the surface. In this paper we analyze diffracted beam profiles from stepped surfaces that are reversible. A reversible surface is one in which the pair correlation function over the surface is symmetric with respect to positive and negative directions. We show that the intensity profile across a diffracted beam can be separated into a sharp central spike due to the limit of the correlation function at large separation plus wings or shoulders due to the finite extent of the step disorder. Simple functional expressions for these angular profiles are obtained by a Markov method of treating a one-dimensional geometric distribution of steps. The result explicitly displays the deep structure found for the general case. The method reduces the calculation to a simple eigenvalue problem so that even the continuously changing step distributions that occur in epitaxial growth can be treated easily. As in the general case, the resulting intensity profile is a sharp central spike plus a step-broadened term which now is a sum of Lorentzians. The widths of the Lorentzians are the logarithms of the eigenvalues of the matrix of probabilities which describe the step distribution over the surface. This matrix method, which treats the surface as a Markov chain, also points the correct way to account for correlations between surface atoms for two-dimensional distributions of steps. For a two-dimensional surface one must consider a Markov Random Field as opposed to a simple multiplication of two one-dimensional results. We compare the results of the general calculation to the Si epitaxy experiments of Gronwald and Henzler. The coverage and momentum transfer dependencies of the shapes of the calculated profiles agree with their measurements. The calculation is also applied to the RHEED measurements of Van Hove et al. during GaAs MBE. The measured intensity oscillations can be accounted for by a cyclically changing one-dimensional geometric distribution of steps among three layers in which the third-layer scattering increases with time.

The angular distribution of the diffracted intensity from a crystal surface in a reflection electron diffraction experiment is sensitive to the defect structure of the surface. For a surface with atomic steps, the diffracted beams can be split or broadened with the shape determined by the distribution of steps. For a surface with a finite number of levels we have previously shown that in general the angular profile across a diffracted beam is the sum of a central spike, due to the long range ofder of the surface, plus one or more broad functions, due to the step disorder. The explicit shapes of the broad functions were previously calculated for a one-dimensional geometric model of the disorder that corresponded to the limiting case of non-interacting steps. In this paper, we extend the calculation to treat any one-dimensional distribution of steps which does not depend upon choice of origin and for which the surface consists of either (1) an infinite number of levels, each with identical terrace length distributions or (2) a finite number of levels, but with different distributions allowed. The results of the calculation are compared to reflection high-energy electron diffraction (RHEED) measurements from vicinal and singular GaAs(001) surfaces prepared by molecular beam epitaxy (MBE). For the vicinal surfaces, the staircase step distribution is observed to order on crossing a surface phase transition. The measured profiles agree with a calculation that assumes a minimum terrace length. For the singular surface, steps are observed to appear after the same transition is crossed. These latter profiles cannot be described by a one-dimensional geometric step distribution. The behavior of these two systems are discussed in terms of the increased evaporation rate at temperature above this phase transition.

The orthogonalized plane wave band structures of GeTe, SnTe, PbTe, PbSe, and PbS are fit with a nearest-neighbor, 18-orbital sp3d5, relativistic tight-binding model that exhibits chemical trends. The band gaps of Pb1−xSnxTe, Sn1−yGeyTe, and Ge1−zPbzTe alloys are predicted as functions of compositions x, y, and z. Bowing of the gap is expected to be substantial for Ge1−zPbzTe, and either Sn1−yGeyTe or Ge1−zPbzTe should exhibit a Dimmock reversal.

We investigate the influence of continuum resonances on the release of electrons by a quantum well. We find that the inclusion of resonance effects leads to a decrease in the rate of scattering by non-polar phonons from bound confined states to unbound continuum states.

The current-voltage characteristics for a constriction in a quantum waveguide channel are calculated. The constriction forms an effective barrier which can be employed as a tunneling injector. We find that such a structure may be useful in providing high-energy electrons in a single mode of the waveguide. We also examine the current in the far-from-linear response regime. Away from the linear region the current through the constriction saturates and the conductance falls to zero.

The doping characters of vacancies, the existence of impurity resonances, Fermi-level saturation and de-saturation, the sensitivity of the resonances to host chemistry, the behavior of In as both a donor and an acceptor in Pb1−xSnxTe, and the occurrence of other apparently anomalous valences of impurities in IV–VI semiconductors are shown to be simple and direct consequences of a covalent defect theory, with several of the important defects necessarily assigned to “incorrect” or “anti” sites.

In addition to bound states, quantum wells also produce resonant states in the continuum. While bound states have received considerable attention and are now the basis of device applications, the corresponding virtual states have hardly been studied. Here, we specifically investigate the influence of these resonant states on hot electron transport in quantum wells. We find that the matrix elements which determine scattering rates exhibit structure at the resonant energies. This leads to suppression of scattering by polar optical phonons relative to non-polar optical and acoustic phonon scattering. We discuss the effect that these states have on the capture and release of carriers by the quantum well.