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It is well known that the regions of aligned spins (magnetic domains) in a ferromagnet can influence the electrical properties of the material, so control of domain structure is a promising avenue for manipulating the spins of electrons in the emerging field of 'spintronics'. Less attention has been paid to the domain structure of antiferromagnets, where neighbouring magnetic spins are opposed rather than aligned (these domains have no net magnetism and so are difficult to probe). Now Ravi Kummamura and Yeong-Ah Soh show that pronounced spin-related effects can also be seen in the electrical properties of the archetypical antiferromagnet, chromium. As the effects are at least as large as those seen in ferromagnets, they might to lead to new applications for chromium, already used in integrated circuits and disk drives. The regions of aligned spins (magnetic domains) in a ferromagnet can influence the electrical properties of the material, but less is known about the domain structure of antiferromagnets, where neighbouring magnetic spins are opposed, rather than aligned. Pronounced spin-related effects can be seen in the electrical properties of the archetypical antiferromagnet, chromium. As the effects are at least as large as those seen in ferromagnets, they might prove of practical value. The role of magnetic domains (and the walls between domains) in determining the electrical properties of ferromagnetic materials1 has been investigated in great detail for many years, not least because control over domains offers a means of manipulating electron spin to control charge transport in ‘spintronic’ devices2. In contrast, much less attention has been paid to the effects of domains and domain walls on the electrical properties of antiferromagnets: antiferromagnetic domains show no net external magnetic moment, and so are difficult to manipulate or probe. Here we describe electrical measurements on chromium—a simple metal and quintessential spin density wave antiferromagnet3—that show behaviour directly related to spin density wave formation and the presence of antiferromagnetic domains. Two types of thermal hysteresis are seen in both longitudinal and Hall resistivity: the first can be explained by the quantization of spin density waves due to the finite film thickness (confirmed by X-ray diffraction measurements) and the second by domain-wall scattering of electrons4,5. We also observe the striking influence of the electrical lead configuration (a mesoscopic effect) on the resistivity of macroscopic samples in the spin density wave state. Our results are potentially of practical importance, in that they reveal tunable electrical effects of film thickness and domain walls that are as large as the highest seen for ferromagnets6,7.

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.

Quantum-Dot Cellular Automata (QCA) is a computational scheme utilizing the position of interacting single electrons within arrays of quantum dots ("cells") to encode and process binary information. Clocked QCA architectures can provide power gain, logic level restoration, and memory features. Using arrays of micron-sized metal dots, we experimentally demonstrate operation of a QCA latch-inverter and a two-stage shift register.

We present an experimental demonstration of power gain in quantum-dot cellular automata (QCA) devices. Power gain is necessary in all practical electronic circuits where power dissipation leads to decay of logic levels. In QCA devices, charge configurations in quantum dots are used to encode and process binary information. The energy required to restore logic levels in QCA devices is drawn from the clock signal. We measure the energy flow through a clocked QCA latch and show that power gain is achieved.

Magnetic domains and the walls between are the subject of great interest because of the role they play in determining the electrical properties of ferromagnetic materials and as a means of manipulating electron spin in spintronic devices. However, much less attention has been paid to these effects in antiferromagnets, primarily because there is less awareness of their existence in antiferromagnets, and in addition they are hard to probe since they exhibit no net magnetic moment. In this paper, we discuss the electrical properties of chromium, which is the only elemental antiferromagnet and how they depend on the subtle arrangement of the antiferromagnetically ordered spins. X-ray measurement of the modulation wavevector Q of the incommensurate antiferromagnetic spin-density wave shows thermal hysteresis, with the corresponding wavelength being larger during cooling than during warming. The thermal hysteresis in the Q vector is accompanied with a thermal hysteresis in both the longitudinal and Hall resistivity. During cooling, we measure a larger longitudinal and Hall resistivity compared with when warming, which indicates that a larger wavelength at a given temperature corresponds to a smaller carrier density or equivalently a larger antiferromagnetic ordering parameter compared to a smaller wavelength. This shows that the arrangement of the antiferromagnetic spins directly influences the transport properties. In thin films, the sign of the thermal hysteresis for Q is the same as in thick films, but a distinct aspect is that Q is quantized.

We present an experimental demonstration of a latch in a clocked quantum-dot cellular automata (QCA) device. The device consists of three floating micron-size metal dots, connected in series by multiple tunnel junctions and controlled by capacitively coupled gates. The middle dot acts as an adjustable barrier to control single-electron tunneling between end dots. The position of a switching electron in the half cell is detected by a single-electron electrometer. We demonstrate “latching” of a single electron in the end dots controlled by the gate connected to the middle dot. This ability to lock an electron in a controllable way enables pipelining, power gain and reduced power dissipation in QCA arrays.

We present the experimental characterization of a leadless (floating) double-dot system and a leadless quantum-dot cellular automata cell, where aluminum metal islands are connected to the environment only by capacitors. Here, single electron charge transfer can be accomplished only by the exchange of an electron between the dots. The charge state of the dots is monitored using metal islands configured as electrometers. We show improvements in the cell performance relative to leaded dots, and discuss possible implications of our leadless design to the quantum-dot cellular automata logic implementation.

A device representing a basic building block for clocked quantum-dot cellular automata architecture is reported. Our device consists of three floating micron-size metal islands connected in series by two small tunnel junctions where the location of an excess electron is defined by electrostatic potentials on gates capacitively coupled to the islands. In this configuration, the middle dot acts as an adjustable Coulomb barrier allowing clocked control of the charge state of the device. Charging diagrams of the device show the existence of several operational modes, in good agreement with theory. The clocked switching of a single electron is experimentally demonstrated and advantages of this architecture are discussed.

We characterize and compare the reaction of alkanethiol with Ag continuous planar thin films and Ag islands on inert substrates. Ag islands generate a significantly larger (3-fold) amount of alkanethiolate than continuous Ag films at comparable conditions. The reaction with planar Ag thin films produces alkanethiol self-assembled monolayers (SAMs), whereas the reaction with Ag islands yields two dissimilar products depending on the size of the islands. Small Ag islands are more likely to be converted into multilayer silver-alkanethiolate (AgSR) crystals, while larger Ag islands form monolayer-protected clusters (MPCs). The AgSR lamellar crystals are initially small having only a few layers. However, during thermal annealing, ripening occurs that generates large AgSR lamellae having diameters of 1 microm and thickness up to 30 layers. Atomic force microscopy shows the single-layer step-heights of individual crystals which match the layer thickness obtained via X-ray diffraction analysis. The crystals have facets and flat terraces with extended area, and have a strong preferred orientation (010) normal to the substrate surface. The MPCs move laterally upon annealing and reorganize into a single-layer network with their separation distance approximately equal to the length of an extended alkyl chain.

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.

Quantum-dot Cellular Automata (QCA) is a device architecture that uses the position of electrons in quantum-dot arrays to implement digital logic. We present the experimental demonstration of a two-stage QCA shift register and an analysis of errors encountered in its operation.

An overview is given of the QCA architecture, along with a summary of experimental demonstrations of QCA devices. Quantum-dot cellular automata (QCA) is a transistorless computation paradigm that addresses such challenging issues as device and power density. The basic building blocks of the QCA architecture, such as AND, OR gates and clocked cells have been demonstrated and are presented. The quantum dots used in the experiments are metal islands that are coupled by capacitors and tunnel junctions. An improved design of the cell is presented in which all four dots of the cell are coupled by tunnel junctions. A noninvasive electrometer is presented which improves the sensitivity and linearity of dot potential measurements. The operation of this basic cell is confirmed by an externally controlled polarization change of the cell.

Presents an experimental demonstration of two novel clocked QCA devices - a QCA latch and a QCA shift register. We demonstrate the operation of the devices, and discuss sources and methods of lowering the digital errors in QCA clocked devices.

Discusses an experiment that demonstrates power gain in a quantum-dot cellular automata (QCA) shift register. Power gain is essential in any electronic system for the restoration of logic levels. The clock signal plays an important role in providing power gain in QCA devices as it can be used as a source of energy for the system. We discuss how this can be achieved in a QCA shift register and experimentally demonstrate a power gain greater than unity.

We present an experimental demonstration of novel Quantum-dot Cellular Automata (QCA) devices based on clocked architecture -- a QCA latch and a two-bit QCA shift register. The operation of the devices is demonstrated, and sources of the digital errors occurring in clocked QCA devices are discussed.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An overview is given of the quantum-dot cellular automata (QCA) architecture, along with a summary of experimental demonstrations of QCA devices. QCA is a transistorless computation paradigm that can provide a solution to such challenging issues as device and power density. The basic building blocks of the QCA architecture, such logic gates and clocked cells have been demonstrated. The experiments to date have used QCA cells composed of metallic islands, and operate only at low temperatures. For QCA to be practical, the operating temperature must be raised, and molecular implementations are being investigated that should yield room temperature operation.

We have investigated the stability and solidification of nanometer size Au-Si droplets using an ultrafast heating/cooling nanocalorimetry and in situ growth techniques. The liquid can be supercooled to very low temperatures for both Au-rich (ΔT ∼ 95 K) and Si-rich (ΔT ∼ 220 K) samples. Solidification of a unique metastable phase δ1 is observed with a composition of 74 ± 4 at. % Au and a b-centered orthorhombic structure (a = 0.92, b = 0.72, and c = 1.35 nm; body-center in the a-c plane), which grows heteroepitaxially to Aus. Its melting temperature Tm is 305 ± 5 °C. There is competition during formation between the eutectic and δ1 phases but δ1 is the only metastable alloy observed. For small size droplets, both the δ1 and eutectic phases show considerable depression of the melting point (size-dependent melting).

The total enthalpy of reaction and heat capacity to 850 °C were measured using differential scanning nanocalorimetry (nano-DSC) for the reaction of a nickel and silicon bilayer at heating rates up to 106 K/s. Exothermic dips in heat capacity attributed to nickel silicide formation were found along with indications of phase changes at 430 and 550 °C. The postreaction phases were identified using electron backscattered diffraction. Samples with a Ni:Si molar ratio of 1.2 heated to 850 °C were a mixture of orthorhombic NiSi and the θ-phase (hexagonal—Ni2Si); samples heated to 790 °C resulted in predominantly NiSi.

In this paper we report on the fabrication of a close proximity shadow mask designed for sputtering into cavities or onto the back surface of freestanding silicon nitride (SiNx) membranes. Sputtering into a well-defined area on a fragile surface is difficult since sputter deposition through a shadow mask separated from the deposition surface typically results in spreading of the deposited material. The area of spreading beyond the desired area of deposition depends on the vertical separation between the shadow mask and the surface of the membrane. In our design, a high degree of accuracy (±5 µm) in the separation (25 µm) between the shadow mask and the deposition surface is achieved. The shadow mask is made from SiNx-coated silicon wafers, using potassium hydroxide (KOH) etching on both sides of the wafer. The design rules chosen to maintain accuracy of the fit between the shadow mask and the deposition surface over various etch conditions, fabrication and methods used for convex-corner compensation and the alignment to the wafer crystal axis, which also contribute to an accurate fit, are discussed. Spreading of deposited material due to sputtering is limited to about 40 µm when this shadow mask is used.

Devices based on the quantum-dot cellular automata (QCA) computational approach (Lent et al, 1993) use interacting quantum dots to encode and process binary information. In this transistorless approach to computation, logic levels are represented by the configurations of single electrons in coupled quantum-dot systems. In the last few years, significant progress has been made towards the realization of basic QCA elements. However, in these devices, power gain needed for the operation of large QCA arrays was not possible since the only source of energy was the signal input. Recent theoretical work (Lent and Tougaw, 1997) proposed clocked control of the QCA circuitry. Clocked controlled QCA systems have many advantages such as power gain, reduced power dissipation, and pipelined architectures. The original theoretical work applied only to semiconductor implementation of clocked QCA arrays, but recently a scheme for clocked control of metallic QCA cells was proposed (Toth and Lent, 1999; Korotkov and Likharev, 1998). Here an extra dot placed between the two dots of the QCA half-cell acts as a tunable barrier controlled by the clock signal. We present the experimental demonstration of a clocked QCA cell. The device consists of two capacitively coupled half-cells, where each half-cell consists of three micron-size Al islands separated by tunnel junctions, and four electrometers to measure the charge state of the half-cells. The half-cells are leadless, with no DC connection to the environment.