There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Carrier concentration profiles of two-dimensional electron gases are investigated in wurtzite, Ga-face AlxGa1−xN/GaN/AlxGa1−xN and N-face GaN/AlxGa1−xN/GaN heterostructures used for the fabrication of field effect transistors. Analysis of the measured electron distributions in heterostructures with AlGaN barrier layers of different Al concentrations (0.15<x<0.5) and thickness between 20 and 65 nm demonstrate the important role of spontaneous and piezoelectric polarization on the carrier confinement at GaN/AlGaN and AlGaN/GaN interfaces. Characterization of the electrical properties of nominally undoped transistor structures reveals the presence of high sheet carrier concentrations, increasing from 6×1012 to 2×1013 cm−2 in the GaN channel with increasing Al-concentration from x=0.15 to 0.31. The observed high sheet carrier concentrations and strong confinement at specific interfaces of the N- and Ga-face pseudomorphic grown heterostructures can be explained as a consequence of interface charges induced by ...

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Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures

Electrons have a charge and a spin, but until recently these were considered separately. In classical electronics, charges are moved by electric fields to transmit information and are stored in a capacitor to save it. In magnetic recording, magnetic fields have been used to read or write the information stored on the magnetization, which 'measures' the local orientation of spins in ferromagnets. The picture started to change in 1988, when the discovery of giant magnetoresistance opened the way to efficient control of charge transport through magnetization. The recent expansion of hard-disk recording owes much to this development. We are starting to see a new paradigm where magnetization dynamics and charge currents act on each other in nanostructured artificial materials. Ultimately, 'spin currents' could even replace charge currents for the transfer and treatment of information, allowing faster, low-energy operations: spin electronics is on its way.

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The emergence of spin electronics in data storage

The fabrication methods of the microelectronics industry have been refined to produce ever smaller devices, but will soon reach their fundamental limits. A promising alternative route to even smaller functional systems with nanometre dimensions is the autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces. This approach combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials.

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Engineering atomic and molecular nanostructures at surfaces

“Spintronics,” in which both the spin and charge of electrons are used for logic and memory operations, promises an alternate route to traditional semiconductor electronics. A complete logic architecture can be constructed, which uses planar magnetic wires that are less than a micrometer in width. Logical NOT, logical AND, signal fan-out, and signal cross-over elements each have a simple geometric design, and they can be integrated together into one circuit. An additional element for data input allows information to be written to domain-wall logic circuits.

http://science.sciencemag.org/content/sci/309/5741/1688.full.pdf

Magnetic Domain-Wall Logic

We report on dual-gate reflectometry in a metal–oxide–semiconductor double-gate silicon transistor operating at low temperature as a double quantum dot device. The reflectometry setup consists of two radio frequency resonators respectively connected to the two gate electrodes. By simultaneously measuring their dispersive responses, we obtain the complete charge stability diagram of the device. Electron transitions between the two quantum dots and between each quantum dot and either the source or the drain contact are detected through phase shifts in the reflected radio frequency signals. At finite bias, reflectometry allows probing charge transitions to excited quantum-dot states, thereby enabling direct access to the energy level spectra of the quantum dots. Interestingly, we find that in the presence of electron transport across the two dots the reflectometry signatures of interdot transitions display a dip-peak structure containing quantitative information on the charge relaxation rates in the double q...

Level Spectrum and Charge Relaxation in a Silicon Double Quantum Dot Probed by Dual-Gate Reflectometry

We report direct measurements of the charging diagram of a nanoscale series double-dot system at low temperatures. Our device consists of two metal dots in series, with each dot capacitively coupled to another single dot serving as an electrometer. This configuration allows us to externally detect all possible charge transitions within a double-dot system. In particular, we show that transfer of an electron between two dots, representing a polarization switch of the double dot, can be most prominently detected by our differential sensing scheme. We also perform theoretical calculations of the device characteristics and find excellent agreement with experiment. We discuss possible applications as an output stage for quantum-dot cellular automata architecture.

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External charge state detection of a double-dot system

According to Landauer’s principle, dissipation of energy is only necessary when information is erased, suggesting that vastly more efficient logical switches than transistors are possible. However, an influential analysis of binary switching suggests that representing information with electric charge is the root of the problem, that Landauer’s principle is fundamentally flawed, and that any movement of charge, such as charging a capacitor, must dissipate at least kBT ln(2). Here, using a RC circuit, an energy loss of much less than kBT ln(2) is demonstrated while delivering energy of 100 kBT ln(2) to the capacitor. This shows that there is no fundamental lower limit to energy dissipation in moving charge.

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Fundamental limits of energy dissipation in charge-based computing

Nanomagnets that exhibit two distinct stable states of magnetization can be used to store digital bits. This phenomenon is already applied in today’s magnetic random access memories (MRAM). In addition, interacting networks of such nanomagnets, with physical spacing on the order of 10 nm between them, have been proposed to propagate and process binary information by means of magnetic coupling. The application of field-coupled nanomagnets for digital logic circuits was described in a concept called magnetic quantumdot cellular automata (MQCA) [1][2]. MQCA offer very low power dissipation and high integration density of functional devices. In addition, it can operate over a wide temperature range from near absolute zero to the Curie temperature of the employed ferromagnetic material. We introduce a logic gate similar to that proposed by Parish and Forshaw [3], which performs majority-logic operation. The nanomagnets are arranged in a cross-geometry as shown in Fig. 1, where the dipole coupling between the nanomagnets produces ferromagnetic and antiferromagnetic ordering of the magnetic states. Consider that the magnetic state of nanomagnets A, B, and C can be set by some inputs, and that a horizontal external magnetic field, called the clock-field, can allow the system to relax to its ground state. Then, majority logic operation can be performed by the central nanomagnet M, and the result can be transferred to another nanomagnet labeled as “out”. The cross-geometry can be extended, and inputs can be provided by adding nanomagnets that are oriented along the clock-field. Varying the position of these horizontally elongated nanomagnets, all eight input combinations in the majority-logic truth table can be tested. Figure 1. (a) An elongated polycrystalline NiFe alloy nanomagnet exhibits two stable magnetic states in the direction of its longest axis. (b) Majority gate geometry built up from nanomagnets. We demonstrate room temperature operation of majority gates made of NiFe alloy and fabricated by electron-beam lithography on silicon. Dipolar ordering in the nanomagnetnetworks is imaged by magnetic force microscopy (MFM), and the operation is explained by means of micromagnetic simulations. Figure 2 introduces a particular majority gate, in which the magnetic state of A is set to be the opposite to that of B and C. In this gate, A, B, and C all exert torque on the central dot’s magnetic moment in the same direction. Simulations show that, as the clock-field is reduced, the switching of the nanomagnets inside the gate begins at the input dots. The central dot switches after A, B, and C, and the switching propagates along the antiferromagnetically-coupled chain to the right. The figure demonstrates the final states after two independent experiments, in which the clock-field was applied in opposite directions. The MFM data shows the correct alignment of the magnetic moments in both cases.

Alexei O. Orlov

Papers

Level Spectrum and Charge Relaxation in a Silicon Double Quantum Dot Probed by Dual-Gate Reflectometry

External charge state detection of a double-dot system

Fundamental limits of energy dissipation in charge-based computing

Magnetic Logic Devices Based on Field-Coupled Nanomagnets

Experimental demonstration of electron switching in a quantum-dot cellular automata (QCA) cell