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Register a new userOne-dimensional Silicon and Germanium Nanostructures With No Carbon Analogues
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In this work we report new silicon and germanium tubular nanostructures with no corresponding stable carbon analogues. The electronic and mechanical properties of these new tubes were investigated through ab initio methods. Our results show that the structures have lower energy than their corresponding nanoribbon structures and are stable up to high temperatures (500 and 1000 K, for silicon and germanium tubes, respectively). Both tubes are semiconducting with small indirect band gaps, which can be significantly altered by both compressive and tensile strains. Large bandgap variations of almost 50% were observed for strain rates as small as 3%, suggesting possible applications in sensor devices. They also present high Youngs modulus values (0.25 and 0.15 TPa, respectively). TEM images were simulated to help the identification of these new structures.
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The lifting of the two-fold degeneracy of the conduction valleys in a strained silicon quantum well is critical for spin quantum computing. Here, we obtain an accurate measurement of the splitting of the valley states in the low-field region of interest, using the microwave spectroscopy technique of electron valley resonance (EVR). We compare our results with conventional methods, observing a linear magnetic field dependence of the valley splitting, and a strong low-field suppression, consistent with recent theory. The resonance linewidth shows a marked enhancement above $Tsimeq 300$ mK.
Piezoresistance is the change in the electrical resistance, or more specifically the resistivity, of a solid induced by an applied mechanical stress. The origin of this effect in bulk, crystalline materials like Silicon, is principally a change in the electronic structure which leads to a modification of the charge carriers effective mass. The last few years have seen a rising interest in the piezoresistive properties of semiconductor nanostructures, motivated in large part by claims of a giant piezoresistance effect in Silicon nanowires that is more than two orders of magnitude bigger than the known bulk effect. This review aims to present the controversy surrounding claims and counter-claims of giant piezoresistance in Silicon nanostructures by presenting a summary of the major works carried out over the last 10 years. The main conclusions that can be drawn from the literature are that i) reproducible evidence for a giant piezoresistance effect in un-gated Silicon nanowires is limited, ii) in gated nanowires a giant effect has been reproduced by several authors, iii) the giant effect is fundamentally different from either the bulk Silicon piezoresistance or that due to quantum confinement in accumulation layers and heterostructures, the evidence pointing to an electrostatic origin for the piezoresistance, iv) released nanowires tend to have slightly larger piezoresistance coefficients than un-released nanowires, and v) insufficient work has been performed on bottom-up grown nanowires to be able to rule out a fundamental difference in their properties when compared with top-down nanowires. On the basis of this, future possible research directions are suggested.
Nonlinear electrical properties, such as negative differential resistance (NDR), are essential in numerous electrical circuits, including memristors. Several physical origins have been proposed to lead to the NDR phenomena in semiconductor devices in the last more than half a century. Here, we report NDR behavior formation in randomly oriented graphene-like nanostructures up to 37 K and high on-current density up to 10^5 A/cm^2. Our modeling of the current-voltage characteristics, including the self-heating effects, suggests that strong temperature dependence of the low-bias resistance is responsible for the nonlinear electrical behavior. Our findings open opportunities for the practical realization of the on-demand NDR behavior in nanostructures of 2D and 3D material-based devices via heat management in the conducting films and the underlying substrates.
Recent years have seen the development of several experimental systems capable of tuning local parameters of quantum Hamiltonians. Examples include ultracold atoms, trapped ions, superconducting circuits, and photonic crystals. By design, these systems possess negligible disorder, granting them a high level of tunability. Conversely, electrons in conventional condensed matter systems exist inside an imperfect host material, subjecting them to uncontrollable, random disorder, which often destroys delicate correlated phases and precludes local tunability. The realization of a condensed matter system that is disorder-free and locally-tunable thus remains an outstanding challenge. Here, we demonstrate a new technique for deterministic creation of locally-tunable, ultra-low-disorder electron systems in carbon nanotubes suspended over circuits of unprecedented complexity. Using transport experiments we show that electrons can be localized at any position along the nanotube and that the confinement potential can be smoothly moved from location to location. Nearly perfect mirror symmetry of transport characteristics about the centre of the nanotube establishes the negligible effects of electronic disorder, thus allowing experiments in precision engineered one-dimensional potentials. We further demonstrate the ability to position multiple nanotubes at chosen separations, generalizing these devices to coupled one-dimensional systems. These new capabilities open the door to a broad spectrum of new experiments on electronics, mechanics, and spins in one dimension.
The idea of quantum computation is the most promising recent developments in the high-tech domain, while experimental realization of a quantum computer poses a formidable challenge. Among the proposed models especially attractive are semiconductor based nuclear spin quantum computers (S-NSQC), where nuclear spins are used as quantum bistable elements, qubits, coupled to the electron spin and orbital dynamics. We propose here a scheme for implementation of basic elements for S-NSQCs which are realizable within achievements of the modern nanotechnology. These elements are expected to be based on a nuclear-spin-controlled isotopically engineered Si/SiGe heterojunction, because in these semiconductors one can vary the abundance of nuclear spins by engineering the isotopic composition. A specific device is suggested, which allows one to model the processes of recording, reading and information transfer on a quantum level using the technique of electrical detection of the magnetic state of nuclear spins. Improvement of this technique for a semiconductor system with a relatively small number of nuclei might be applied to the manipulation of nuclear spin qubits in the future S-NSQC.
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