Can sound wave remove electrons from atoms
In one-dimensional electron systems, the movement of electrons is restricted to one spatial direction. A new phenomenon occurs: the electrons lose their “identity” at low temperatures.
In such one-dimensional systems at low temperatures there are only collective excitations of the electrons that resemble sound waves and can be understood as charge or spin “sound waves”. Spin and charge then move completely independently of one another at different speeds, although they are actually inextricably linked with the “normal” electrons. This astonishing fact is called spin-charge separation in a Luttinger liquid, named after the discoverer J. M. Luttinger.
Such one-dimensional structures can be produced with the help of various techniques. For example, electron beams can be used to process the surfaces of layer semiconductors in such a way that the electrons in a layer near the surface can only move in one direction parallel to the surface. But nature also provides us with one-dimensional electronic materials: carbon nanotubes are tubular aggregates of carbon and also have the characteristic properties of one-dimensional electrical conductors with great application potential.
In this context, the metallic nanowires, which have become the focus of interest due to the requirements of technical applications, are also very interesting. Nowadays, on mass-produced memory chips, metallic conductor tracks that are less than two hundred nanometers wide and thin are used to electrically connect the individual components. For comparison: a human hair is roughly fifty thousand times as thick. Any further downsizing of these wires will have ramifications for both their manufacture and their electronic properties as we move into the area where quantum effects become important.
Lithographically produced nano bridge
In the transport of electricity through small metallic structures, diverse new phenomena occur, which essentially have two causes. The first is the quantization of the charge, the elementary charge of the electron being the smallest unit. The phenomena based on this are summarized under the term charge effects. An important electronic component, the functionality of which is based on charge quantization, is the single-electron transistor. The circuits of the single charge electronics contain a small metallic island as an essential component, which is only weakly coupled to the environment. The small size of the island means that a large amount of electrostatic energy has to be applied in order to bring an additional electron onto the island. This makes it possible to control the number of electrons on the island. The attraction of single charge electronics is to store information with the help of individual electrons. You can therefore work with very low currents and, as a result, with very low energy losses.
Single-electron transistors have been studied for about ten years, and the first technical applications have been developed, for example as a highly sensitive electrometer for measuring charges. An extension is the single electron pump, in which several separately controllable islands are arranged one behind the other. Above all, it has metrological applications: One is considering introducing the single electron pump as a new current standard with which the unit of electrical current, the ampere, can be determined much more precisely than before. The three electrical units of the international system of units (SI), namely the ohm (determined by the quantum Hall effect), the volt (determined by the Josephson effect) and the ampere, would be completely reduced to natural constants.
The second cause of new phenomena in electronic transport in nanosystems is the wave nature of the electrons. It can be observed directly if the quantum mechanical coherence of the electrons is maintained across the dimensions of the samples, i.e. if the phase of the electrons remains intact. Interference then occurs between the various orbits that an electron can travel through to get to its target. When the electron waves are in phase, they constructively interfere. However, if they are out of phase, they can cancel each other out. In principle, interference is always present, even in macroscopic solids. However, they usually remain unobservable there, since their consequences are averaged out due to the large number of possible paths and phase positions. In metallic nanostructures, however, the number of possible paths is limited. Therefore, the interference at low temperatures, if the coherence conditions are observed, can be observed as an increase or decrease in the electrical resistance.
The wave properties of the electrons are also expressed in the fact that they cannot be transmitted through structures of any size without resistance. One consequence of this is the conductance quantization. Analogous to hollow waveguides such as glass fibers, which only transmit certain "oscillation modes" of the light wave field, a narrow constriction in a metallically conductive system only allows certain modes of the electron wave field to pass through.
The decisive factor here is the relationship between the wavelength of the electrons and the size of the constriction, which is called point contact. The wavelength depends on the properties of the metal according to the rule of thumb: the fewer electrons, the greater the wavelength. That is why the phenomenon of conductance quantization was first discovered in two-dimensional electron systems, which can arise at the interface between two semiconductor layers and which contain relatively few electrons, which leads to a wavelength of around 200 to 500 nanometers. At a point contact that had an initial width of about 250 nanometers, BJ van Wees and coworkers from Delft University of Technology (Netherlands) and D. Wharam and coworkers from Cambridge University (England) observed that the electrical conductance - the reciprocal of the resistance - only in multiples of the conductance quantum e2/ h occurs.
Elemental metals such as copper, aluminum or lead contain so many electrons that their wavelength is roughly the same as the distance between the atoms in the solid. An observable effect of the conductance quantization is only expected for “wires” or contacts that are only a few atoms thick. Even if the production of such tiny structures is extremely difficult, it is by no means impossible! As early as 1990, Don Eigler and colleagues at IBM in San Jose (California) succeeded in pushing atoms to a desired position with a fine metal tip on an atomically smooth surface. If you approach an atom or molecule with a tip, either attractive or repulsive forces act between this particle and the tip, depending on the materials involved and the distance between the two. If the particle is now on a smooth surface and it is not held too tightly in a certain place by its chemical bond to the base, it can be moved anywhere on the surface with the help of the tip. This enables atomic nanostructures to be created.
With further miniaturization, electronic circuits in computer chips will soon contain functional elements that only consist of a few atoms. This requires a fundamental change in both the technology and the physical description: the previously used structuring "from above", i.e. the successive reduction in size of a larger initial structure, will possibly give way to a structure "from below" in which the desired component atom for Atom composed. The question arises as to how the current flow works in such small structures. This gives rise to further questions: "Which atomic physical and chemical properties determine the transport of electricity when atoms are assembled into conductive structures?" and "Which atoms have to be put together and how in order to achieve the desired electronic properties?" To find answers to these questions, one investigates single-atom contacts. Such contacts can be made using Eigler's methods or using self-supporting metallic nano bridges, which are thinned out by pulling so that they only contain one atom at their thinnest point (see Fig. 4). Even atomically thin chains, in which the atoms are lined up like a string of pearls, can be produced in this way.
One-atom contacts can carry a current of up to ten microamps. This may seem like little, but it is more than the typical amperage in today's semiconductor chips. Compared to their tiny size, atoms are excellent conductors. If a standard household copper cable with a cross-section of one and a half square millimeters and approved for sixteen amperes were to transport as much current per area as a one-atom contact, this would correspond to a current of four hundred million amperes! This comparison also clearly shows that the flow of current on an atomic scale obeys different laws than that through a macroscopic solid.
The investigations described are a first step towards using less atomic components in the electronics of the future. An interesting aspect is that a single atom can be used to determine the properties of a macroscopic circuit. Before speculating about the specific application possibilities of such components, however, one must first solve some fundamental problems that arise "from below" during construction, such as the chemical stability of individual atoms or precise control of the atomic positions.
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