Artificial particles, superfluid and supersolid will be characterized by the flow of electrons with zero friction, at relatively high temperatures
What are atoms made of? Of course, from protons, neutrons and electrons - no? Well, not necessarily: these are the components of natural atoms, but there are also artificial atoms made of only electrons, with exotic and surprising properties.
What is the question? How is it possible to create artificial atoms made of only electrons?
The field of quantum materials has been at the forefront of physics in recent years. "Quantum materials" are materials in which the surprising properties of quantum particles, often revealed at the level of the individual particle, are expressed as a collective behavior of many particles together. A well-known example of such behavior is superconductivity, i.e. zero electrical resistance, which characterizes certain materials at temperatures close to absolute zero. Another example is superfluidity: there are substances that, when cooled very much, become superfluids, meaning they flow without viscosity and without friction. If we mix a cup of superfluid helium with a spoon, it will continue to circulate forever, unlike the coffee we know from everyday life. Superconductivity and superfluidity have enormous engineering potential, but the need to cool the materials almost to absolute zero is of course a significant limitation
Superconductivity and superfluidity have enormous engineering potential, but the need to cool the materials almost to absolute zero is of course a considerable limitation.
Prof. Ronen Rappaport from the Hebrew University of Jerusalem aims to achieve these properties at higher temperatures, and if the natural particles do not provide the goods - perhaps artificial particles will.
Prof. Rappaport and his group therefore build artificial particles, or quasi-particles. First, a single-atom-thick layer is built from natural materials - such as alloys of semi-conducting materials such as gallium arsenide, or graphene-like semi-conducting materials (graphene is a single layer of the graphite we know from pencils). This layer is a crystal with a fixed and repeating structure, and it has many electrons that originate from the atoms of the crystal. These electrons are not bound to a specific atom, but form a kind of "electron pool".
If we add enough energy to the system, one electron will rise to a higher energy level and thus create two "pools": the higher one has one electron, while the lower one has a "hole", i.e. a lack of an electron. Since the electron has a negative charge, the "hole" acts as a positively charged particle. Positive charge and negative charge are attracted to each other; Let's recall that the simplest atom in nature, the hydrogen atom, is made up of a nucleus with a proton that has a positive electric charge, and one electron that has a negative electric charge. Similar to the positive proton and the negative electron in the hydrogen atom, the electron and the "hole" in our system are attracted to each other, creating a kind of artificial atom, or quasi-atom. But a hydrogen atom can exist almost forever, while the quasi-atom before us is a transient phenomenon: the electron in the high pool tries to return to its place in the low pool, and after a very short time the electron returns to its place while emitting light. Such a system is called an exciton.
These artificial atoms are large and very light, so the collective quantum phenomena appear in them at a relatively high temperature. There is no need to cool the material to a temperature close to absolute zero - it is possible to be satisfied with a few degrees Kelvin, and in the future maybe even room temperature for certain materials.
Is it possible to create an electric dipole in such a system, and if so - what will be the result? To create an electric dipole, the electron at the high energy level must be separated in space from the "hole" from which it came, so that they are in adjacent atomic layers. Such separation creates a layer of electrons and a layer of "holes". Each pair of an electron and a "hole" is a quasi-atom characterized by an electric dipole, and all the dipoles in the system are in the same direction.
Such an artificial system allows scientists to ask new questions about interactions in multi-particle quantum systems. This is a field that challenges physicists today
"Such an artificial system allows us to ask new questions about interactions in multi-particle quantum systems. This is a field that challenges physicists today," says Prof. Rappaport. "We want to test whether we can actually achieve two fascinating theoretical types of materials: a stable superfluid and a supersolid."
The dark side of the force
Electric dipoles, like single electrons, also have a magnetic property called spin (which can be in the "up" or "down" direction). A dipole whose sum of spin, together with its angular momentum ("generalized spin") is 1, can fade and disappear while emitting or absorbing a photon. Therefore it is called "illuminated dipolar exciton". On the other hand, a dipole whose generalized spin is 2 cannot decay through light emission, so it is called a "dark dipolar exciton".
These two types of excitons are able to behave as a collective, in which it is impossible to distinguish between the individual particles. Many excitons together flow without friction at a relatively high temperature - a desirable feature with enormous potential in the construction of computer chips. The obstacle is their stability, or rather, the lack of it: the bright excitons fade and disappear. Apparently, the dark excitons will be more stable and solve the problem; But when they collide with each other, they become bright and fade - so the dark superfluid is also unstable.
Prof. Rappaport's research - which won a research grant from the National Science Foundation - offers a solution to this obstacle: if you create a single layer of dipoles, all in the same direction, the particles repel each other and cannot come close and collide. Using such a system of dark particles, it is probably possible to achieve a stable superfluid.
Flaws flow
A second target is, as you remember, a "supersolid", meaning a crystal that flows like a superfluid. If possible, by analogy with an atomic crystal, produce an ordered crystal of dipoles; And if the structure of such a crystal lacks a "dipole atom" at one point in a million, and a nearby atom "jumps" to fill it, and another atom jumps to fill the vacated position - then a flow of "holes" has been created. A normal crystal consists of atoms, which are heavy and bound in place, so such flow does not occur. Quasi-atoms, on the other hand, are almost weightless, and therefore can flow. If the entire system is cold enough, it is expected to become a superfluid - and therefore the flow will never stop. If the entire system is cold enough, it is expected to become a superfluid - and therefore the flow will never stop
"These are fascinating systems," says Prof. Rappaport. "In addition to their surprising properties, which expand the limits of what we know about the material, they also have enormous practical potential."
Life itself:
Prof. Ronen Rappaport is engaged in rock climbing, and enjoys climbing mountains around the world.
For the article on the Voice of Science website
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Great article