An experiment conducted in the physics department at Bar-Ilan, in the research group of Prof. Bina Kalisky, using a unique sensor developed by the researchers, documented a phenomenon that breaks conventions in the field of the transition of a material from a conductor to an insulator. The observation of the sensitive instrument showed that if you look very closely, you discover that a law that is considered universal ceases to exist
Technology for experimental research that exists in a few laboratories in the world allowed the researchers in Prof. Bina Kalisky's laboratory to show that laws, which until now were considered universal, are broken when they are observed at the microscopic level. When a material approaches a phase transition, i.e. a transition from a state of conductor to a state of insulator, it is expected that the electric current passing through it will have the shape of a fractal, meaning that it will look the same when zoomed in and zoomed out. Fractals with similar structures appear in systems very different from each other, starting with the passage of an electric current through a conductive material, to the percolation of water in the soil or of coffee through a filter, to the progress of a forest fire. The study was published in the scientific journal Nature Communication.
The universal meaning of this law is that the fractal structure is not related to the microscopic properties of the system but to more general things. It can be compared to water seepage that looks the same in two types of soil. The current bypasses obstacles in the shortest way and winds on. Research led by doctoral student Elon Persky, in collaboration with researchers from Delft and Stanford universities, showed that at the microscopic level the universal law may cease to exist.
Persky's experiment was carried out in two-dimensional electronic systems - a very thin layer of material, from which electrons can be gradually subtracted. When the electron density is low enough, electrical conduction stops. This is a well-known method for controlling the conductivity of materials, during which what is known in physics as a "phase transition" takes place, that is, a transition of the material from one behavior - a conducting metal to another behavior - an insulating material. The laboratory researchers expected that at the critical density, in the transition phase between the phases, interesting things would happen. They were right.
The medium through which the current passes in the experiment is an interface between two materials that are a type of ceramic, each one is isolated by itself. For the purpose of observation, create an interface between two different ceramics. The interface is formed by growing individual atomic layers of one material on the other material. The electrons accumulate in the seam line between the two materials and thus a conductive layer is formed. The one-dimensional flow is created because the electrons in the sandwich of the ceramic layers are affected by their crystal structure. The one-dimensional stripes are defects in the crystal. "It's a special type of defect," Persky explains. "You can imagine them like the slits you make in bread before baking and then when it's ready there's a dent in it. These things create a transition that is not universal. This is different from what was expected because it means that the transition depends on the microscopic properties of the system and not just on general properties that are true for many types of materials."
Is electron flow not universal but dependent on the microscopic details of the system?
"The discovery is that in the system we looked at this universality was broken. From a distance the transition looks universal, but up close it doesn't," Persky explains. "We discovered that something else was going on. Instead of a fractal, which is a twisted structure, we saw that the current in many places flows in straight lines. These lines are created because of microscopic defects in the crystal - in the ceramic - that hosts the electrons. This means that the electron flow is not universal but depends on the microscopic details of the system. There is a transition from two-dimensional flow to flow in straight one-dimensional lines." You can imagine it like a leaf floating on water in a stream whose course is natural. He sailed slowly through the fractal windings of the channel, and suddenly reached a straight channel, caused by some disturbance, in which the flow was fast.
"Regarding the types of materials, there were several open questions following our previous experiments and those of others," Persky explains. "By and large, there was a pretty neat picture of how the transition should look as a result of other measurements (for example, from the measurement of the model's resistance), but within this picture there were details that did not add up - for example, different models behave in different ways, each model has its own critical point, depending on its size and more. Our motivation was to try to give a microscopic picture that would explain all these details. And really the discovery - that the transition is not universal and depends on the defects - solves the problem: in all materials the defects are arranged in a different way and it turns out that this greatly affects the transition.
An electric current produces magnetic fields, and a highly sensitive sensor for magnetic fields developed in the Kalisky laboratory takes microscopic images of the magnetic field, from which the image of the electric current could be deduced. "This is an ability that very few groups in the world have, which allows for close and accurate monitoring of the changes in the leading layer up to the critical stage. At any point we can tell how much current has passed. It's something unique that allowed us to reach conclusions," says Persky.
The meaning of the discovery is that if you plan electronic devices that make use of the ceramic layers you have to take into account that one-dimensional flow will take place. It may be possible to exploit the one-dimensional flow to produce components that are more efficient, or with unique properties. On a more general level, this means that each phase transition should be carefully examined at the microscopic level and see if it is universal or if it has unique properties.
In systems such as the progress of fire in a forest fire affected by the density of trees, the percolation of water in the soil affected by the size of the grains or the passage of coffee in a filter that depends on the fibers of the paper may be an area of unexpected and faster flow. "In the forest it is easy to identify, but in systems of electrons it is more complicated. It is not enough to know whether the system is conductive or not - you have to go into the area and photograph the electrons. Microscopic details may change behavior on a global level," Persky concludes.
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