New findings present a more accurate picture of electron flow in quantum states

The phrase "on the edge" takes on a new meaning when it comes to experiments in the quantum states of matter. Weizmann Institute of Science scientists studied electrons moving around the edges of a unique system. Their measurements - the most accurate and sensitive ever conducted - revealed new details about the behavior of electrons at the quantum level. These findings were recently published in the scientific journal Nature

The phrase "on the edge" takes on a new meaning when it comes to experiments in the quantum states of matter. Weizmann Institute of Science scientists studied electrons moving around the edges of a unique system. Their measurements - the most accurate and sensitive ever conducted - revealed new details about the behavior of electrons at the quantum level. These findings were recently published in the scientific journal Nature.

The scientists, from the laboratory of Prof. Eli Zaldov from the Department of Condensed Matter Physics at the institute, conducted the experiments together with members of the research group of Prof. Andrei Geim from the University of Manchester. Prof. Geim is one of the inventors of graphene - a two-dimensional lattice of carbon atoms - an achievement that won him and his partner, Prof. Konstantin Novoslov, the Nobel Prize in Physics for 2010. Graphene is of central importance in the study of the quantum Hall effect, due to the fact that, in addition to being a conductor of electricity, It is also free of impurities - which allows excellent electron mobility, along with low energy loss.

Prof. Zeldov and the research group conducted the measurements using a unique system developed in the laboratory, based on superconducting devices for quantum interference (SQUID). The tiny system consists of a thin quartz needle coated with superconductors, with a tiny ring at the end that scans the properties of the material very close to the surface, to display various phenomena on the nano scale.

In a previous experiment conducted in collaboration with Prof. Geim's group, with graphene layers cooled to very low temperatures, the scientists showed that their experimental system is so sensitive that it is able to measure the tiniest of thermal signals: heat released by electrons in their flow as a result of "car accidents" - collisions with atomic defects isolated in the material through which they flow, and "backscattered" from their original direction. This study allowed researchers to identify any atomic defect in matter that causes energy loss.

In the current study, the researchers took it a step further and used the graphene system to create a quantum Hall system. In such a system, electrons move in a two-dimensional plane cooled to a very low temperature and exposed to a strong magnetic field. This experimental system, first demonstrated in 1980, led to many discoveries about the fundamental nature of particle behavior in "topologically protected" quantum systems. In these discoveries, Prof. Zaldov explains, the importance of the edges of the system became clear, since in this situation the electrons move in a loop along the edges; What is between the ends serves as an insulator, separating the sides and "protecting" the electrons from "losing their path".

The patterns of energy loss found in the previous experiment did not prepare the scientists for the results of the new experiment. They expected confirmation of theories describing when and how quantum Hall systems can lose energy. In practice, the researchers noticed a waste of energy in places where it was not supposed to be wasted - and did not find a loss of energy where they expected a loss.

For example, they expected the system to act as an ideal conductor - one that does not allow energy loss at the edges, but the resulting heat map showed a significant amount of energy loss. Did these measurements somehow disprove the laws of energy loss in electrons in quantum Hall systems?

The graphene in the experiments was placed between insulating layers with silicon underneath, for the purpose of controlling the electron density, and the SQUID needle above. Apart from documentation, this needle can also be used to "adjust" the experimental conditions: for example, to add or remove electrons, or to change the effective shape of the edges - that is, the way the electrons "see" it. The operation of the needle allowed the researchers to create two different maps - one of energy loss and the other of resistance - and to discover an overlap between the two phenomena, although not absolutely and not in all conditions. This led to the discovery of another strange phenomenon: instead of one loop of electrons, which move clockwise along the edges of the graphene, the researchers discovered three loops - two clockwise, and the third, close to them, counterclockwise.

"We were able to see this because the local and spatial information resolution provided by our thermal imaging method is at the nano level, and is several thousand times more sensitive than any other method," says Prof. Zeldov. "In global measurements that have been made so far, the opposite contributions of the clockwise and counterclockwise loops have offset each other, so it seems that one loop exists instead of three." The existence of several loops flowing in opposite directions is the cause of energy waste. "Electrons are like train cars on a one-way track," explains Prof. Zeldov. "They can 'jump' to one of the neighboring tracks - including the one that was directed in the opposite direction - while losing energy."

"This is the first time that someone has been able to spatially decipher such a process in the quantum Hall state in any material and discover the microscopic mechanism that leads to energy loss," he adds. And while the research team showed that the findings do not disprove the basic theories of energy loss in quantum Hall states, it laid the foundations - theoretical as well as experimental - for new discoveries in the field.

Dr. Arthur Margherita, Amit Aharon-Steinberg, Dr. Dori Halbertal, Dr. Kusik Begani, Ido Markus and Dr. Yuri Miasovidov from the Department of Condensed Matter Physics at the Weizmann Institute of Science also participated in the study; and John Birkbeck and Prof. David Perlow of the University of Manchester.

To read one "bit" of information on our computer or mobile phone, we currently spend about 100 million times more energy than required by the laws of thermodynamics.

for the scientific article

More of the topic in Hayadan:

• A 140-year-old physics mystery has been solved: the Hall effect of light using a subject
• A connection between two research groups at the Technion gave rise to a new scientific field: quantum metamaterials
• The electronic properties of the graphene material were verified