All because of a small electron

Yigal Meir from Ben-Gurion University of the Negev solved the mystery of the "0.7 anomaly"

By Yael Laban

Prof. Yigal Meir, from the Department of Physics and the Ilza Katz Institute for Nanoscale and Mesoscopic Research at Ben-Gurion University, has solved a mystery that has occupied researchers in the field of nanoelectronics for about twenty years.

Nanoelectronics is found in the field of mesoscopic physics - the field between classical physics and quantum physics. Classical physics deals with large bodies - from a grain of sand to stars and galaxies. Quantum physics describes the behavior of atoms and molecules. In recent years, methods have been found to build systems in the intermediate range, which include hundreds or thousands of atoms and whose size is measured in tens to hundreds of nanometers. These systems present interesting phenomena, which in the past could only be hypothesized and described theoretically. One such system is the "quantum point contact" (QPC).

"Quantum bottleneck" is the basic unit of various nanoelectronic devices. To build it, two very smooth semiconductor materials are attached to each other. By matching the properties of the semiconductors, a layer of electrons is created in the rim between them that can move freely only parallel to the rim. This layer of electrons moving on one plane is called "two-dimensional electron gas". The movement of electrons in the plane is controlled by means of gates that block the passage of electrons when a negative voltage is applied to them. The "bottleneck" is created when an opening several hundred nanometers wide is left between the two barriers, and it works similar to an hourglass: the wider the opening, the more electrons cross the barrier, which means the conductivity of the device increases. The size of the opening is controlled by the voltage applied to the gates (Vg): the lower the voltage, the greater the gain and the higher the conductivity. This feature turns the QPC into a tiny electronic switch.

As mentioned, in a system of this size, the laws of quantum physics work. According to quantum theory, the current of electrons passing through the transition has a wave character (similar to a light beam). As a result, the energy levels of the system are not continuous but change in "jumps". The system moves to a new energy level every time the width of the opening reaches a value that is an integral multiple of the wavelength of the electrons. This phenomenon, called quantization of the energy levels, means that the electrical conductivity of the QPC does not change continuously with the change of the gate voltage, but is characterized by steps of uniform height marked G0. Theoretical physicists found that this quantity is a universal quantity that depends only on fundamental constants of nature: the electron charge e, and Planck's constant, h according to the equation: G0 = 2e2/h. Experiments did verify the theoretical prediction, but with one change: an additional, unexplained step appeared in them, approximately equal to 0.7G0. The phenomenon, which was named the "0.7 anomaly", was first observed in 1988 but until recently remained one of the central and unsolved mysteries in the field.

Prof. Yigal Meir, a theoretical physicist from Ben-Gurion University, and his colleague Prof. Ned Wingreen from Princeton University proposed a solution a few years ago. They argued that a single electron trapped in the bottleneck of the QPC repels other electrons and causes a lower than expected first degree. The theory predicted that the magnetic moment accompanying the trapped electron changes the conductivity of the system at low temperatures - a phenomenon known as the "Kondo effect". Experiments conducted by Professor Charles Markus and his group at Harvard University did verify the theory of Meier and Weingrin, but it was still necessary to explain how an electron, whose charge, as we know, is negative, was captured in the first place, precisely at the point where a negative electric voltage was applied that was supposed to repel the electron. This is similar to the formation of a puddle of water at the top of a hill.

Meir and his postdoctoral student Dr. Thomas Reitz recently published an explanation of the phenomenon in the prestigious journal Nature. Meir and Reitz showed that if you take into account the wave nature of all the electrons in the system and the interactions between them, you find that when the "bottle neck" opens, the total energy at the point of contact will be lower precisely if an electron is indeed trapped there. In doing so, Meir and Reitz proved the correctness of the solution to the mystery. Now it is the turn of the experiments again, and there is already indirect evidence of the correctness of the theoretical solution.

The QPC may be a fundamental component in the creation of "quantum computers" - one of today's popular research directions in the field of nanoelectronics, which if realized will cause a revolution in the field of computing. The interaction of the electron trapped in the QPC with the other electrons may disrupt the calculations and cause the system to lose the quantum memory on which it is based. The theoretical work of Meir and Reitz provides researchers with tools to understand the phenomenon and deal with it. Such collaborations are the essence of science: theorists come up with ideas; The experimental research people test them in the laboratory; The results of the experiments reveal problems that require new theoretical solutions that are also, in turn, verified (or not) in the laboratory; And the accumulated knowledge serves as a basis for both further research and practical applications.