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An international team of physicists has solved one of the last fundamental problems of superconductivity.

Physicists from the Skolkovo Institute of Science and Technology (Skoltech), together with colleagues from European scientific organizations, have experimentally demonstrated the non-stationary phase slip effect, making an experimental discovery of a fundamental physical phenomenon. This was reported in the press service of Skoltech. The discovery promises to be as promising as the Josephson effect, thanks to which we have a voltage standard and ultra-sensitive magnetic field detectors.

The phase slip effect manifests itself in the form of steps of electric current flowing through superconducting nanowires under the action of microwave radiation. A nanowire made of a superconductor plays the role of a tunneling barrier for magnetic flux quanta in the same way that a Josephson junction, a thin layer of insulator between two superconductors, serves as a tunneling barrier for electric charges.

From the point of view of classical physics, such a transition is no different from a complete break in the circuit, but the quantum mechanical tunneling effect allows current to flow through such gaps without any resistance. Similarly, a magnetic flux cannot "jump" through a wire in classical physics (a superconductor shields a magnetic field), but can "tunnel" through it thanks to the laws of quantum mechanics.

The use of the nonstationary phase slip effect promises to be as promising as the Josephson effect. Based on Josephson junctions, supersensitive magnetic field detectors are made, for example, to study very weak magnetic fields of the brain. Another practical application of these contacts is based on the fact that the current flowing through them may not change smoothly, but may form voltage steps if the contact is irradiated with microwave radiation.

On the basis of the last effect, called Shapiro steps, quantum metrology is based: today the volt standard is determined not with the help of any chemical battery from the chamber of measures and weights, but with the help of devices based on Josephson contacts. Based on the effect of non-stationary coherent phase slip, it will also be possible to make a quantum ampere standard.

“This makes it possible to achieve unprecedented accuracy, since the size of each step in both effects is determined by the fundamental laws of nature: in principle, it does not depend on the material or on external conditions: the main thing is the presence of superconductivity,” commented Oleg Astafiev, head of the study, on the significance of the discovery.

An international team led by Skoltech professor and head of the MIPT Laboratory of Artificial Quantum Systems Oleg Astafiev published an article in the journal Nature, where they described one of the few remaining unrealized effects of fundamental physics, which has so far been in the status of “theoretically predicted” - the non-stationary phase slip effect, which manifests itself in the form of reverse Shapiro steps in superconducting nanowires: in the current-voltage characteristics, steps are formed in current with a change in voltage, in contrast to steps in voltage in the long-known Shapiro effect on Josephson junctions.

And although Soviet physicists Konstantin Likharev, Alexander Zorin and Dmitry Averin from Moscow State University predicted the appearance of such steps back in 1990, they could not be directly observed in practice for a long time. An international group of scientists led by Oleg Astafiev used a new approach in their work. The key to success turned out to be the right material for the nanowire — thin films of niobium nitride — as well as a special and unusual electrical circuit: next to the nanowire, compact micron-sized inductive elements were made from the same material.

The observation of Shapiro's backward steps does more than just confirm the existence of this fundamental physical phenomenon. It also opens the way to the creation of new types of devices, and this is in demand both in fundamental scientific research and in the creation of new devices, in particular, current metrological standards.

An international team of physicists has solved one of the last fundamental problems of superconductivity.