Physicists have found an explanation for the paradoxical behavior of "very dirty" superconductors at low temperatures. These promising materials can be used to create a quantum computer. By understanding why such substances do not obey the standard theory of superconductivity, scientists will be able to create the most isolated qubits - the elementary computing units of quantum computers. The work of a team of researchers with the participation of employees of the L. D. Landau RAS was published in the journal Nature Physics.
Superconductors are materials in which, under certain conditions, electrical resistance completely disappears. This means that electric current can flow through wires that are made of this material without loss, while in conventional wires some of the energy is dissipated as heat. Superconductivity was discovered at the beginning of the 20th century, but the first phenomenological theory, which explained many of its properties, was developed in 1950 by Lev Landau and Vitaly Ginzburg. Seven years later, the Americans Harry Bardeen, Leon Cooper and John Schrieffer created a general theory of superconductivity (the so-called BCS theory), which immediately won the Nobel Prize - so obvious was the colossal significance of the phenomenon.
Among other things, the BCS theory predicted how superconductors should behave in a magnetic field. When the fields are small, such substances "push" them out of themselves, while remaining superconducting. This fundamental property is called the Meissner effect. If we continue to increase the field, at some point the superconducting properties abruptly disappear. The value at which the magnetic field suppresses superconductivity in the material is called the critical magnetic field. It depends on temperature: the colder, the greater the critical field. That is, when a superconductor is at a temperature close to the critical one, even small magnetic fields are sufficient to bring it out of the superconducting state,however, with very strong cooling (up to 1/5 of the critical temperature and below) this regularity disappears and the critical magnetic field ceases to depend on temperature. Now, in order to remove a material from a superconducting state, it is necessary to apply a magnetic field of the same magnitude - it does not matter whether the superconductor remains at this temperature or even cools down.
“This classical picture of dependence does not hold for“very dirty”superconductors,” explains one of the authors of the article, Mikhail Feigelman from the Institute of Physics named after L. D. Landau. - This term denotes superconductors made of metal alloys with a highly damaged crystal lattice, almost amorphous. The critical magnetic field continues to increase approximately linearly with decreasing temperature to arbitrarily low values that can be achieved experimentally. This fact was known for a long time, but he had no clear explanation."
In the new work, scientists were able to understand what is the nature of the atypical behavior of "very dirty" superconductors. The key experiment that made it possible to understand this was the measurement of another most important parameter of superconductors - the critical current. This is the maximum value of sustained current that can flow in a superconductor without energy loss for dissipation into heat. At higher currents, the substance loses its superconducting properties, that is, resistance appears in it, and the sample of the substance begins to heat up. Physicists have measured how the critical current in a superconducting indium oxide film depends on the magnetic field. The scientists passed a current through the film, which was in a magnetic field, the value of which was slightly less than the critical value, and observed at what value of the current in the sample the superconducting behavior would be destroyed.
Similar experiments have been carried out before. The uniqueness of this work is that the dependence of the maximum superconducting current on the magnetic field in "very dirty" superconductors was measured at magnetic fields close to critical and very low temperatures. “Surprisingly, it turned out that the critical current in a very simple way depends on how close the magnetic field is to the critical value. It's a power-law relationship, the degree is 3/2,”says Feigelman. In addition, scientists have determined how the critical field in an indium oxide film depends on temperature.
“By looking at the results of these two experiments, we were able to understand how they are related,” says Feigelman. - A stable increase in the critical magnetic field at low temperatures in "very dirty" superconductors occurs due to the fact that in the superconducting state, which is realized in a strong magnetic field, there are thermal fluctuations of the so-called Abrikosov vortices (quantum supercurrent vortices that appear in superconductors under the effect of an external magnetic field, which penetrates the superconductor in this way). And we found a way to describe these fluctuations. " The predictions of the theory created by the authors describe well the experimental data obtained.
"Very dirty" superconductors, also called highly disordered superconductors, are an active area of research in modern physics. Usually, the more "disorder" a metal has, the worse it conducts an electric current. With decreasing temperature, the conductivity of disordered metals increases. "Very dirty" superconductors behave differently: in the normal state, they are weak dielectrics and, when cooled, conduct current worse and worse, but upon reaching a critical temperature, they suddenly transform into superconductors. "A superconductor and a dielectric are opposite states in their properties, which is why it is surprising that in such substances they can transform into one another," explains Feigelman. - Although "very dirty" superconductors have been studied for 25 years, a full-fledged theory,which would explain all their oddities, is still not present."
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In recent years, interest in disordered superconductors has additionally increased due to the emergence of new areas where such substances are in great demand. For example, "very dirty" superconductors are ideal for isolating superconducting quantum bits from all kinds of interference - the elementary computing units of a quantum computer. It is most convenient to isolate them from the outside world using elements with very high inductance. It determines how strong the magnetic flux will be created by the electric current flowing in the system. The inductance of a substance is the greater, the lower the density of conducting elements in it, and this parameter decreases with the growth of "dirt" in superconductors.
