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Impossible superconductors gone live

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The scientists from the Faculty of Physics of the Lomonosov Moscow State University conducted a study evaluating the appearance of the superconducting state in the iron-based superconductors with two energetic gaps. The report on the study was published in the latest issue of the Journal of Superconductivity and Novel Magnetism. http://dx.doi.org/10.1007/s10948-016-3386-5

A team of Russian scientists lead by the Lomonosov Moscow State University physicists for the first time in history managed to measure, reliably and directly, the energetic gaps of a number of superconductors (first of all — iron-containing). According to Svetoslav Kuzmichev who leads the research project the results of the work would allow to solve some questions concerning the appearance of the superconductivity in the iron-containing materials.

The main thing interesting for the physicists in this experiment was a chance to measure the temperature dependences of the two energetic gaps. The term "superconducting gap" refers to denoting a range of energies that is forbidden for the conducting electrons.

Since 1957, when American physicists John Bardeen, Leon N. Cooper and John Robert Schrieffer developed a theory explaining the superconductivity phenomena (the BCS theory, awarded with the Nobel Prize in 1972), there was only one such band: from the temperature of the transition to the superconductivity state to zero. But in 1959 a probable existence of the two-gap superconductors was assumed by a Soviet physicist V.A. Moskalenko and his US colleague G. Suhl. The two scientists independently deriveded sets of equations, describing mechanisms of such superconductivity, however experimentally the first two-band superconductor was found only at the beginning of the present century, in 2001. It was quite a simple in composition magnesium diboride.

By that time physicists doubted the possibility of the two-gap superconductivity. Something new, standing out of the common frameworks, always appears as a heavy psychological burden for researchers of any scientific fields. To lighten this 'burden', the scientific community preoccupied with superconductivity problems treated magnesium diboride as in exception, confirming the rule.

Though only seven years later, in 2008, the phenomenon of two-gap superconductivity was found and experimentally confirmed in the iron-containing materials. Many laboratories all over the world started to use the superconductive 'ferrum', the score of the two-gap materials went to dozens, and the exception became a rule. Such a surprise made by iron-based superconductors turned out to be not the only one: eight years ago, by the moment of the discovery, it was considered that they could not exist at all, as a magnetic field kills the superconductivity. Since the appearance of the BCS theory the absence of the magnetic atoms in a superconductor seemed to be an indisputable condition.

According to this theory, the superconductivity appears because of the interaction of electrons and the crystal lattice vibrations, which results in building the so-called Cooper pairs of two electrons with opposite spins (the resulting spin is hence absent), so the electrons have a chance to move without colliding with the lattice.

As a spin is a magnetic moment of a particle, in a presence of magnetic interactions it seems impossible to preserve the zero resulting spin. According to Svetoslav Kuzmichev, the first author of the article and the senior research fellow of the physical faculty, MSU, this fact was multiply confirmed in experiments with the common superconductors. Addition of a tiny magnetic admixture or replacing any of the atoms with a ferromagnetic one in a superconductor lead to drastic decrease in superconductivity, up to its total disappearance.

After the discovery of the iron-based conductors the new class of materials came into spotlight among all physicists dealing with the superconductivity. Previously they used to show higher interest to high-temperature cuprates (cuprum-containing superconductors) and two-gap magnesium diboride. During the next eight years the amount of the superconductors based on ferrum compounds with arsenic or selenium, as well as the amount of the possible explanations of the 'iron-superconductivity', outnumbered all the superconductive cuprates, though a certain understanding of the phenomenon nature did not come.

'It was found that the ferrum-arsenic or ferrum-selenium blocks are responsible for the appearance of the superconductivity,' Svetoslav Kuzmichev comments. 'Almost all the scientists agree that though the outer magnetic field is suppressed, inside the blocks its fluctuations may exist in a form of magnon quasiparticles and with a high probability they take part in the developing of the superconductive state. However the matter is so novel and our knowledge is so limited, that almost none of the suggested mechanisms of reaching superconductivity was neither confirmed, nor refuted yet.'

The complexity is increased by the fact that the iron-based superconductors are multi-band. This circumstance significantly complicates the understanding of the already intricate processes accompanying the superconductivity phenomenon, regardless the existence of the above mentioned Moskalenko and Suhl's equations.

Basing on this equations scientists calculated the tendency of the temperature behavior of the two superconductive gaps for a number of the iron-based superconductors and the 'non-iron' magnesium diboride (with a partial replacement of magnesium with aluminium), then, for the first time in history, researchers conducted the direct experimental measurements of those dependences and as the result detected a convincing correlation of the calculations and the experimentally gained data. Moreover, they managed to evaluate what contributes more to the superconductive state — an interband or an intraband pairing. In other words, they established how strong is the connection within a Cooper pair, which is formed by coupling electrons from the same or two different bands. According to Kuzmichev, that is particularly important for the understanding of the mechanisms of the 'iron' superconductivity.

'Up to now such estimations of the gaps' characteristics were based on the indirect measurements, the scientist says. For example, the correlation with the temperature and other parameters of the superconductive state was measured, with the further extrapolation of the results for distinguishing the energetic gaps. Those were quite approximate measurements, and in case of two gaps their certainty appears to be, so to say, by eye. The MSU Physical faculty professor Yaroslav Ponomarev (1938-2015) developed a "break-junction" technique that helped us for the first time to measure directly the energetic gaps of the high-temperature superconductors under the temperatures up to the critical temperature of the superconductive transition, avoiding the procedure of the indirect measurement. That is our key 'know-how', which let us estimate the magnitude of the interband and the intraband electron pairing. As the result we have shown that the crucial role in the mechanism of the magnesium diboride superconductivity is played by the intraband pairing. The condensates interact weakly, and in the magnesium diboride interband interaction is far weaker, than in the iron-based superconductors.'

Kuzmichev hopes that this work would clarify the situation with the developing of the 'iron' superconductivity. Though nowadays in terms of critical temperatures such superconductors give way to cuprates (while the maximal temperature of the superconductive transition, observed in ferrum-selenium films, was about 85 K, for cuprate superconductors it reaches up to 135 K), as the main advantage of the 'iron' superconductors Kuzmichev pronounces an unprecedented current density that they are able to carry.

'They can conduct the current from ten to a hundred times greater than cuprates and even niobium with its alloys, that are applied today in the superconductive magnets for generating extremely high fields for the powerful accelerators and tokamaks. No other superconductor can be compared to them today, except high purity magnesium diboride, which is able to carry currents with the density up to a million ampere per square centimeter. In laboratory conditions those numbers are impossible to prove directly, of course, though according to the existing estimations such densities are absolutely attainable with the 'iron' species. Well, I suppose, soon we will have no alternative to them,' the scientist concludes.

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Vladimir Koryagin
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http://www.msu.ru

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