Physicist Develops an Alternative Concept of Single-Band Symmetry of the P-Wave

13:24, October 29, 2020

Theory explains in a new way superconductivity in iron compounds

A new concept explaining the nature of the superconductivity of iron-based compounds was developed by Evgeny Talantsev, a senior researcher at the NANOTECH Center of the Ural Federal University and the Laboratory of Precision Alloys and Intermetallics of the Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences. An article describing the concept was published in the journal Results in Physics.

The emergence of superconductivity is associated with the formation of the so-called "Cooper pairs", when an electric current is carried by a quasiparticle consisting of two paired electrons (when current is transferred by a single electron, electrical resistance is always observed). Cooper pairs (the term was formed on behalf of the American physicist Leon Cooper, who discovered this phenomenon) behave coherently, that is, interconnected, like a single quantum system. How intensely the electrons interact in a Cooper pair is quantitatively expressed by an indicator of the strength of the energy connecting them, which is called the energy gap.

Until now, the most consistent concept explaining the energy gap phenomenon in iron-based superconductors was the concept of multiband symmetry of s-waves, that is, the symmetry of the directions of interaction of two charges in a Cooper pair, proposed in 2009. In this case, "s" means sphericity: according to the ideas of the creators of the concept of s-waves, two (in some cases, three) spherical slits are embedded in each other like nesting dolls and interact with each other, while reacting differently to external influences by pressure and temperature.

The concept of s-waves has been applied to explain superconductivity in all 13 types of iron-based superconductors. At the same time, the problem is that, firstly, this concept was put forward when physics did not yet have the ability to conduct high-precision experiments and obtain high-quality samples of materials. Secondly, the concept of multiband symmetry of s-waves contains more than a dozen parameters, which makes the analysis of experimental data either difficult or simply impossible.

“I have developed an alternative concept of one-zone symmetry of the p-wave, which schematically represents one sphere, resembling a balloon, several times pulled by a thread. The bulges are only outside the sphere, inside it is empty and smooth. So, from my point of view, the energy gap looks, but not in real space, but in the space of quasi-momenta - physical vector quantities characterizing the motion of quasiparticles along the X, Y, Z axes. The space of the energy gap makes it possible for a Cooper pair to exist while maintaining its coherence, the larger the space, the more “viable” the Cooper pair, and vice versa: in the places of “constrictions”, where the space of the energy gap narrows to zero, the Cooper pair cannot exist. The presence of such places distinguishes the concept of single-band symmetry of the p-wave from the concept of multi-band symmetry of s-waves, says Evgeny Talantsev. "The concept of single-band symmetry of the p-wave manages with only three or four parameters, these are the energy gap, the magnitude of the thermal conductivity jump during a superconducting transition, the transition temperature and either the London penetration depth or the coherence length, depending on the experiment."

The minimum number of parameters is the main advantage of the concept of one-zone symmetry of the p-wave, which makes the analysis of experimental data the most reliable.

The article, published in Results in Physics, develops the publication of Evgeny Talantsev in Scientific Reports (Nature Research publishing house), which, at the end of 2019, was included in the top 100 most popular articles in physics. If the 2019 article describes the concept of a p-wave applicable to the analysis of experiments with iron-based superconductors in critical currents, then the second article talks about the analysis of experiments in a different measuring technique - the temperature dependence of the upper critical field.

“The upper critical field is the maximum magnetic field in which superconductivity is maintained. If the magnetic field is higher, superconductivity is completely destroyed. The upper magnetic field is described by the coherence length - a nanoscale value, ”explains Evgeny Talantsev.

Which of the concepts - s-waves or p-waves - most accurately describes the phenomenon of the energy gap in superconductors based on iron compounds, remains to be verified in subsequent experiments.

On Superconductivity

Since the discovery at the beginning of the 20th century of superconductivity - the ability of some materials to conduct electric current without resistance - and for almost a hundred years, it has been believed that iron as a ferromagnet (a substance, at a certain temperature, below the Curie point, which has magnetization without the application of an external magnetic field) is incompatible with superconductivity, since superconductors, on the contrary, are diamagnets, that is, when trying to magnetize, they generate a magnetic field of the opposite positive direction relative to the applied external magnetic field. In experiments, "classical" superconductors, even with a small admixture of ferromagnetic iron, completely lose their superconducting properties.

However, in 2006, a group of Japanese scientists led by physicist Hideo Hosono obtained an iron-based superconductor at a temperature close to the temperatures of liquid helium (boils at a temperature of 4.2 kelvin, −268.9 ℃). Two years later, the same group achieved superconductivity in iron compounds at the boiling point of liquid hydrogen (20.28 kelvin, -252.87 ℃). In the scientific world, competition has begun and is developing in the search for iron compounds that go into a state of superconductivity at the highest possible temperatures. Thus, Hideo Hosono's group, having investigated a total of more than 5 thousand compounds, discovered superconductivity in only two dozen of them.

In 2015, a research team led by Mikhail Eremets achieved superconductivity in compressed hydrogen sulfide at a temperature of -72 ℃. In 2018, two groups of scientists - the same Mikhail Eremets, as well as Madduri Somayazulu - with an interval of one day reported that the transition temperature had reached −25 ℃, however, not in iron compounds, but in lanthanum superhydride (LaH10), and under conditions of colossal pressure - 2 million atmospheres. On October 14, 2020, a group led by Rang Diaz reported superconductivity with a critical temperature of + 15 ℃ in a carbon-sulfur-hydrogen compound that was compressed at a pressure of over 2 million atmospheres.