Posted in | Quantum Physics

Researcher Finds that Symmetrical Bipolarons Form the Basis of High-Temperature Superconductivity

According to Viktor Lakhno, a Russian physicist from Keldysh Institute of Applied Mathematics, RAS, symmetrical bipolarons form the basis of high-temperature superconductivity.

The theory describes the latest experiments where superconductivity was achieved in lanthanum hydride (LaH10) at extra-high pressure at almost room temperature. The outcomes of the research have been reported in Physica C: Superconductivity and its Applications.

Superconductivity means a complete absence of electric resistance in the material upon being cooled below a critical temperature. Heike Kamerlingh Onnes was the first to detect that when the temperature of mercury went down to −270 °C, there was a decrease in its resistance by a factor of 10,000. If researchers can find out ways to realize this at higher temperatures, it would have a revolutionary impact on technologies.

In 1957, Bardeed, Cooper, and Schrieffer came up with the first theoretical explanation of superconductivity at the microscopic level in their BCS theory. But this theory does not offer an explanation for superconductivity above the absolute zero. Toward the end of 2018, two research teams found that lanthanum hydride (LaH10) exhibits superconductivity at record-high temperature.

According to the first team, the temperature of transition into the superconducting state is Tc = 215 K, or −56 °C. The second team has reported that the temperature is Tc = 260 K, or −13 °C. In both instances, the samples were subjected to a pressure of over one million atmospheres.

As there is no theory for explaining the mechanism of high-temperature superconductivity, it is found in new materials almost at random. In his new study, Viktor Lakhno proposes the use of bipolarons as a basis.

A polaron is a quasiparticle formed of electrons and phonons. Polarons have the ability to form pairs by the electron-phonon interaction, which is so strong that they become as small as an atomic orbital and are known as small-radius bipolarons in this case. The issue with this theory is that the mass of small-radius bipolarons is extremely large than an atom. Their mass is governed by a field accompanying them when they move. Moreover, the mass has an impact on the temperature of a superconducting transition.

Viktor Lakhno developed an innovative translation-invariant (TI) bipolaron theory of high-temperature superconductivity. This theory states that the formula for establishing the temperature does not involve a bipolaron mass but a normal effective mass of a band electron, which can be less or greater than the mass of a free electron in vacuum and nearly 1000 times less than that of an atom.

Upon squeezing the crystal lattice in which an electron moves, there is a change in the band mass. A decrease in the distance between the atoms reduced the mass as well. As a result, the transition temperature can exceed the relevant temperature in normal bipolaron theories several times.

I have focused on the fact that an electron is a wave. If so, there is no preferable place in a crystal where it would be localized. It exists everywhere with equal probability. On grounds of the new bipolaron theory one can develop a new theory of superconductivity. It combines all the best features of modern conceptions.

Viktor Lakhno, Physicist, Keldysh Institute of Applied Mathematics, RAS


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