Superconducting, why does it make it so cold?

Why does it always have to make it so cold? We now know a whole range of materials that – under certain conditions – conduct the electric current without any resistance. We call this phenomenon superconducting. All these materials, however, have a common problem: they only become superconductors at extremely low temperatures. The search for theoretical calculation methods to represent and understand this fact has been going on for many years. To date, no one has been able to fully find the solution. However, TU Wien has developed a new method for a better understanding of superconducting.

Many particles, complex calculation

"In fact, it is surprising that superconducting occurs only at extremely low temperatures," says Professor Karsten Held of the Institute of Solid Physics at TU Wien. "If you consider the energy released by the electrons involved in superconducting, you expect superconducting to be possible at much higher temperatures as well."

In response to this riddle, he and his team set out to find a better way to theoretically represent superconducting. Dr. Motoharu Kitatani is the lead author of a new publication that brings significant improvements and allows for a deeper understanding of high temperature superconductivity.

It is not possible to understand superconducting by imagining electrons in the material as tiny spheres following a distinct path, such as balls on a pool table. The only way to explain superconduction is to apply the laws of quantum physics. "The problem is that many particles are involved in the superconducting phenomenon, all at the same time," explains Held. "This makes the calculations extremely complex."

The individual electrons in the material can not be considered as independent objects; they must be treated together. Yet, this task is so complex that it would not be possible to solve it accurately even using the world's largest computers. "However, there are different methods of approximation that can help us represent the complex quantum correlations between electrons," according to Held. One of them is the "Dynamic Mean Field Theory", ideal for situations in which calculating quantum correlations between electrons is particularly difficult.

Better representation of interactions

The TU Wien research group is currently adding an addition to the existing theory based on a new "Feynman diagram" calculation. Feynman's diagrams – designed by Nobel laureate Richard Feynman – show the interactions between particles. All possible interactions – for example during particle collision, but also particle emission or absorption – are represented in diagrams and can be used to perform very accurate calculations.

Feynman has developed this method to study individual particles in a vacuum, but it can also be used to describe complex interactions between particles in solid objects. The problem of solid physics lies in the fact that a large number of Feynman diagrams must be taken into account because the interaction between the electrons is very intense. "In a method developed by Professor Toschi and myself, we no longer use Feynman's diagrams solely to describe interactions, but also use a complex time-dependent peak," says Held. "This vertex itself consists of an infinite number of Feynman diagrams, but, using an intelligent trick, it can still be used for calculations on a supercomputer."

A thorough detective work

This has created an expanded form of dynamic mean field theory that allows a good approximation of the complex quantum interaction of particles to be calculated. "The interesting thing about physics is that we can show that it is actually the temporal dependence of the vertex that means that superconductivity is only possible at low temperatures." After much detailed detective work, Motoharu Kitatani and Professor Held have even been able to identify Feynman's orthodox diagram that shows why conventional materials only become superconducting at -200 ° C and not at room temperature.

In conjunction with ongoing experiments at the Institute of Solid State Physics in a working group chaired by Professor Barisic, the new method should contribute significantly to a better understanding of superconductivity and thus allow the development of superconducting materials even further. best. Identifying a material that is also superconducting at room temperature would be a major breakthrough and allow for a host of breakthrough technological innovations.

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More information:
Motoharu Kitatani et al. Why is the critical temperature of high Tc cuprate superconductors so low: The importance of the dynamic vertex structure, Physical examination B (2019). DOI: 10.1103 / PhysRevB.99.041115