Scientists from the University of Cambridge believe they have solved the longstanding mystery of where superconductivity emerges in high-temperature superconductors. Equipped with this knowledge, scientists may be able to tap into the astronomical potential of these materials which could have applications in a wide variety of technologies, from magnetic levitating trains to supercomputers.
Through understanding where superconductive properties originate, scientists can now search for similar characteristics in other materials which should significantly speed up the search for new superconductors.
In this recent report, scientists reveal that superconductivity emerges from twisted pockets of electrons in the material which are the result of charge density waves, or ripples of electrons. The study has been published in Nature.
Superconductivity was first discovered in 1911 by a scientist called Heike Kamerlingh Onnes whilst he was investigating the properties of metals at low temperatures. Superconductivity is a phenomenon that occurs in certain materials characterized by zero electrical resistance. In the majority of instances, the materials need to be cooled to nearing absolute zero (-273oC) before the superconductive properties appear. These materials are known as low-temperature superconductors.
However, some materials exist that display superconductive properties at much higher temperatures, around -135oC (-211oF). These materials are therefore much more useful in modern technology as they can be utilized in a wider range of scenarios but unfortunately, unlike low-temperature superconductors, little was known about the perfect recipe for these materials.
“One of the problems with high-temperature superconductors is that we don’t know how to find new ones, because we don’t actually know what the ingredients are that are responsible for creative high-temperature superconductivity,” said lead author Dr Suchitra Sebastian in a news-release.
Unlike your average electronic device, the current in superconductors is carried by electrons that travel in tight pairs. When they travel in this conformation they can move smoothly through the material, which is why there is no resistance. Lonely electrons, however, travel more haphazardly and often bump into each other, creating resistance. This phenomenon of zero resistance will occur in superconductors provided that they are kept below a certain critical temperature.
Scientists knew that something in the superconducting material was behaving as an adhesive, causing the electrons to pair up, but they didn’t know what. All they knew was that the glue can be weakened by exposing these materials to increases in temperature or magnetic field strength, which separates the electron pairs and thus abolishes superconductivity.
The scientists used a reverse approach in order to discern what causes the electrons to pair up, starting with materials in their non-superconducting state.
“We’re trying to understand what sorts of interactions were happening in the material before the electrons paired up, because one of those interactions must be responsible for creating the glue,” said Sebastian. “Once the electrons are already paired up, it’s hard to know what made them pair up. But if we can break the pairs apart, then we can see what the electrons are doing and hopefully understand where the superconductivity came from.”
The researchers knew that in the majority of materials, superconductivity tends to abrogate other properties that the material has in its normal state, such as magnetism. It is therefore possible to produce superconductivity by suppressing these normal state properties, and vice versa.
Through the use of strong magnetic fields, the team successfully suppressed the superconductivity of materials belonging to a family of copper oxide compounds known as cuprates. This allowed the researchers to finally locate the source of electrons in the natural-state material that pair up when the material becomes a superconductor. Much to their surprise, it turns out that the electron pockets are located where superconductivity is weakest, as opposed to the strongest. These pockets originate from undulations of electrons called charge density waves.
“By identifying other materials which have similar properties, hopefully it will help us find new superconductors at higher and higher temperatures, even perhaps materials which are superconductors at room temperature, which would open up a huge range of applications,” said Sebastian.