Jan 30 2019
Latest discoveries are redefining the way people think about superconductivity. At TU Wien (Vienna), experiments underscore the unique role of immobile charge carriers that act as a “glue,” which, in turn, makes superconductivity feasible.
Different cuprates which are being studied at TU Wien (Image credit: TU Wien)
Some amount of electric resistance exists in every standard wire, every cable, and every electronic device. However, there are superconducting materials that are capable of conducting electrical current with exactly zero resistance—at least at extremely low temperatures. If a material behaving as a superconductor at room temperature is identified, it would represent a scientific innovation of remarkable conceptual and technological significance. It may possibly result in an array of novel applications, ranging from levitating trains to innovative imaging technologies for medicine.
The quest for high-temperature superconductors is highly challenging because the majority of the quantum effects associated with superconductivity are yet to be completely understood. Cuprates are a group of materials. Neven Barišić, professor for solid state physics at TU Wien (Vienna) is conducting experiments with these materials, which act as a superconductor at ambient pressure and at record temperatures as high as 140 K.
Barišić and his team have now developed a new understanding and an incredible set of results that could considerably alter the way people think about these intricate materials as well as high-temperature superconductivity on the whole.
The Quest for the Holy Grail
The phenomenon of high-temperature superconductivity has been thoroughly investigated for decades, but nobody has cracked the problem yet. Quite a few materials show superconducting behaviour at temperatures close to absolute zero, and we understand why this happens in some of them. But the real challenge is to understand superconductivity in cuprates, where this states persists at much higher temperatures. A material which behaves as a superconductor at room temperature would be the Holy Grail of solid state physics—and we are getting closer and closer.
Neven Barišić, Professor, Institute of Solid State Physics, TU Wien (Vienna)
Barišić and his coworkers have demonstrated that two essentially different kinds of charge carriers are present in cuprates, and proposed that superconductivity mainly relies on the slight interaction between them.
Some amount of the electrical charge is localized—that is, all the charge carriers sit at a specific set of atoms and can only shift away upon heating the material. Other charge carriers can shift, hopping from one atom to another atom. The mobile charge is the one that eventually becomes superconductive; however, to explain superconductivity, the immobile charge carriers also need to be taken into consideration.
“There is interaction between the mobile and the immobile charge carriers, which governs the properties of the system”, stated Barišić. “Apparently, the immobile charges act as the glue, binding pairs of mobile charge carriers together, creating so-called Cooper pairs, which are the basic idea behind classical superconductors. Once paired the charge carriers can become superconducting and the material can transport the current with zero resistance.”
In other words, there needs to be a subtle balance of immobile and mobile charge carriers in order to realize superconductivity. Very few localized charge carriers means that sufficient “glue” is not available to couple the mobile charge carriers; by contrast, very few mobile charge carriers means that nothing is available for the “glue” to pair. In both the cases, superconductivity is either weakened or ceases altogether.
However, superconductivity exists at incredibly high temperatures at an optimal middle ground. It was difficult to figure out that the balance between immobile and mobile charges is altered, as a function of doping or temperature, in a slow way.
We have performed many different experiments with cuprates, collecting large amounts of data. And finally, we can now propose a comprehensive phenomenological picture for superconductivity in cuprates.
Neven Barišić, Professor, Institute of Solid State Physics, TU Wien (Vienna)
Recently, Barišić published his findings in a number of journals—most recently in “Science Advances”—that reveals that superconductivity also materializes in a slow way. This represents a vital step towards the aim of interpreting cuprates and offering a means to look for novel and relatively better superconductors.
If materials are developed which stay superconductors even at room temperature, this would have important implications for technology. It is possible to develop electronic devices that do not utilize much energy at all. With the help of very strong superconducting magnets, levitating trains can possibly be developed, so that affordable and ultrafast transportation would become feasible.
“We are not yet near this goal,” stated Neven Barisic. “But deep understanding of high-temperature superconductivity would pave the way to get there. And, I believe, that we have now taken several important steps in this direction.”