'Raindrops on the roof-technique' reveals new quantum liquid

Koen Bastiaans. Credit: Leiden University
Koen Bastiaans, his colleagues and others have discovered a quantum liquid unlike any other. The technique they used can be described as hearing raindrops fall on a roof.

You may still be able to recall those old-fashioned lightbulbs. They gave off a warm glow and were very expensive. What's the reason? These were made from a material that has a high electrical resistance. Every material has a certain resistance to electricity.

Are all materials acceptable? Except for superconductors, all materials? They can transport electricity at low temperatures without resistance. It is a mystery of physics how they work.

Textbook theory

One theory explains why certain materials are superconducting. When the material is cooled below its transition temperature, electrons pair up to form Cooper pairs. These Cooper pairs then condense to form a quantum liquid which flows easily, with no resistance, and this results in superconductivity.

Scientists believed that Cooper pair formation and condensation into the resistance-free state are simultaneous processes. Koen Bastiaans (physicist), is the lead author of this study.

Koen Bastiaans explaining the raindrops-on-the-roof technique. Credit: Leiden University

Mysterious quantum liquid

The big surprise is here: Bastiaans, colleagues discovered that Cooper pairs can still exist above superconducting transition temperatures, creating a new mysterious quantum fluid state. Bastiaans says that Cooper pair formation is different from condensing. This has completely transformed my thinking about superconductivity.

In close collaboration with Delft University of Technology (SRON), research began on superconductor TiN (titanium nutride). This superconductor is also used in highly sensitive optical detectors for space exploration.

Raindrops falling on the roof

The Leiden group has developed a method that allows them "listen" to electrons in superconductors to detect Cooper pairs. These single electrons produce a different noise profile than when they form Cooper pairs. Bastiaans explained in an earlier video that it's similar to listening to raindrops fall on the roof. "When the drops get fatter, the sound effects of rain change."

Researchers measured noise in superconducting TiN and found that the material was composed of Cooper pairs at low temperatures.

Image of the TiN from STM. Credit: Koen Bastiaans et al, Science 2021

"The best is yet"

The superconductivity disappeared as expected when the temperature was raised to 2.95 Kelvin (or 2.95 degrees above -273 Celsius). The Cooper pairs remain until the temperature reaches 7.2 Kelvin. Bastiaans says that Cooper pairs can be created without superconductivity and condensation. Although there had been some indications that this was possible, it has never been proven. This means that we have discovered a new type of quantum liquid. It is a state of matter which is not only resistive but also completely composed of Cooper pairs.

This discovery was published in Science and provides new insights to physicists trying to understand superconductivity. It is clear that condensing and forming Cooper pairs are not always the same thing.

Bastiaans, now a Delft University of Technology postdoc, says that "it's funny" and that the most important measurements were made on the night before my Ph.D. defense at Leiden. My thesis advisor stated that the best was yet to come during that defense. I was certain he was correct about that.

Continue reading Experiments reveal the formation of a new state in matter: Electron quadruplets

Further information: Koen Bastians et al. Direct evidence for Cooper pairing in disordered superconductors above Tc. Science (2021). Information from Science Koen Bastiaans and colleagues, Direct evidence of Cooper pairing in disordered superconductors above Tc (2021). DOI: 10.1126/science.abe3987