A ‘breath of nothing’ provides a new perspective on superconductivity

Zero electrical resistance at room temperature? A material with this property, that is, a superconductor at room temperature, could revolutionize energy distribution. But so far, the origin of superconductivity at high temperatures is only incompletely understood. Scientists from the University of Hamburg and the Cluster of Excellence “CUI: Advanced Images of Matter” have managed to observe strong evidence of superfluidity in a central model system, a two-dimensional gas cloud for the first time. Scientists report their experiments in the journal. Science, which allow to investigate key questions of high temperature superconductivity in a very well controlled model system.

There are things that are not supposed to happen. For example, water cannot flow from one glass to another through the glass wall. Surprisingly, quantum mechanics allows it, as long as the barrier between the two liquids is thin enough. Due to the quantum mechanical tunnel effect, particles can penetrate the barrier, even if the barrier is higher than the liquid level. Even more remarkable, this stream can even flow when the level on both sides is the same or the stream should flow slightly uphill. For this, however, the fluids on both sides must be superfluid, that is, they must be able to flow around the obstacles without friction.

This surprising phenomenon was predicted by Brian Josephson during his doctoral thesis, and it is so important that he was awarded the Nobel Prize for it. The current is driven only by the wave nature of the superfluids and, among other things, it can ensure that the superfluid begins to oscillate between the two sides, a phenomenon known as Josephson oscillations.

The Josephson effect was first observed in 1962 between two superconductors. In the experiment, in direct analogy to the flow of water with no level difference, an electric current could flow through a tunnel contact without an applied voltage. With this discovery, impressive proof has been provided that the wave nature of matter in superconductors can be observed even at the macroscopic level.

Now, for the first time, scientists in Professor Henning Moritz’s group have managed to observe Josephson’s oscillations in two-dimensional (2-D) Fermi gas. These Fermi gases consist of a “puff of nothing,” that is, a gas cloud of only a few thousand atoms. If they cool to a few millionths of a degree above absolute zero, they become superfluid. They can now be used to study superfluids in which particles interact strongly with each other and live in only two dimensions, a combination that appears to be central to high-temperature superconductivity, but is still incompletely understood.

“We were surprised by the clarity with which the Josephson oscillations were visible in our experiment. This is clear evidence of phase coherence in our ultra-cold 2D Fermi gas,” says first author Niclas Luick. “The high degree of control we have over our system has also allowed us to measure the critical current above which superfluidity breaks.”

“This advancement opens up many new opportunities for us to gain insight into the nature of strongly correlated 2-D superfluids,” says Professor Moritz, “they are of great importance in modern physics, but very difficult to simulate theoretically. We are delighted to contribute to a better understanding of these quantum systems with our experiment. ”