Under high pressure, a quantum material can become superconducting at a significantly higher temperature than without pressure. Researchers at the Paul Scherrer Institute PSI have now investigated this using muons. Their study provides new insights into the emergence of unconventional superconductivity. This could contribute to the search for superconductors that function at temperatures suitable for practical applications: an important prerequisite for the development of energy-efficient technologies.

Superconductors have long been considered a promising technology for the energy systems of the future. They can conduct electricity without resistance, thus eliminating both conduction losses and waste heat. Up to now, however, superconductors have only been applied in special cases, as in the immensely powerful magnet coils of particle accelerators such as the Large Hadron Collider at CERN. This is because superconductors must be well cooled, down to extremely low temperatures for some materials. In the future, novel materials with special quantum properties are expected to make superconductivity possible at less frosty and more easily achievable subzero temperatures. A research team led by Zurab Guguchia at the Paul Scherrer Institute PSI has now provided the first comprehensive characterisation of such a quantum material. This should contribute to a detailed understanding of these processes and facilitate the search for technologically usable superconductors. The results are published in the journal Nature Communications.

“Currently, research is being conducted worldwide on novel, unconventional superconductors that exhibit robust superconductivity even at higher temperatures or in strong external magnetic fields,” Guguchia says. The physicist is a research group leader in the PSI Center for Neutron and Muon Sciences and works with his team on the materials of the future.

Layered material with surprising properties

For their new experiments, Guguchia and his team chose a material with an impressive range of unusual quantum properties. Tantalum disulfide belongs to a class of materials made up of extremely thin layers. Although it does not exhibit high-temperature superconductivity, its interesting properties offer exciting opportunities for experimentation. “Its chemical formula sounds very simple: for every tantalum atom there are two sulfur atoms,” says Guguchia. “But inside it is an enormously complex material with almost paradoxical properties.”

If tantalum disulfide is produced in the right way, two alternating layers with different atomic arrangements are always formed. “This means that the electronic properties of these two layers behave in completely opposite ways,” the researcher explains. Both layers are metallic at high temperatures and can conduct electrons. When it cools down, something strange happens: one layer becomes an insulator, while the other becomes superconducting. The tantalum disulfide then only conducts current in the superconducting layer, in one plane, because the insulating layers do not allow electrons to pass through.

But if you cool the material to an extremely low temperature, to just over one degree above absolute zero, something unusual happens: “Suddenly the entire material becomes superconducting, so the insulating layers also become conductive and take part in superconductivity,” Guguchia says. If you put the material under high pressure, the temperature at which this happens actually increases. The exact reason for this was not yet known because the interaction of electrons at the atomic level is not well understood.

Muons provide deep insights into materials

This is precisely where the PSI team’s experiments come in. The researchers have access to state-of-the-art experimental methods. One important technique is muon spin spectroscopy.

Muons are elementary particles – similar to electrons, but about 200 times heavier and with a lifetime of only a few millionths of a second. Implanted in materials, muons react to the magnetic properties of their environment with extreme sensitivity. This allows researchers to probe what happens inside a material on a microscopic scale. PSI is particularly well equipped for such experiments: with the Swiss Muon Source SμS, it operates the world’s most powerful muon source.

“Since muons are exceptionally sensitive probes for magnetic and superconducting properties, we can gain unique insights into quantum materials here at PSI,” Guguchia says.

In addition to muon measurements, the team used other methods to investigate how electrons move within the material. This combination of techniques enabled a breakthrough in the understanding of tantalum disulfide.

What happens when the material is squeezed

The researchers conducted a series of experiments in which they subjected the material to varying levels of pressure and analysed the behaviour of electrons within the material at very low temperatures.

Two factors play a role here. At very high pressure – several hundred times higher than in an car tyre – the crystal layers of tantalum disulfide are tightly squeezed together. This leads, first, to the superconducting layers coming into closer contact with each other, so that the separating, insulating atomic layer has a less disruptive effect. And second, some of the electrons in the insulating layer are released and can then also participate in superconductivity. Guguchia summarises the measurements: “Due to these effects, high pressure causes tantalum disulfide to become superconducting in all three dimensions at temperatures approximately three times higher.” Furthermore, a sevenfold increase was observed in the number of electrons participating in superconductivity.

“So, pressure not only raises the temperature at which superconductivity can occur, but also changes the very nature of the superconducting state,” the researcher explained. “It alters the way electrons pair up and move together through the material, resulting in a more robust form of superconductivity.”

Superconductivity under more practical conditions

These precise results will be a valuable aid for theoretical physicists, enabling them to better describe such quantum materials in the future. This will bring the research closer to a long-term goal: tailor-made materials that are superconducting at high temperatures – ideally at room temperature – and under atmospheric pressure. The path to this goal still presents some challenges, but research is advancing. “By investigating important quantum materials, we want to uncover the fundamental mechanisms underlying superconductivity,” Guguchia says. “This will allow us to find ways to optimise the temperatures at which superconductivity occurs.”

In the future, researchers at PSI will be able to delve even deeper into the fascinating world of superconducting quantum materials. After an upgrade of the muon source within the framework of the IMPACT project in the coming years, muon beams hundreds of times more powerful will be available (IMPACT stands for: Isotope and Muon Production using Advanced Cyclotron and Target technologies). PSI also leads the Swiss National Centre of Competence in Research (NCCR) Muoniverse. Building on PSI’s muon source, this project brings together muon research from leading institutions in Switzerland. “We are already looking forward to the new perspectives these two developments will offer,” Guguchia concludes. “Especially for work on superconducting quantum materials, this opens up unimagined experimental possibilities.”

Text: Dirk Eidemüller