The quantum artists: Atoms under pressure

Materials scientists aren’t exactly squeamish. They clamp materials tightly and push or pull at them with great force until the samples break or shatter. Things aren’t quite so rough in Zurab Guguchia’s lab, though. The physicist at the PSI Center for Neutron and Muon Sciences isn’t trying to destroy anything, but rather to create something new. For example, exotic substances that conduct electricity without any losses, even at high temperatures, or that exhibit novel magnetic and electronic properties. If his experiments are successful, they could not only provide new insights in quantum physics but also open doors to practical applications – such as energy-efficient power grids or electric motors that don’t require rare-earth magnets. These metals aren’t exactly rare, but mining them is complex and expensive.

The sample that Guguchia is currently examining at the Laboratory for Muon Spin Spectroscopy is invisible to lab visitors. The small metal fragment is hidden in a tube, immersed in an oily liquid that exerts gentle, all-round pressure. As the hydrostatic pressure gradually increases, the measuring instruments display surprising values. Suddenly, superconductivity sets in: electric current flows without any resistance – triggered solely by the pressure and without the need for extreme cooling of the sample, as is normally required for superconductivity.

Guguchia discovers fascinating effects in his data almost every week. He regularly publishes in prestigious scientific journals, including a 2022 article in Nature – a great honour for any researcher. In conversation, he hints that further publications are already in the works.

His latest study involves a material with a layered structure, like puff pastry, where each layer is only one atom thick. Place the sample on a lab bench, and nothing happens. But if you clamp it in a movable frame and pull gently, the layers stretch and move closer together – just like pastry dough being rolled out. Alternatively, you can place the sample in an oilfilled tube where the hydrostatic pressure acts on it evenly from all sides. If all the parameters are right, something magical happens: the electrons – negatively charged particles that orbit atomic nuclei – begin to sense the electrons in neighbouring layers. This creates so-called quantum phases: the material loses its electrical resistance and becomes superconducting, or develops magnetic properties, or forms a charge-ordered structure in which the charge carriers arrange themselves in regular patterns. In quantum materials, these three ordered states often coexist and interact in complex ways.

Conflict of the quantum phases

Physicists know of many different quantum phases such as these, each producing distinct types of electron interactions. If the sample is stretched or compressed, it can also exhibit properties contrary to the expected results. Therefore, Guguchia precisely adjusts the forces in the compression or tension device to suppress undesirable phases and enhance desirable ones, such as superconductivity. The desired effect only occurs when the atomic layers are distorted in a very specific direction. For example, Guguchia discovered that applying a tensile force to cuprates – Nobel Prize-winning high-temperature superconductors – can increase their superconducting temperature fivefold. These findings underscore the potential of such mechanical distortion. The research has been published in two highly regarded journals.

Guguchia envisions a material that can be switched into various desired phases – or even modulated continuously – by external forces. It could serve as a kind of switch that shifts from zero resistance (superconducting) to a metal’s normal resistance. In combination with other materials, products with novel technical properties are conceivable – such as electric motors that do not require rare-earth magnets.

Atomic bamboo baskets

Guguchia is continuing his experiments with hydrostatic pressure and directed tension. In the scientific community, however, he is known worldwide for another major discovery – one that has earned him invitations to leading international conferences: kagome. An Internet search for this term will produce images of traditional Japanese woven bamboo baskets. In the quantum world, there are atomic lattices that replicate this pattern of hexagons surrounded by triangles at each edge, which in turn are connected to other hexagons – in an endlessly repeating structure.

Scientists have long suspected that flat, two-dimensional atomic lattices can exhibit charge ordering. This arises from the collective behaviour of the electrons, through spontaneous currents, without external excitation. Guguchia was the first to discover this experimentally in the laboratory, in a kagome lattice made of potassium, vanadium, and antimony atoms. The breakthrough came thanks to the powerful SμS muon source at PSI. In the experiment, a muon – an electrically charged elementary particle 200 times heavier than an electron – serves as a highly sensitive microscopic measuring instrument. It is implanted into the kagome lattice and observed as it decays. This provides information about the local magnetic field and thus about the spontaneous currents flowing in the ring of six atoms.

While a suitable material typically needs to be cooled to around minus 240 degrees Celsius to become superconducting, in the kagome lattice this effect already occurs at around minus 190 degrees Celsius – a temperature that can be achieved by relatively inexpensive cooling with liquid nitrogen. Researchers also suspect that such spontaneous currents could exist in cuprates – the high-temperature superconductors whose discovery was awarded the Nobel Prize in Physics in 1987. There is growing hope that applying the kagome architecture to these materials could further increase their critical temperature.

Wanted: Room-temperature superconductors

This hope has intensified in recent months. Guguchia recently discovered charge ordering in a Kagome lattice at temperatures up to 527 degrees Celsius – a finding published in the renowned journal Advanced Materials. While superconductivity in the same material typically occurs at low temperatures, Guguchia’s pressure experiments showed that it does not follow conventional rules. This raises a key question: Could the charge ordering be suppressed at high temperatures? And might this reveal a state near room temperature in which current flows without resistance and without cooling?

As someone engaged in fundamental research, Guguchia remains modest: “This quantum system is very promising.” But the implications are far-reaching. A superconductor that works at room temperature would revolutionise the energy landscape: cables with zero electrical resistance could potentially save 40 percent of global energy consumption. 

“My main interest is to understand the fundamental mechanisms behind unusual quantum phenomena and learn how they can be optimised,” says Zurab Guguchia. “With its unique combination of large research facilities and strong theoretical and computational research groups, PSI is the perfect environment to combine theoretical and applied research.”