Hard as Honey

Prof. Tilman Pfau had to wait for almost 20 years for this, as he put it in retrospect, “special moment”. He was appointed as an experimental physicist at the University of Stuttgart in the year 2000, where he heads up the 5th Institute of Physics. Since then, he has been working on a question to which his team has now found a definitive answer: yes, supersolidity, a phenomenon first predicted in the ’50s, does actually exist!

To understand the concept, one can picture familiar treats such as honey or ice cream: “As honey ages”, explains Pfau. “it begins to crystallize in the jar. So you have a mixture of set and liquid (clear) honey”. Anyone who eats ice cream too slowly will experience a similar situation: it will start to melt. Supersolid substances are similar; they exist simultaneously in solid and liquid forms. But, as the physicist points out, there is one important difference: “The individual atoms in honey and icecream are either in a solid or liquid state. In a suprasolid substance, on the other hand, each atom is in both states: the atoms are, so to speak, in a solid and liquid state at the same time.”

“But”, as Pfau explains, “the quantum world also affects potential aggregate states of matter”. We usually distinguish between solid, liquid and gaseous states. However, physicists are familiar with other states of matter that they are able to create in the laboratory, which require low – very low – temperatures ; less than a millionth of a degree above absolute zero, which is -273.15 °C: nothing in the universe could get any colder. It was at this low temperature that Pfau and his team came across supersolidity: “Suitable materials consist of atoms with the properties of a crystal, i.e., a solid, and simultaneously that of a liquid.” This is basic research in its purest form – it is far too early for questions concerning technical applications. This can be illustrated by comparison with a completely different, but also “cryogenic” field of research – the field of superconductivity. Since 1911, it has been known that at extremely low temperatures certain materials can conduct electricity without loss. However, only in recent times has there been any appreciable progress in the technical application of this knowledge. The situation concerning supersolidity could be similar.

In their search for supersolids, physicists around the world have long been using helium, a material which had already been shown to be good for another phenomenon in low-temperature physics. Since the start of the 20th century, physicists have been able to cool helium to a point where it becomes liquid at temperatures close to absolute zero. However, because of the quantum world, this liquid state of aggregation is a special one: in this state, helium no longer evinces any internal friction, which is expressed in such a way that it flows over obstacles, that it could not have overcome at all according to the laws of macrophysics. Physicists refer to this state of aggregation as superfluidity, in allusion to a fluid substance. So, strictly speaking, when Tilman Pfau talks in terms of supersolidity, he is referring to a state of aggregation in which atoms are simultaneously part of a crystal and a superfluid.

“However”, says Pfau, “helium proved unsuitable for this type of research, as supersolid helium doesn’t seem to exist”. So, researchers turned to other elements. Chromium was regarded as a source of hope for a long time, including in Stuttgart. “Two types of interactions between the atoms play a role at temperatures close to absolute zero”, says Pfau. “When two atoms come very close, they bounce off one another like billiard balls. At the same time, chromium atoms interact magnetically over long distances and can attract or repel each other.”

However, one should not imagine the material sample with which the Stuttgart-based team is currently experimenting as a piece of metal. Instead, the objective is to combine the largest possible number of individual atoms in a controlled manner at low temperatures to form a condensate on which the actual experiments to find out whether it is a supersolid or not can be performed. All the atoms in such a condensate, whose density is lower than that of air, must move in perfect harmony: only then can the desired quantum effect occur, i.e., the indistinguishability of the atoms. This can be specifically adjusted by finding the right balance between short-distance billiard ball collisions and long-distance magnetic interactions. “However”, says Pfau, “this didn’t work. The chromium condensate kept collapsing: we couldn’t measure anything”.

The team continued to refine the process used to generate the dysprosium droplets. “In 2018, we succeeded in slowly and carefully combining the atoms into a condensate such that they remained still for the requisite measurements.” By then, time was running out, as the Stuttgart-based team was no longer the only one on the trail of supersolids. To be unambiguously considered a supersolid, a condensate must meet three criteria: a crystal structure must form, which must be embedded within a superfluid, and sound waves must propagate through the condensate in a characteristic manner. Measuring the speed of the sound waves, in particular, was a hard nut to crack, which nobody had been able to crack before. The Stuttgart-based team succeeded in doing so at the start of this year. “Ultimately, three or four droplets proved sufficient for the measurement”, says Pfau.

They immediately took their results to their colleague Prof. Hans Peter Büchler, head of the Institute of Theoretical Physics III [de]. His job was to verify whether the experimental results actually corresponded with the theoretical expectations. “Only after extensive discussions with him was I convinced that we had actually proven the existence of supersolidity.”

Text: Michael Vogel

This text was published in the magazine “forschung leben”.