An international collaboration between researchers from Denmark, Spain and Austria shows that heating does not always melt a solid material. Their discovery reveals how increasing the temperature of an ultracold quantum fluid can trigger its phase transition to a solid state.

At the beginning of springtime, snow and ice start to melt as temperatures rise. This is in line with our common intuition that heat turns a solid into a liquid and eventually vaporizes the formed fluid. A recent discovery, reported in Nature Communications, now turns this notion upside down and established that heating a fluid can also cause its transformation into a solid state of matter. The considered system, however, is not your ordinary kind of solid and – unlike ice cubes in water – only forms under extreme conditions where effects of quantum mechanics start to play a predominant role. 

In fact, the laws of quantum mechanics allow for the emergence of quite unusual forms of matter that defy the simple categorization into solids, fluids and gases. One of such exotic states is the so-called supersolid. In a supersolid, particles arrange themselves to freeze into an ordered state and, yet, at the same time can flow through the formed solid without any friction. This seeming contradiction has fascinated researchers for many decades, dating back to the first conjecture of supersolidity more than 50 years ago.

However, scientists only recently found ways to explore these questions in actual experiments. This has become possible through a quantum mechanical version of so-called ferrofluids. They consist of tiny magnetic particles suspended in a carrier fluid. Invented at NASA in the early 60s, these magnetic colloids have numerous surprising properties that have since impacted applications in electronics, mechanical engineering, and other industries. In a quantum ferrofluid, the magnetic particles come in the form of single atoms that carry a large magnetic dipole. In the laboratory, such dipolar quantum fluids are tiny droplets that only contain a few 10.000 atoms, cooled down by laser light to astonishingly low temperatures near absolute zero. Extreme conditions like that can force all the atoms to retreat into a single quantum state and thereby form a so-called Bose Einstein condensate. Quantum mechanically, such a Bose-Einstein condensate can be thought of as a quantum fluid composed of a single giant wave that can propagate without resistance – a superfluid with a vanishing viscosity. In a dipolar superfluid, the magnetic interaction between the atoms can trigger the emergence of regular patterns in the Bose Einstein condensate. The resulting state corresponds to a supersolid and has been observed a few years ago in a series of ground-breaking experiments.

Building on these findings, a collaboration between researchers from Aarhus University, BarcelonaTech, the University of the Balearic Islands, and the University of Innsbruck set out to understand the role that temperature plays in the phenomenology of dipolar supersolids. While most prior measurements were performed at the lowest achievable temperatures, the experiment at the University of Innsbruck has been designed to study the supersolid melting behaviour upon a controlled variation of the temperature. To everyone’s surprise, the data revealed that increasing the temperature could also trigger the formation of a supersolid instead of the anticipated melting into a superfluid. The developed theory offered an intuitive explanation for this seemingly paradoxical behaviour. Raising the temperature usually increases fluctuations in a system and speeds up the thermal motion of particles. If this random motion becomes too large, a material melts or a fluid vaporizes. Raising the temperature of a Bose Einstein condensate also increases fluctuations and propels atoms out of the condensate, which remain part of the quantum fluid. Remarkably, the magnetic interaction of even a small fraction of these expelled atoms can have a dramatic effect on the Bose Einstein condensate and induce the formation of the supersolid phase.

“The physics of dipolar quantum fluids have already revealed many surprises, but we did not at all expect that these fluids could solidify upon heating.”, says Thomas Pohl, who led the theoretical efforts at Aarhus University.  “Clarifying this paradox has been an exciting puzzle to solve and makes another crucial step towards better understanding these rich and fascinating systems.” Indeed, the authors’ findings could initiate more detailed investigations into the thermodynamics of dipolar superfluids, which has remained largely unexplored scientific territory until now. “Studying this effect has raised many more questions”, explains first-author Juan Sánchez Baena, “and I hope we will be able to use our recent insights and resolve some of these remaining mysteries.”

Read the research article here