Studies show that heat can freeze liquids in the quantum world

May 23, 2023

(Nanowerk News) In early spring, snow and ice begin to melt as temperatures rise. This phenomenon exemplifies the transition from solid to liquid when heat energy is added to matter. We can easily observe this in nature. Continuous heat will cause liquid to evaporate, according to the laws of physics that we all know.

Now, the latest discoveries, published in Nature Communications (“Heating a dipolar quantum liquid to a solid”), reversed this idea and determined that extreme heating of a fluid can also cause it to transition to a solid state. However, the system that was created was not the usual solid type that we are familiar with and, unlike an ice cube in water, only formed under extreme conditions, where the effects of quantum mechanics started to play a decisive role. Illustration showing the solidification of a liquid with increasing temperature. (Image: UPC)

In fact, the laws of quantum mechanics allow for the emergence of unusual forms of matter, which defy simple categorization into solids, liquids, and gases. One such exotic state is what is called superdense. In a supersolid, the particles arrange themselves to freeze into an ordered state and are, however, at the same time able to flow through the structure formed without any friction. Thus, they have solid and superfluid properties.

This apparent contradiction has fascinated the scientific community for decades, since the first allegations of supersolidity more than 50 years ago. However, scientists are only recently finding ways to explore these questions in actual experiments. This is made possible through a quantum version of so-called ferrofluids: microscopic magnetic particles suspended in a liquid.

Experimenting with quantum ferrofluids

Invented at NASA in the 1960s, ferrofluid is a magnetic colloid that has many surprising properties and applications in electronics, mechanical engineering, and other industries. In quantum ferrofluids, magnetic particles correspond to single atoms. In the laboratory, such a dipolar quantum fluid is a microscopic droplet containing about 10,000 atoms, which is cooled by a laser beam to an extremely low temperature close to absolute zero.

Such extreme conditions could force all atoms back into a single quantum state and form what is called a Bose-Einstein condensate. This state can be thought of as a fluid that can propagate unhindered—a superfluid with zero viscosity. In dipolar superfluids, magnetic interactions between atoms can trigger the appearance of regular patterns in the condensate. The resulting state corresponds to an exotic, extremely dense state of matter observed several years ago in a series of groundbreaking experiments.

New knowledge to solve scientific puzzles

Based on these findings, a collaboration between researchers from Universitat Politècnica de Catalunya – BarcelonaTech (UPC), University of the Balearic Islands (UIB) in Palma de Mallorca, University of Aarhus in Denmark and University of Innsbruck in Austria set out to understand the role that temperature plays in the phenomenology of dipolar superdensities.

While most of the previous experimental measurements have been carried out at the lowest achievable temperature, the experiment at the University of Innsbruck was designed to study the melting behavior of supersolids under controlled temperature variations. To everyone’s surprise, the data revealed that increasing temperature could trigger the formation of supersolids, not the expected melting behavior.

The theory was developed by Juan Sánchez Baena, a postdoctoral researcher at UPC’s Computer Simulation in Condensed Matter Group (SIMCON), during a research visit at Aarhus University, together with Aarhus professor Thomas Pohl and UIB professor Fabian Maucher.

The researchers offer an intuitive explanation for this seemingly paradoxical behavior. Raising the temperature usually increases the fluctuations in a system and speeds up the thermal motion of the particles. If this motion becomes too great, then the solid will melt or the liquid will evaporate. Raising the temperature of the Bose-Einstein condensate also increases the fluctuations and pushes the atoms out of the condensate, which remain part of the quantum fluid. The magnetic interaction of this ejected fraction of atoms can induce the formation of a supersolid phase.

“The physics of dipolar quantum fluids has revealed many surprises, but we did not expect these liquids to solidify when heated,” said researcher Thomas Pohl, who leads the theoretical effort at Aarhus University. “Clarifying this paradox has been a fascinating puzzle to solve and makes another important step towards better understanding this rich and amazing system.”

Indeed, the authors’ findings may initiate more detailed investigations into the thermodynamics of dipolar superfluids, which remains an unexplored area so far. “Studying this effect has raised even more questions,” explained first author Juan Sánchez Baena. “I hope we can use our recent insights to solve some of these lingering mysteries,” the researcher added.

Raising the temperature of a dipolar quantum liquid can trigger a phase transition to solid matter. Heating usually tends to melt or vaporize a material, but in a dipolar quantum liquid it can produce a superdense, consisting of mesoscopic quantum droplets arranged in an ordered structure.

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