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Solids can be melted by heating, but in the quantum world it can also be the other way around: In a joint effort, an experimental team led by Francesca Ferlaino in Innsbruck, Austria, and a theoretical team led by Thomas Pohl in Aarhus, Denmark, show how a quantum liquid forms supersolid structures through heating. The scientists obtained a first phase diagram for a supersolid at finite temperature.

Supersolids are a relatively new and exciting area of research. They exhibit both solid and superfluid properties simultaneously. In 2019, three research groups were able to demonstrate this state for the first time beyond doubt in ultracold quantum gases, among them the research group led by Francesca Ferlaino from the Department of Experimental ÃÈÃÃÉçÇøics at the University of Innsbruck and the ÖAW Institute for Quantum Optics and Quantum Information (IQOQI) in Innsbruck.

In 2021, Francesca Ferlaino's team studied in detail the life cycle of supersolid states in a dipolar gas of dysprosium atoms. They observed something unexpected: "Our data suggested that an increase in temperature promotes the formation of supersolid structures," recounts Claudia Politi of Francesca Ferlaino's team. "This surprising behavior was an important boost to theory, which had previously paid little attention to thermal fluctuations in this context."

The Innsbruck scientists joined forces with the Danish theoretical group led by Thomas Pohl to explore the effect of thermal fluctuation. They developed and published in Nature Communications a theoretical model that can explain the experimental results and underlines the thesis that heating the quantum liquid can lead to the formation of a quantum crystal. The theoretical model shows that as the temperature rises, these structures can form more easily.

"With the new model, we now have a phase diagram for the first time that shows the formation of a supersolid state as a function of temperature," Francesca Ferlaino says. "The surprising behavior, which contradicts our everyday observation, arises from the anisotropic nature of the dipole-dipole interaction of the strongly magnetic atoms of dysprosium."