Some solids can be stirred. Researchers have shown that a bizarre quantum state of matter that is both a solid and a frictionless fluid at the same time can be agitated, apparently creating a series of tiny vortices inside the material. The experiment is evidence of the progress being made in understanding the strange properties of so-called “supersolids”.
“I’ve never done, in all my life, such a challenging experiment,” says at the University of Innsbruck in Austria. In 2022, she and her colleagues became the first researchers in the world to create a chunk of supersolid big enough to form into a pancake, about 0.01 millimetres in diameter. But having done this, they wanted to see how it would respond to being stirred.
Ordinary solids cannot be stirred, but supersolids are extraordinary – they exist at extremely low temperatures where quantum effects endow them both with a crystal-like structure akin to atoms in a diamond and with zero viscosity, so they are “superfluid”. Physics tells us that any superfluid always develops tiny vortices when stirred, says team member , also at the University of Innsbruck.
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The researchers wanted to know whether they could create – and image – similar vortices in their supersolid.
They made their supersolid from several tens of thousands of atoms of the rare earth element dysprosium, which the researchers had to cool to temperatures close to absolute zero before carefully controlling how they interacted with each other. They then had to work out how to stir the supersolid without destroying its special properties – something that hadn’t previously been tackled in experiments, says at the University of Innsbruck who worked on the project.
Using rotating magnetic fields as “stirrers” proved successful. But imaging the stirred supersolid was a challenge, because the vortices that this stirring created settled into supersolid regions where the density was low. This meant there was little visual contrast between vortices – which are empty in the middle – and the rest of the supersolid. To take images that showed signatures of the vortices, the researchers devised a way of tuning the interactions between the atoms while illuminating them with more lasers, which effectively “melted” some of the supersolid and made the vortices more visible.
at the University of Stuttgart in Germany, who was not involved in the research, thinks the experiment presents promising evidence that supersolids do indeed develop vortices when stirred. But he says there is still room for more precise measurements, like measuring the speed at which the vortices rotate or getting images with even better contrast.
“This is really a new state of matter,” says at the University of Trento in Italy, whose theoretical work partly inspired the researchers. He says there are many more experiments with supersolids that are just now becoming possible to perform, and they could uncover previously unseen, fundamentally quantum effects even more complex than superfluid vortices.
For Bland, the research could have an unexpectedly far reach. He says there may be similar states of matter – complete with vortices – inside superdense neutron stars.
arXiv
