"By Kate McAlpine

 

"The first supersolid -- a ghostly, quantum form of matter in which a solid flows, frictionless, through itself -- was reportedly made in 2004. But a debate has raged ever since over whether the researchers involved had simply misinterpreted their results. Now two new studies suggest that genuine supersolids have been made after all.

 

"According to quantum theory, supersolidity should kick in at very low temperatures. In a solid, atoms are bound together in a regular lattice, keeping their structure rigid under normal circumstances. But if you cool some solids close to absolute zero, they should become frictionless, flowing supersolids, while retaining their lattice structure. In the original experiment, Eunseong Kim -- now of the Korea Advanced Institute of Science and Technology in Daejeon, South Korea -- and Moses Chan of Pennsylvania State University in University Park cooled and pressurised liquid helium until the atoms were forced into a crystal lattice. They then made a cylinder filled with this solid helium spin one way and then the other, over and over again. As they cooled it, the cylinder switched direction more frequently. The researchers concluded that some of the helium was standing completely still, reducing the mass that was rotating along with the cylinder and allowing it to switch more quickly. They assumed that this was because some of the helium had become frictionless due to supersolidity....

 

"Earlier this year, however, this interpretation was challenged by John Reppy of Cornell University in Ithaca, New York. He suggested that the reason the cylinder switched more quickly at lower temperature was because the helium had become a wobbly 'quantum plastic', a previously unknown phase of matter that is distinct from a supersolid. The increased elasticity of this new material allowed the cylinder to more easily reverse its rotation, he said.

 

"To test whether Reppy was right, Kim spun the larger apparatus in which the cylinder sits: the apparatus spun in just one direction, while the cylinder spun one way, then the other, as it had before. He reasoned that elasticity should affect only how quickly the cylinder switched direction, not its actual spinning rate. Therefore if Reppy was right, and the solid helium was a quantum plastic, adding a constant underlying rotation should not change the results. His team found, however, that it did. Unlike in the original experiment, the direction switches did not get faster with a falling temperature. The best way to explain this, says Kim, is if the helium is indeed supersolid. That’s because, in a supersolid, the constant rotation should cause vortices to form, rather as in a liquid, disturbing the material’s quantum properties, and reducing the supersolidity....

 

"In a tantalising coincidence, Yaroslav Lutsyshyn of the Polytechnic University of Catalonia in Barcelona, Spain, and his colleagues have just found further evidence of supersolidity. Theory suggests that supersolid helium flows because holes form in the crystal lattice. Lutsyshyn's team experimented with how likely these holes were to form under different pressures; and it turned out that the pressure under which holes formed most easily matched that at which Kim and Chan identified the largest proportion of supersolid helium in their system.

 

"'It's like rabbit ears sticking out of the grass,' says Lutsyshyn of his team's results. He also holds that the experiment by Kim's team is strong evidence -- but not full proof -- that the solid helium contains a supersolid. Reppy, however, remains unconvinced that Kim's most recent work rules out quantum plasticity as the cause of the apparent supersolid effects. He disputes Kim's assumption that the extra energy added by rotating the whole apparatus would not affect the rate at which a quantum plastic switches. 'I'm now fairly certain that the large supersolid signals that they were seeing are manifestations of the elastic properties,' he says." [Quoted from here; accessed 10/10/2016. Quotation marks altered to conform with the conventions adopted at this site. Several paragraphs merged.]