In school, we are taught that solid, liquid, and gas are the basic states of matter. What is often skipped is that they're not the only ones, and nature has a lot more variety than what we can experience in our everyday life.
When pressure and temperature are taken to the extreme, matter develops incredible properties. ETH Zurich and Massachusetts Institute of Technology (MIT) physicists have just proven the existence of a new one.
The new state is a supersolid that combines the regular structure of a solid with the lack of viscosity of a superfluid. The state, as reported in the two Nature papers (ETH’s and MIT’s), has been described as the first supersolid with ultracold quantum gas.
“It is counterintuitive to have a material which combines superfluidity and solidity,” Professor Wolfgang Ketterle, senior author on the MIT paper and team leader, said in a statement. “If your coffee was superfluid and you stirred it, it would continue to spin around forever.”
The MIT team started with a state known as a Bose-Einstein Condensate (BEC), which was co-discovered by Ketterle and earned him a Nobel Prize in 2001. BEC can be achieved at incredibly low temperatures, a few billionths of a degree above absolute zero in an ultrahigh vacuum.
The researchers manipulated the BEC using two lasers to control their motions and create the crystalline structure, while the quantum gas continued to exhibit superfluidity. This was called the stripe phase as the density of the supersolid is not constant but changes and propagates inside the supersolid like a ripple.
“The recipe for the supersolid is really simple,” lead author Jun-Ru Li added, “but it was a big challenge to precisely align all the laser beams and to get everything stable to observe the stripe phase.”
The ETH team also used a BEC to obtain the supersolid, but instead of having two lasers, they used a single one and a special device with two chambers, where the light of the laser could create resonance. This resonance trapped the BEC’s atoms into a regular shape.
Scientists have been trying to achieve this state for decades. In 1969, British physicists David Thouless suggested that superfluids could be turned into supersolids, and many tried to achieve that state using helium atoms, with no luck. The two successful supersolids steered clear of the complexity of superfluid helium.
“Our work has now successfully implemented Thouless’s ideas,” explained Tobias Donner, ETH's senior author, in a statement. “We didn’t use helium, however, but a Bose-Einstein condensate.”
There’s no technical application for such a curious state, but scientists think it might help us understand superconductivity and superfluids better.