ETH Zurich/Julian Leonard

Physicists Say They've Created an 'Impossible' New Form of Matter: Supersolids

The most compelling evidence yet that supersolids exist.

FIONA MACDONALD
3 MAR 2017
 

Our world just got a little stranger, with physicists claiming they've successfully created a brand new 'impossible' form of matter in the lab - supersolids, which have properties of both liquids and solids at the same time.

Scientists have predicted that this exotic state of matter could exist for more than 50 years, but no one had been able to demonstrate that it's actually possible. Now, two independent teams of physicists have used different techniques to achieve the same odd result - what they claim are the first examples of supersolid matter.

 

There's sure to be controversy over whether these new experiments unequivocally demonstrate a supersolid state - especially after a similar claim in 2004 went on to be debunked -  but this is the most compelling evidence yet that supersolids could actually exist.

For those who aren't familiar with just how bizarre that is, a 'supersolid' is a strange sate of matter that has the crystalline structure of a solid, while flowing like a liquid - something that's pretty contradictory when it comes to traditional physics.

Usually, matter exists in just four simple states: solid, liquid, gas, and plasma. These states arise depending on conditions such as temperature and pressure, and are defined by the arrangements of particles within the matter.

What's weird about the supersolid state of matter is that the particles are arranged in a rigid, solid structure, but then it can also flow without viscosity - or 'stickiness' - which is a key characteristic of a superfluid

"It is counterintuitive to have a material which combines superfluidity and solidity," said lead researcher of one of the teams behind the discovery, Wolfgang Ketterle from MIT.

"If your coffee was superfluid and you stirred it, it would continue to spin around forever."

 

Supersolids were first predicted by Russian physicists back in 1969, who hypothesised that a helium-4 isotope could display solid and liquid properties simultaneously, under certain conditions.

For a long time, researchers generally assumed that it would be impossible to create such a structure - but that didn't stop some from trying.

A breakthrough came in 2004, when Pennsylvania State University researchers cooled helium to less than one-tenth of a degree above absolute zero (around -273 degrees Celsius) and stumbled upon what might have been a supersolid state.

As Bec Crew reported for us last year, the team wasn't confident enough to say they'd actually made a supersolid, seeing as they couldn't rule out the possibility that a thin layer of liquid had snuck inside the container and skewed their results.

Several experiments in the decade that followed further debunked the idea that a supersolid had been made, by showing that helium-4 has a type of 'quantum plasticisity' under certain situations, which isn't caused by supersolidity.

Most of the science community pretty much decided that the 2004 sample wasn't an example of a real supersolid, and for the past few years the field has been pretty quiet on the subject.

But then in November, not one, but two independent teams both declared in pre-print papers that they'd done it - they'd managed to create supersolids in the lab.

The researchers are from MIT in Cambridge, Massachusetts, and ETH Zurich in Switzerland, and although they both had different processes, the team had both used used a strange type of gas known as a Bose-Einstein condensate to create their supersolids.

Bose-Einstein condensates are a fifth state of matter that appear at ultra-cold temperatures, where atoms behave like waves. 

They have unique properties of their own, but what's good about using a Bose-Einstein condensate to create a supersolid is that it's already a superfluid, so it's halfway there.

The team took these ultra-cold gases and used slightly different techniques to coax them into a quantum phase of matter with a rigid structure like a solid, but the ability to flow like a superfluid.

At the time, the two teams published their results on the pre-print server arXiv.org. And now they've both been peer-reviewed and published in Nature (here and here)offering the most substantial evidence to date that supersolids are real.

The Swiss researchers were able to achieve this by taking a small amount of rubidium gas and putting it in a vacuum chamber, where they cooled it to a few billionths of a kelvin about absolute zero, causing them to form a Bose-Einstein condensate.

The team then put this condensate in device with two optical resonance chambers, each consisting of two tiny opposing mirrors. Using lasers, the particles eventually adopted a regular, crystal-like structure, indicative of a solid.

But the condensate also retained its superfluid properties - they were able to flow without any energy input, which isn't possible in a normal solid.

"We were able to produce this special state in the lab thanks to a sophisticated setup that allowed us to make the two resonance chambers identical for the atoms," one of the ETH Zurich team, Tilman Esslinger, told Phys.org.

The MIT team took a different approach - they used a combination of laser and evaporative cooling methods to turn sodium atoms into a Bose-Einstein Condensate.

They then used lasers to also manipulate it into a crystalline solid arrangement by creating density variations in the atoms.

Although the process was different, the end result was the same as the Swiss team's - solid matter that flowed like a superfluid.

The fact that the results has been verified by two teams at the same time makes it even more compelling that these supersolids are the real deal.

"It's certainly the first case where you can unambiguously look at a system and say this is both a superfluid and a solid," Sarang Gopalakrishnan from the City University of New York, who wasn't involved in the research, told Science News back in November.

It's likely there will now be a new round of independent testing and verifying, to make sure that what has been produced can really be called a supersolid. 

There's the argument that because the team used Bose-Einstein Condensates rather than helium-4 to create the state of matter, it could be seen as 'cheating'. But it's definitely our most compelling evidence to date that supersolids exist. 

So what does a potential new state of matter for the rest of us? Right now, not much. The fact that these materials can only exist at extremely low temperatures in ultrahigh-vacuum conditions means they're not very useful at the moment.

But a further understanding of the strange state of matter could lead to improvements in superconductors - incredibly useful materials that conduct electricity without resistance.

"With our cold atoms, we are mapping out what is possible in nature," said Ketterle.

"Now that we have experimentally proven that the theories predicting supersolids are correct, we hope to inspire further research, possibly with unanticipated results."

Although the fact that two teams have both made this claim at the same time might sound competitive, the reality is that both research groups welcome the validation and feedback from each other.

"The simultaneous realisation by two groups shows how big the interest is in this new form of matter," Ketterle added.

The two papers have been published in Nature here and here.

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