Can Helium-4 Transition to a Supersolid?

For forty years supersolid behavior has been predicted, and since the 1970s, theories about supersolid behavior involving helium-4 have been developed. However, it wasn’t until 2004 that some evidence of supersolid behavior was found. But, when a group of scientists from the Helsinki University of Technology tested the thermodynamical properties of solid helium-4, they found something different.

“We found some unexpected behaviors in entropy at low temperatures,” Igor Todoshchenko, one of the experimenters, tells PhysOrg.com. “This is an anomaly that no one thought of.” The results of the experiment are published in an article titled “Melting Curve of 4He: No Sign of a Supersolid Transition down to 10 mK” in Physical Review Letters.

“In solid helium-4,” explains Todoshchenko, “the absences of atoms — called vacancies — are very mobile and delocalized, being the property of the whole solid. At very low temperatures, the idea was that they may condense to form a superfluid.” He pauses before continuing. “If you think about the motion of the vacancy system in one direction, then the mass of the solid is moving in the opposite direction, which means that crystalline solid moves without friction. The solid behaves like a superfluid. This is the theoretical prediction that many people have tried for years to find.”

And, while this was thought to have been achieved in 2004, through observation of supersolid-like characteristics, the mechanism was not understood, and no knowledge of how the helium-4 supersolid was formed came out of it. “We started to investigate the behaviour of entropy of the [helium-4] solid by measuring its melting pressure,” Todoshchenko says. “Entropy can be used to verify theoretical models of solid. In particular, it should drop at the supersolid transition.”

And what did the Helsinki University of Technology team find when they began to measure the entropy of solid helium-4? “It is not behaving as predicted by classical model of solid,” Todoshchenko explains. “Below the temperature 0.1K, the entropy stopped decreasing with decreasing the temperature. It stayed constant. This is a deviation from classical behavior.”

Todoshchenko does not rule out the possibility of a helium-4 supersolid, however. “What does this mean? We do not see a drop of the entropy at the supposed supersolid transition, but we see that it stays constant, probably indicating some low-energy degrees of freedom in solid. Maybe that it isn’t connected to a supersolid, maybe it is something else. Or maybe it is supersolid, but its nature is more complicated than thought before.” He points out this anomaly requires further study: “We are interested in this anomaly because it is not what anyone expected… Our accurate measurements of the entropy of the solid show there is some different system, consisting of defects, or impurities, or something we don’t know.”

In order to improve their knowledge of the properties of solid helium-4 at low temperatures, the Helsinki team plans to run experiments with different concentrations of helium-3 impurities. “Our findings might be related to impurities,” Todoshchenko suggests. “We will do this measurement again, but with ultra-pure helium-4, and we will compare the results with what we measured in helium-4 of commercial purity. If the anomaly is present also in ultra-pure helium-4, it is not connected to impurities but it is intrinsic property of solid helium-4.”

For now, the findings in Helsinki seem to indicate that there is no transition in a helium-4 solid to a supersolid. But the search for a new quantum phase of matter continues. “We will keep working on this,” Todoshchenko says. “Our measurements could give a better idea of what is solid helium-4 at low temperatures, a better understanding of the characteristics of what might make a supersolid.”

By Miranda Marquit, Copyright 2006 PhysOrg.com

Related stories:

Can quantum antiferromagnets reveal secrets of bosonic supersolids?
“One of the fundamental issues in physics right now – and for the past many years – is whether or not bosons can form a supersolid phase,” Frédéric Mila tells PhysOrg.com. Mila is a scientist at the Institute of Theoretical Physics at École Polytechnique Fédérale in Lausanne Switzerland. “We show how a supersolid phase may be achieved in a quantum antiferromagnet.”
Quantum Criticality Found in a Simple Liquid
A team from the Low Temperature Laboratory, Royal Holloway, University of London, has discovered a breakdown of the standard theoretical model of strongly interacting fermions in liquid 3He films.
Probing Question: Are there upper and lower limits to temperature?
Most people have heard absolute zero described as the lowest possible temperature, but what does that mean? Is it really the coldest cold, or just the lowest temperature that we can measure? Is there a corresponding highest temperature? According to Moses Chan, Evan Pugh professor of physics at Penn State, answering these questions requires understanding the meaning of temperature.
High-quality helium crystals show supersolid behavior
High-quality, single-crystal, ultra-cold solid helium exhibits supersolid behavior, suggesting that this frictionless solid flow is not a consequence of defects and grain boundaries in poor-quality, polycrystalline, solid helium, according to a team of Penn State researchers.
When is a supersolid not quite so super?
A deceptively simple experiment, recently published in the journal Science, has moved physics one step closer to explaining the odd behavior of supersolid helium. The unusual state of matter – in which a portion of the atoms are able to flow through a solid crystal with no resistance – was predicted as early as 1969 but not observed until recently.
'Supersolid' or melted 'superfluid' film: A quantum difference
New calculations support an alternative to "superfluidity" of a solid as the explanation for the behavior of an isotope of helium, ⁴He, at temperatures approaching Absolute Zero, according to a report in Physical Review Letters.
Probing Question: What is a supersolid?
"Imagine you have an orchestra together, but everyone is playing their own tune, until they begin to follow a conductor. In a normal solid, every atom has its own behavior until very close to absolute zero. Then quantum mechanics takes over and dictates everyone to play the same tune."
That's physics professor Moses Chan's musical metaphor for his discovery that atoms in a solid can condense into what he likes to call "one giant atom," a new phase of matter called a supersolid. Together with post-doctoral associate Eun-Seong Kim, Chan found that when a particular type of helium gas has frozen into a crystal at a fraction of a degree above absolute zero, it exhibits a property only seen before in superfluids: no friction.
Probable observation of a supersolid helium phase
Just last year we have seen a Nobel Prize in physics awarded to Abrikosov, Ginzburg and Leggett for "pioneering contributions to the theory of superconductors and superfluids"
And now Nature publishes an article by E. Kim and M. Chan (vol 427, p. 225) who claim to observe a 'supersolid' He phase.

News discussion:

Physics news