Searching for a solid that flows like a liquid

Searching for a solid that flows like a liquid
Hans Lauter with the sample environment cell within which he grows the solid helium samples used in his neutron scattering experiments at ORNL's Spallation Neutron Source

( -- A series of neutron scattering experiments at Oak Ridge National Laboratory and other research centers is exploring the key question about a long-sought quantum state of matter called supersolidity: Does it exist?

"The goal of our experiments is to find this new quantum state of supersolidity. This is a challenge for theory as well as experiment," says principal investigator Hans Lauter of ORNL. "The superfluid transition we have observed in solid helium may lead to supersolid helium as a new quantum state."

Whether there's such a thing as supersolidity isn't an issue apt to cause much of a stir outside the physics community. But in the world of , discovering a new quantum state would be like sighting a new species would be for a biologist, or a for an .

Quantum superstates—stranger than fiction

Quantum mechanics describes the way matter behaves at the nanoscale (the atomic/molecular level), where "particles" of matter are simultaneously waves (at least in quantum theory). Observations from the world of quantum mechanics seem completely bizarre to us because all our experience and intuitions argue against them: a particle/wave can, at the same time, move and not move, it can be in more than one place at the same time.

We don't see these behaviors in everyday-size objects because quantum states are exquisitely fragile, easily crushed by the comparatively huge forces in what physicists call the macro world. In fact, only two "superstates" resulting from quantum mechanics have been conclusively observed at the macro level: superconductivity, in which electrons pair up and flow without resistance through a material, and superfluidity, in which liquid helium loses all viscosity and can flow through the finest pores in any material. And both of these states so far are seen only at very low temperatures at which there is little energy to disrupt them. The supersolid state would be even more elusive, existing only in helium-4 cooled to barely above absolute zero (0 kelvin, the temperature at which atoms lose all their energy) and subjected to extreme pressure.

Understandably, many people hearing the word "supersolid" assume it means something really hard. Actually, it indicates almost the opposite ("super" in this case means beyond solid rather than more solid). It's a profoundly difficult concept. Picture a puzzle with several pieces missing. Imagine the vacant spaces beginning to flow through the tiny seams between the puzzle pieces, eventually pooling together and seeping through the entire picture as a wave. That's something like how physicists conceive of a supersolid: the atoms in the material have spatial order (i.e., each atom occupies a particular position in space), but under certain conditions, vacancies in the structure begin to flow through the solid without resistance, like a superfluid.

The existence of supersolid helium has been predicted theoretically but never observed conclusively. In 1969, physicists in Russia theorized a state of solid matter in which vacancies in the crystal structure of solid helium-4 could condense into something like a single entity and flow without resistance through the atoms. Experimenters at Penn State University claimed to have verified the existence of supersolid helium in 2004 in experiments using an oscillator, but others questioned whether they actually observed a supersolid, or just a superfluid confined in solid helium.

Hunting supersolids with neutrons

A paper published recently in Physical Review Letters adds significant knowledge to the debate, although it doesn't settle it. A research team led by Lauter, in experiments at the Institut Laue-Langevin neutron scattering center in France, obtained data indicating a superfluid state incorporated in a sample of solid helium-4, but not necessarily a supersolid state.

A series of follow-on experiments conducted at the Spallation Neutron Source at Oak Ridge National Laboratory showed "deviations from known structures" in solid helium, but data from those studies are still being analyzed, Lauter says. He is preparing yet another set of experiments early in 2012 at SNS, along with computational calculations, that he hopes will establish precisely what happens inside solid helium under extreme temperature and pressure.

The Cold Neutron Chopper Spectrometer at SNS is the best instrument available for measurements at the momentum and energy ranges at which superfluid effects manifest, says Lauter. And SNS has the sample environment capabilities to make the experiments feasible. "Superflow effects appear below a temperature of about 100 mK and in the pressure range from 25 to 60 bar. To look for these effects within the same range, a special sample cell and a dilution refrigerator are necessary," Lauter notes. A one-of-a-kind sample cell had to be designed and fabricated from an alloy that meets stringent requirements for heat conduction and ability to withstand pressure, and produces little background scattering. A dilution refrigerator had to be adapted for neutron scattering and for unusual stepwise increases and decreases in temperature.

The ILL experiments by Lauter's team studied a 3 by 5 centimeter sample of solid helium-4 powder condensed in a matrix of highly porous aerogel. Previous experiments have indicated that the supersolid effect would appear only in a crystal that is perturbed (i.e., made to deviate from its ideal structure). The aerogel matrix generates numerous dislocations, which act as nucleation sites for imperfections.

The question was whether a supersolid state would emerge in the sample, not just a superfluid component occurring within solid helium, Lauter explains. "The latter is a coexistence, like ice and water. In this case, it is some sort of coexistence of liquid in a quasi 2-dimensional state with 3-dimensional solid helium around it."

The scattering results showed both a pattern indicating solid helium and a liquid dispersion curve somewhat different from the pattern characteristic of liquid helium under pressure, says Lauter. "So we have these two signs—the sign from the solid and from the liquid. But we do not have signs of a supersolid." The dispersion curve of the liquid revealed energy excitations (called "rotons") that were like those seen in superfluid bulk helium but displaying different parameters. "Therefore, they must originate from the development of quasi 2-dimensional superfluid helium within the ," Lauter says.

The experiments at SNS are performed with solid helium-4 without the constraint of a confinement. This will help determine the microscopic (atomic-level) origin of the transition to superflow and, the researchers hope, show a transition from solid to supersolid behavior. Unlike oscillator experiments, neutron scattering can show the actual atomic interactions, Lauter points out.

The experiments are arduous and time-consuming. The helium crystals must be grown inside a sophisticated sample environment at a temperature barely above 0 K (approximately -459°F or -273°C) and a pressure many times atmospheric pressure.

Growing the crystal in the desired state can take half the experiment time, says Lauter. A researcher using neutrons never actually sees the solid sample, he points out, because it can exist only inside the special sample environment. The atomic-level origin of supersolidity can be observed only indirectly through experimental means.

Many experiments are needed to resolve whether scattering will provide evidence of a supersolid , or only superfluid layers inside a solid, Lauter says, but only roughly one can be done per year. "Just creating the sample is an effort. There are lots of difficulties in doing the experiment, but it's the challenge that makes it fun."

Explore further

A crack in the case for supersolids

Journal information: Physical Review Letters

Citation: Searching for a solid that flows like a liquid (2012, February 3) retrieved 17 August 2019 from
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Feb 03, 2012
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Feb 03, 2012
Many materials are behaving like supersolid, just in limited extent. For example the well known crunching of snow is an indicia of ballistic transport of the molecules of water at the ice grain boundaries. http://aetherwave...ity.html

Feb 03, 2012
Your want a solid that flows like a liquid?
How about cash inside the Beltway?

Feb 03, 2012
For example the well known crunching of snow is an indicia of ballistic transport

Didn't you claim that the crunching of snow is an example of a quantum effect in the other thread? Make up your mind.

Feb 03, 2012
Don't subscribe to his pamphlet, it's a cult.

Feb 03, 2012
Didn't you claim that the crunching of snow is an example of a quantum effect in the other thread
I do appreciate your attention - it is really so. Nevertheless, it's still correct, simply because ballistic transport is a quantum effect. Ballistic conduction uses the quantum mechanical properties of electron wave functions and it's coherent in wave mechanics terms.

Feb 03, 2012
I'm sick of rawa/callippo's stupid aether spam on every article. It takes a pathetic person to create a sockpuppet account just so they can pretend people agree with them.

Feb 03, 2012
Um... based on reading that article, ballistic conduction is just conduction when the length of the conductor is less than the mean scattering length. So it's similar to charge flowing in a vacuum, but with symmetric arrangement of wave nodes?

I guess I have no idea why you think it has anything to do with supersolidity. Or with crunching snow. It seems quite unlikely to me.

EDIT: I looked a bit more and now I'm quite certain they are unrelated.

Feb 04, 2012
ballistic conduction is just conduction when the length of the conductor is less than the mean scattering length
The ballistic transport arises, when the mean scattering length is smaller than the deBroglie wavelength of quantum fluctuations. The fermions in 3D undergo quantum oscillations and when their motion is constrained into thin 2D layer, they're doing quantum jumps like anyons (angels described with Thomas Aquinas). On the similar mechanism the ballistic motion of electrons along thin graphene layers is based. This gives a high mobility to charge carriers. At the surface of crystal grains the water molecules are in partially molten state, which is constrained into thin layer too. As the resuls, the water molecules are becoming a superfluous for brief time intervals. The supersolidity of solid helium works in the same way, it's just more permanent. The recrystallized samples of He-4 are losing their supersolidity, because their dislocations and grain boundaries are healed.

Feb 04, 2012
We could say, that the ballistic motion is a temporal superfluidity, which is limited just to brief time intervals. Note the strong isotopic effect of supersolidity - the He-3 atoms are asymmetric and they're shaking too much even at the absolute zero temperatures - so they cannot form a superfluous layer at the surface of helium crystals. Actually, just a tiny impurity of He-3 in He-4 samples kills the whole effect effectively, because these atoms are polar due the unpaired neutrons inside of their nuclei: they're adsorbed at the surface of grains and dislocations preferentially and they prohibit their free motion there.
I'm sick of rawa/callippo's stupid aether spam on every article
Which aether? We are discussing the quantum effects. You're too sensitive to all tiny particles, just admit it. It's sorta dust allergy.

Feb 04, 2012
I wrote possibly the first theoretical paper on this in 1969. (1969). "Destruction of Superflow in Unsaturated 4He Films and the Prediction of a New Crystalline Phase of 4He with Bose-Einstein Condensation", Physics Letters, Vol. 30, No. 5, November 3, 1969, pp. 300301. Jack Sarfatti

Feb 04, 2012
Yes, it's cited for example in the (unsuccessful) search for supersolidity in 1981 http://adsabs.har...4.2844B. Commonly the supersolidity prediction is attributed to Andreev and Lifshitz article from 1969 (Sov. Phys. JTEP 29, 1107 (1969)) and to Chester and Legget (Nobel prize winner of 2003 exactly by his works on superfluidity and superconductivity) in two different papers Phys. Rev. A 2, 256 (1970) and Phys. Rev. Lett. 25, 1543 (1970) Actually, Chester does not cite Andreev and Lifshitz's work, but Legget explicitly cites Chester's work. Andreev-Lifshitz theory of supersolids implicitly neglects uniform bulk processes that change the vacancy number, between others.

Feb 04, 2012
Did you just seriously quote Thomas Aquinas as a source for physical science?

Feb 04, 2012
"Some have maintained that the local movement of an angel is instantaneous. They said that when an angel is moved from place to place, during the whole of the preceding time he is in the term "wherefrom"; but in the last instant of such time he is in the term "whereto." Nor is there any need for a medium between the terms, just as there is no medium between time and the limit of time. But there is a mid-time between two "nows" of time: hence they say that a last "now" cannot be assigned in which it was in the term "wherefrom," just as in illumination, and in the substantial generation of fire, there is no last instant to be assigned in which the air was dark, or in which the matter was under the privation of the form of fire: but a last time can be assigned, so that in the last instant of such time there is light in the air, or the form of fire in the matter. And so illumination and substantial generation are called instantaneous movements."[Summa Theologiae, Part I, Question 53]

Feb 04, 2012
IMO it's first reference to quantum noise and quantum tunneling in peer-reviewed literature. It gives a meaning, if you imagine this quantum noise as a observable traces (low dimensional projection) of omnipresent and omnipotent hyperdimensional reality, which is forming us. Angels are considered as a messengers of God and this concept is widespread in most of monotheistic religions.

Feb 05, 2012
"The atomic-level origin of supersolidity can be observed only indirectly through experimental means."

Much like the existence of life can only be inferred indirectly.

I'm still waiting.

Feb 06, 2012
Hey guys, don't be so quick to rate Callippo down. It's hard to read, but a surprising amount of what he said actually made sense to me. Enough sense that I'm quite hesitant to say he's wrong without doing a lot of investigation into the subject.

Don't be too reflexive in considering people's ideas.

NOTE: I am referring to the 2 posts after mine, not the angels and God and stuff.

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