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Helium-4
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Study in Science casts new light on 'supersolid' effects in helium-4

Does helium-4 become a "supersolid" near absolute zero? What previous researchers thought might be a supersolid transition is better explained by changes in the solid's resistance to shearing, according to new research by J. C. Sťamus Davis, the J.G. White Distinguished Professor in the Physical Sciences.
The research is reported in the May 13 issue of Science.

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RE: Solid Helium
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Supersolidity or quantum plasticity?

Superfluidity - the frictionless flow of a liquid analogous to lossless current flow in a superconductor - is well known in liquid 4He, but "supersolidity" - coexistence of crystalline order and superflow - seems counterintuitive. However, quantum mechanics allows atomic exchange even in a solid, especially if the atoms are light and the interatomic potential is weak. This makes helium the most quantum of solids and the possibility of supersolidity in helium was suggested more than 40 years ago. Despite earlier searches, it was not until 2004 that Kim and Chan's torsional oscillator experiments provided convincing evidence of supersolidity. The torsional oscillator technique has been a cornerstone of our understanding of superfluidity since Andronikashvili's classic experiment in which he attached immersed disks in liquid 4He and set them into torsional oscillation. Liquid moving with the discs contributed to the oscillator's moment of inertia and so reduced its frequency (increased its period). Since a frictionless fluid is not dragged along with the disks, the oscillator frequency provided a direct measurement of the superfluid density in liquid helium. During the 1970s, John Reppy and co-workers at Cornell University in the US refined the "high Q torsional oscillator" into an exquisitely sensitive tool for studying superfluids. Now, Reppy reports new experiments in Physical Review Letters that throw into question our current understanding of supersolidity.
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Frozen helium-4 may be an unusual 'superglass'
Frozen helium-4 that flows without friction may be a superglass rather than a crystalline supersolid, according to Cornell researchers.

When helium is cooled to around 4 degrees above absolute zero, it turns liquid. Make it a couple of degrees cooler, and it becomes a "superfluid" that flows without resistance from its container, just as electrons flow without resistance in a superconductor.
Now pressurise the helium to about 50 atmospheres until it solidifies, and then cool it a lot more to about two-tenths of a degree above absolute zero, and it becomes -- well, there's a lot of argument about what it is. Perhaps a supersolid, or a solid with some superfluid moving through it.
In fact, it may be a superglass, report J.C. Sťamus Davis, the James Gilbert White Distinguished Professor in the Physical Sciences at Cornell and colleagues in the May 1 issue of the journal Science.

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RE: Solid Helium
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At very low temperatures, helium can be solid and a perfect liquid at the same time. Theoreticians, though, have incorrectly explained the phenomenon for a long time. Computer simulations at ETH Zurich have shown that only impurities can make this effect possible.
Theoreticians first predicted the "supersolidity" phenomenon in 1969. Their explanation was incorrect, but this escaped notice for some time. The first evidence for "Supersolidity" was measured in an experiment only in 2004. This involved attaching a disc-shaped helium crystal to a spring and rotating it to and fro. In this arrangement, the vibration frequency depends on the rotating mass. The researchers found that the frequency became higher if they cooled the apparatus down to below 0.2 Kelvin - almost down to absolute zero. Part of the mass no longer participated in the rotation; it behaved as a superfluid, meaning it behaved like a friction-free liquid. In other words, it had become "supersolid".

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Physicists at the University of Alberta have made a major advance in the understanding of what appears to be a new state of matter.
Working in the highly specialized field of quantum fluids and solids, professor John Beamish, chair of the Department of Physics, and PhD student James Day, report their findings in a paper published in the science journal Nature today. Beamish and Day are the only researchers in Canada conducting experimental research in this area of fundamental physics.
At very low temperatures, helium gas turns into a liquid. Put under extreme pressure the liquid turns into a solid. Physicists have been manipulating solid helium so they can study its unusual behaviour.
In 2004, a research team at Penn State University, led by Moses Chan, startled the physics world when it announced that it may have discovered an entirely new state of matter - supersolidity. The team made its discovery by cooling solid helium to an extremely low temperature and oscillating the material at different speeds. They found that the particles behaved in a way not seen before, which suggested it might show the "perpetual flow" seen in superfluids like liquid helium.
Day and Beamish have taken this research a different direction. In an experiment not done before, they cooled the solid helium and manipulated the material another way - by shearing it elastically. In doing so, they found that the solid behaved in an entirely new and unexpected way - it became much stiffer at the lowest temperatures.

"The experimental results from the University of Alberta are remarkable. This is an important breakthrough, since the original discovery" - Moses Chan.

Other physicists around the world are also studying the implications. Through this discovery, Beamish and Day have significantly added to the body of knowledge about the fundamental states of matter allowed by nature.

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Supersolid
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Evidence is mounting for the existence of a strange new state of matter called a "supersolid", in which a small fraction of ultracold helium decouples from the rest of the solid and flows effortlessly through the material as if it were not there. Although the first clear signs of supersolidity were obtained three years ago by Moses Chan and colleagues at Pennsylvania State University in the US, subsequent research cast doubt on those findings. Now, however, Chan has measured the specific heat of several helium-4 samples and has found a peak in the data that he says is a "probable" signature of the supersolid phase

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Supersolids
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The mysterious saga of "supersolid" helium continues this week. If you recall, there were some new results a little while back showing that the effect depends on disorder in the samples, followed by neutron scattering studies that didn't show the expected distribution of states in the sample. These results suggest that something else is going on in these samples, and the explanation of the observed effects isn't all that simple.

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Solid Helium
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High-quality, single-crystal, ultra-cold solid helium exhibits supersolid behaviour, 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.
In 2004, Penn state physicists -- Eunseong Kim, then-graduate student and Moses Chan, the Evan Pugh professor of physics-- announced the observance of frictionless superflow in solid helium at nearly absolute zero. This new phenomenon is a cousin of Bose-Einstein condensate observed in gases in 1995 and in liquid helium in 1938.

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