enters the superfluid state and flows without
friction below 2.176 K. Thin liquid films adsorbed on solid substrates
undergo the same transformation, although at a lower temperature. When the
substrate is subjected to oscillatory motion a portion of the film, known as
the superfluid fraction, decouples from the oscillation.
A similar phenomenon has been observed1,
in solid 4He, in which a fraction of the solid seems to decouple
from the motion of the surrounding lattice. Although this observation has been
replicated in various laboratories3,
no thermodynamic signature of the possible supersolid
transition has been seen. Here we report the finding of a heat capacity peak
that coincides with the onset of mass decoupling. This complementary
experimental evidence supports the existence of a genuine transition between
the normal solid and supersolid phases of 4He.
Probable heat capacity signature of the supersolid transition, X. Lin, A. C. Clark & M. H. W.
Nature 449, 1025-1028 (2007)
The apparent observation
of supersolid helium in 2004 generated much interest
in the physics community, and curiosity outside of it. But the thermodynamic
signature expected to accompany the transition was absent. Lin et al. present new evidence on the
matter. The phenomenon, which shows similarities to that seen when liquid
helium enters the superfluid state, occurs when a
fraction of the solid appears to decouple from the motion of the surrounding
lattice. The new experiment reveals a heat capacity peak coincident with the
onset of mass decoupling, which supports the existence of a genuine transition
between the normal solid and supersolid phases of 4He.
Supersolid signature, Nature, Editor's Summary, 25 October 2007
In recent years, no topic
in condensed matter physics has been hotter than the study of ultracold solid helium. Subtle experiments suggest that as
temperatures dip toward absolute zero, crystalline helium can bizarrely flow
like a liquid with no viscosity, a phenomenon known as supersolidity.
Now, a new experiment lends credence to that controversial claim by revealing a
possible second sign of the transition.
The first evidence for supersolidity emerged in 2003 and 2004.
Moses Chan and Eun-Seong Kim of Pennsylvania
in State College filled a tiny can with liquid
helium and squeezed the ultracold stuff to greater
than 25 times atmospheric pressure to make it solidify. They then set the can
twisting back and forth on top of a thin shaft. Below a temperature of about
0.2 kelvin--2/10 of a degree
above absolute zero--the frequency of the twisting shot up as though the can
had become less massive. That implied that some of the helium had let go of the
can and was standing stock-still while the rest of it continued to twist back
and forth. That in turn suggested that the solid helium was flowing through
itself without any resistance, a phenomenon known as supersolidity
that had been hypothesized in the 1960s (Science,
1 July 2005, p. 38).
But some believed that Kim
and Chan had observed less-mysterious "superfluid"
liquid helium wending its way through cracklike
defects in the crystal. That alternative interpretation got a shot in the arm
in 2006, when Ann Sophie Rittner and John Reppy of Cornell
University claimed that
they, too, had seen the phenomenon but could make it go away by gently heating
and cooling the solid helium to smooth over defects in the crystal (ScienceNOW,
15 March 2006).
Now, Chan and colleagues
Xi Lin and Anthony Clark have new results that suggest supersolidity
may be a property of the solid crystal after all. If that were true, then the
onset of supersolidity ought to be a
"thermodynamic phase transition," much like the freezing of water
into ice or the emergence of magnetism in hot iron as it cools. During such
transitions, the heat capacity of a material--the amount of heat required to
raise the temperature of the stuff--increases dramatically. And that's what
Chan and colleagues see in solid helium, they report this week in Nature.
Technically, this was no
small feat. The heat capacity of the helium is much smaller than that of the
metal container holding it. So to spot the peak, the researchers had to use a
special container fashioned out of silicon and take precautions to prevent any
unaccounted heat leaks. Chan cautions that the team hasn't yet proved that the
heat-capacity signal, which actually occurs at a slightly lower temperature
than the onset of flow, is tied to supersolidity,
however. "That's why we use the word 'probable,' " he says, "in
case there is another explanation."
But if the peak is really
there, it bolsters the case that supersolidity
involves a real phase transition, says experimenter Norbert Mulders
of the University of Delaware, Newark.
However, he adds, "I have a pretty good idea of how difficult these
measurements are, so it would probably take [confirmation by] some competing
experiment to completely convince me."
Evidence for "Supersolidity"
Becomes More Solid, Adrian Cho, ScienceNOW Daily News, 24 October
Adrian Cho <email@example.com>
Subject: Nature 449,
Date: 24 November
Drs M H W Chan, X Lin & A C Clark
Department of Physics
Dear Learned Researchers,
Probable heat capacity signature of the supersolid transition
In reference to your above Letter, please be good enough to
access (1) and (2)
for perusal. You may find them to be of fundamental interest and an
indispensable backdrop to your continuing research.
The heat capacity peak you report
here (in the transition between the normal solid and supersolid
phases of 4He) should also correspond to an equivalent peak in the mass content of the material. This would
not only be affirmative to your report but also be in accordance with the final
perspective on the nature of things, as introduced in (2). (The individual 4He
atoms in the supersolid would act as perfect ball
bearings, with their centers at relative rest in the crystal lattice.) Hope
there's a way of precisely measuring this mass (or weight) difference here as
A further verification that follows
from your experiment is that of absolute space, as defined in (2). Decoupling
from its surroundings, the supersolid would eventually
come to rest in the lab frame but oriented in the direction the lab is moving
in absolute space, or the absolute reference frame; and this orientation
should ideally revolve through a full cycle every 24 hours in the lab!
Finally, it may also be noted that the response from Physics World to the submission of (2)
has since been negative, leaving the monetary offer therein for its refutation
still open! Would your physics department head at PSU kindly consider being the
moderator here (holding the cash in hand!)? Moderator's fee, too, will be paid
by me in advance. No joke; no scam; just an earnest appeal to you to resolve
this matter for me early with your profound knowledge and expertise. (The money
offered is the least I could do in gratitude. It would be worth every
penny for me considering the time I would otherwise be wasting in trying to
flog a fundamentally flawed worldview.)
Any feedback would be gratefully received.
Thank you and all the very best in your continued quest for
End of Letter
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