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★★☆
SUPERSOLID, QUANTUM CRYSTAL, A BOSE-EINSTEIN CONDENSATE IN SOLID

FORM
---all of these expressions apply to a weird substance observed
in a Penn State experiment in which a solid made of helium-4 atoms
appears to behave like a superfluid. Moses Chan and Eun-Seong Kim
look for signs of bizarre quantum behavior in a tiny disk hung from
a slender rod. The disk is filled with a porous glassy material
(Vycor), into which helium-4 atoms are inserted. Then the sample is
chilled down to a temperature of 2 K and subjected to a pressure of
63 atmospheres. This turns the helium into a solid. The disk
containing the now-solid helium residing within the spongelike Vycor
is set in motion. The disk gently oscillates like a pendulum and
its resonant frequency is recorded. Next the helium-filled disk is
cooled further. Below a temperature of about 175 mK a phase change
seems to occur. Without losing its status as a solid, the helium
now acts like a superfluid. Evidence for this consists in the
lowering of the resonant frequency. The oscillation will shift (its
spring constant changes) depending on the mechanical property of the
disk, and below the special temperature there is an abrupt drop in
the rotational inertia of the solid. The solid behaves like a
superfluid.
It is one thing to visualize a superfluid gliding frictionlessly
through the porous Vycor, another thing to imagine a solid moving in
this way. How can one solid (the helium) pass through another solid
(the Vycor), however porous it might be? Moses Chan
(chan@phys.psu.edu)
 invokes quantum theory to explain what might be
going on in the sample. The motion of the supersolid is facilitated
by the fact that at very low temperatures atoms in a solid still
possess a certain minimum amount of motion, allowed to them by the
quantum uncertainty principle. For lightweight atoms like helium,
this "zero-point energy" is even larger, and in the porous
Vycor,
there are lots of vacancies into which helium atoms can shuttle,
courtesy of the quantum fluctuations. The quantum way of looking at
the crystal of He-4 atoms is to say that they are governed by a
single wave function, just as vapor atoms cooled to a Bose-Einstein
condensate (BEC) form participate in a single quantum state. The
Penn State researchers look for alternative explanations by
performing lots of control tests---with an empty disk, with disks
filled with helium-3 (the solid effect goes away), and with helium-4
samples with helium-3 admixtures---without altering the supersolid
interpretation. (Nature, 15 January 2004.)

COLOR GLASS CONDENSATE (CGC) is the name for an extreme form of
nuclear matter that may have been created in recent experiments at
Brookhaven's Relativistic Heavy Ion Collider (RHIC). At this week's
Quark Matter 2004 conference in Oakland, California,
experimentalists presented possible preliminary evidence for this
novel state of matter. While nuclear physicists are debating the
evidence for a CGC, the concept itself is an accepted, if evolving,
theoretical idea that may describe a universal form of matter at
high energies. In RHIC experiments, researchers ordinarily collide a
beam of gold ions with another beam of gold ions. But during the
first quarter of 2003, they studied the collision of gold ions with
deuterons, nuclei which each consist of a proton and neutron. They
used a deuteron beam precisely to avoid making the coveted
quark-gluon plasma (QGP), the hypothetical soup of individual quarks
and gluons that the RHIC researchers hope to recreate in their
future experiments. They do this in order to better observe the CGC
state, which many believe would be a precursor to QGP
So what is a color glass condensate? According to Einstein's
special theory of relativity, when a nucleus travels at near-light
(relativistic) speed, it flattens like a pancake in its direction of
motion. Also, the high energy of an accelerated nucleus may cause
it to spawn a large number of gluons, the particles that hold
together its quarks. These factors--relativistic effects and the
proliferation of gluons--may transform a spherelike nucleus into a
flattened "wall" made mostly of gluons. This wall, 50-1000
times
more dense than ordinary nuclei, is the CGC (see
www.bnl.gov/bnlweb/pubaf/pr/2003/colorglasscondensate-background.htm
for a letter-by-letter explanation of the CGC's name). How does the
gluon glass relate to the much sought quark-gluon plasma? The QGP
might get formed when two CGC's collide.
Reporting their gold-deuteron data at the Quark Matter conference,
researchers in the BRAHMS collaboration (Jens Jorgen Gaardhoje,
gardhojenbi.dk)
 observed fewer-than-usual high-momentum particles
emitted transverse (sideways) to the direction of the collision.
According to Gaardhoje, the data, which includes BRAHMS's ability to
detect particles at small angles to the beam, provided evidence that
the deuteron nucleus formed a CGC. Meanwhile, the PHOBOS
collaboration (Gunther Roland, MIT, gunter.rolandcern.ch)
 confirms
the experimental effect seem by BRAHMS, though Roland cautions that
direct calculations that confront the CGC theory with the observed
effect need to be performed. According to Brookhaven theorist Larry
McLerran (mclerran@quark.phy.bnl.gov),
 the BRAHMS and PHOBOS
observations provide evidence for this new state of matter.
However, Columbia theorist Miklos Gyulassy
(gyulassy@mail-cunuke.phys.columbia.edu),
 disagrees. BRAHMS
spokesperson Gaardhoje points out there are conflicting theoretical
views, but considers the suppressed production of high-momentum
particles to be "a necessary feature" of a CGC. Whether it
is
sufficient evidence is another story, he says, and the next RHIC
runs should provide further insights. Nonetheless, Gyulassy
believes that CGC is a valid concept and that the RHIC researchers
should actively search for signs of it, just as they continue to try
to create and study the QGP (which, incidentally, he believes RHIC
has already produced--see Update 642). (Gaardhoje adds that evidence
for the existence of a CGC state has already appeared in
electron-positron collisions at HERA in Germany.) According to
McLerran, the CGC has the potential to explain many things in
high-energy nuclear physics such as the mechanisms by which
particles are produced in nuclear collisions as well as the
distribution of gluons inside nuclei. (For more information, see
Brookhaven news release at
http:// )
 

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