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Physics News Update
Number 669, January 14, 2004 by Phil Schewe, James Riordon, and Ben Stein

Supersolid, Quantum Crystal, A Bose-Einstein Condensate in Solid

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

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 did 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 Brookhaven page 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 Jørgen Gaardhoje, gardhoje@nbi.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.roland@cern.ch) confirms the experimental effect seen 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-proton 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; for more background on RHIC, see October 2003 Physics Today article)