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.)