Classroom demonstrations of the weird behavior of helium-4 have roused many physics students out of their early morning stupor. When cooled below 2.18 K, liquid 4He enters a zero-viscosity, superfluid phase and can creep up and out of its container or shoot up like a fountain. Now, 4He appears to have at least one more trick in its bag: It exhibits superfluid-like properties even when compressed into its solid phase, according to an experiment done by Eun-Seong Kim and Moses Chan at Pennsylvania State University. 1
How can one get resistance-free flow in a solid? In the normal picture of a solid, one atom occupies each site of a periodic lattice, and the possibility of mass flow should be negligible. But 4He does not form a typical solid: It stubbornly resists solidification until forced into submission at pressures of at least 25 bar and temperatures below 1.3...
