Professor Wayne Saslow,
Physics Department, Texas A&M University 

Title: Superflow in Solid ⁴He

 Abstract

In 1969 Andreev and Lifshitz suggested that solid 4He might display unusual lattice flow properties that could be associated with superfluidity.  In 1970 Leggett suggested that solid 4He might display Non-Classical Rotational Inertia (NCRI), due to superflow of the solid, without motion of the lattice.  No unusual lattice flow properties have yet been observed, but in the January issue of Nature, Kim and Chan report observing NCRI with a T=0 superfluid fraction on the order of 1.5% and a Tc on the order of 200 mK.  We summarize their experiments, and discuss the state of the theory in terms of predicting the superfluid fraction and Tc.  The low observed critical velocities can be interpreted as evidence for quantum vortex lines whose cores lie between the lattice sites, and certain hysteresis effects suggest the presence of quantum vortex rings.  Such topological excitations change the way we think about solids: solid ⁴He is as much a localized liquid as it is a delocalized solid.