Colloquium: Criticality and superfluidity in liquid He-4 under nonequilibrium conditions

We review a striking array of recent experiments and their theoretical inte rpretations on the superfluid transition in He-4 in the presence of a heat flux Q. We define and evaluate a new set of critical point exponents. The s tatics and dynamics of the superfluid-normal interface are discussed, with special attention to the role of gravity. If Q is in the same direction as gravity, a self-organized state can arise, in which the entire sample has a uniform reduced temperature, on either the normal or superfluid side of th e transition. Finally, we review recent theory and experiment regarding the heat capacity at constant Q. The excitement that surrounds this field aris es from the fact that advanced thermometry and the future availability of a microgravity experimental platform aboard the International Space Station will soon open to experimental exploration decades of reduced temperature t hat were previously inaccessible.