Statistical Mechanics of Superfluidity

For the description in terms of a flowing ground state plus excitations see Lecture 15 and problem 3 of Homework 7 for Ph127a. This approach has some disadvantages. Conceptually, it has the problem that we calculate superflow in terms of what it isn’t, i.e. the excitations which reduce the mass flow! The superflow is slipped in by Galilean boost arguments. Practically, the approach is limited to translationally invariant situations, such as a bulk fluid, where Galilean invariance applies. The most dramatic manifestations of superfluidity—the flow through porous material with pores of a few Å in size such as vycor or aerogel, or in thin films of a few atomic layers thick (or even subatomic layers) on substrates, cannot be addressed.