Teaching superfluidity at the introductory level

Abstract

Standard introductory modern physics textbooks do not exactly dwell on superfluidity in 4He. Typically, Bose-Einstein condensation (BEC) is mentioned in the context of an ideal Bose gas, followed by the statement that BEC happens in 4He and that the ground state of 4He exhibits many interesting properties such as having zero viscosity. Not only does this approach not explain in any way why 4He becomes a superfluid, it denies students the opportunity to learn about the far reaching consequences of energy gaps as they develop in both superfluids and superconductors. We revisit superfluid 4He by starting with Feynman's explanation of superfluidity based on Bose statistics as opposed to BEC, and we present exercises for the students that allow them to arrive at a very accurate estimate of the superfluid transition temperature and of the energy gap separating the ground state from the first excited state. This paper represents a self-contained account of superfluidity, which can be covered in one or two lessons in class.