Teaching Superfluidity at the Introductory Level

L. Ruzhitskaya and W. Montfrooij

University of Missouri

Standard introductory modern physics textbooks do not exactly dwell on superfluidity in $^4$He. Typically, Bose-Einstein condensation (BEC) is mentioned in the context of an ideal Bose gas, followed by the statement that BEC happens in $^4$He and that the ground state of $^4$He exhibits many interesting properties such as having zero viscosity. Not only does this approach not explain in any way why $^4$He becomes a superfluid, it deprives students of the opportunity to learn about how the symmetry requirements on the wavefunction for bosonic systems lead to the emergence of energy gaps, which in turn lead to superfluidity and superconductivity. We revisit superfluid $^4$He by starting with Feynman's explanation of superfluidity, and we present exercises for the students that will allow them to arrive at a very accurate estimate of the superfluid transition temperature. This paper represents a self-contained account of superfluidity, which can be covered in one or two lessons in an introduction to modern physics class.