Theory for superfluidity in a Bose system

We present a microscopic theory for superfluidity in an interacting many-particle Bose system (such as liquid $^4$He). We show that, similar to superconductivity in superconductors, superfluidity in a Bose system arises from pairing of particles of opposite momenta. We show the existence of an energy gap in single-particle excitation spectrum in the superfluid state and the existence of a specific heat jump at the superfluid transition. We derive an expression for superfluid particle density $n_s$ as a function of temperature $T$ and superfluid velocity ${\bf v}_s$. We show that superfluid-state free energy density $F$ is an increasing function of $v_s$ (i.e., $\partial F/\partial v_s > 0$), which indicates that a superfluid has a tendency to remain motionless (this result qualitatively explains the Hess-Fairbank effect, which is analogous to the Meissner effect in superconductors). We further speculate the existence of the equation {\bf j}=-\Lambda\nabla\times \text{\boldmath \omega}, where ${\bf j} = n_s{\bf v}_s$ is the superfluid current density, \text{\boldmath \omega}=\nabla\times {\bf v}_s the superfluid vorticity, and $\Lambda$ a positive constant (with the help of this equation, the Hess-Fairbank effect can be quantitatively described).