Theory of Superfluidity

Abstract

A new structure for the microscopic theory of superfluid helium is presented which is motivated by analogy with coherent radiation of ideal lasers and with the BCS theory of superconductivity. The concept of off-diagonal long-range order is employed to put these three phenomena on a common basis. The unifying concept is that the ground state is that which completely factors the reduced density matrix appropriate to the "basic group." It is shown how the model predicted by this motivation can also be derived from the microscopic Hamiltonian by a quasiparticle self-consistent-field approximation. The superfluid ground state can be viewed as energy-lowering "coherent" excitations of the normal fluid. The normal and superfluid components are the direct analog of "noise" and "signal" in partially coherent radiation fields.