Bose-Einstein condensation and the λ transition in liquid helium

Integrating seminal ideas of London, Feynman, Uhlenbeck, Bloch, Bardeen, and other illustrious antecessors, this paper continues the development of an ab initio theory of the λ transition in liquid 4He. The theory is based upon variational determination of a correlated density matrix suitable for description of both normal and superfluid phases, within an approach that extends to finite temperatures the very successful correlated wave-functions theory of the ground state and elementary excitations at zero temperature. We present the results of a full optimization of a correlated trial form for the density matrix that includes the effects both of temperature-dependent dynamical correlations and of statistical correlations corresponding to thermal phonon/roton and quasiparticle/hole excitations-all at the level of two-point descriptors. The optimization process involves constrained functional minimization of the associated free energy through solution of a set of Euler-Lagrange equations, consisting of a generalized paired-phonon equation for the structure function, an analogous equation for the Fourier transform of the statistical exchange function, and a Feynman equation for the dispersion law of the collective excitations. Violation of particle-hole exchange symmetry emerges as an important aspect of the transition, along with broken gauge symmetry. In conjunction with a semi-phenomenological study in which renormalized masses are introduced for quasiparticle/hole and collective excitations, the results suggest that a quantitative description of the λ transition and associated thermodynamic quantities can be achieved once the trial density matrix is modified-notably through the addition of three-point descriptors-to include backflow effects and allow for ab initio treatment of important variations in effective masses.