Microscopic theory of liquid 4He

A finite temperature theory is given of the temperature variation of the sound velocity and of the structure factor of liquid helium. The sound velocity is found to decrease slightly as the temperature is increased from absolute zero. The first peak of the structure factor is higher for higher temperature below the λ point. The internal energy is characterized by the quasiparticle distribution function with the Bogoliubov-Zubarev type excitation energy. The structure factor and the energy are determined by the effective interaction which depends on the average density, the density of the excited particles, and temperature in addition to the interaction potential.