Quantum Monte Carlo study of dynamics : from weakly interacting Bose gas to helium-like Bose fluid

We have studied dynamic properties of a system of hard-sphere bosons at zero temperature through ab-initio estimations of the dynamic structure factor, S(q,w), from the dilute regime up to a strongly interacting regime, paroganable to overpressurized liquid helium. To this extent, we have used the Genetic Inversion via Falsification of Theories (GIFT) algorithm [1], which perform accurate analytic continuations of imaginary time correlation functions under ill-posed conditions. The imaginary time correlation functions have been computed with an exact numerical method, the path integral ground state (PIGS) method [2]. With increasing density, we observe a crossover of the dispersion of elementary excitations from a Bogoliubov-like spectrum to a phonon-maxon-roton curve, characteristic of superfluid helium, and the emergence of a broad multiphonon contribution accompanying the quasi-particle peak. References: [1] E. Vitali, M. Rossi, L. Reatto and D.E. Galli, Phys. Rev. B 82, 174510,(2010). [2] A. Sarsa, K. E. Schmidt, and W. R. Magro, J. Chem. Phys. 113, 1366 (2000)