Roton excitations and the fluid-solid phase transition in superfluid 2D   Yukawa Bosons
  We compute several ground state properties and the dynamical structure factor of a 0-temperature system of Bosons interacting with the 2D screened Coulomb (2D-SC) potential. We resort to the exact shadow path integral ground state (SPIGS) quantum Monte Carlo method to compute the imaginary-time correlation function of the model, and to the genetic algorithm via falsification of theories (GIFT) to retrieve the dynamical structure factor. We provide a detailed comparison of ground state properties and collective excitations of 2D-SC and 4He atoms. The roton energy of the 2D-SC system is an increasing function of density, and not a decreasing one as in 4He. This result is in contrast with the view that the roton is the soft mode of the fluid-solid transition. We uncover a remarkable quasi-universality of backflow and of other properties when expressed in terms of the amount of short range order as quantified by the height of the first peak of the static structure factor. 
S. Molinelli, D. E. Galli, L. Reatto, M. Motta
cond-mat.other