Dynamic many-body theory: Dynamic structure factor of two-dimensional   liquid $^4$He
  We calculate the dynamic structure function of two-dimensional liquid $^4$He at zero temperature employing a quantitative multi-particle fluctuations approach up to infinite order. We observe a behavior that is qualitatively similar to the phonon-maxon-roton-curve in 3D, including a Pitaevskii plateau (L. P. Pitaevskii, Sov. Phys. JETP 9, 830 (1959)). Slightly below the liquid-solid phase transition, a second weak roton-like excitation evolves below the plateau. 
Eckhard Krotscheck and Thomas Lichtenegger
cond-mat.other