Pinning of quantized vortices in helium drops by dopant atoms and molecules

Using a density functional method, we investigate the properties of liquid He-4 droplets doped with atoms (Ne and Xe) and molecules (SF6 and hydrogen cyanide). We consider the case of droplets having a quantized vortex pinned to the dopant. A liquid-drop formula is proposed that accurately describes the total energy of the complex and allows one to extrapolate the density functional results to large N. For a given impurity, we find that the formation of a dopant + vortex + He-4(N) complex is energetically favored below a critical size N-cr. Our results support the possibility to observe quantized vortices in helium droplets by means of spectroscopic techniques.