Layer- and bulk roton excitations of ⁴He in porous media

Layer-roton excitations of ⁴ He in porous media, such as aerogel, Geltech or Vycor, depend on the number of atoms in the active liquid layer, which, in turn, depends on the substrate potential. Hence, by measuring or calculating the energetics of the layer modes, one can get first-hand information about the substrate potential. Layer rotons can have energies as high as 8 K, while calculated two-dimensional rotons never seem to reach 6 K. Further indication that layer modes are actually an intermediate case between two and three dimensions is given by the way layer modes show up in neutron scattering data of helium in porous materials. We argue that layer rotons should be regarded as a third, completely independent kind of excitation.