Excitations in Quantum Boson Fluids
Excitations in the superfluid 4He have been an interesting subject of studies for decades.’ Yet, recently precise measurements of excitations in thin films2have created new interest in the possible excitation mechanisms in reduced dimensions. A thin 4He film adsorbed on strongly binding substrates like lithium, solid hydrogen, magnesium or solid helium on graphite forms a pronounced layered structure with one atomic layer thickness.3,4 The fluid within the first layer can well be approximated with a two-dimensional fluid. The energy gap between the lowest two single particle states perpendicular to the surface is between 7 and 13 Kelvins on these substrates. That makes it possible to observe clearly the collective, two-dimensional phonon - roton mode within the layer5 With the increasing layer density that mode turns into the ripplonlike surface mode. In the case of very thick films the lowest energy mode at small k is the ripplon mode and the higher lying layer modes merge into the three-dimensional phonon-roton mode.
KeywordsSaturation Density Pair Distribution Function Solid Hydrogen Single Particle Density Condensed Matter Theory
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