STRUCTURE AND EXCITATIONS OF QUANTUM LIQUID CLUSTERS

Abstract

We review recent theoretical progress in the evaluation of collective excitation spectra of quantum liquid clusters, with specific application to 4 He N. An excitation operator theory together with the variational Monte Carlo method has provided a powerful means of calculating the excited state wavefunctions and energies. The compressional excitations of 4 He N clusters calculated from the excitation operator theory agree well with the results of a quantum liquid drop theory based on accurate knowledge of microscopic density correlations in the ground state. The compressional excitation spectra evolve toward the spectrum of bulk He II as N increases, with the roton structure first appearing at N≈70. The implication for the finite size scaling of superfluid behavior in these clusters is discussed. These techniques now allow microscopic study of clusters containing foreign species, and can also be extended to quantum solid clusters.