Roton excitations in Bose–Einstein condensates and a fluid–solid transition

Abstract

Several authors have recently discussed the existence of roton excitations in Bose–Einstein condensates (BECs) and considered how a roton dip in the dispersion curve of elementary excitations may be related to the formation of a spatially modulated ground state. Here attention is drawn to a theoretical study of Minguzzi et al. on interatomic correlations in a BEC from dipole–dipole interactions induced by laser light of increasing intensity. Attractive interactions in superfluid ⁴He and repulsive interactions in ‘untuned’ BECs are then compared and contrasted, the experiments of Woods and Cowley and of Greiner et al. providing the focus respectively. It is stressed that, contrary to a very recent assertion by Nazario and Santiago, ⁴He is crucially different from BECs at the lowest temperatures.

Keywords:

• Bose–Einstein condensates
• Dipole–dipole interactions
• Elementary excitations

Acknowledgments

This work was partially supported by an Advanced Research Initiative of Scuola Normale Superiore (SNS). NHM wishes to acknowledge that his contribution to this article was brought to fruition during a visit to SNS: it is a pleasure to acknowledge the very stimulating atmosphere in which the work was carried out, and the generous hospitality afforded by SNS.