title: Roton-type mode softening in a quantum gas with cavity-mediated long-range interactions.
abstract: Long-range interactions in quantum gases are predicted to give rise to an excitation spectrum of roton character, similar to that observed in superfluid helium. We investigated the excitation spectrum of a Bose-Einstein condensate with cavity-mediated long-range interactions, which couple all particles to each other. Increasing the strength of the interaction leads to a softening of an excitation mode at a finite momentum, preceding a superfluid-to-supersolid phase transition. We used a variant of Bragg spectroscopy to study the mode softening across the phase transition. The measured spectrum was in very good agreement with ab initio calculations and, at the phase transition, a diverging susceptibility was observed. The work paves the way toward quantum simulation of long-range interacting many-body systems.
authors.affiliation: Institute for Quantum Electronics, ETH Zürich, Zürich, Switzerland.
authors.forename: R
authors.initials: R
authors.lastname: Mottl
authors.forename: F
authors.initials: F
authors.lastname: Brennecke
authors.forename: K
authors.initials: K
authors.lastname: Baumann
authors.forename: R
authors.initials: R
authors.lastname: Landig
authors.forename: T
authors.initials: T
authors.lastname: Donner
authors.forename: T
authors.initials: T
authors.lastname: Esslinger