Density Fluctuations across the Superfluid-Supersolid Phase Transition in a Dipolar Quantum Gas

TL;DR

This study provides the first direct measurement of density fluctuations during the superfluid-supersolid phase transition in a dipolar quantum gas, revealing the emergence of roton modes and crystalline order.

Contribution

It introduces a new method to analyze density fluctuations and static structure factors, linking them to elementary excitations like roton modes in a supersolid.

Findings

Observation of a strong static structure factor response

Detection of a roton minimum in the excitation spectrum

Identification of both superfluid and crystal phonons

Abstract

Phase transitions share the universal feature of enhanced fluctuations near the transition point. Here we show that density fluctuations reveal how a Bose-Einstein condensate of dipolar atoms spontaneously breaks its translation symmetry and enters the supersolid state of matter -- a phase that combines superfluidity with crystalline order. We report on the first direct in situ measurement of density fluctuations across the superfluid-supersolid phase transition. This allows us to introduce a general and straightforward way to extract the static structure factor, estimate the spectrum of elementary excitations and image the dominant fluctuation patterns. We observe a strong response in the static structure factor and infer a distinct roton minimum in the dispersion relation. Furthermore, we show that the characteristic fluctuations correspond to elementary excitations such as the roton modes, which have been theoretically predicted to be dominant at the quantum critical point, and that the supersolid state supports both superfluid as well as crystal phonons.