Probing the roton excitation spectrum of a stable dipolar Bose gas

Abstract

We measure the excitation spectrum of a stable dipolar Bose--Einstein condensate over a wide momentum-range via Bragg spectroscopy. We precisely control the relative strength, ${\epsilon }_{\mathrm{dd}}$, of the dipolar to the contact interactions and observe that the spectrum increasingly deviates from the linear phononic behavior for increasing ${\epsilon }_{\mathrm{dd}}$. Reaching the dipolar dominated regime ${\epsilon }_{\mathrm{dd}}>1$, we observe the emergence of a roton minimum in the spectrum and its softening towards instability. We characterize how the excitation energy and the strength of the density-density correlations at the roton momentum vary with ${\epsilon }_{\mathrm{dd}}$. Our findings are in excellent agreement with numerical calculations based on mean-field Bogoliubov theory. When including beyond-mean-field corrections, in the form of a Lee-Huang-Yang potential, we observe a quantitative deviation from the experiment, questioning the validity of such a description in the roton regime.

11 pages, 8 figures