Probing the roton excitation spectrum of a stable dipolar Bose gas.

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(dd) of the dipolar to the contact interactions and observe that the spectrum increasingly deviates from the linear phononic behavior for increasing epsilon(dd). Reaching the dipolar-dominated regime epsilon(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(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.