Probing the roton excitation spectrum of a stable dipolar Bose gas
arXiv:1811.12115 · doi:10.1103/PhysRevLett.122.183401
The paper reports Bragg spectroscopy measurements of the excitation spectrum of a stable dipolar Bose‑Einstein condensate, revealing a roton minimum that softens as dipolar interactions dominate, and compares the results with mean‑field and beyond‑mean‑field theories.
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, $ε_{\rm dd}$, of the dipolar to the contact interactions and observe that the spectrum increasingly deviates from the linear phononic behavior for increasing $ε_{\rm dd}$. Reaching the dipolar dominated regime $ε_{\rm dd}>1$, 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 $ε_{\rm dd}$. 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