Long-Range Order in Laser-Cooled, Atomic-Ion Wigner Crystals Observed by Bragg Scattering

Abstract

We report the first observation of Bragg scattering from atomic ions confined in an electromagnetic trap. The results reveal long-range order and give evidence for bulk behavior in a strongly coupled collection of laser-cooled $^{9}\mathrm{Be}^{+}$ ions in a Penning trap. Long-range order emerges in approximately spherical clouds with as few as 5 \ifmmode\times\else\texttimes\fi{} ${10}^{4}$ ions (cloud radius ${r}_{0}\ensuremath{\approx}37a$ where $a\ensuremath{\equiv}\mathrm{W}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{e}\mathrm{r}\ensuremath{-}\mathrm{S}\mathrm{e}\mathrm{i}\mathrm{t}\mathrm{z}\phantom{\rule{0ex}{0ex}}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{s}$). Bulk behavior is evident with 2.7 \ifmmode\times\else\texttimes\fi{} ${10}^{5}$ trapped ions (${r}_{0}\ensuremath{\approx}65a$), with Bragg scattering patterns characteristic of a body-centered cubic lattice.