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

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 × ${10}^4$ ions (cloud radius $r_0\approx 37a$ where $a\equiv \mathrm{W}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{e}\mathrm{r}-\mathrm{S}\mathrm{e}\mathrm{i}\mathrm{t}\mathrm{z}\mathrm{r}\mathrm{a}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{s}$). Bulk behavior is evident with 2.7 × ${10}^5$ trapped ions ($r_0\approx 65a$), with Bragg scattering patterns characteristic of a body-centered cubic lattice.