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 ⁹Be⁺ ions in a Penning trap. Long-range order emerges in approximately spherical clouds with as few as 5 ×{} 10⁴ ions (cloud radius r₀≈37a where $a{≡}Wigner{-}Seitz{{0ex}{0ex}}radius). Bulk behavior is evident with 2.7 ×{10}⁵$ trapped ions (${r}₀{≈}65a$), with Bragg scattering patterns characteristic of a body-centered cubic lattice.