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Physical Review Letters

Structural Properties of Two-Component Coulomb Crystals in Linear Paul Traps

Liv Hornekær, Niels Kjærgaard, A. M. Thommesen, Michael Drewsen

Published Mar 5, 2001• 150 citations
Publisher

Abstract

We report on structural properties of two-component Coulomb crystals in a linear Paul trap. The crystals consist of two laser cooled ion species, 24Mg+ and 40Ca+. The lighter 24Mg+ ions form an inner cylindrical crystal structure surprisingly similar to that of an infinitely long single component crystal, while the outermost shell of the surrounding 40Ca+ ions have a spheroidal shape, which is highly insensitive to the presence of the 24Mg+ ions. Observed changes in the radial separation of the two ion species with the radius of the inner cylindrical crystal is explained by a simple model.

DOI
10.1103/physrevlett.86.1994

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We report on structural properties of two-component Coulomb crystals in a linear Paul trap. The crystals consist of two laser cooled ion species, 24Mg+ and 40Ca+. The lighter 24Mg+ ions form an inner cylindrical crystal structure surprisingly similar to that of an infinitely long single component crystal, while the outermost shell of the surrounding 40Ca+ ions have a spheroidal shape, which is highly insensitive to the presence of the 24Mg+ ions. Observed changes in the radial separation of the two ion species with the radius of the inner cylindrical crystal is explained by a simple model.
We obtain bi-component Coulomb crystals using laser-cooled 40Ca+ ions to sympathetically cool 9Be+ ions in a linear Paul trap. The shell structures of the bi-component Coulomb crystals are investigated. The secular motion frequencies of the two different ions are determined and compared with those in the single-component Coulomb crystals. In the radial direction, the resonant motion frequencies of the two ionic species shift toward each other due to the strong motion coupling in the ion trap. In the axial direction, the motion frequency of the laser-cooled 40Ca+ is impervious to the sympathetically cooled 9Be+ ions because the spatially separation of the two different ionic species leads to the weak motion coupling in the axial direction.
We report on the observations of three-dimensional long-range ordered structures in the central 40^{40}40Ca+^++ ion component of 40^{40}40Ca+^++--44^{44}44Ca+^++ two-species ion Coulomb crystals in a linear Paul trap. In contrast to long-range ordering previously observed in single species crystals, the structures observed are strikingly persistent and always of one specific type in one particular orientation. Molecular dynamics simulations strongly indicate that these characteristics are a hitherto unpredicted consequence of the co-axial cylindrical symmetry of the central ion component of the Coulomb crystal and the radio frequency quadrupole trapping field.
We study the spatial structure and sympathetic cooling of the bi-component Coulomb crystal (CC), which consists of approximate 450 9Be+ ions and 450 40Ca+ ions with a mass ratio of 0.225 in a segmented linear ion trap. By two-dimensional imaging of the bi-component CC, the 9Be+ ions are found to be surrounded by the 40Ca+ ions in the radial direction with a separation ratio of ∼ 2.0, and the axial length of the 9Be+ ions occupied area is much larger than that of the 40Ca+ ions occupied area. Combined with the previous experimental results, the structure of the 9Be+–40Ca+ CC shows the larger the difference in the mass–charge ratio, the larger the separation between the two species. The comparison of the fluorescence spectra of the 9Be+ ions in the bi-component CC and the pure CC indicates that the 9Be+ ions can be sympathetically cooled and stably localized by the laser-cooled 40Ca+ ions during the recording of the fluorescence spectrum.
In this contribution, we present results on the structural properties of single‐component as well as two‐component ion Coulomb crystals confined in a linear Paul trap. Metastable long range ordered structures as the face‐centered cubic (fcc) and body‐centered cubic (bcc) lattices are observed in single‐component ion crystals of sizes down to ∼ 1000 ions. In the case of two‐component ion crystals we observe a stable long range ordered structure in one of the trapped components, which is compared with a Molecular Dynamics (MD) simulation of a two‐component crystal.
Coulomb crystals, containing up to a few hundred ions of which more than 50% were cooled sympathetically by the Coulomb interaction with laser cooled Mg+ ions, have been produced in a linear Paul trap. By controlling the balance of the radiation pressure from the two cooling lasers, the Coulomb crystals could be segregated according to ion species. Previous studies of ion crystals and molecular dynamics simulations suggest that the temperature may be around 10 mK or lower. The obtained results indicate that a wide range of atomic and molecular ions, which due to their internal structures are not amenable to direct laser cooling, can be effectively cooled and localized (crystallized) in linear Paul traps. For high resolution spectroscopy of such ions this may turn out to be very useful.
We present studies of mixed Coulomb crystals of 40Ca+ and 40Ca2+ ions in a linear Paul trap. Doubly charged ions are produced by photoionisation of trapped 40Ca+ with a vacuum ultraviolet laser source and sympathet-ically cooled via Doppler cooled 40Ca+ ions. We in-vestigate experimentally and theoretically the struc-tural configurations and the vibrational modes of these mixed crystals. Our results with 40Ca2+ are an impor-tant step towards experimental realization of the pro-posals for mode shaping in a linear crystal and spin-dependent configuration changes from zigzag to linear as proposed by Li et al., Phys. Rev. A 87, (2013) 052304 and Phys. Rev. Lett 108, (2012) 023003 using ions excited to Rydberg states. 1