Laser-cooled 9 Be ϩ ions confined in two-dimensionally extended lattice planes were directly observed, and the images were used to characterize the structural phases of the ions. Five different stable crystalline phases were observed, and the energetically favored structure could be sensitively tuned by changing the areal density of the confined ions. The experimental results are in good agreement with theoretical predictions for the planar (infinite in two dimensions) one-component plasma. Qualitatively similar structural phase transitions occur, or are predicted to occur, in other experimentally realizable planar systems.The one-component plasma (OCP) has been a model of condensed matter in statistical physics for over 30 years, and it is used to describe such diverse systems as dense astrophysical matter (1) and electrons on the surface of liquid helium (2). Laser-cooled trapped ions (3) are an excellent experimental realization of the OCP. The phase structures of spatially homogeneous (infinite) (4) and cylindrical (infinite in one dimension only) (5) OCPs have been explored previously. Here, images of individual ions that were confined in twodimensionally extended lattice planes are presented and used to characterize the structural phases; the observed structures agree well with the predictions of an analytic theory for the planar OCP.The OCP model consists of a single charged species embedded in a uniform, neutralizing background charge. In Paul (6) or Penning (6, 7) traps, which are used to confine charged particles, a (fictitious) neutralizing background is provided by the confining potentials. The thermodynamic properties of the infinite classical OCP are determined by its Coulomb coupling parameter, ⌫ ϵ, which is the ratio of the Coulomb potential energy of neighboring ions to the kinetic energy per ion; ⑀ 0 is the permittivity of the vacuum, e is the charge of an ion, k B is Boltzmann's constant, T is the temperature, and a WS is the Wigner-Seitz radius [defined by 4(a WS ) 3 /3 ϭ 1/n 0 , where n 0 is the ion density]. The onset of shortrange order for the infinite OCP is predicted (8) at ⌫ Ϸ 2, and a phase transition to a body-centered cubic (bcc) lattice is predicted (8, 9) at ⌫ Ϸ 170. With an OCP in a planar geometry (infinite in only two dimensions), boundary effects are predicted to cause the formation of a variety of additional structural phases, such as the hexagonal close-packed (hcp) and face-centered cubic (fcc) phases (10, 11). Qualitatively similar structural phase transitions occur, or are predicted to occur, in other planar systems with varied interparticle interactions, such as plasma dust crystals (12), colloidal suspensions (13), semiconductor electron bilayer systems (14), and hard spheres (15).The crystallization of small numbers (total number N Ͻ 50) of laser-cooled ions into Coulomb clusters (16) was first observed in Paul traps (17). With larger numbers of trapped ions, concentric shells (18) were directly observed in Penning (19) and Paul traps (5,20). Recently, Bragg diffractio...
Complexes of Fe(III) that contain a triazacyclononane (TACN) macrocycle, two pendant hydroxyl groups, and a third ancillary pendant show promise as MRI contrast agents. The ancillary group plays an important role in tuning the solution relaxivity of the Fe(III) complex and leads to large changes in MRI contrast enhancement in mice. Two new Fe(III) complexes, one with a third coordinating hydroxypropyl pendant, Fe(L2), and one with an anionic non-coordinating sulfonate group, Fe(L1)(OH2), are compared. Both complexes have a deprotonated hydroxyl group at neutral pH and electrode potentials representative of a stabilized trivalent iron center. The r1 relaxivity of the Fe(L1)(OH2) complex is double that of the saturated complex, Fe(L2), at 4.7 T, 37 °C in buffered solutions. However, variable-temperature 17O-NMR experiments show that the inner-sphere water of Fe(L1)(OH2) does not exchange rapidly with bulk water under these conditions. The pendant sulfonate group in Fe(L1)(OH2) confers high solubility to the complex in comparison to Fe(L2) or previously studied analogues with benzyl groups. Dynamic MRI studies of the two complexes showed major differences in their pharmacokinetics clearance rates compared to an analogue containing a benzyl ancillary group. Rapid blood clearance and poor binding to serum albumin identify Fe(L1)(OH2) for development as an extracellular fluid contrast agent.
We report the precise control of the rotation frequency of strongly coupled non-neutral plasmas by rotating electric fields. These plasmas of up to 10 6 9 Be 1 ions are trapped in a Penning trap and laser cooled into crystallized structures which undergo a rigid-body rotation. Bragg diffraction shows that the crystalline lattice can be stable for longer than 30 min (ϳ10 8 rotations), and that the plasma rotation can be phase locked to the applied field without any slip. These corotating plasmas are in a novel global thermal equilibrium whose asymmetric surface shape (triaxial ellipsoid) has been measured.
Rotating asymmetric electric fields have been applied to control the rotation frequency ͑and hence the density͒ of non-neutral plasmas, which are confined in Penning-type traps and have relaxed close to thermal equilibrium characterized by a global rigid-body rotation. ''Infinite'' confinement times and density compression were first reported for uncorrelated plasmas of ϳ10 8 Mg ϩ ions with temperatures ranging from 1 K to 5ϫ10 4 K (4 eV͒ ͓Huang et al., Phys. Rev. Lett. 78, 875 ͑1997͔͒. In this paper, the rotating field technique has been applied to control strongly coupled plasmas of ϳ10 5 9 Be ϩ ions which are laser-cooled to millikelvin temperatures so that the plasma freezes into a solid with a crystalline lattice. Here, Bragg diffraction peaks from crystals provide an accurate way of measuring the rotation frequency, and it is observed that the plasma rotation can be phase locked to the applied rotating field without any slip. In essence, these corotating plasmas have reached thermal equilibrium with the rotating field, and the azimuthally asymmetric boundaries of the equilibrium states have been measured experimentally. Both rotating dipole and quadrupole fields have been used to provide this precise control of the plasma rotation. However, the effectiveness of the dipole field depends on the presence of multiple ion species. With the rotating dipole field, density compression to near the Brillouin limit and increase of the rotation frequency to near the cyclotron frequency have been achieved. ͓S1070-664X͑98͒91105-3͔ *Paper gTuaI2-2 Bull. Am. Phys. Soc. 42, 1876 ͑1997͒. † Invited speaker.
We report sympathetic cooling and compression of a few thousand positrons by laser-cooled 9 Be ϩ ions in a Penning ion trap. The observed centrifugal separation of the two species implies approximate rigid rotation of the positrons and 9 Be ϩ ions, and a positron density comparable to the 9 Be ϩ ion density of տ4ϫ10 9 cm Ϫ3 . We use the sharpness of the separation to place a 5-K upper limit on the positron temperature of motion parallel to the magnetic field. The positron lifetime is greater than two weeks in our room-temperature Penning trap.
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