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J. L. Bohn, D. Blume, D. C. E. Bortolotti, and S. Ronen
We have calculated various properties of zero-temperature dipolar Bose gases in harmonic traps. For ground states, we employ the standard Gross-Pitaevskii (GP) approach, and compare it in detail to an essentially exact diffusion Monte Carlo (DMC) calculation. Using the DMC method verifies that the GP equation is adequate to describe the gas, but with a proviso: the interparticle interaction must incorporate the correct dipole-dependent scattering length. When this scattering length is accounted for, we find instabilities in the gas whenever the scattering length is negative and of sufficiently large magnitude, similar to what is seen in atomic Bose-Einstein condensates with s-wave interactions. In a second work, we have explored the stability of the gas as a function of trap aspect ratio and dipole moment, setting the scattering length to zero. We find that the condensate still becomes unstable for large enough dipole moments, even in highly oblate traps. We calculate the Bogoliubov spectrum and find that it shows a discrete roton-like feature. In certain isolated regions of the parameter space we find a novel ground state in which the condensate density profile resembles that of a blood-platelet, with a depression in its center. At high dipole moment,this platelet state becomes unstable to a collective mode with azimuthal quantum number m>0. |
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