Instabilities and the roton spectrum of a quasi-1D Bose-Einstein condensed gas with dipole-dipole interactions

We point out the possibility of having a roton-type excitation spectrum in a quasi-1D Bose-Einstein condensate with dipole-dipole interactions. Normally such a system is quite unstable due to the attractive portion of the dipolar interaction. However, by reversing the sign of the dipolar interaction using either a rotating magnetic field or a laser with circular polarization, a stable cigar-shaped configuration can be achieved whose spectrum contains a 'roton' minimum analogous to that found in helium II. Dipolar gases also offer the exciting prospect of tuning the depth of this 'roton' minimum by directly controlling the interparticle interaction strength. When the minimum touches the zero-energy axis the system,is once again unstable, possibly to the formation of a density wave.