Polarized dipolar Bose gas with strong interactions

We present calculations of properties of a polarized dipolar Bose gas, trapped in the polarization direction by a harmonic potential while translationally invariant in the perpendicular direction. This system is of particular interest because the dipolar interaction is not only anisotropic, but also long-ranged and it already showed an interesting behaviour in the weakly interacting limit (Santos et al. in Phys. Rev. Lett. 90:250403, 2003), where a roton-maxon like excitation spectrum was found. Here we stabilize the dipolar Bose gas by a repulsive core of the form (σ/r) ¹² to avoid a collapse of the system. For our calculation we use the hypernetted-chain Euler-Lagrange method which is not limited to weakly interacting or dilute systems, but is also valid for strongly interacting systems. We find strong evidence that under certain conditions a quantum phase transition occurs to a state where pairs of dipoles bind to form dimers. Close to this phase transition a roton-maxon like excitation spectrum is observed.