Instability of Rotationally-Tuned Dipolar Bose-Einstein Condensates.

The possibility of effectively inverting the sign of the dipole-dipole interaction, by fast rotation of the dipole polarization, is examined within a harmonically-trapped dipolar Bose-Einstein condensate. Our analysis is based on the stationary states in the Thomas-Fermi limit, in the co-rotating frame, as well as direct numerical simulations explicitly accounting for the rotating polarization. The condensate is found to be inherently unstable due to the dynamical instability of collective modes. This ultimately prevents the realization of robust and long-lived rotationally-tuned states. Our findings have major implications for experimentally accessing this regime.