Thermal friction on quantum vortices in a Bose-Einstein condensate

We investigate the dissipative dynamics of a corotating vortex pair in a highly oblate axisymmetric Bose-Einstein condensate trapped in a harmonic potential. The initial vortex state is prepared by creating a doubly charged vortex at the center of the condensate and letting it dissociate into two singly charged vortices. The separation of the vortex pair gradually increases over time and its increasing rate becomes higher with increasing the sample temperature $T$. The evolution of the vortex state is well described with a dissipative point vortex model including longitudinal friction on the vortex motion. For condensates of sodium atoms having a chemical potential of $\mu\approx k_B\times 120$ nK, we find that the dimensionless friction coefficient $\alpha$ increases from 0.01 to 0.03 over the temperature range of 200 nK $<T<$ 450 nK.