Title: Superfluidity and relaxation dynamics of a laser-stirred 2D Bose gas,
Abstract: We investigate the superfluid behavior of a two-dimensional (2D) Bose gas of
$^{87}$Rb atoms using classical field dynamics. In the experiment by R.
Desbuquois \textit{et al.}, Nat. Phys. \textbf{8}, 645 (2012), a 2D
quasicondensate in a trap is stirred by a blue-detuned laser beam along a
circular path around the trap center. Here, we study this experiment from a
theoretical perspective. The heating induced by stirring increases rapidly
above a velocity $v_c$, which we define as the critical velocity. We identify
the superfluid, the crossover, and the thermal regime by a finite, a sharply
decreasing, and a vanishing critical velocity, respectively. We demonstrate
that the onset of heating occurs due to the creation of vortex-antivortex
pairs. A direct comparison of our numerical results to the experimental ones
shows good agreement, if a systematic shift of the critical phase-space density
is included. We relate this shift to the absence of thermal equilibrium between
the condensate and the thermal wings, which were used in the experiment to
extract the temperature. We expand on this observation by studying the full
relaxation dynamics between the condensate and the thermal cloud.