Decaying superfluid turbulence near an anomalous non-thermal fixed point

Abstract: Anomalously slow coarsening in a dilute two-dimensional (2d) superfluid -- associated with a non-thermal fixed point in the closed system's universal dynamics and driven by three-vortex collisions which initiate vortex-pair annihilations -- is found to exhibit spatial scaling characteristics of Kraichnan-Kolmogorov turbulence. During a universal interval, when the characteristic length scale related to the inter-defect distance grows as $\ell_\text{v}\sim t^{\,\beta}$, with $\beta\approx1/5$, moments of the superfluid velocity circulation around an area of extent $r$ scale as predicted by classical turbulence theory, $\Gamma^{p}(r)\sim r^{4p/3}$. Intermittency corrections seen for higher values of $p$ are found to be consistent with values measured for fully developed, classical turbulence. Hence, we link the decaying quantum turbulence cascade in a closed superfluid to universal dynamics close to a non-thermal fixed point. The decay exponent $\beta$ clearly deviates from values known for classical systems.