Evidence of a fractional Debye-Stokes-Einstein law in supercooled o-terphenyl

The rotational dynamics of nearly spherical and cylindrical tracers dissolved in supercooled o-terphenyl is studied via the Electron Spin Resonance spectroscopy. On decreasing the temperature, a sharp crossover is observed from the regime where the Debye-Stokes-Einstein law (DSE) holds to a new one which is accounted for by a fractional DSE (FDSE) $ D_{\rm r} \propto (\eta /T)^{- \xi} $ with $ \xi \leq 1 $. $ D_{\rm r} $ is the rotational diffusion constant, η the shear viscosity and T the temperature. Tumbling and spinning of the cylindrical tracer exhibit distinct crossovers. A simple model of FDSE based on the presence of dynamic heterogeneities is presented