The isotope effect, E = (D_#alpha#/D_#beta#-1)/((m#beta#m_#alpha#~(1/2))-1), of cobalt diffusion in the deeply supercooled liquid state of the bulk metallic glass formers Zr_(46.7)Ti_(8.3)Cu_(7.5)Ni_(10)Be_(27.5) and Zr_(65)Cu_(17.5)Ni_(10)Al_(7.5) has been measured employing the radiotracters ~(57)Co and ~(60)Co in conjunction with serial sectioning by means of ion-beam depth profiling. In both alloys a small mass dependence with E approx= 0.10 is observed. This dependence has no temperature dependence in a range of 120 K in the supercooled liquid state encompassing four orders of magnitude in the diffusivity. We suggest that these results show that long-range diffusion in a deeply supercooled liquid is not mediated by viscous flow but proceeds by collective hopping processes involving about 10 atoms.
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