  Two-dimensional(2D) vortex dynamics was studied in an YBa_2Cu_3O_7/
PrBa_2Cu_3O_7 superlattice by measuring the I-V characteristics. In the high
current limit, 2D collective creep was observed with an activation energy
characterized by U(j)=a j^u. A dislocation mediated vortex melting happened
when the temperature increased. In the low current limit, the exponential
growth of energy barrier for elastic motion was prohibited by the plastic
deformation of vortices. A plateau in the resistive transition was observed,
which was attributed to possible quantum tunneling of vortices. Our results
suggest that a 2D vortex glass can not exist at any temperature, including T=0
K.
