Thermally activated current transport in MgB₂ films

Thermally-activated flux flow (TAFF) resistivity above the irreversibility field B_i is reported for two different c-axis textured MgB₂ superconducting films. Transport measurements at different perpendicular magnetic fields 0<B<9 T and temperatures from 5 to 40 K reveal TAFF Ohmic resistivity ρ(T,B) for B>B_i described by the Arrhenius law, ρ=ρ₀ exp(-U∕T) with the quadratic field dependence of the activation energy, U(B,T)=U₀(T)[1−B∕B_c2(T)]². Our transport measurements on bulk MgB₂ ceramic samples also show the TAFF behavior, but do not show the quadratic field dependence of U(T,B). We explain our data in terms of thermally-activated drift of pre-existing quenched dislocations in the vortex lattice. Our results indicate that thermal fluctuations can be essential in determining the irreversibility field in MgB₂ though to a much lesser extent than in high-temperature superconductors.