Order–disorder transition in the flux line lattice of superconducting MgB2 single crystals with artificially introduced defects: comparison with theory

We report on the order-disorder transition line (Bod) in the flux line lattice-illustrated by the fishtail effect-of pure and carbon doped superconducting MgB2 single crystals. Neutron irradiation offers a unique opportunity for varying the defect density (nd) in a controlled way and to determine Bod not only as a function of temperature (T), but also as a function of nd. Experimental data are compared with theoretical predictions and found to be in good agreement over most of the available parameter range, both for Bod(T) and Bod(nd). Including two-band effects in MgB2 in quite a rough way leads to even better agreement.