Effect of disorder on the peak effect and structure of flux line lattice in 2H-NbSe2

The effect of disorder on flux line lattice (FLL) melting is studied via the peak effect phenomenon. On the upper branch of the peak effect curve, where this effect is more robust, we observe a stepwise disordering of the FLL, suggesting two first order transitions spanning the peak effect region. The lower, reentrant part of the curve is found to be strongly affected by disorder. Both the branch and the nose, i.e. the turnaround of the curve, disappear in systems with stronger disorder and the observed results are consistent with the line separating a disentangled liquid or a glass from an entangled flux liquid. (author)