Is the irreversibility line in conventional superconductors a melting boundary?

Full text: The magnetic phase diagram of high temperature superconductors (HTSs) is quite complicated. The irreversibility line (IL) marking the boundary between reversible and irreversible magnetic behaviour in the field-temperature (H-T) plane, has been extensively measured in HTSs. Various models have been proposed to account for this boundary including depinning, melting and vortex-glass transitions. However, there has been limited investigation of the same phenomena in conventional low temperature superconductors (LTSs) and the interpretation of results on the irreversibility line in LTSs appear to be inconsistent, with some authors arguing for a melting transition and others for a depinning transition. We have measured the magnetic behaviour of bulk samples of conventional superconductors V3Si and Nb3Sn with different pinning strengths. We have found good matching to the flux-lattice-melting formula of Houghton et al. but other data suggests that this should be interpreted as a depinning boundary where the characteristic length of the typical pinning centre is proportional to the lattice spacing, a0. Such an interpretation can help resolve the apparent inconsistencies of previous reports