J. Figueras, T. Puig, X. Obradors, W. K. Kwok, L. Paulius, G. W. Crabtree and G. Deutscher
Nat. Phys. 2, 402-407 (2006)
In high-temperature superconductors, magnetic field lines penetrate the samples through vortices arranged in an Abrikosov vortex lattice. In a magnetic field Hm(T ) below the upper critical field Hc2(T ) that destroys bulk superconductivity, the vortex lattice melts to a liquid vortex state, in which each vortex line must be ‘pinned’ individually to prevent dissipation. Linear and planar defects have been found to be effective for pinning the vortex liquid because they trap an entire vortex within a single extended defect. However, up to now it is not known how far into the liquid state this pinning process can be effective. Here, we show that there is a universal magnetic field line Hl(T ) betweenHm(T ) and Hc2(T ), where thermodynamic fluctuations of the order parameter can cause vortex unpinning from extended defects. This magnetic field Hl(T ) sets an upper limit to the irreversibility line Hirr(T ) marking the onset of dissipation. For that reason it determines a new magnetic-field–temperature region in which a superconductor can remain useful.