Nature of the Irreversibility Line in High-Temperature Superconductors

We have performed molecular dynamic simulations of an ensemble of rigid flux lines interacting according to the London model for different pinning strengths. We have studied the equilibrium state and calculated $E\left(J\right)$ curves over a range of temperatures and fields. Analysis of the resulting data shows that the unpinned lattice undergoes a genuine melting transition at the irreversibility line, while weak pins turn the low-temperature phase into a glass. In the case of strong pins, collective effects cease to be important and the irreversibility line simply marks the onset of observable thermally activated depinning.