Vortex melting and superconducting transition in high-T_c oxides

The existence of an order-disorder phase transition which can be induced in the vortex matter of type II superconductors by increasing T or H is now well established. At high temperatures the location of the so-called vortex melting line can then be shifted either downwards or upwards by introducing point or columnar defects. We show that the introduction of the latter in (K,Ba)BiO ₃ single crystals has also a strong influence on the transition from superconducting to normal state.