Entropy, vortex interactions and the phase diagram of heavy-ion irradiated Bi₂Sr₂CaCu₂O_8+δ

Using dynamic and thermodynamic magnetization measurements, we analyze the phase diagram of Bi₂Sr₂CaCu₂O_8+δ single crystals containing amorphous columnar defects created by heavy-ion irradiation. Reversible magnetization experiments yield the respective magnitudes of the pinning energy and entropy contributions to the free energy of the vortex lattice. It appears that the entropy contribution in the London regime is relatively minor in both unirradiated and irradiated crystals, except in the case of high density of columns and inductions B that are smaller than the interaction field H_int≈B_φ/6. The dependence of the entropy contribution on vortex and defect density correlates well with measurements of the irreversibility line (IRL) H_irr(T), which shows a sharp increase at H_int.