Evidence for a Two Step Vortex Matter Melting in Irradiated Bi2Sr2CaCu2O8+δ Single Crystals

Abstract Vortex pinning mechanism has been studied in single crystals shaped as thick microbridges using a laser method. Current-Voltage characteristics V(I) have been recorded in the vicinity of the critical temperature T c , and at very low magnetic field B (from 5.10 -3 to 3.10 -2 T) parallel to the columnar defects for the irradiated sample (matching field B Φ = 1.5T parallel to its c axis). Our results clearly show a two steps depinning mechanism of the vortices with two different critical current values (I c1 c2 ): I c1 for “interstitial” vortex, and I c2 for strongly localized ones. Using the temperature dependence of the critical current values I c1 , we recover the irreversibility line B IRR (T/T c ) already determined by magnetization measurements. More, our results clearly evidence that, despite I > I c1 , the vortex matter (VM) remains partially localized on columnar defects (CDs) for I c2 . And, from I c2 (T), a new line B TL (T/T C ) has been determined above which the VM is totally liquid and CD's are no more efficient. Finally, we conclude that a composite vortex structure exists for very low filling factor (i.e. B/B Φ 0.7 T c ) independently of the vortex dimensionality.