Low-field vortex melting in a single crystal of Ba0.6K0.4Fe2As2

Theoretically, the vortex melting phenomenon occurs at both low and high magnetic fields at a fixed temperature. While the high-field melting has been extensively investigated in high-T-c cuprates, the low-field melting phenomena in the presence of disorder has not been well explored. Using bulk magnetization measurements and a high-sensitivity differential magneto-optical imaging technique, we detect a low-field vortex melting phenomenon in a single crystal of Ba0.6K0.4Fe2As2. The low-field melting is accompanied with a significant change in local magnetization, similar to 3 G, which decreases with increasing applied field. The observed vortex melting phenomenon is traced on a field-temperature phase diagram, which lies very close to theoretically predicted Lindemann criteria based low-field melting line. Our analysis shows a Lindemann number c(L) = 0.14 associated with the low-field melting. Imaging of low-field vortex melting features shows that the process nucleates via formation of extended fingerlike projections which spreads across the sample with increasing field or temperature, before entering into an interaction-dominated vortex solid phase regime. Magnetization scaling analysis and angular dependence of bulk magnetization hysteresis loop shows extended pins naturally present in the sample. These defects create a low-field glassy vortex phase sustaining a finite critical current present in the phase diagram below the low-field liquid phase. We construct a vortex matter phase diagram, which identifies boundaries demarcating a low-field glassy vortex state from a dilute vortex liquid phase and a weakly interacting solid phase above it. All these phases are shown to be present well below the interaction dominated vortex state shown in the phase diagram.