Low-field vortex melting in a single crystal of 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Ba</mml:mi><mml:mrow><mml:mn>0.6</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi mathvariant="normal">K</mml:mi><mml:mrow><mml:mn>0.4</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>Fe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>As</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>

Ankit Kumar, Sayantan Ghosh, Tsuyoshi Tamegai, S. S. Banerjee

doi:10.1103/physrevb.101.014502

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 ${\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. The low-field melting is accompanied with a significant change in local magnetization, \ensuremath{\sim}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 ngular 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.

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