Thermodynamics of Meissner effect and flux pinning behavior in the bulk of single-crystal La2−xSrxCuO4 ( x=0.09 )

Abstract

We have studied the evolution of magnetic flux pinning behavior in the Meissner phase and the mixed state for the high-${T}_{\mathrm{c}}$ single-crystal ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ ($x=0.09$) superconductor using the polarized neutron imaging method with varying magnetic field and temperature. In the Meissner state expulsion of magnetic field (switched on during the measurements) is visualized, and the signatures of a mixed state with increasing temperature are observed. However, for flux pinning behavior in the range $5\phantom{\rule{0.28em}{0ex}}\mathrm{K}\phantom{\rule{0.28em}{0ex}}\ensuremath{\le}\phantom{\rule{4pt}{0ex}}T\phantom{\rule{4pt}{0ex}}\ensuremath{\le}\phantom{\rule{0.28em}{0ex}}15\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and ${H}_{\mathrm{ext}}=63.5\phantom{\rule{0.16em}{0ex}}\mathrm{mT}$ (switched off during the measurements), the evolution of the fringe pattern indicates magnetic flux pinning inside the bulk of the sample. At $25\phantom{\rule{0.28em}{0ex}}\mathrm{K}\phantom{\rule{0.28em}{0ex}}\ensuremath{\le}\phantom{\rule{4pt}{0ex}}T\phantom{\rule{4pt}{0ex}}\ensuremath{\le}\phantom{\rule{0.28em}{0ex}}32\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, a continuous decrease in inhomogeneously distributed pinned magnetic flux is observed, with the sample reaching a normal conducting state at ${T}_{\mathrm{c}}$ ($\ensuremath{\approx}32$ K). The flux pinning behavior is also explored as a function of ${H}_{\mathrm{ext}}$ at $T=5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. As expected, with increasing ${H}_{\mathrm{ext}}$ an increase in fringe density is observed, indicating an increase in magnetic flux pinning in the bulk of the sample. A comparison between calculated and experimentally visualized pinned magnetic fluxes shows good agreement. This implies quantification of pinned magnetic flux inside the sample, which is not possible with any other technique for bulk samples.