In-plane anisotropy of vortex-lattice melting in large YBa 2 Cu 3 O 7 single crystals

摘要: The vortex-lattice melting transition of a large untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ crystal ${(T}_{c}=91.6 \mathrm{K})$ in the case $H\ensuremath{\Vert}c$ axis appears as jump dc magnetization ${M}_{\mathrm{dc}}$ at ${T}_{m}$ and an abrupt ac susceptibility ${\ensuremath{\chi}}_{\mathrm{ac}}$ above ${T}_{m}.$ Similarly, shows $H\ensuremath{\Vert}b$ $H\ensuremath{\Vert}a$ configurations, indicating that first-order-melting occurs highly compressed vortex lattice. This is reinforced by lattice softening below sharp lines fitted ${H=H}_{0}(1\ensuremath{-}{T/T}_{c}{)}^{1.5}$ give relations ${H}_{0}^{\ensuremath{\Vert}a}{/H}_{0}^{\ensuremath{\Vert}c}=7.24\ifmmode\pm\else\textpm\fi{}0.09,$ ${H}_{0}^{\ensuremath{\Vert}b}{/H}_{0}^{\ensuremath{\Vert}c}=8.93\ifmmode\pm\else\textpm\fi{}0.12,$ ${H}_{0}^{\ensuremath{\Vert}b}{/H}_{0}^{\ensuremath{\Vert}a}=1.23\ifmmode\pm\else\textpm\fi{}0.01.$ These are excellent agreement with anisotropy parameters ${\ensuremath{\gamma}}_{\mathrm{ca}}=\sqrt{{m}_{c}{/m}_{a}}=7.44\ifmmode\pm\else\textpm\fi{}0.25,$ ${\ensuremath{\gamma}}_{\mathrm{cb}}$ $=\sqrt{{m}_{c}{/m}_{b}}=8.79\ifmmode\pm\else\textpm\fi{}0.21,$ ${\ensuremath{\gamma}}_{\mathrm{ab}}=\sqrt{{m}_{a}{/m}_{b}}=1.18\ifmmode\pm\else\textpm\fi{}0.05.$