In-plane anisotropy of vortex-lattice melting in large ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ single crystals

The vortex-lattice melting transition of a large untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ crystal ${(T}_c=91.6\mathrm{}\hspace{0.5em}\mathrm{K})$ in the case of $H\Vert c$ axis appears as a jump in dc magnetization $M_{\mathrm{dc}}$ at a melting transition $T_m$ and an abrupt transition in the ac susceptibility ${\chi }_{\mathrm{ac}}$ above $T_m.$ Similarly, $M_{\mathrm{dc}}$ shows a jump at $T_m$ in the $H\Vert b$ and $H\Vert a$ configurations, indicating that a first-order-melting transition occurs in a highly compressed vortex lattice. This is reinforced by a lattice softening in ${\chi }_{\mathrm{ac}}$ below $T_m$ and a sharp transition above $T_m.$ The melting lines fitted by ${H=H}_0(1\mathrm{}-{T/T}_c{)}^{1.5}$ give the relations $H_0^{\Vert a}{/H}_0^{\Vert c}=7.24\mathrm{}\pm 0.09,$ $H_0^{\Vert b}{/H}_0^{\Vert c}=8.93\mathrm{}\pm 0.12,$ and $H_0^{\Vert b}{/H}_0^{\Vert a}=1.23\mathrm{}\pm \mathrm{0.01.}$ These are in excellent agreement with the anisotropy parameters ${\gamma }_{\mathrm{ca}}=\sqrt{m_c{/m}_a}=7.44\mathrm{}\pm 0.25,$ ${\gamma }_{\mathrm{cb}}$ $=\sqrt{m_c{/m}_b}=8.79\mathrm{}\pm 0.21,$ and ${\gamma }_{\mathrm{ab}}=\sqrt{m_a{/m}_b}=1.18\mathrm{}\pm \mathrm{0.05.}$