INPLANE ANISOTROPY OF VORTEX-LATTICE MELTING IN LARGE YBA2CU3O7 SINGLE-CRYSTALS

The vortex-lattice melting transition of a large untwinned YBa2Cu3O7 c rystal(T-c=91.6 K) in the case of H parallel to c axis appears as a ju mp in de magnetization M-dc at a melting transition T-m and an abrupt transition in the ac susceptibility chi(ac) above T-m. Similarly, M-dc shows a jump at T-m in the H parallel to b and H parallel to a config urations, indicating that a first-order-melting transition occurs in a highly compressed vortex lattice. This is reinforced by a lattice sof tening in chi(ac) below T-m and a sharp transition above T-m. The melt ing lines fitted by PI = H-0(1-T/T-c)(1.5) give the relations H-0(para llel to a)/H-0(parallel to c) = 7.241+/-0.09, H-0(parallel to b)/H-0(p arallel to c) = 8.93+/-0.12, and H-0(parallel to b)/H-0(parallel to a) = 1.23+/-0.01. These are in excellent agreement with the anisotropy p arameters gamma(ca) = root m(c)/m(a) = 7.44 +/- 0.25, gamma(cb) = root m(c)/m(b) = 8.79 +/- 0.21, and gamma(ab) = root m(a)/m(b) = 1.18 +/- 0.05.