EVIDENCE FOR VORTEX LATTICE MELTING AND SOFTENING IN UNTWINNED YBA2CU3O7 SINGLE-CRYSTAL

Real part of the ac susceptibility chi(1)' of an untwinned YBa2Cu3O7 s ingle crystal jumps at the onset temperature in the narrow width (0.1 K in 0.7 T), suggesting the first-order melting transition. A sharp ch i(1)' dip and a sharp chi(1) '' peak can be interpreted as synchronous trapping of vortices due to lattice softening prior to melting. At lo wer temperatures, the chi(1)'-chi(1) '' chart shows a single-parameter behavior while the melting occurs as a strong deviation from this.