SEQUENTIAL VORTEX STATES UPON LATTICE MELTING IN UNTWINNED SINGLE-CRYSTAL YBA2CU3O7

Simulatenous measurements of the ac susceptibility and the dc magnetiz ation of an untwinned single-crystal YBa2Cu3O7 has been carried out to clarify the nature of vortex lattice melting. The first-order melting transition appears as a jump in the dc magnetization. Evidence of eff ectual pinning at temperatures above the melting transition is obtaine d as a sharp transition in chi' and a sharp peak in chi '' of width 0. 2 K in 10 kG. In addition, a prescursor lattice softening phenomenon c an be seen in chi' and chi '' at temperatures below the melting temper ature. This sequential change is more visible when the ac susceptabili ty is given the chi'-chi '' representation. It has been confirmed that there exists a melting transition of YBa2Cu3O7 even below 10 kG.