AC SUSCEPTIBILITY AND VORTEX LATTICE MELTING IN (Y,LU)NI2B2C

We investigated the mixed-state ac-response of the (Y, Lu)Ni2B2C syste m. We observed multiple peaks in the imaginary part and a dip structur e in the real part of the ac susceptibility under a magnetic field exc eeding a threshold value. This anomaly was accompanied by a thermal hi story effect and a narrower transition width in the ac susceptibility as well as by a kink or dip structure in the de magnetization. These a nomalies were interpreted to be a result of a three-dimensional vortex lattice melting. We constructed a H-T phase diagram with the vortex l attice melting line H-m proportional to (1 -T/T-c)(1.30).