Investigation of the vortex order-disorder phase transition line in Bi ₂Sr₂CaCu₂O_8+x using ac techniques

ac magnetic measurements were employed in the study of the vortex phase diagram in Bi2 Sr2 Ca Cu2 O8+x. The Bragg-glass to vortex-glass (solid-solid) and the Bragg-glass to liquid (melting) transitions are manifested in these measurements by an increase or a decrease in the screening signal, respectively. The ac measured solid-solid transition line extends to high temperatures where a melting transition is observed in dc measurements. Increasing the frequency of the ac field can further enlarge this region of the solid-solid transition. These results confirm that the solid-solid and the solid-liquid transition lines are two segments of the same order-disorder phase transition line.