Magneto-optical investigation of 'supercooled' disordered vortex states in Bi2Sr2CaCu2O8+δ

Magnetic induction profiles across the width of a Bi2Sr2CaCu2O8þd crystal were measured while the external magnetic field was ramped down from above the vortex solid–solid phase transition field down to zero. These profiles indicate dynamic coexistence of two vortex phases: a disordered phase in the sample interior and a quasi-ordered phase near the sample edges. The border between these two phases,marked by an abrupt change in the slope of the profiles, progresses towards the sample center and appears at the same induction, Bf ,independent of the location. By increasing the sweep rate of the external field, Bf can be monotonically decreased to values well below the transition field. These results emphasize the role of the electric field in the vortex solid–solid phase transition,and question the interpretation of such ‘‘supercooling’’ experiments as signifying the first order nature of this transition. 2003 Elsevier Science B.V. All rights reserved.