Coexisting ordered and disordered vortex phases in Bi 2 Sr 2 CaCu 2 O 8þd B. Kalisky * , Y. Wolfus, A. Shaulov, Y. Yeshurun Institute of Superconductivity, Bar-Ilan University, Ramat-Gan 52900, Israel Abstract Ahightemporalresolutionmagneto-opticalsystemwasemployedtoimagetheinductiondistributiononthesurface ofaBi 2 Sr 2 CaCu 2 O 8þd crystal while the external magnetic field was ramped up at a constant rate. These data reveal coexistence of quasi-ordered and disordered vortex phases, near the order–disorder phase transition line. The coexis- tence region in the B–T phase diagram narrows down with increasing temperature or decreasing sweep rate. These observationsclarifypreviousinterpretationsofphenomenaassociatedwiththefishtail,e.g.theshiftofthefishtailonset to higher inductions for slower sweep rates and the absence of a fishtail at low temperatures. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Vortex states; Vortex phase transitions; Bi 2 Sr 2 CaCu 2 O 8þd ; Magneto-optics Studies of the magnetic phase diagram of Bi 2 Sr 2 - CaCu 2 O 8þd (BSCCO)atlowtemperatureshaverevealed the existence of two distinct vortex solid phases: A quasi-ordered lattice at low fields, and a highly disor- dered solid at high fields. Dynamic coexistence of these two phases, near the phase transition line, was observed in time-resolved magneto-optical measurements [1–4] andwasinferredfromglobalmagneticmeasurementsat different sweep rates of the external field H ext [5]. In the present work we exploit high temporal resolution mag- neto-opticalsystem[1]todefinethecoexistenceregionin the B–T phase diagram at different sweep rates. Measurements were performed on a 1:55  1:25  0:05 mm 3 BSCCO single crystal with T c ¼ 92 K, at dif- ferent temperatures between 18 and 26 K. Field was swept up from zero to about 850 Oe (above the order– disorder transition field B ss  450 G), at a constant rate ofeither6or47.6Oe/s.Whiletheexternalmagneticfield was swept up, the induction distribution on the surface ofthecrystalwasimagedsuccessivelyataconstantfield intervalof6Oe,usingirongarnetindicatorsandaCCD video camera with exposure time of 36 ms. Fig. 1 shows the induction profiles deduced from the magneto-optical images, taken at T ¼ 22:5 K, while the externalfieldwassweptatarateof47.6Oe/s.Theinitial Bean profiles change gradually in the interior of the sample into dome-shape profiles, characteristic of a quasi-ordered state. As H ext increases toward a value of about 400 Oe, a break appears in the profile, separating betweenahighpersistentcurrentregionnearthesample edgesandalowpersistentcurrentregionnearthecenter. This break moves toward the sample center, and the induction at the break increases. In this experiment, coexistence of order and disorder phases appear for H ext ¼ 400 Oe and disappears for H ext > 850 Oe. De- creasing sweeping rate, or increasing temperature, nar- rows down the range of external magnetic fields for which the coexisting phases are observed. The induction at the break, being time and location dependent, cannot signify the thermodynamic transition field B ss . We therefore interpret the first appearance of the break as resulting from injection of transient disor- dered state through inhomogeneous surface barriers [6]. FollowingtheapproachofPaltieletal.[7],thistransient state anneals over a characteristic Ôhealing lengthÕ * Corresponding author. E-mail address: ph50@mail.biu.ac.il (B. Kalisky). 0921-4534/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4534(02)02540-6 Physica C 388–389 (2003) 737–738 www.elsevier.com/locate/physc