INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER J. Phys.: Condens. Matter 17 (2005) 2723–2732 doi:10.1088/0953-8984/17/17/021 Formation and behaviour of macrovortices during a turbulent relaxation process in high-T c superconductors M R Koblischka 1 and T H Johansen 2,3 1 FR Experimentalphysik, Universit¨ at des Saarlandes, PO Box 151150, D-66041 Saarbr¨ ucken, Germany 2 Department of Physics, University of Oslo, Blindern, N-0316 Oslo, Norway 3 Texas Center for Superconductivity and Advanced Materials, University of Houston, Houston, TX 77204-5002, USA Received 14 December 2004, in final form 31 March 2005 Published 15 April 2005 Online at stacks.iop.org/JPhysCM/17/2723 Abstract The time dependence of flux patterns obtained on an untwinned YBa 2 Cu 3 O 7−δ single crystal showing the ‘meandering instability’ is observed at T = 65 K using magneto-optical imaging. When applying a reversed external field to a remanent state, along the front of invading antiflux, macrovortices or droplets of flux are formed, and eventually separate from the flux front in a spiral-like motion. The time-dependent behaviour of these macrovortices is investigated in detail. (Some figures in this article are in colour only in the electronic version) 1. Introduction The critical state of a type-II superconductor is normally considered to be stable,or quasi-stable, once it has been formed. However, recent experiments [1–4] have shown that flux turbulence or macroturbulence can occur in a narrow temperature window provided the sample is in a magnetic state containing domains of vortices and antivortices. Applying a reversed field to a previously generated remanent state with trapped flux causes the formation of a domain boundary, or vortex annihilation zone, between the pinned flux and the newly entering antiflux. In the case of YBa 2 Cu 3 O 7−δ (YBCO) single crystals above a temperature of approximately 47 K, the annihilation zone shows unstable behaviour as the invading flux causes it to form irregular meandering patterns. This behaviour persists until an upper limit in temperature (≈80 K) is reached; above this limit, the irregular patterns vanish and the flux front again becomes regular. In several magneto-optic experiments, such flux turbulence was demonstrated in various samples, including twinned YBCO single crystals [3], underdoped YBCO single crystals [5] and NdBa 2 Cu 3 O 7−δ (NdBCO) single crystals [5, 6]. Untwinned YBCO crystals were found to display an even more drastic effect, as along the front of penetrating antivortices, 0953-8984/05/172723+10$30.00 © 2005 IOP Publishing Ltd Printed in the UK 2723