Low-temperature vortex structures of the mixed state in underdoped Bi2Sr2CaCu2O8+delta

We report muon spin rotation studies of the magnetic held distribution n(B- mu) of the low-temperature (T = 5 K) vortex state of the high-T-c supercond uctor Bi2Sr2CaCu2O8+delta (Bi-2212) in an underdoped state (T-c = 77 K). In agreement with previous reports we observe a pronounced change in n(B-mu) with increasing field. The corresponding reorganization of the vortex struc ture with increasing vortex density is related to a disorder-induced destru ction of the vortex lattice. Furthermore, we find evidence for a partial re storation of the vortex lines at higher fields, which may be related to the increase of the vortex-vortex interaction and the subsequent change from s ingle-vortex pinning to collective pinning. (C) 2000 Elsevier Science B.V. All rights reserved.