Investigations of the Vortex Matter in Bi2Sr2Cal Cu208+6 Single Crystals T. Blasius ', Ch. Niederrnayer ', J. Schiessling ', U. Bolz 1 J. Eisenmenger ', B.-U. Runge , P. Leiderer ', J.L. Tallon 2, D.M. Pooke 2, A. Golnik 3, C.T. Lin and C. Bernhard ' Fukultiit Jiir Physik, Uniuersitat Konstar~t, 0- 75457 Konstanz, Gemuny The New Zealnd Institute for Insdustrial Resorch, Lower Huit, Xew Zealand 3iMo3: - Planck - Institut fiir Festk&ye7.forscht~ng: D- 70569 Stuttgort, Gemtnny Abstract. We report t,ransverse-field muon spin rotation studies of the magnetic field distribution n(B) in the vortex state of the high temperature supercoucluctor Bi2Sr2CaI CU~O~+~ (Bi-2212) and present data on three sets of overdoped, nearly optimized and underdopd single crystals which to our bet knowledge provide the first evidence that a two-stage melting transition of tlie vortex matter occuls under equilibrium conditions. In addition, we compare the TF-pSR resulk with those from DC-magnetization measurements and magneboptic experiments on the same crystals. INTRODUCTION The vortex matter in the cuprate high-T, superconductors exhibits a complex (H, T)-phase diagram where thermal fl~ictuatiorls dominate the vortex slate well below the upper critical field Hc2(T) [I]. Early on; a melting transition of the flux-line lattice (FLL) was proposed [2] and confirmed experimentally [3]. How- ever, the details of this melting transition are not yet fully u.nderstood. When a magnetic field is applied perpendicular to the CuOz planes the flux-lines can be viewed as stacks of pancake vortices [4]. Both the intra-planar vortex order and the inter-planar coupling between the pancake vortices can be overcome by thermal fluctuations. In principle, these processes may occur independently and at differ- ent temperatures. Based on these considerations a two-stage meltirlg transition has been pcoI)osed [5]. Recently, exyerime11ta.l evidence for this twestage melting scenario has been obtained from non-equili brium experiments [6]. The equilibrium vortex structure, however, can be studied with the technique of transverse-field muon spin rotation (TF+S R) which has already provided valuable information on the vortex state, in particular; on the vortex matter transitio~l at the irreversibil- jt~ line (IL) and the so called 'dimensional crossover' lrom a 3D to a 2D vortex h~ctu re [7,8]. CP483, High Temperature Superconductivity, edited by S. E. Barnes et al. Q 1999 American Institute of Physics 1 -56396-880-0/99/$15.00 First publ. in: High temperature superconductivity [AIP Conference Proceedings], Vol. 483 (1999), pp. 201-206 Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2007/2847/ URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-28475