FLUX LATTICE MELTING AND DIMENSIONAL CROSSOVER IN BI-2212 SINGLE-CRYSTALS

Flux lattice melting in Bi2Sr2CaCu2O8+x (Bi-2212) single crystals has recently been studied by various techniques. In this contribution to t he I. Schegolev Memorial Volume we review some experiments and compare their results. Muon-spin rotation and small angle neutron scattering (SANS) gave the first indications for an abrupt change of the flux lat tice structure along a line in the B-T phase diagram. Experiments with Hall-sensor arrays proved that the flux line lattice experiences a fi rst order phase transition (FOT) at this line. Measured as a function of field the resistance shows an abrupt kink at the FOT line and flux how through narrow channels reveals a sudden drop of the shear modulus . Subsequent studies showed that the location of the FOT line in the p hase diagram strongly depends on oxygen stoichiometry. For decreasing temperature the FOT line terminates at a critical point and continues into an almost field independent line which represents the location of a sharp peak in the magnetization (second peak). The second-peak line coincides with a crossover region in the low temperature range of the B-T diagram where both the muon spin rotation and SANS experiments re veal a transition from a lattice of flux lines to a lattice of pancake vortices. We compare our results with theoretical scenarios.