Flux lattice melting and dimensional crossover in Bi-2212 single crystals

Flux lattice melting in Bi₂Sr₂CaCu₂O_8+x (Bi-2212) single crystals has recently been studied by various techniques. In this contribution to the 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 lattice structure along a line in the B - T phase diagram. Experiments with Hall-sensor arrays proved that the flux line lattice experiences a first 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 flow through narrow channels reveals a sudden drop of the shear modulus. Subsequent studies showed that the location of the FOT line in the phase 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 reveal a transition from a lattice of flux lines to a lattice of pancake vortices. We compare our results with theoretical scenarios.