Indication of confirmation of transition and formation boundary from ordered to disordered flux vortex chain state in high-T-c superconductors Y1Ba2Cu3O7-delta and Bi2Sr2Ca2Cu3O10 and new low-field data delineating magnetic transition in Gd1Ba2(Fe0.02Cu0.98)(3)O-7-delta

Data are presented herein that support a phase boundary or quasi-phase-boundary delineating in YiBa2Cu3O7-δ and in Bi2Sr2Ca2Cu3O10 ceramic materials a transition from a vortex solid lattice to a line-flux disordered state that has been referred to as representing flux lattice melting to a flux liquid, but herein is interpreted not in terms of a liquid but in the form of a less-mobile 'polymer'-like or entangled chain species. These data are derived from electrical resistance (r) versus applied magnetic field (H) measurements at various isotherms (T) corresponding to the zero resistance state of yttrium-barium-cuprate, and the mixed state foot regime of bismuth-strontiumcalcium-cuprate. We interpret significant slope changes in r versus B at constant T in these materials to be indicative of the H-T conditions for a second-order or weakly first-order phase transition delineating the disordering of a flux lattice vortex solid. We believe that this technique is in ways more direct and at least as accurate as the conventional mechanical oscillator and vibrating magnetometer method to study the flux state. Additional very-low-field studies in Gd1Ba2(Fe0.02Cu0.98) 3O7-δ, from 1 to 1000 mT, yield indication for what appears to be a magnetic transition at ca. 77 K at 575 mT, and possibly a second transition at 912 mT (also at ca. 77 K). These data points correspond well with the extrapolated low-field experimental magnetic phase transition boundary curve described at higher field herein (and by others using the mechanical technique), and also correspond well to theoretically predicted work regarding transition involving the vortex state. © 1996 The Royal Society.