The effect of vortex dynamics on the phase diagram of YBa2Cu3O7-8 as determined by resistivity measurements

We present the results of detailed magneto-resistivity measurements on a number of high quality, twinned and detwinned, YBa2Cu3O7-δ single crystals. By annealing at different temperatures (in flowing O2) we have carefully varied the oxygen concentration of these samples about the optimally doped point of 7- δ = 6.93 (the critical temperature Tc has a maximum of 93.6 K at this point). In this way we have been able to produce a detailed study on the effect that oxygen has on the thermodynamic and dynamic properties in the region 6.91 ≤ 7 - δ ≤ 6.95. Our results fall into two main categories: thermodynamic properties and the effect the oxygen concentration has upon the vortex phase diagram and resistive properties and the effect that the concentration of quenched disorder has on the flux-flow resistivity.As far as the thermodynamic properties are concerned, we find, in accordance with a number of other groups, that there is a sharp drop in resistivity as a function of temperature p(T) at approximately 0.1 ρn, where ρn = ρ (100K). This kink has been associated with a first-order melting transition of the vortex lattice into the vortex liquid state. We show for the first time that this sharp drop in resistivity is also present in measurements taken as a function of angle θ between the applied field B and the crystalline c-axis. Increasing θ in a constant applied field is equivalent to decreasing the effective field Beff = B (cos2 θ+ε2 sin2θ)1/2, where ε = (mab /mc)1/2 is anisotropy factor. So there is a sharp drop in the resistivity when the effective field has decreased enough for the vortices to solidify. We show, by varying the applied field B or angle θ that if the effective field Beff is held constant the melting temperature Tm will also remain constant. Using this result we have been able to extract the anisotropy for the crystals. Above the multi-critical point (the multi-critical field Bmc in p (T) measurements and the multi-critical temperature Tmc in p(Beff) measurements) the first order melting transition is destroyed and the resistivity curves decreases smoothly to zero. We show that as the amount of disorder is increased, by decreasing the oxygen concentration, the multi-critical point is decreased in field. In addition to this we show that the multi-critical field can be scaled within the Beff vs T vortex phase diagram.