Continuity and temperature dependence of the vortex-phase boundary of Bi < inf > 2 < /inf > Sr < inf > 2 < /inf > CaCu < inf > 2 < /inf > O < inf > 8+δ < /inf >

Abstract

Local magnetization data of Bi2Sr2CaCu2O8+δ crystals measured using a Hall sensor array are reported for underdoped, optimally doped, and overdoped samples. Magnetization data just inside the sample edge provide the most reliable values for the vortex-phase transition field in the magnetic phase diagram among those at other positions. The first-order melting line (ML), which manifests itself as a step in magnetization M(H), is detected at relatively high temperatures for all the samples. The position in the field of the second peak in magnetization at low temperatures can be determined unambiguously from the relaxed magnetization after a sufficiently long waiting time. It is found that in all the samples the onset field of the second peak, which is interpreted as an entanglement line (EL), connects continuously with ML as the temperature is increased. At intermediate temperatures, for the underdoped and optimally doped samples, either the second peak or the magnetization cusp, whose position connects with that of the step, is observed at the same temperature and the same field but in different experimental time scales. The ML for the three samples corresponds to a single curve when a scaling of (Tc2 - T2)/TCTλ2γ2 (TC is the superconducting transition temperature, λ is the penetration depth, and γ is the anisotropy constant) is used against the induction B, consistent with the decoupling theory. The EL is well fitted by Bα(1 + cT2)/γ2, with c > 0, suggesting a presence of dimensional crossover. The positions of the depinning line and the irreversibility line, measured by temperature sweep and field sweep, respectively, are found to depend on the time scale of the sweep rate, suggesting that they represent crossovers between two different vortex-creep mechanisms. Only the ML-EL is suggested to be the real phase boundary. © 2000 American Physical Society.