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Probe the vortex state of YBa2Cu3O7−δ under small magnetic field using short time magnetic moment relaxation method (abstract)

At the present time, there is a large amount of convergent experimental evidence pointing toward the existence of a vortex‐glass state in the high temperature superconductor YBa2Cu3O7−δ in the presence of a high magnetic field, as predicted by Fisher1 and Fisher et al.2 However, the situation at low magnetic field is not so clear. The nature of the vortex state separating the Meissener state at low field and the vortex‐glass state at very high field remains unsolved both theoretically and experimentally. To shed some light to this problem, we have measured the remnant magnetic moment relaxation of melt‐textured bulk YBa2Cu3O7−δ samples in various magnetic fields from ∼10−4 to ∼300 s. Even at zero field, the time dependence of the relaxation follows the vortex‐glass model prediction m(t)∝1/[ln(t)]1/μ but with a μ=1.4±0.2, contradicting the vortex‐glass model. As the field is increased, the μ value decreases until it reaches a value of ∼0.2–0.3 above 1 T. The larger than one value of μ seems to point to a c...