Strongly nonlinear magnetization above T c in Bi ₂ Sr ₂ CaCu ₂ O 8 + δ

Using high-resolution magnetometry we have investigated in detail the magnetization M above the critical temperature Tc in Bi₂Sr₂CaCu₂O8+δ. In a broad range of temperature T above Tc, we find that $M(T,H)$ is strongly non-linear in the field H. We show that as T→ Tc, the susceptibility χ(T,H) diverges to very large values (χ → -1) if measured in weak H. In addition, $M(H)$ displays an anomalous non-analytic form M~ H1/δ in weak fields with a strongly T-dependent exponent δ(T). These features strongly support the proposal that, above Tc, the pair condensate survives to support significant London rigidity.