Strongly nonlinear magnetization above T_c in Bi ₂Sr₂CaCu₂O_8-δ

Using high-resolution magnetometry we have investigated in detail the magnetization M above the critical temperature T_c in Bi ₂Sr₂CaCu₂O₈. In a broad range of temperature T above; T_c, we find that M(T, H) is strongly non-linear in the field H. We show that as T → T_c 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 ∼ H ^1/δ in weak fields with a strongly T-dependent exponent δ(T). These features strongly support the proposal that, above T _c, the pair condensate survives to support significant London rigidity.