Magnetisation as a probe of the pairing symmetry in Bi 2Sr 2CuO 6 + δ

Abstract

We have measured the magnetisation in the mixed-state and the specific heat of Bi 2.08Sr 1.92CuO 6+δ (Bi-2201). The wide reversible domain in the H− T plane allowed us to compare the data with existing predictions for different symmetries of the order parameter. We observed in the limit of low temperatures: (a) a high ratio γ ∗/γ s between the linear term of the specific heat γ ∗ = C/T| T → 0 and the Sommerfeld constant γ s, (b) a quadratic variation in temperature of the inverse of the London penetration depth squared ( Δλ −2 = λ −2( T) − λ −2(0) |∼ T 2), and (c) a logarithmic dependence of the second derivative of magnetisation ( ϖ 2 M/ ϖT 2 |∼ ln H). All these features are consistent with new calculations, also reported in this paper, predicting the effects of non-magnetic impurities on d-wave superconductors. The isotropic and anisotropic s-wave symmetries in the pure and impure limits and the d-wave symmetry in the pure limit seem to be ruled out.