Evidence for nodes in the superconducting gap of La2-xSrxCuO4. T2 dependence of electronic specific heat and impurity effects

The electronic specific heat Cel of La2-xSrxCuO4 (0.16 x 0.22) exhibits a T2 dependence at T Tc. Substitution of as little as 0.3-0.5% Zn for Cu changes the T2 dependence of Cel into a quite different T dependence, described by the sum of T-linear and T3 terms, which is characteristic of a superconductor with lines of nodes in the gap such as a d wave superconductor. The relation between Tc/Tc0 and γ/γN for La2-xSrxCu1-yZnyO4 can be explained in terms of strong impurity scatterings in a d wave superconductor. The superconducting gap 2Δ0 of La2-xSrxCuO4, estimated from the T2 term of Cel on the basis of a nodal superconductor, decreases markedly with Sr content at x0.16; this reduction of 2Δ0 seems to correlate with the suppression of a spin-singlet formation in the normal state. The ratio 2Δ0/T0(= 7.5 for the optimum superconductor (x = 0.16) is consistent with that reported in a recent Raman scattering experiment.