Evidence for nodes in the superconducting gap of La(2-x)Sr(x)CuO4. T-2 dependence of electronic specific heat and impurity effects

The electronic specific heat C-el of La2-xSrxCuO4 (0.16 less than or similar to x less than or similar to 0.22) exhibits T-2 dependence at T << T-c. Substitution of as little as 0.3-0.5% Zn for Cu changes the T-2 dependence of C-el into a quite different T dependence, described by the sum of T-linear and T-3 terms, which is characteristic of a superconductor with lines of nodes in the gap such as a d wave superconductor. The relation between T-c/T-c0 and gamma/gamma N for La2-xSrxCu1-yZnyO4 can be explained in terms of strong impurity scatterings in a d wave superconductor. The superconducting gap 2 Delta(0) of La2-xSrxCuO4, estimated from the T-2 term of C-el on the basis of a nodal superconductor, decreases markedly with Sr content at x greater than or similar to 0.16; this reduction of 2 Delta(0) seems to correlate with the suppression of a spin-singlet formation in the normal state. The ratio 2 Delta(0)/T-0(= 7.5) for the optimum superconductor (x = 0.16) is consistent with that reported in a recent Raman scattering experiment.