Evidence for anisotropic pairing in YBa2Cu3O/sub 7-//sub δ/ from the Landau theory of fluctuation specific heat

Recently, Inderees et al. have observed inverse-square-root behavior of specific heat of YBa2Cu3O/sub 7-//sub δ/ near T/sub c/ corresponding to Gaussian fluctuations. We analyze their results using the most general Landau theories of superconductivity that are consistent with either orthorhombic or tetragonal crystal symmetry. In particular we calculate the amplitude ratio C/sub +//C/sub -/, where C(T) = C/sub +- /T-T/sub c//sup -1/2/ and the coefficients C/sub +/ and C/sub -/ are associated with the behavior of C(T) above and below T/sub c/, respectively. We conclude that the value of the amplitude ratio observed experimentally is inconsistent with ordinary s-wave pairing. For an orthorhombic crystal only triplet superconductivity is possible. Assuming tetragonal symmetry we find that either triplet pairing or some d-wave singlet states are allowed. The latter must have an order parameter transforming like d/sub x//sub z/,d/sub y//sub z/. This singlet case lies within the experimental bounds only if the anisotropic gradient terms satisfy certain conditions, which we discuss in terms of microscopic models