The temperature dependence of the London penetration depth \lambda was measured for an untwined single crystal of YBa_2Cu_3O_{7-\delta} along the three principal crystallographic directions (a, b, and c). Both in-plane components (\lambda_a and \lambda_b) show an inflection point in their temperature dependence which is absent in the component along the c-direction (\lambda_c). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of s+d-wave symmetry whereas it is exclusively s-wave along the c-direction. In conjunction with previous results it is concluded that coupled s+d-order parameters are universal and intrinsic to cuprate superconductors.
@article{arxiv.0708.2374,
title = {Universal observation of multiple order parameters in cuprate superconductors},
author = {R. Khasanov and S. Strässle and D. Di Castro and T. Masui and S. Miyasaka and S. Tajima and A. Bussmann-Holder and H. Keller},
journal= {arXiv preprint arXiv:0708.2374},
year = {2009}
}