Universal observation of multiple order parameters in cuprate superconductors

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.