Multiple gap symmetries for the order parameter of cuprate superconductors from penetration depth measurements

The temperature dependence of the London penetration depth lambda was measured for an untwinned single crystal of YBa2Cu3O7-delta along the three principal crystallographic directions (a, b, and c). Both in-plane components (lambda(-2)(a) and lambda(-2)(b)) show an inflection point in their temperature dependence which is absent in the component along the c direction (lambda(-2)(c)). The data provide convincing evidence that the in-plane superconducting order parameter is a mixture of (s+d)-wave symmetry whereas it is mainly 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.