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

The temperature dependence of the London penetration depth λ was measured for an untwinned single crystal of YBa2Cu3O7-δ along the three principal crystallographic directions (a, b, and c). Both in-plane components (λa-2 and λb-2) show an inflection point in their temperature dependence which is absent in the component along the c direction (λc-2). 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