Transition from a mixed to a pure d-wave symmetry in superconducting optimally doped YBa2Cu3O7-x thin films under applied fields.

We have probed the Landau levels of nodal quasiparticles by tunneling along a nodal direction of (110) oriented YBa2Cu3O7-x thin films with a magnetic field applied perpendicular to the CuO2 planes, and parallel to the film's surface. In optimally doped films and at low temperature, finite energy nodal states are clearly observed in films thinner than the London penetration depth. Above a well defined temperature, the order parameter reverts to a pure d-wave symmetry.