THERMAL FLUCTUATIONS IN D-WAVE LAYERED SUPERCONDUCTORS

We study the thermal fluctuations of anisotropic order parameters (OF) in layered superconductors. In particular, for copper oxides and a d- wave OP, we present some experimental consequences of fluctuations in the direction normal to the layers. It is shown that the c-axis penetr ation depth lambda(c) can have a ''disorder-like'' quadratic temperatu re dependence at low temperature. The fluctuations are analyzed in the framework of a Lawrence-Doniach model with an isotropic Fermi surface . Anisotropies pin the orientation of the OP to the crystallographic a xes of the lattice. Then we study an extended t-J model that fits Ferm i surface data of bilayers YBCO and BSCCO. This leads to a d-wave OP w ith two possible orientations and, including the thermal fluctuations, yields the announced temperature dependence of lambda(c). Furthermore a reservoir layer is introduced. It implies a finite density of state s at the Fermi energy which is successfully compared to conductance an d specific heat measurements.