EXTRINSIC OR INTRINSIC CONDUCTION IN CUPRATES - ANISOTROPY, WEAK, ANDSTRONG LINKS

The layered structure of superconducting cuprates is the origin of the anisotropic conductivity in the normal state and the anisotropic ener gy gap and anisotropic critical current density j(c) in the supercondu cting state. The anisotropic, quasi-two-dimensional conduction in the CuO2 planes is close to the metal-insulator transition. Defects can ei ther initiate insulating or conducting behavior depending on the posit ion of the defects and orientation of the current. For example, defect s may enhance the small conductivity sigma(perpendicular-to) perpendic ular to the planes, whereas defects reduce the strong conductivity sig ma(parallel-to) along the planes. That is, defects make the cuprate su perconductors more isotropic. Due to the layered structure defects are often organized in planes, usually of reduced conductivity, which are named ''weak links'' (WL). The effect of WL on the dc conductivity an d superconductivity, especially as function of temperature and field, allows their classification and quantification. Their grain-boundary r esistances R(bn)parallel-to greater-than-or-equal-to 10(-7)-10(-9) OME GA cm2 are several orders of magnitude larger than the metallic Sharvi n resistance, proving that WL are insulating interruptions. They conta in localized states carrying current across by resonant tunneling. Suc h WL deteriorate the metallic conductivity sigma(parallel-to), the sup erconducting critical current j(c)parallel-to, and the energy gap DELT A(s)parallel-to, but enhance the leakage current j(bl)parallel-to. Loc alized states between CuO2 double planes enhance the ''insulating'' si gma(perpendicular-to) and j(c)perpendicular-to and deteriorate DELTA(s )perpendicular-to. For Y cuprates such states are due to oxygenation i n the Cu chains. For Bi cuprates such states are caused by oxidation o f the Bi oxide layers (overdoping). The intergranular sigma(perpendicu lar-to) and j(c)perpendicular-to in grain-aligned material are differe nt for Y and Bi cuprates because Y cuprate surfaces decay above 70 K w hereas the Bi-O surface stays stable up to 400 K containing even local ized states. So sigma(perpendicular-to) and j(c)perpendicular-to are e nhanced in grain-aligned Bi cuprates, which is the base of the brick-w all model. For all cuprates the sigma(parallel-to) and j(c)parallel-to degradation by WL is similar, due to the common two-dimensional natur e.