GROWTH AND DOPING OF MBE PREPARED CUPRATE FILMS

The superconductivity of cuprates is highly anisotropic owing to their lamellar structure, the coherence length being typically of the order of 15A within the layer plane (ab direction) and of 2 to 5A in the perpendicular direction (c direction). The c axis coherence length is sufficient to couple coherently the CuO2 planes within a single (MCuO2)n superconducting block, while the coupling between two neighbouring superconducting blocks depends strongly on the doping level and on the nature of the reservoir block between them. Among the different families (Table I), the critical temperature increases with the number n of CuO2 planes up to n=3 and decreases slightly for n larger than 3. In 1993 the discovery of the Hg family 1,2 allowed to synthesize bulk compounds under high pressure3 with n values as high as 6. In these compounds Tc varies from 94 K for n=1 to 135K when n=3, and decreases to 106K for n=6 under atmospheric pressure.