ON SUPERCONDUCTIVITY IN (YBA2CU3O7-DELTA)(M) (PRBA2CU3O7-DELTA)(N) SUPERLATTICES - A QUANTITATIVE-ANALYSIS BASED ON INDIRECT-EXCHANGE PAIRING/

We undertake a theoretical analysis of critical temperatures T-C(m, n) observed in superlattices composed of m YBa2Cu3O7-delta (YBCO) and, o r alternating with, n PrBa2Cu3O7-delta (PrBCO) unit cells, as a functi on of mt and n. A survey of suggested explanations, and of different p henomenological analyses based on Kosterlitz-Thouless transitions, pro ximity effects, and charge transfer processes, shows that no conclusiv e interpretation of these phenomena exists. Instead, we adopt a micros copic approach based on indirect-exchange Cooper pairing between condu ction electrons (quasi-particles) via oxygen anions. On this basis, an d due to structural and electronic similarities between the components , an (m,n) YBCO;PrBCO supercell is treated as an ''extended'' YBCO sys tem. Assuming that only the YBCO unit cell adjacent to PrBCO is (elect ronically) directly affected upon forming an (m,n) supercell, essentia l agreement with experiment is obtained for all values of m and n, wit hout any parameter adjustment. In conformity with the results of condu ctance experiments by Cieplak ct al. (Phys. Rev. B 50 (1994) 12 876) w e establish a direct correlation between Tc(m, n) and a normal-state p roperty of the system, the Fermi vector length k(F), as a function of rn and n. The results do not support the (widespread) assumption of in terlayer (intercell) coupling in the YBCO system, neither between diff erent m-YBCO units nor intercell coupling within the same unit. (C) 19 98 Elsevier Science B.V. All rights reserved.