Resonant pair tunnelling and singular effects of c-axis disorder in cuprate and organic superconductors

In the interlayer pair tunnelling theory of superconductivity, the large scale of T-c has its origin in the k-space locality of the pair tunnelling matrix elements. We give a complementary interpretation of the same physics as a process of resonant pair tunnelling and illustrate it through Cooper pair analysis. This interpretation is used to give a mechanism which leads to a singular suppression of T-c as a function of c-axis (off-plane or off-axis) disorder. In this mechanism the nonresonant tunnelling processes arising from the c-axis disorder in general contributes a pair binding energy which is reduced by a factor of the order of T-J/epsilon(F), where T-J is the interlayer pair tunnelling matrix element and epsilon(F) is the Fermi energy. This leads to a simple theorem which states that the scale of T-c is controlled by the space average value of the bare one-electron interlayer hopping matrix element. After briefly discussing the fact that the organic superconductors based on ET and tetramethyltetraselenafulvalene molecules are strongly correlated narrow-band systems, the dramatic reduction in T-c due to anion disorder in organic superconductors is explained by our mechanism. Off-plane disorder effects in some of the cuprates are also discussed.