  In this study, the possible pairing mechanism based on attraction between
electrons from adjacent CuO2 layers is proposed. Initially, each CuO2 layer was
found to expand the Fermi sphere owing to ridged geometry. When the two layers
are close enough for tunneling, it becomes energetically advantageous to form
correlated quantum states (CQS), reducing the Fermi sphere volume. Cooper
pairs, comprising inter-tunneling electrons, occupy the CQS. The image force is
responsible for the electron-electron attraction. Energy exchange between the
paired electrons happens through photons. Pair-binding energy and the
corresponding effective mass vary in a wide range. At T>0, some heavy pairs do
not condense. Such pairs are responsible for pseudogap. Light pairs get Bose
condensed and are responsible for superconductivity. The proposed mechanism
provides possible explanation of two energy gaps and two characteristic
temperatures in cuprates. It also provides clarification on the unconventional
isotopic effect, Fermi surface pockets, anisotropy of charge transport, and
other properties of HTS cuprates. The pseudogap, calculated within the model,
is close to the experimental values for the two-layer cuprates, such as YBCO,
Bi2212, Tl2212, and Hg1212. It has been shown that the model can be extended to
multiple and single-layer cuprates.

Superconductivity