Pairing mechanism and superconducting state parameters in YBa2Cu3Oδ

The nature of pairing mechanism and the superconducting state of bi-layer optimised hole doped yttrium based cuprates have been theoretically investigated using the strong coupling theory. An interaction potential is set up which incorporates various interactions as hole-hole, hole-phonon and hole-plasmon with proper layered structure of the system. For the admissible value of interlayer phonons the deduced value of coupling strength favours the strong coupling regime and screening parameters infers the effective screening of holes as carriers. The transition temperature with the use of model coupling parameters is estimated as TcPh = 73.45 K. Incorporating the 2-D acoustic plasmons the Tc enhances to 90 K, which is in the measured range. The approach is further applied to evaluate the isotope exponent and the reduced total isotope exponent suggests a transition from phonon mediated superconductivity to dual phonon-electronic boson mechanism in high-Tc yttrium based systems. The energy gap ratio is substantially larger than BCS ratio and infers that the strong coupling effects are important. The implications of the co-operative phonon-plasmon approach are discussed. (author)