Role of spin-density fluctuations in the pairing theory of superconductivity in high T_c layered cuprates

Abstract

Within the framework of a generalized spin-singlet pairing theory of superconductivity in layered metals, the role of intralayer anti-ferromagnetic spin-fluctuations in cuprates has been examined. For this purpose, a two-dimentional one-band Hubbard model is used to obtain the intralayer effective interaction due to spin-fluctuations. It is already known that a simple perturbative calculation involving the exchange of one spin-fluctuation excitation between electrons of opposite spins leads to a repulsive interaction for small energy transfers(w-o). This prevents the possibility of nodeless s-wave type of gap function observed experimentally in cuprates. However, in a more exact non-perturbative analysis based on very general considerations, we show that in highly correlated metals like cuprates, the static part of the effective interaction can become attractive in the presence of strong residual spin-fluctuations close to the magnetic ordering. Thus spin-fluctuations may be playing a leading role in suppressing the long range Coulomb repulsion. In addition, the intralayer interaction is made more attractive because of interlayer interaction between the conducting CuO₂ layers and nearby nonconducting layers. Due to significant interlayer interactions among the conducting layers, T_c is enhanced further if there is more than such CuO₂ layer per unit cell.