Effects of hole concentration on binding energy of holes in the two π–d chains

The two chains composed of d electrons with localized spin 1/2 and itinerant π electrons have been studied to research the possibility of organic superconductors with spin gaps. The binding energy, E B , of two holes as a function of hole concentration, x, has been computed at strong coupling by using a mean-field approximation and periodic boundary conditions. E B with negative value as x increases from 0.0125 to 0.1125, while it increases as x increases from 0.1125 to 0.25. The charge and spin densities distribute more orderly as x increases from 0.0125 to 0.1125, while these distribute more disorderly as x increases from 0.1125 to 0.25. The holes on rungs form the bound state as E B < 0. Two holes bind in the spin-singlet state. It turns out that the bound states of holes are stabilised mostly at x=0.1 125.