Spin Fluctuations Can Turn Fermions Into Bosons - Superconductivity in Strong-Coupling Paramagnets

We study the motion of electrons across atoms which have only two permissible valence states: empty or singly occupied. The model we use is the t-J model. For the geometry composed of an infinite chain of edge-sharing squares, we show that, at low concentrations of electrons, their spin and charge degrees of freedom 'separate'. Even in the absence of Heisenberg interactions the spin degrees of freedom yield the Heisenberg ground state of the linear chain. The charge degrees of freedom may be modelled by spinless fermions. When two electrons meet in a relative spin singlet, they are locally converted into spinless hard-core bosons. This possibility promotes Heisenberg correlations and simultaneously yields an attraction between the spinless fermions. This physical mechanism is clearly a candidate for an explanation to perovskite superconductivity.