Spin-free quantum chemistry. XXVI. The Ising, small-biopolaron theory of cuprate superconductivity

The Ising, small-biopolaron (ISB) theory is a strong-coupling theory of cuprate superconductivity which is based in the negative-U, Hubbard Hamiltonian. Its ground state is composed of (small) biopolarons and (small-biopolaron) holes with a vibronically induced, biopolaron-hole exchange interaction, JBH, between them. The energy gap, Δ(0), is taken to be equal to the dissociation energy of a small biopolaran and which, since it is defined spectroscopically, is not an order parameter. The application of the Ising meanfield theory to the highly degenerate ground-state yields a second-order phase change with kTc/2=JBH and a real order parameter, Ω(T), which is valid over the entire temperature range from zero to Tc. Near Tc, the Ising free energy functional takes the same form as does the Landau. In the presence of an electromagnetic field, the Ising functional is a generalization of the Ginzburg-Landau functional which employs a complex order parameter and which is invariant under the electromagnetic gauge transformation. The breaking of the gauge invariance yields the London theory of superconductivity. 6 refs., 9 figs