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Low spin correlations between copper spins in oxygen hole superconductors

The author compares three basic physical mechanisms which induce correlations between the copper spins of a single strong-coupling plane of copper oxide in the perovskite superconductors. Motions of holes by virtual Cu3+ excitations induces ferromagnetic correlations, hole motion by Cu+ excitations induces paramagnetism, and Heisenberg interactions induce antiferromagnetism. They study the smallest clusters which exhibit the phenomena and examine the competition between these effects by exact diagonalisation. The paramagnetism and antiferromagnetism are quite similar and readily coexist locally. The ferromagnetic correlations are rapidly destroyed by increasing the Heisenberg interactions which stabilise the paramagnetic state locally. The mechanism which leads to paramagnetism via hole motion by virtual Cu+ excitations is investigated by solving a one-dimensional chain topology using short-range valence bonds. The paramagnetic correlations seem the only phenomenon which is likely to be relevant in the experimental systems. The inclusion of the Heisenberg interactions may be considered a minor perturbation to the paramagnetic correlations induced by the hole motion, and so may be neglected to a first approximation for this limit.