The Resonating Valence Bond State and High-Tc Superconductivity in La2CuO4+

Abstract

A novel mechanism is proposed to explain the high-Tc superconductivity in oxide materials obtained from La2CuO4 by doping it sufficiently strongly. The insulating state of pure La2CuO4, an odd electron insulator, is proposed to be the long sought “resonating valence bond” state or “quantum spin liquid”, hypothesized in 1973. This state is visualized as consisting of a linear combination of pairwise singlet states, and has strong antiferromagnetic correlations, without however, displaying Neil long ranged order. It is a variational state relevant to the Hubbard model in the limit of strong coulomb repulsions with one electron per site -or equivalently the spin 1/2 Heisenberg model -and has no energy gap for spin channel excitations. This leads to interesting Pauli-like low temperature thermodynamics despite its insulating character. The formal similarity of this state to the BCS state is pointed out. Such a state is favoured by low dimensionality, low spin (s = 1/2) and magnetic frusration. Doping the insulator sufficiently strongly creates mobile carriers and hence a metal. The preexisting singlets in the insulating state now become charged superconducting pairs. This mechanism for superconductivity should be common to several oxide superconductors and is predominantly magnetic and electronic in origin, although moderately strong electron-phonon interactions may favour this state. Extremely strong phonon interactions leading to spin-Peierls distortions however, are not favourable. Several unusual properties are predicted, especially in the insulating state.