Abstract
Cuprates with corner-sharing CuO4 plaquettes have received much attention owing to the discoveries of high-temperature superconductivity and exotic states where spin and charge or spin and orbital degrees of freedom are separated. In these systems spins are strongly coupled antiferromagnetically via superexchange mechanisms, with high nearest-neighbour coupling varying among different compounds. The electronic properties of cuprates are also known to be highly sensitive to the presence, distance and displacement of apical oxygens perpendicular to the CuO2 planes. Here we present ab initio quantum chemistry calculations of the nearest-neighbour superexchange antiferromagnetic (AF) coupling J of two cuprates, Sr2CuO3 and La2CuO4. The former lacks apical oxygens, whilst the latter contain two apical oxygens per CuO2 unit completing a distorted octahedral environment around each Cu atom. Good agreement is obtained with experimental estimates for both systems. Analysis of the correlated wavefunctions together with extended superexchange models shows that there is an important synergetic effect of the Coulomb interaction and the O–Cu hopping, namely a correlated breathing-enhanced hopping mechanism. This is a new ingredient in superexchange models. Suppression of this mechanism leads to drastic reduction in the AF coupling, indicating that it is of primary importance in generating the strong interactions. We also find that J increases substantially as the distance between Cu and apical O is increased.
Highlights
Cuprates with corner-sharing CuO4 plaquettes have received much attention owing to the discoveries of high-temperature superconductivity and exotic states where spin and charge or spin and orbital degrees of freedom are separated
Key questions concern the reasons behind the very large AF couplings observed in cuprates, and how they can be accommodated within the accepted superexchange mechanism[1,2], as well as the role of apical oxygens
The choice of the active space will be discussed shortly, but let us note that, the solution of the CAS WF is still an exponentially scaling problem, it is manageable with novel quantum chemistry methods, namely with full configuration interaction quantum Monte Carlo (FCIQMC)[15,16] and density-matrix renormalization group (DMRG)[14,17], as long as n and m are not too large
Summary
Cuprates with corner-sharing CuO4 plaquettes have received much attention owing to the discoveries of high-temperature superconductivity and exotic states where spin and charge or spin and orbital degrees of freedom are separated. An exact wavefunction (WF)-based calculation[11] within the NN J model would involve correlating ~100 electrons among ~300 orbitals, leading to an eigenvalue problem in a Hilbert space of 10115 determinants. Since problems on such a scale are out of reach, we use the complete active space self-consistent field (CASSCF) method[12,13,14] together with multi-reference perturbation theories to systematically approximate the correlation energy.
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