Materials Theory and Simulation

Predictive calculations of cuprate magnetic properties

(a) Crystal structure of (Ca/Sr)2CuO3. (b) The minimum total energies for the ferromagnetic (FM) and antiferromagnetic (AFM) states is found with QMC for U=1-3. (c) Superexchange coupling, J, from density functional theory (DFT) (open), QMC (filled), and experiments (gold bands). QMC results fall within the range of experimental estimates.

Magnetic couplings in a realistic cuprate system have been correctly predicted for the first time with highly accurate Quantum Monte Carlo (QMC) calculations.  Effective magnetic models of superconductivity (previously reliant on experiment) can now be derived with confidence from theory, which could lead to better fundamental predictions of superconductor behavior.

QMC methods accurately describe many-body systems, but at a high computational cost.  High-performance computers, such as those at the Oak Ridge Leadership Computing Facility, allow application of QMC to problems that have remained unsolved for decades, such as magnetic phenomena associated with the remarkable high-temperature superconductivity in cuprate materials. 

We have used QMC to study magnetic properties of Ca2CuO3, an effectively one-dimensional counterpart of the famous superconducting cuprates.  The obtained magnetic superexchange coupling, J, is in very good agreement with experiment, demonstrating the predictive capability.  This development could lead to better predictions of superconductor behavior derived from fundamental laws. The dependence of J on initial approximations is reduced compared to other approaches, due to the variational character of the energy within the QMC approach.

K. Foyevtsova, J. T. Krogel, J. Kim, P. R. C. Kent, E. Dagotto, and F. A. Reboredo, “Ab initio quantum Monte Carlo calculations of spin superexchange in cuprates: the benchmarking case of Ca2CuO3,” Physical Review X (accepted 2014).

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