Structure and magnetism in bulk and uniaxially strained LaCoO_3-x through ab-initio diffusion quantum Monte Carlo

Advances in high-performance computing (HPC) is allowing the use of ambitious methods in materials science with unprecedented accuracy. Density Functional Theory (DFT) accuracy is not sufficient for challenging problems, as it depends critically on the empirical corrections applied, such as Hubbard-U and exchange mixing. For example, DFT has not yet been able to discern the microscopic origin of the ferromagnetic (FM) state in uniaxially strained LaCoO₃ thin films. In contrast, Diffusion quantum Monte Carlo (DMC) method treats electrons explicitly, solving the many-body Schrodinger equation with minimum approximations. DMC has been applied to an increasingly larger set of materials with excellent agreement. In this presentation, we report ground state energies, magnetism, defect formation energies in uniaxially strained LaCoO_3-x using DMC. DMC yields an antiferromagnetic ground state for bulk uniaxially strained LaCoO₃which agrees with the recent experiments. A transition between high-spin AFM and FM structures is also identified in the uniaxially strained structures, which may help explain the spin transition in thin films.