Electronic and magnetic state of LaMnO3 epitaxially strained on SrTiO3 : Effect of local correlation and nonlocal exchange

Abstract

Motivated by the puzzling report of the observation of a ferromagnetic insulating state in LaMnO3/SrTiO3 heterostructures, we calculate the electronic and magnetic state of LaMnO3, coherently matched to a SrTiO3 square substrate within a "strained-bulk" geometry. We employ three different density functional theory based computational approaches: (a) density functional theory (DFT) supplemented with Hubbard U (DFT+U ), (b) DFT + dynamical mean field theory (DMFT), and (c) a hybrid functional treatment of the exchange-correlation functional. While the first two approaches include local correlations and exchange at Mn sites, treated in a static and dynamic manner, respectively, the last one takes into account the effect of non-local exchange at all sites. We find in all three approaches that the compressive strain induced by the square substrate of SrTiO3 turns LaMnO3 from an antiferromagnet with sizable orbital polarization to a ferromagnet with suppressed Jahn-Teller distortion in agreement with experiment. However, while both DFT+U and DFT+DMFT provide a metallic solution, only the hybrid calculations result in an insulating solution, as observed in experiment. This insulating behavior is found to originate from an electronic charge disproportionation. Our conclusions remain valid when we investigate LaMnO3/SrTiO3 within the experimental set-up of a superlattice geometry using DFT+U and hybrid calculations. arXiv:1907.11979v1 [cond-mat.mtrl-sci]

Understanding the role of exchange and correlations in complex oxides under strain and oxide heterostructures

“…In our previous studies 14,15 we show the immense importance of exchange and correlation in case of LMO/STO heterostructures. In these studies we investigated the case of superlattice structure with comparable thicknesses of LMO and STO and we find that we can qualitatively reproduce the experimental result of a ferromagnetic insulating ground state.…”

“…This effectively proves that correlations are significantly altered by the effect of strain on oxides, which is thought to have far reaching effects on emergence of exotic properties. In our previous study 14 we have already shown the effect of epitaxial strain on LaMnO 3 which is a Jahn Teller driven correlated oxide. We have shown that compressive epitaxial strain exerted by epitaxial matching of LMO to a square substrate significantly reduces the Jahn Teller distortion in LMO which in turn favours a ferromagnetic (FM) ground state instead of the antiferromagnetic state in bulk LMO.…”

Emergence of a ferromagnetic insulating state in LaMnO$_3$/SrTiO$_3$ heterostructures: The role of strong electronic correlations and strain

“…However, symmetry lowering in the geometry of strained LMO [15] seems unlikely in superlattices because LMO is sandwiched between cubic STO. Concerning orbital polarisation, experiments show a significant suppression of the JT distortion, further supported by DFT+U studies [16,17], which in turn reduces orbital polarisation [12] in the superlattice geometry that hosts the ferromagnetic insulating behavior. A further attempt suggests electronic phase separation leading to the nucleation of metallic nanoscale ferromagnetic islands embedded in an insulating antiferromagnetic matrix.…”

“…For instance, a large value of effective U = 8 eV was required to obtain a band gap of 1.4 eV [32]. A previous study shows that calculations using hybrid functionals with HF exchange of 25% yield a value of 1.7 eV band gap [17]. We want to note that the actual value of the band gap of bulk LMO is also not clear experimentally.…”

“…The observed coexistence of ferromagnetism and insulating behavior in the LMO/STO heterostructure thus remains a mystery. Saha-Dasgupta et al studied the problem considering bulk LMO, but epitaxially strained to the lattice constants of a square substrate of STO [17]. They consider the general framework of DFT, which is expected to capture the structural changes that happen upon epitaxial straining of LMO correctly, supplemented with Hartree-Fock based hybrid functional calculations.…”